@completeaerogeek, thanks as well for the sail explanation. I would just like to clarify that the sail 'luffs' when parallel to the apparent wind ie when an attempt is made to use it similarly to a solid airfoil. It is only through the turning of the air and momentum exchange that it becomes a curved surface. Also the stalled condition for a wing 'is' the parachute condition for a sail.
@completeaerogeek, thanks as well for the sail explanation. I would just like to clarify that the sail 'luffs' when parallel to the apparent wind ie when an attempt is made to use it similarly to a solid airfoil. It is only through the turning of the air and momentum exchange that it becomes a curved surface.
@completeaerogeek, thanks, that's a much more satisfying explanation than 'the wing turns the flow'. Yes the pressure before the leading edge is of particular interest, becoming a dynamic extension of the wing's leading edge. The shearing information and the collapse behind the wing are both functions of the air (as you said) leading to differences in response as the wing approaches sound speed. Hence the need for different profiles.
@g1geo1g -No worries. Yes that right. The shape of a wing can vary. All airliners use supercritical wings which are flatter on top and curved on the bottom with a reflex curve at the underside trailing edge. This increases the critical Mach number and reduces the size of the shock wave and the associated drag allowing for a higher MMO. Helicopters and aerobatic aircraft use symmetrical aerofoils which of course blows up the 'Venturi explanation and the equal transit time one as well.
Also 'lift' can be used out of context.. Any solid object produces 'lift' in an airstream. It is a component of the net force at right angles to the flow. A sailboat, for instance, produces 'lift'. It also produces 'sailboat drive' and 'heel' both 'relevant' to the boats motion. 'lift' only coincides with 'sailboat drive' when the wind is across the beam. A sail still works in full drag when the 'lift' tends to zero.
@g1geo1g 0 - In a sail boat the turning flow produces lift force that causes the boat to heel in close hauled conditions. This is opposed by the keel and rudder not unlike side slip/crabbing in an aircraft, When the wind is anywhere from 30 deg to 90 deg off the bow the wing is producing turning flow. On a broad reach the sail produces lift/turning flow with reversed airflow When running the sail acts more like a parachute thus is has modes aircraft do not. A sail cannot work if stalled-it luffs
@completeaerogeek, thanks for the input. Mmmmm. Not convinced. If the wing pushes the air up to create the low pressure then that pushes the wing down (Newton' third). Also air cannot 'turn a corner' unless acted upon by an external force (Newton's first). Are you comfortable about the truths of Newton's laws wrt non relativistic physics?
@g1geo1g - If you look at the streamlines you can see that the bow wave of the wing is affecting the streams before the air reaches the wing.then air has to divide as the wing pushes through. If you look at pressure distribution the over an aerofoil there is in fact a high pressure at the stagnation point ( and slightly above it) so you are partly correct.. As the air turns the corner is is pulled into the lower pressure area created by the forward movement of the wing= rearward downward vector
@g1geo1g -Also remember that the air is not moving the wing is. The passage of the wing displaces air and creates a void which begins to collapse as the highest thickness/chord passes through, This collapse is caused by the normal air pressure acting on the shrinking void this shrinking and air's natural viscosity which causes it to follow a curved surface (Coanda) lowers the pressure below static You can see this as the streamlines spread out slightly. NASA knows it's stuff.
Part of the key to what the video shows is that the air above is not dramatically affected- (watch the free stream reference lines) rather the pressure wave created by the lower surface has a huge influence on the pressure differential as it pushes the air forwards and down-much like a bow wave. Pushing the air out of the way creates a semi void (LP) on the upper surface (hence the need for an AOA for lift to be created) the air turns the corner and follows the upper surface = downward vector..
Lift is caused by turning a fluid.(NASA) The only difference between a glider/sailplane and a powered aircraft is that gravity is used to provide the forward motion via a descent angle. A hang glider and a sail boat also use the exact same principle of creating lift by turning flow and they are single surface aerofoils. Aerofoils can also be flat such a missiles or even paper planes and work perfectly well (but stall earlier) or supercritical (flatter on top and curved on the bottom. Fun Huh?
@jetplaneflyer, a glider is towed to it's 'unhook' point by a vehicle with an engine. This imparts the glider with a combination of potential and kinetic energy, from which the pilot can trade, one for the other within the flight envelope in a 'controlled descent'. If meteorologically produced 'rising' air can be found to fly through, then altitude can be gained. A glider has a high lift/drag ratio to extent the descent time.
You will not find a single, set in stone description for how a wing works. There are many profiles. The flow has to follow the profile to minimise drag. After that the wing is for the plane. It's tuned to provide the best efficiency at the cruising thrust and altitude to provide level flight and be controllable and stable at this and all lower levels.
An airfoil with a steep leading edge DEFINITELY projects engine (or gravitational) power through direct or glancing impingement into a pressure increase in front of the wing ceasing on the peak of the width, whereby the environment (ie the wing) releases this pressure in favour of the downward component. The net is a turning of the airflow downwards.
As for the Bernoulli Principle, there is a wonderful, and relevant, expression that goes, "Come on, this isn't rocket science!" but in this case, it is! Take a look at the instrument that's always shown, where a pressure stable vessel vents through a pipe, turning pressure into flow, then realise that it produces thrust. It is DIRECTED MOMENTUM.
As an addition, a planes engines are the force that ultimately generate lift. The wings then become thrust vectors. The wing propagates 'directed' energy from the thrust into the air which interferes with enough air to produce the required moment about the centre of mass to direct the thrust. The thrust then controls the attitude. Flaps give more control where needed but with massive drag as a consequence.
The physics of the airfoil requires no more than Newtonian mechanics and a particulate view of the air. The wing is designed to exchange energy and momentum with the air and it's profile is designed to 'select' velocities that produce maximum differences in momentum exchange between it's upper and lower surfaces whilst minimising drag.
I have a degree in aeronautical engineering. Its Bernoulli's Principle that creates on average 60% of the lift. The air DOESN'T have to be diverted downward to create lift. Re-flexed trailing edges can actually deflect the air upwards and STILL create lift. Actually because there's low pressure on top of the wing, air far upstream is actually pulled upwards anyways so the whole deflecting air theory isn't very strong in the aero communit
@necedm it still looks more logical and possble then bernoulli's principle. also , i read many places that the popular explanation based on bernoullis principle of equel transit times, is incorrect and the lift is created not by the shape of the wing but by angle of attack and speed, which both effect on the deflected air downwards.
@davidds0 The equal transient times theory IS incorrect. Thats not what supports Bernoulli's equations. Your right though that angle of attack and speed have a lot to do with it but not in the sense that most people would think. When you analyze the pressure around a airfoil the strongest forces (or lack thereof) occur on the top side (usually near the nose). Some people confuse deflecting air with "vorticity" a theory that DOES hold and is actually how we design airfoils from scratch.
@necedm Watch a wind tunnel video with smoke steams (e.g. Cambridge University) and tell me that equal transit time is true. The segmented airflow streams show that the air below the wing is significantly forced to slow and move down while the air above continues on its merry way. They never come close to meeting again.See NASA Glenn research Centre pages on this.
@necedm Sorry, me bad!.. That's what I get for speed reading messages! Looks like we agree 100% then, equal transit is not, and never was, a theory that had any support from anyone who knew what they were talking about!
@jetplaneflyer I can see how people, prior to experimenting, would believe it to be true. But nothing beats building, testing, and recording. When you do that you realize that Bernoulli's equation (essentially Pressure = F(velocity)) matches the pressure field around a wing pretty well for adiabatic, inviscid, and incompressible flow. Divide by the area of the wing, and you have the lift.
If equal transit time is correct then how come this video clearly shows that the transit time is far from equal?
No one with any grounding in fluid dynamics EVER subscribed to the equal transit theory, it's just a simple, easy to grasp (but wrong) explanation for 'Boys own encyclopaedia' type publications.
how can i generate same type of smoke for my wind tunnel. can anyone help please, i want to generating many think filaments of smoke for flow visualization.. i tried a fog machine but its hard to make into many thin lines... can anyone help please ?
Wings do not have to be "designed" for inverted flight. A wing with a cambered surface on the top (which this not) will fly inverted just fine. It is a function of angle attack and the area of low pressure would then be over the flat surface. This is a symmetrical airfoil though.
@SubitC The pitch of the wing is altered to provide the proper angle of attack in order to generate the right amount of lift. The point on the wing where the air splits will not be the same point. It's less efficient and creates drag, so additional engine thrust may be necessary, as well as wings that are designed for inverted flight (e.g., built from low-drag materials).
@SubitC the angle of attack creates a more curved airflow whilst inverted resulting in venturi effect and generates lift... Now critical AOA is decreased rendering the wing more susceptible of stalling if AOA is changed abruptly or at slow speeds... So for example the aerobatic planes have special symmetric or near symmetric airfoil, To increase critical AOA and fly more extreme without stalling to early. Hope it's clear...
Just in case: critical AOA: is the angle of attack where the wing Stalls
@charlieechovictor More recent articles I've read, suggest wings generate most lift by diverting air downwards
i.e Newtons law - equal and opposite forces - so volume of air diverted downwards equals force or lift upwards. The force generated by the reduced pressure above the wing due to increased airflow is apparently very small in comparison - interested tohear thers thoughts on this ?
@EFCthebest Yep I was actually presenting an exposition on aerodynamics forces, and included this in my expo but I still think that the low pressure area gives a fairly good amount of Lift, especially in level flight... I've spoken to several aerospace engineers, and they all agreed that this downwards force did significantly come into account in the capwings capability to create Lift, but planes and helicopters still fly with Bernoulli's law. :-)
@HipHopUploadz it's cause the bottom is straight while the top is curved like a dome, so although the horizontal distance is the same, the top side air has to travel across more distance cause there's no straight path available ! so it HAS to speed up ! :D
@pashaahsj Thanks for your help =) still don't understand why exactly it HAS to speed up and why the air has to meet again at the end of the wing, but I'll just accept it :P thanks for the vid !
@HipHopUploadz well ther's no rule etched in stone that it HAS to meet up... it's just that if the top air doesn't speed up... then there will be a vaccum on the top side where the wing ends.... so this vaccum is what actually pulls the air in faster to fill it up :)
that's why if the air is really thin, the strength of the vaccum is not that high, so it doesn't pull the air on top quickly enough for the aircraft to get any significant lift... so that's why when they are at 40k feet,SPEEDmatters
sorry people but it is not this simple, bernoullis equation has been proved not to be the cause, it is caused by the magnus effect, caused by initial starting vortex at the trailing edge, this causes an oposite circulation around the aerofoil known as the bound vortex, due to newtons third law. this is why a pressure difference is created thus causing lift, it can be proven by the use of rotating cylinders in fluids, it also creates lift. research and dont belive what every youtube video tells u
Why isn't it the deflection of the wind moving underneath the wing? I don't understand this. All my life I've thought it was simple, and now I'm clueless.
the air moving under the wing reaches the trailing edge, this causes a "starting vortex" for a micro second. once this occurs it starts a clockwise circulation of air around the wing ( in principle) this causes an oposing force to the air moving under the wing and therefore its velocity decreases so static pressure increases, causing static pressure bellow the wing to be lower than that above the wing, hence we hav lift, its a rather complex system, if you turn an airofoil upside down it still
@jaytonbye Air flowing on top of the above airfoil has "flow attachment," so it curves downwards while lifting airfoil upwards. If the upper flow should detach from the surface and flow straight back, the wing goes into stall mode. Less lift during stall.
In the above vid, the air going below the airfoil does contribute lots of lift.
this was done in an actual wind tunnel. The title says "smoke streamlines around an airfoil" - they mean real smoke, so this is a real wind tunnel rather than a computer simulation.
Hello everybody! My name is Gustav and I am making my senior year of pilotstudies. I am currrently working on a big assignement about Principles of flight (differnet theories like Newton and Bernoullis). I just wonder if there's someone out there who knows any good pages about these theories (A little bit more about Newton is what I really need) I would be very greatful for some help:) Thanks
M. this has all been lots of fun but I'm calling it quits. Number one, it's getting soarable this afternoon. Number two, too much energy for an audience of one. I'd rather put my spare time into my upcoming series of articles on some aspects of flight dynamics for the national hg and pg magazines. Watch for it! S.
Sure the Cd of PG includes a parasitic component from the pilot. So? If all forces and moments are in balance at AoA X (measured from ANY arbitrary reference section), then they will remain in the same balance, w/ the same Cd, Cl, and L/D, when the total weight is increased. OK to be fair the delta weight would need to be evenly distributed but I don't think that's what we're arguing about. PG's are less of a "special" case than you think and the flaws in your theories are fundamental.
M. if you expose yourself to some cross-fertilization by taking a serious look at some of the best, technical sources on sailplane flight, you will be forced to see the core errors in your understanding of gliding flight in general, and will be in a better position to create a correct theory of PG flight.
Sailplane pilots have been using polar curves for decades, exactly as I described.
M. a polar curve can be constructed from actual in-flight tests and DOES include parasitic drag from ALL sources and DOES reflect the real properties of the actual aircraft w/ its finite span, washout, etc. (It could also be created by putting the whole aircraft in a wind tunnel.) The general theory of polar curves and ballasting as I described applies WELL to paragliders, somewhat less well to flex-wing hang gliders due to the way their washout changes under load.
-- I'm not the least bit surprised at your response M. I'll say it again: you have constructed a do-it-yourself theory of gliding flight that is riddled with core errors. The most glaring evidence of this is in your inability to understand the significance of the polar curve and how it is modified to reflect ballasting. No knowledge of the special case of paragliding is required to see the flaws in your general ideas about gliding flight.
M...y at a high power setting an a/c could be climbing at the stall and we have to rotate the movie frame to show that the flight path is rising rather than horizontal. At a low power setting the a/c will be descending at the stall and we have to rotate the movie frame to show that the flight path is descending. No problem, the movie isn't meant to show the pitch attitude of the aircraft at stall, only the aoa at stall. They are not the same, but the movie applies just fine to gliding flight.
Quote 'at a high power setting an a/c....' (or ' a low POWER setting....)
So in other words it's Ok when looking at a POWERED or 'FOUR Force' aircraft (which was my point in the first place!) but of course a gliding PG wing is a 'THREE Force' aircraft...
M...y you are missing the point. The situation at low power (thrust<drag) is essentially the same as the situation at no power (thrust=0). In both cases the wing can be at equilibrium, "held in place" (i.e. no acceleration) by the closed vector triangle of weight, lift, and drag or (drag-thrust). If you don't understand what I'm talking about you need to study up on the basic theory of gliding flight. There is nothing special about the gliding case, be it gliding at low power or without power.
M..y, don't you know that the Wright brothers refined the performance of their airfoils using both wind tunnels and gliders and saw no discontinuity in theory and practice between the two, long before they ever got around building a powered airplane?
NOTE: It is VITAL any paraglider pilots looking at this UNDERSTAND that what is shown is a FOUR force (powered) wing in simulated flight (NB. the wing is held in a fixed position i.e. has thrust, AGAINST the airflow) this MUST NOT, for safety, be considered representive of THREE force soft aerofoil gliding flight... This application of FOUR force flight (good) information to paragliding (3 force) flight is one of the reasons the accident rates still remain so high.
Nonsense--addition or subtractn of thrust doesn't change the basic physics of flight. Even w/ powered a/c, thrust can be less than drag giving a closed vector triangle of gravity, lift, and (drag-thust), just like the GLD closed vector triangle that we have during unpowrd flght. Did you mean that outside the wind tunnel there is no guarantee that the direction of the flight path and airflow will always be horizontal? True but the movie still works; there is no horizon line included in the movie.
There are SIGNIFICANT safety issues involved in trying to base 'How PG's fly' on wind tunnel testing which of course adds 'Thrust' to the equation....
The last ten years of RE-training BHPA PG pilots has shown that ONLY sure fire way to avoid 'normal' PG accidents rates is to avoid the common INSTRUCTION errors!
The 'basic physics' of GLIDING (un-powered) flight is NOT the same as POWERED flight..
I disagree. If any powered pilot thinks his flight path will always be horizontal, he has big problems too. The physics of gliding flight are the keystone for the physics of powered flight. I don't know much about what errors PG instructors are making these days but I know the power instructors are telling some whoppers....
And PS I would be willing to bet anything, that powered parachute pilots can get into every problem that unpowered PG pilots can, if they fly the identical wings in the identical conditions. S.
In fact with a PPG (Paramotor) there are an additional set of problems (Example 'Flying at the back end of the power curve'), but at least the wind tunnel test is accurate while there IS a forth force during the powered stages of flight! NOTE: if flying in 'identical conditions' to a GLIDING PG then again my statment holds true (NO '4th force)
My comment has nothing to do with flight being (or not being) 'horizontal', it is all to do with the BASIC physics, in wind tunnel testing (or simulations where the following is true) when the wing section is held in a (left/right) fixed position against the drag of the air you have a 'Four force' type aircraft i.e. Thrust. Another common misconception is due to 'loose' use of english, i.e. refering to 'Pressure' when what is being measured is STATIC Pressure.
Any given frame in the wind tunnel mvie shows how the air flows around the wing at a given angle-of-attack. Any aoa that can be obtained in gliding flight, corresponds to a specific frame in the movie. Nothing about the movie is erroneous when applied to gliding flight. If we want to know our glide ratio at 12 degrees aoa, we can go to the wind tunnel, calculate the lift and drag coefficients from the scale reading, and from there we can calculate our glide ratio.
(Continued) The wind tunnel picture will be an accurate picture of the airflow around our gliding wing, except we have to tilt the wind tunnel picture so the airflow is no longer horizontal. Please don't tell me they are rewriting British paragliding textbooks to obscure these facts. Remember that that the brothers Wright obtained airfoil data from a wind tunnel and then went out and validated it by building gliders and measuring their glide ratios, etc.
You've a number of very basic 'application' errors here when trying to apply this to 'study' THREE force gliding flight, the fact remains that to SUSTAIN gliding i.e. to say study a 'Glide Ratio' (which MUST be a ratio taking DISTANCE [time] vert/horiz moved in the REAL world, the situation MUST match the ballance of forces in 3 force flight, you can NOT do this by ADDING a 4th force where you test the (non-gliding) wing at UN-sustainable (gliding) AoA.. i.e. a real wing won't maintain airspeed!
M...y your fundamental mistake is that you say wind tunnel testing "of course adds thrust into the equation". It does not. Any frame in the movie is a realistic snapshot of how the air flows around the airfoil at a given angle of attack. To apply the snapshot to any real-world steady-state situation where thrust does not exactly equal drag--i.e. where we are climbing or gliding with respect to the airmass--we have to rotate the movie frame so the airflow is no longer horizontal. No problem.
Steve it is VERY simple, if no 'thrust' is added the wing section will NOT remain in place in the tunnel, of course the force comes from the fact the wing section is held in place... A basic look at Newton's 3rd Law will help you correct your error (equal and opposite force).
How simple can we make it? In the tunnel the section stays in place because it is nailed to the wall. In steady-state flight (be it climbing, horizontal, or gliding) the section stays in place (i.e. does no accelerate) because the lift, drag, weight, and thrust (if present) form a closed vector triangle, square, or other polygon. Nothing mysterious happens when thrust happens to go to zero! Steve
The ONLY time 'thrust happens to go to zero' is when the wind tunnel is switched off.. You clearly need, as I've already highlighted, to spend a little time in study of some basic physics, i.e. Newtons 3rd Law, in a WORKING wind tunnel with a 'nailed' wing section and airflow there is always the 'Forth Force' i.e. 'Thrust'. If you still don't understand this just ask yourself what would happen if the 'nails' are removed! IF you never fly a GLIDING aicraft, lack of understanding less important.
1) An aircraft is in equilibrium whenever acceleration is zero, i.e. whenever net force is zero, i.e. whenever lift, weight, drag, and thrust form a closed vector polygon. If thrust is zero and lift, weight, and drag form a closed vector triangle, the aircraft is in equilibrium. This is the case whenever we glide at a constant airspeed. The competing force vectrs hold the airfoil "still" (no acceleratn)-the net force is zero just as it is in the wind tunnl.
Go back to basics: in the wind tunnel (fan running/wing fixed remain in the flow) you ALWAYS have added 'thrust' into the equation so you do NOT match the physics of gliding flight. In addition as I have already pointed out the win can be HELD at a PITCH that is NOT sustainable in real flight (also often a AoA that can not be maintained!)
M..y here is the root of your problem. A wind tunnel is not meant to model the pitch attitude of a wing in actual flight. A wind tunnel is also not meant to tell us what AoA's can be sustained by the aircraft's pitch control system. A wind tunnel only tells us the flow, Cl, and Cd for a given AoA. A wind tunnel does NOT model ALL the forces present in free flight. A wind tunnel no more models ALL the forces present in free flight in an airplane, than in a glider.
Rather than saying "Wind tunnel applies only to 4 force aircraft", you should be saying "wind tunnel is valid for any airfoil on any aircraft but wind tunnel is a tool to explore Cl and Cd and is not a simulation of the aircraft's pitch attitude in actual flight."
Which should be obvious--the wing section is held in the tunnel by a mechanical pivot, so the pitch attitude is not determined by the interplay of AoA and flight path as it is in actual flight!
In actual flight the pitch attitude is the sum of the AoA plus the glide or climb angle. The glide/climb angle is determined by the vector sum of weight, lift, drag, and thrust. Of all these forces, only weight acts in a fixed direction and thus constrains the flight path to a single possible trajectory. In the wind tunnel weight is irrelevant and so obviously we are not modeling an actual flight path or pitch attitde. Your problem with the wind tunnel has nothing to do with 3 vs 4 force arcrft.
Example: AoA +5 degrees, glide angle in still air -10 degrees, pitch attitude ends up at -5 degrees. If the airmass happens to be rising or falling or translating sideways this won't affect these angles. I should've stated that I was speaking of the still-air glide or climb angle.
Quote: 'I should've stated that I was speaking of the still-air xxxx or CLIMB angle.' Yep, once again it is perfectly clear you don't understand the basic physics involved in 1) Gliding Aircraft 2) Paragliders... in 'STILL-AIR' a glider does NOT climb!
NOTE: (once again!) due to the wing shape a PG NEVER has a single AoA! In addition as a significant part (parasitic only) of the total drag is 'outside' the aircraft your addition (the values given) is of course incorrect!
M. you are too eager to find fault. We both know that a glider can have a positive climb angle with respect to the airmass as it trades kinetic enery for altitude. And we both know that only a powered aircraft can have a positive climb angle with respect to the airmass in sustained equilibrium flight. Naturally, my comment was inclusive of the powered case. It IS accurate to say that for any aircraft, the pitch attitude equals the AoA plus the glide or climb < with respect to the airmass.
Yes, it can climb in a DYNAMIC (short term) situation... this is NOT under study in the wind tunnel, yet again you are going off at a tangent and ignoring your core errors.
REF: 'It is accurate... ANY aircraft'..
INCORRECT this is NOT true in the case of PG flight where a significant proportion of the drag (PARASITIC only) is situated well 'outside' the aircraft!
Further NOTE: a PG NEVER has a single AoA (nor can it be said to have a single value to its PITCH)
"INCORRECT this is NOT true in the case of PG flight where a significant proportion of the drag (PARASITIC only) is situated well 'outside' the aircraft!"
What are you smoking? Why not include the pilot and lines as part of the aircraft? And why is any of this germain to the idea that pitch attitude = AoA+ glide < ? Pick one particular wing section (rib) to measure if from if it bothers you that the AoA varies across the wing and likewise for pitch.
PG flight your statment is wrong! Pitch will NOT = AoA + glide.
Paraglding: Remember with no 'bodywork' the only reasonable refrence to work from is the PG wing i.e. 'The Aircraft', as such the pilot IS clearly 'outside' the aircraft,
Note: While the same is true of (most) HG's (NB PG's are Class 3 HG's) having the HG pilot (slightly) 'outside' the aircraft is less of an issue as it's helped by 1) higher airspeeds 2) more aerodynamic pilot position 3) less 'draggy' pilot/wing connection.
M . you say "due to the wing shape a PG NEVER has a single AoA!" So? This is true of nearly all aircraft but that shouldn't give us such distractions in figuring out the basic physics. Start with the basics then add the details. We can measure the Cl and Cd of a model in a wind tunnel at various AoA's using some reference line like the mean chord line, without assuming that the whole wing is at the same AoA. These Cl and Cd values allow us to predict the still-air glide angle in free flight.
Quote: 'shouldn't give us such distractions in figuring out the basic physics', I certainly causes me no problems, nor any of the students. YOU can measure a (single) wing section for the said values at various AoA's that are produced in POWERED flight... Try to remember we are intrested in NON-powered (gliding) flight of a paraglider.
The vaules generated in POWERED simulations will NOT gaive valid results for UN-powered flight, NB. most of the AoA's are NOT sustainable in gliding flight!
From a pilots viewpoint, distinction between "3 force aircraft" and "4 force aircraft" is contrived. If we slowly retard the throttle on an airplane to fully closed, at what point does it suddenly turn into a "3 force aircraft"? The fundamental flight dynamics are the same with power as without. Now, if you have paraglider pilots who think the wind tunnel shows them the pitch attitude with respect to the horizon that they can maintain in actual flight, sure they need some re-education.
One reason a pilot might get confused about wind tunnel results: if you think of pitch attitude as being more fundamental than AoA, you might wonder why different pitch attitudes give you different airspeeds depending on power setting. For a given pitch attitude, reducing the power could increase the AoA to the stall AoA. In most aircraft the flight controls determine AoA not pitch attitude and for a given AoA the aircraft's fundamental dynamics are the same regardless of power setting.
Food for thought: is there a distinction between aircraft that trim to a given AoA and aircraft that trim to a given pitch attitude. There would be some fundamental differences in dynamics especially as the flight path becomes steeply descending at the stall, etc. When you hang a pilot below the wing on flexible lines that pull on different parts of the airfoil depending on the aircraft's pitch attitude, this is a pitch-based stability system not an AoA based stab. system at least in part.
The Basics: In PG flight changes in airspeed alter the Parasitic drag (pilot&lines) this directly effects the pitch of the wing, AND ALSO (feedback) alters the airspeed airspeed... hence the AoA.
10 years of study & teaching PG aerodynamics to RE-traning pilots has show me that even the designers of PG's have core errors in their understanding of PG flight! NOTE: As a PG in flight does NOT have a single AoA... fixating on AoA is NOT a good way to gain understanding!
I think you are off track on a lot of basic points. In a glide, drag doesn't control airspeed. Drag controls sink rate. Lift coefficient controls airspeed and that's why AoA will always be very important. If you really want to offer the PG community a more enlightened theory of flight you should collaborate with someone with more training in basic aerodynamics. Still there are some fascinating aspects to a pitch-based trim system.
Wrong, as I've pointed out before you need to sort out your basic, core errors.
Drag, in it's various forms effects the airspeed of PG's, in the case of Parasitic drag of the pilot INCREASING drag results in an INCREASE of airspeed, while increasing parasitic drag at the wing (distorting training edge mainly) results in a REDUCTION of airspeed.
Re 'sink rate', slowly increasing drag at the wing results in first a REDUCTION in sink rate then of course an INCREASE in sink rate.
M..y you said "Drag, in it's various forms effects the airspeed of PG's, in the case of Parasitic drag of the pilot INCREASING drag results in an INCREASE of airspeed, while increasing parasitic drag at the wing (distorting training edge mainly) results in a REDUCTION of airspeed." M...y those effects are 100% due to the effects of the way the drag changes the Cl either by changing the wing shape or changing the wing AoA. Fundamentally Cl is the driving factor in airspeed control, as I posted.
Looking at the big picture that started this discussion: it's a mistake to say a wind tunnel movie doesn't apply to PG flight just because it doesn't represent all the force vectors at play in actual flight. It was never meant to. I still say the interesting difference between airplanes and PG's isn't "3 force" systems vs "4 force" systems. The interesting difference is the contrast between a purely AoA-based trim system, vs a trim system that is to some extent directly based on pitch attitude.
Looking at the big picture some more: your comments suggesting wind tunnels results apply only to "3 force aircraft" and not "4 force aircraft" would have the Wright brothers rolling over in their graves. You should try to get some feedback on your ideas from other knowledgable and qualified folks, and try present your ideas about PG dynamics in ways that don't make the larger glider community scratch their heads and say "Say what?"!
Eh no! My comments STATE that you can not correctly apply these wind tunnel 'results' to PG flight as the forces are NOT the same, specificaly as a FOURTH force is added, NOTE you CAN (to a limited extent) apply it to~ PPG (power on)& Tow Launch. It is clear there is a sever lack of 'knowledgable/qualified' this is abundantly clear from the trully appalling 'normal' PG accident rate.
NOTE: I have been pointing out, for 10 years, EXACTLY how it is EASY to cut PG accident rates by up to 95%!
Congrats on making practical improvements to paragliding technique. But you need to collaborate someone who can help you be less of a loose cannon in your approach to theory. It's completely ridiculous to claim that wind tunnel results only apply to aircraft flying under thrust and not to gliding aircraft. A wind tunnel is a tool for exploring flow, Cl, and Cd at a given AoA. It does not pretend to be a model of stability and control in free unconstrained flight, either for power OR gliding.
The 'practical improvments' MAINLY stem from the fact that 95% of the THEORY I teach DIRECTLY CONTRIDICTS the sort of nonsense you are posting! The THEORY I teach results in SIGNIFICANT changes in the way my students act in controling their paragliders both when flying 'within' and ESP. when flying 'out-with' the normal PG 'Flight Envelope'.
You can NOT have a 'given AoA' in a aircraft that NEVER has a single AoA, due to the wing (3D) shape! You badly need to correct your CORE errors.
M..y I'm not trying to be overly harsh on you buy I do encourage you to try to present your ideas in a way that don't seem so incongruous with established theory of gliding flight (meaning the discoveries of Cayley and the Wrights, etc.) Any sailplane+power pilot with a solid background in basic aerodynamics and a good understanding of the static balance of forces in gliding flight will have multiple problems w/ the idea that a wind tunnel represents only the powered situation.
Also I still say the core difference between PG's and other a/c lies in the difference between a pitch attitude based trim system (PG) and an AoA based trim system (conventional a/c and sailplanes, and hang gliders.) W/ the AoA based trim system, there is no real fundamental differences between dynamics under power and dynamics in a glide. W/ the pitch attitude based trim system, the stab. and control will be substantially different under power-- consider things like the "parachutal" mode etc.
Re. 'Harsh'... Physics when poorly understood/applied and so resulting in numerous needless deaths & injuries is not 'harsh' it simply is a fact of aviation.. as I have ZERO desire for my students to suffer from the 'normal' accident rates I will NEVER teach what you appear to consider to be 'correct', I can only suggest you go back and look at your basic, core errors. Bottom line, a wind tunnel looks at FOUR force (i.e. with thrust) flight.
M. a thought experiment: We place a sailplane model in a wind tunnel. The tunnel is wide compared to the modl's span. We measure the Cl and Cd in the tunnel, at a range of AoA's. We use windspeeds that will give realistic Re's. We now take the model up in a balloon and drop it in still air. We fly it at various AoA's. We note that glide ratio just happens to be equal to the Cl / Cd for each AoA as measured in the tunnel. We discard the idea that a wind tunnel only models "4 force aircraft."
IF you are intrested in science... and want to run the test (sailplane/wind tunnel) you need a TALL tunnel... (not 'wide') you also need the sailplane to be FREE to move i.e. NOT attached to the tunnel... so no FOURTH FORCE (THRUST) applied to the aircraft...
As I've pointed out before you REALLY need to work on correcting your core errors otherwise you will keep making the same mistakes!
M. don't be ridiculous, this isn't a discussion about ground effects and you know it. Finite-span effects are very important (hence the stipulation of the wide tunnel) but that's really not the point either. If you think a model has to be free to move around in a wind tunnel to accurately measure Cl and Cd at a given AoA, you don't understand wind tunnels at all. All this stuff was all well understand by the time of the Wright brothers-- who were glider pilots and wind-tunnel users!
Ground effect, yet another pointless comment from you, remember that with PG line lengths the wing, even with the pilot on the ground, is too high for any (significant) 'ground effect' to take place!
YOU clearly can't grasp the basic issue that in THREE force gliding flight, the wing is NOT subject to 'thrust' by being fixed against the airflow as it is in wind tunnel testing, so until you finaly, (if ever!) correct your core error you will not progress in understanding of gliding flight.
M. if you don't want to make paragliding look like the intellectual backwater of aviation, start by gaining a solid understanding of the general theory of gliding flight as has been well-established by the sailplane community many decades ago and then expand to the special case of paragliding with its unique stability and control issues, rather than trying to re-invent the wheel while making multiple "core errors" (to use your own favorite phrase) along the way.
A starting point: "The Science of Flight : Pilot-oriented Aerodynamics" by W.N. Hubins (1992) is a good place to start understanding how lift and drag coefficients apply to actual flight. So is "Model Aircraft Aerodynamics" by Martin Simons (1987). Google your way to the biblio page of the superb "See How it Flies" website for more. "The Complete Soaring Handbook, revised" by Anne and Lorne Welch and Frank Irving (1968) is one good source for the basic physics of gliding flight.
PS if you want to teach only practical things with no theory that's ok too. I just met a military transport pilot who had no understanding whatsoever of a very basic stability and control issues (dihedral creates a roll torque when pilot applies rudder, due to a difference in AoA between the left and right wings) but I guess he still flew just fine, and was even involved in some level of flight test to create procedures for dealing w/ unusual attitudes.
In 'normal' PG training the lack of any significant (compared to say PPL) amounts of theory training (esp. HP&L training) is part of the reason for the appalling BHPA accident rate, however the single largest factor in the 'normal' PG accident rates IS the theory used to teach.
IF you want to slash PG accident rates 'practical' training MUST be done based on correct application of science to understanding of who PG wings work, any other approach WILL result in avoidable injuries & deaths!
Fine, so find someone you trust with a solid knowledge of the physics of flight to give you an opinion on your wind tunnel theories, they need some help.
Steve, one of us has been teaching paragliding (and you say you don't fly PG's!) for over 10 years and as I do NOT use the nonsense you keep repeating my students do NOT suffer from the accident, injury & death rates that RESULT from 'normal' PG theory/training!
The bottom line is that it is the 'side' that needs help is the side that is teaching dangerous nonsense!
Well, I don't have any horse in this race, I don't know exactly what you are teaching or what the other side is teaching, I just see some mistakes on your comments and most specifcally your comments about wind tunnels, which is what this page is supposed to be about. Just trying to help!
Steve you need to get a grip of some basic science!
The REASON for the appalling accident rates in paragliding is it IS the 'intellectual backwater of aviation', as it is in fact the incompetence of normal training 'terory' (and so practice) that keeps injuring and killing pilots...
Simply REMOVE those errors, as I did over 10 years ago, and you get a 95% DROP in accident rates!
ADDING a 'forth force' means results are NOT the same as UN-powered, THREE force gliding flight.
M. your philosphy that a wind tunnel models a powered situation only, is dead wrong. You don't seem to understand how wind tunnels are used to measure lift and drag coefficients which can be then be taken away from the wind tunnel context and used to determine the complete balance of force vectors in free flight. Only at this point--and not in the tunnel--do the thrust and weight vectors enter the picture.
Steve the bottom line is you still fail to grasp some VERY basic physics, ANY results generated in a wind tunnel REQUIRE:
1) the fans to be running (air forced to move)
2) the wing to be held AGAINST the flow of air.
So the results produced will NOT match that of a THREE force, NON-powered gliding flight.
Another KEY point directly relevant to paragliders is that a PG in flight NEVER EVER has a single Angle of Attack (AoA across the span) even when flying straight!
M. if you want to explore the differences in stability and control in paragliders versus powered paragliders, that's fine and interesting, but you come across a bit silly with this generalized theory of "4 force aircraft versus 3 force aircraft". As a sailplane pilot and power pilot and hang glide pilot and enthusiastic student of the physics of flight, I can tell you that there are no truly fundamental differences in the stability and control of most aircraft in the power on vs off situations.
1) You DON'T fly PARAGLIDERS! (so you have some limited excuse for your errors)
2) You don't understand the core diffrence between UN-powered and POWERED flight (3 & 4 force flight)
3) By DESIGN (most, but not all) powered aircraft will naturaly PITCH down on loss of power due to the vertical offset (corrected by trimers) of the thrust (force 4) and drag lines.
The error you make is not in stability/control but rather in basic flight physics forces.
M. this mantra of "3 forces vs 4 forces" is just plain silly and I'm calling you on it. Tell me in a few words what is fundamentally different about sailplane flight versus powered airplane flight? The response to control inputs is fundamentally the same. To the extent that your theories hold water they apply only to paragliders with their peculiar stability and control attributes. You present them as if they are much more general. Do you think we need to re-educate the sailplane community too?
M. I'll grant that it would be uniquely challenging to test a complete model of a paraglider (including pilot) in a wind tunnel. One problem is that the "dangle angle" (d.a.) of the pilot and suspension lines influences the wing shape, and in free flight the d.a. is strongly influenced by the direction of the weight vector, which in free flight is determined by the glide < with respect to the airmass, which is determined by the wing shape! So we have a recursive problem on our hands.
(Cont.) The solution would appear to be to change the fore/aft tilt of the pilot in a way that is keyed to the Cl and Cd values that we are measuring on the scale, so that the tilt of the pilot's body is keyed to whatever would be the true direction of vertical in free gliding flight. That may be on oversimplification but it touches at the issues involved with the flexible wing/pilot system.
(Continued 2) I can also see that these complications go away when we happen to be modelling the case of precisely horizontal (i.e. powered) flight with respect to the airmass.
Once we begin climbing under power, or descending even slightly, the problems all come back.
So I can see that you are on to something.
I still say all the interesting aspects of the situation arise from the uniquely flexible nature of the aircraft and are NOT generalizable to sailplanes or hang gliders.
MY POINT EXACTLY... Your errors are (fairly) unimportant in teaching most branches of aviation but those errors KILL in paragliding! NB. The issue is NOT the 'flexiable' (soft wing) nature of the aircraft as such but the PHYSICS of PG flight... i.e. if you could fill a PG with [semi-rigid] expanding foam the PHYSICS issues will still remain.
OK thank you for educating me. You fill the wing with foam and nothing changes. The "dangle angle" of the pilot doesn't matter because the suspension lines converge to a single point (or 2 single points). So... I've come full circle. There's no reason whatsoever that you can't model this system in a wind tunnel. Include the pilot and the lines. No problem. The pilot is fixed in place. The wing is free to move. For a given setting of the lines thing will find the right position above the pilot.
(cont) So there's no reason after all that you can't model the PG in a wind tunnel. Include the pilot and the lines. No problem. The pilot is fixed in place. The wing is free to move. For a given setting of the lines the wing will find the right position above the pilot. Measure the Cl and Cd with scales in the usual way. There's your glide ratio in free flight, for that particular setting of the lines. In free flight the pitch attitude of everything will be different but that doesn't matter.
You still don't understand the core science, UNBERSTANDING the 'angle of dangle' in PG flight is VITAL to PG safety (this shows most in PG launch incidents/accidents)
'Pilot fixed in place'... Fine if you are only intrested in FOUR FORCE flight NOT in three force PG gliding flight.
Quote: 'so that the tilt of the pilot's body is keyed to whatever would be the true direction of vertical in free gliding flight.'... (PG) this is Ok PROVIDED you are intrested in study of a PG entering a STALL! The very last place a PG pilot should want to be is vertically below the CoP of the wing!
Quote: 'One problem is that the "dangle angle" (d.a.) of the pilot and suspension lines influences the wing shape'... again you have a problem as you DON'T fly paragliders... as far as the wing x-section is concerned the pilot/suspension lines (excluding B-line stall etc.) do NOT effect the wing shape, rather it is the 'cut' of the wing ribs that do that!
PS. Brake fan (pulled) will of course alter the back of the x-section.
PPS. Some (not all) speed systems will alter the x-section.
Quote: 'plain silly'.... is trying to say that four force POWERED experiments is that same as THREE force UN-powered PG flight!
NOTE: while normal sailplane flight theory (for simplicity) contains a number of core errors these errors, due to the design with drag being 'inside' the aircraft rather than like a PG significant amounts of [parasitic] drag being 'outside' the aircraft... are NOT injuring & killing lots of pilots.... it matters little if the errors (for sailplanes) go uncorrected.
Well for a start the concept of 'Low pressure above the wing/high pressure below the wing'... this is of course some what simplistic and while it IS handy in teaching pilots in general is 'bad science' (WRONG!)
Note in (steady state) gliding flight the UNDERSIDE of the wing mainly generates 'lift' that is DOWNWARDS (and slighly 'back')
This BASIC physics is VERY important when it comes to teaching 'soft' wing (PG) flight IF you want a 95% REDUCTION in accident rates..
Let's talk about the true state of the science not what you might read in some handbook someone wrote. My specific assertion is that for sailplanes and hang gliders the Cl and Cd across a given spectrum of AoA's can be acdurately measured with a model or full-scale aircraft in a wind tunnel and this can be used to accurately predict the glide angle in free flight.
Your "4 force vs 3 force" theory is way overgeneralized, it applies only to paragliders.
the 'true state of science'... is you KEEP making the same old core errors... NOTE: (as pointed out before (most) HG's will also have the pilot positioned 'outside' (below) the aircraft (wing) so is subject to similar issues to PG flight / wind tunnels.
M there is a core error in your approach, you don't want to face the fact that there is no problem with measuring the Cl and Cd of a "normal" (non-paragliding) aircraft in a wind tunnel and then predicting the still-air power-off glide angle from this data, at any desired AoA.
The fact that the hang glider pilot is outside the wing is no problem, he still just part of the total parasitic drag of the aircraft and we can still derive a polar curve just as for a more conventional sailplane.
Where did all this start anyway? The wind tunnel movie shows flow around the wing at various AoA's. Obviously if a paraglider wing happens to be the same shape as a the tunnel model we'll get the same flow for the same AoA. That's not rocket science. Sure I agree that actual paragliders are floppy and difficult to work with in wind tunnels because there is really no easy way to hold them at a given desired AoA. Ok?
Anyway it's interesting to think about the pitch stab and control system of a paraglider which has something in common with any a/c where the CG is well below the wing. Take the wing's aerodynamic center as our pivot point. The airfoil generates a nose-down pitch moment. Pilot's drag adds to this. Thrust (if present) applied at pilot applies nose-up moment. In a glide, the component of the pilot's weight vector that acts parallel to the flight path gives a nose-up pitch moment.(to be continued)
(PG stab and cntrl cont.) Conversely, in a climb the pilot's weight vector contributes a nose-down pitch moment, especially at steep climb angles. That summarizes all the key moments at play. The angle of the flight path plays a key role in the dynamics. In a glide, if we change the wing in a way that steepens the flight path, one consequence of this is that the component of the pilot's weight vector that acts parallel to the flight path gets bigger, which contributes a nose-up pitch moment.
(PG stab and cntrl cont. II) So the glide or climb angle (w/ respect to the airmass) plays a role in determining the angle-of-attack. That makes for an interesting feedback effect: the glider has some tendency to retain a constant pitch attitude and does not freely vary it's pitch attitude to retain a constant AoA in the manner that a sailplane would if we opened the spoilers, for example. Clearly the stab and control system of the PG is partly based on pitch attitude and not purely AoA.
NOTE: In a STEADY state of lift (or sink) i.e. no acceleration, a GLIDING (THREE force) aircraft will fly (ignoring TOTAL pressure/density changes due to altitude etc.) in a similar manner to when in 'still air'.
As I have been pointing out to you for a L O N G time to UNDERSTAND PG flight you need to account for ALL the forces in play.
Quote: 'stab & control systems...' (PG's) has almost NOTHING to do with AoA... if you like AoA is the 'tail' NOT the 'dog'... (Tail wagging the dog)
M. I'm about to give up here. First of all, I don't see how you can get anywhere with your analysis of PG's till you get over your blind spot involving wind tunnels and gliding flight in general. Sure I agree that you can't adequately control the AoA of a flexible PG wing in a wind tunnel but that pertains only to PG's.
Also I understand that PG stability is based on pitch attitude as well as AoA, but to say that AoA plays only a minor role in PG flight dynamics is quite an exaggeration!
FIRST I teach paragliding, hence I comment on PARAGLIDING and PG flight!
Wind tunnel (FOUR force) testing has issues (varying amounts) when looking at all THREE force (gliding) situations, and of course also DYNAMIC situations i.e. most of gliding flight!
Remember as PG wings NEVER EVER have a single AoA 'Fixating' on AoA is one of the BIGGEST errors pilots make! PG stability is mainly related to a combination of pilots MASS and pilots (parasitic) drag... other factors of course exist!
Even in a 'simple' (straight) flight the 'pivot point' is highly unlikely to be 'in' the aircraft, rather generaly it is some (small) distance below the wing, NOTE: it can also easily be ABOVE the wing...
Quote: 'In a glide, the component of the pilot's weight vector that acts parallel to the flight path'... what a load of nonsense, PLEASE learn some basic physics!
"'In a glide, the component of the pilot's weight vector that acts parallel to the flight path'... what a load of nonsense..."
If you prefer you can divide the pilot weight vector into a component that acts through a line drawn to the wing's MAC and a component that acts perpendicular to this line. The results will be nearly the same; only the latter component will contribute a pitch torque about the MAC. At the end of the day it's still a pitch-attitude-based control system, in part.
It started because incompetent pilots/instructors use the sort of nonsense you keep spouting to 'teach' / fly paragliders and as a result the spot has a LOT of injuries & deaths.
Re: Tunnel/airflow, to be realistic in modeling PG flight would REQUIRE as a MINIMUM many 100's of thousands of wing sections... for EVERY single (steady state) configuration of PG flight.
In practice I doubt computer/W'tunnel modeling will ever be more than a VERY rough guide regarding PG wing sections/airflows.
No, with HG and sailplanes you can just put the whole thing in the tunnel, or a model thereof. Then you measure the Cl and Cd for a given AoA and this gives you the L/D which is the still-air glide ratio for that AoA. Now you are done with the tunnel. Now determine the weight and you can calculate the actual L and D vectors, and the airspeed.
No problems about the "4th force".
Specifically which part of the above don't you follow?
In the wind tunnel you can generate data on how the whole aircraft (including the pilot 'outside' the HG/PG) flys in FOUR force POWERED flight.
In the end you are making the same old core errors, and PLEASE try to stick to the topic which is using a POWERED wind tunnel to 'model' GLIDING i.e. UN-powered PG wings!
"In the wind tunnel you can generate data on how the whole aircraft...flys in FOUR force POWERED flight."--simply not true. In a wind tunnel there is no effort to achieve ANY balance of forces. You just measure L and D on the scales and then compute Cl and Cd. Lift is NOT equal to weight. Looking at the wind tunnel results you have no idea what climb angle will be achieved in actual flight until you know the actual weight and thrust, neither of which plays any role at all in the wind tunnl test.
@completeaerogeek, thanks as well for the sail explanation. I would just like to clarify that the sail 'luffs' when parallel to the apparent wind ie when an attempt is made to use it similarly to a solid airfoil. It is only through the turning of the air and momentum exchange that it becomes a curved surface. Also the stalled condition for a wing 'is' the parachute condition for a sail.
g1geo1g 2 days ago
@completeaerogeek, thanks as well for the sail explanation. I would just like to clarify that the sail 'luffs' when parallel to the apparent wind ie when an attempt is made to use it similarly to a solid airfoil. It is only through the turning of the air and momentum exchange that it becomes a curved surface.
g1geo1g 2 days ago
@completeaerogeek, thanks, that's a much more satisfying explanation than 'the wing turns the flow'. Yes the pressure before the leading edge is of particular interest, becoming a dynamic extension of the wing's leading edge. The shearing information and the collapse behind the wing are both functions of the air (as you said) leading to differences in response as the wing approaches sound speed. Hence the need for different profiles.
g1geo1g 2 days ago
@g1geo1g -No worries. Yes that right. The shape of a wing can vary. All airliners use supercritical wings which are flatter on top and curved on the bottom with a reflex curve at the underside trailing edge. This increases the critical Mach number and reduces the size of the shock wave and the associated drag allowing for a higher MMO. Helicopters and aerobatic aircraft use symmetrical aerofoils which of course blows up the 'Venturi explanation and the equal transit time one as well.
completeaerogeek 16 hours ago
Also 'lift' can be used out of context.. Any solid object produces 'lift' in an airstream. It is a component of the net force at right angles to the flow. A sailboat, for instance, produces 'lift'. It also produces 'sailboat drive' and 'heel' both 'relevant' to the boats motion. 'lift' only coincides with 'sailboat drive' when the wind is across the beam. A sail still works in full drag when the 'lift' tends to zero.
g1geo1g 2 days ago
@g1geo1g 0 - In a sail boat the turning flow produces lift force that causes the boat to heel in close hauled conditions. This is opposed by the keel and rudder not unlike side slip/crabbing in an aircraft, When the wind is anywhere from 30 deg to 90 deg off the bow the wing is producing turning flow. On a broad reach the sail produces lift/turning flow with reversed airflow When running the sail acts more like a parachute thus is has modes aircraft do not. A sail cannot work if stalled-it luffs
completeaerogeek 2 days ago
@completeaerogeek, thanks for the input. Mmmmm. Not convinced. If the wing pushes the air up to create the low pressure then that pushes the wing down (Newton' third). Also air cannot 'turn a corner' unless acted upon by an external force (Newton's first). Are you comfortable about the truths of Newton's laws wrt non relativistic physics?
g1geo1g 2 days ago
@g1geo1g - If you look at the streamlines you can see that the bow wave of the wing is affecting the streams before the air reaches the wing.then air has to divide as the wing pushes through. If you look at pressure distribution the over an aerofoil there is in fact a high pressure at the stagnation point ( and slightly above it) so you are partly correct.. As the air turns the corner is is pulled into the lower pressure area created by the forward movement of the wing= rearward downward vector
completeaerogeek 2 days ago
@g1geo1g -Also remember that the air is not moving the wing is. The passage of the wing displaces air and creates a void which begins to collapse as the highest thickness/chord passes through, This collapse is caused by the normal air pressure acting on the shrinking void this shrinking and air's natural viscosity which causes it to follow a curved surface (Coanda) lowers the pressure below static You can see this as the streamlines spread out slightly. NASA knows it's stuff.
completeaerogeek 2 days ago
Part of the key to what the video shows is that the air above is not dramatically affected- (watch the free stream reference lines) rather the pressure wave created by the lower surface has a huge influence on the pressure differential as it pushes the air forwards and down-much like a bow wave. Pushing the air out of the way creates a semi void (LP) on the upper surface (hence the need for an AOA for lift to be created) the air turns the corner and follows the upper surface = downward vector..
completeaerogeek 3 days ago
Lift is caused by turning a fluid.(NASA) The only difference between a glider/sailplane and a powered aircraft is that gravity is used to provide the forward motion via a descent angle. A hang glider and a sail boat also use the exact same principle of creating lift by turning flow and they are single surface aerofoils. Aerofoils can also be flat such a missiles or even paper planes and work perfectly well (but stall earlier) or supercritical (flatter on top and curved on the bottom. Fun Huh?
completeaerogeek 3 days ago
@jetplaneflyer, a glider is towed to it's 'unhook' point by a vehicle with an engine. This imparts the glider with a combination of potential and kinetic energy, from which the pilot can trade, one for the other within the flight envelope in a 'controlled descent'. If meteorologically produced 'rising' air can be found to fly through, then altitude can be gained. A glider has a high lift/drag ratio to extent the descent time.
g1geo1g 3 days ago
You will not find a single, set in stone description for how a wing works. There are many profiles. The flow has to follow the profile to minimise drag. After that the wing is for the plane. It's tuned to provide the best efficiency at the cruising thrust and altitude to provide level flight and be controllable and stable at this and all lower levels.
g1geo1g 1 month ago
@g1geo1g - Hi there - Google - NASA incorrect lift theories. It's pretty definitive.
completeaerogeek 3 days ago
An airfoil with a steep leading edge DEFINITELY projects engine (or gravitational) power through direct or glancing impingement into a pressure increase in front of the wing ceasing on the peak of the width, whereby the environment (ie the wing) releases this pressure in favour of the downward component. The net is a turning of the airflow downwards.
g1geo1g 1 month ago
As for the Bernoulli Principle, there is a wonderful, and relevant, expression that goes, "Come on, this isn't rocket science!" but in this case, it is! Take a look at the instrument that's always shown, where a pressure stable vessel vents through a pipe, turning pressure into flow, then realise that it produces thrust. It is DIRECTED MOMENTUM.
g1geo1g 1 month ago
As an addition, a planes engines are the force that ultimately generate lift. The wings then become thrust vectors. The wing propagates 'directed' energy from the thrust into the air which interferes with enough air to produce the required moment about the centre of mass to direct the thrust. The thrust then controls the attitude. Flaps give more control where needed but with massive drag as a consequence.
g1geo1g 1 month ago
@g1geo1g
So how do gliders fly when they have no engines?
jetplaneflyer 3 days ago
The physics of the airfoil requires no more than Newtonian mechanics and a particulate view of the air. The wing is designed to exchange energy and momentum with the air and it's profile is designed to 'select' velocities that produce maximum differences in momentum exchange between it's upper and lower surfaces whilst minimising drag.
g1geo1g 1 month ago
W25G!!!!
coinslotcb 3 months ago
I have a degree in aeronautical engineering. Its Bernoulli's Principle that creates on average 60% of the lift. The air DOESN'T have to be diverted downward to create lift. Re-flexed trailing edges can actually deflect the air upwards and STILL create lift. Actually because there's low pressure on top of the wing, air far upstream is actually pulled upwards anyways so the whole deflecting air theory isn't very strong in the aero communit
necedm 7 months ago
@necedm it still looks more logical and possble then bernoulli's principle. also , i read many places that the popular explanation based on bernoullis principle of equel transit times, is incorrect and the lift is created not by the shape of the wing but by angle of attack and speed, which both effect on the deflected air downwards.
davidds0 6 months ago
@davidds0 The equal transient times theory IS incorrect. Thats not what supports Bernoulli's equations. Your right though that angle of attack and speed have a lot to do with it but not in the sense that most people would think. When you analyze the pressure around a airfoil the strongest forces (or lack thereof) occur on the top side (usually near the nose). Some people confuse deflecting air with "vorticity" a theory that DOES hold and is actually how we design airfoils from scratch.
necedm 6 months ago
@necedm Watch a wind tunnel video with smoke steams (e.g. Cambridge University) and tell me that equal transit time is true. The segmented airflow streams show that the air below the wing is significantly forced to slow and move down while the air above continues on its merry way. They never come close to meeting again.See NASA Glenn research Centre pages on this.
completeaerogeek 3 weeks ago
@completeaerogeek Read my comment again... i said the transient theory is INCORRECT. I never support it.
necedm 3 days ago
@necedm Sorry, me bad!.. That's what I get for speed reading messages! Looks like we agree 100% then, equal transit is not, and never was, a theory that had any support from anyone who knew what they were talking about!
jetplaneflyer 3 days ago
@jetplaneflyer I can see how people, prior to experimenting, would believe it to be true. But nothing beats building, testing, and recording. When you do that you realize that Bernoulli's equation (essentially Pressure = F(velocity)) matches the pressure field around a wing pretty well for adiabatic, inviscid, and incompressible flow. Divide by the area of the wing, and you have the lift.
necedm 3 days ago
@necedm
If equal transit time is correct then how come this video clearly shows that the transit time is far from equal?
No one with any grounding in fluid dynamics EVER subscribed to the equal transit theory, it's just a simple, easy to grasp (but wrong) explanation for 'Boys own encyclopaedia' type publications.
jetplaneflyer 3 days ago
@jetplaneflyer Read my comment again... i said the transient theory is INCORRECT. I never support it.
necedm 3 days ago
how can i generate same type of smoke for my wind tunnel. can anyone help please, i want to generating many think filaments of smoke for flow visualization.. i tried a fog machine but its hard to make into many thin lines... can anyone help please ?
Hafaistchen87 11 months ago
Wings do not have to be "designed" for inverted flight. A wing with a cambered surface on the top (which this not) will fly inverted just fine. It is a function of angle attack and the area of low pressure would then be over the flat surface. This is a symmetrical airfoil though.
doug9driver 1 year ago
argh
sequentialproduction 1 year ago
Read this article for a better understanding of the cause of lift and why the popular equal transit time theory does not work :-
ww.allstar.fiu.edu/aero/airflylvl3.htm
SubitC 1 year ago
If airplanes DO rely on air flowing faster over the longer top surface of its wings, then how can they fly inverted ???
SubitC 1 year ago
@SubitC God did it!
smalltownatheist 1 year ago
@SubitC The pitch of the wing is altered to provide the proper angle of attack in order to generate the right amount of lift. The point on the wing where the air splits will not be the same point. It's less efficient and creates drag, so additional engine thrust may be necessary, as well as wings that are designed for inverted flight (e.g., built from low-drag materials).
fashnek 1 year ago
@SubitC the angle of attack creates a more curved airflow whilst inverted resulting in venturi effect and generates lift... Now critical AOA is decreased rendering the wing more susceptible of stalling if AOA is changed abruptly or at slow speeds... So for example the aerobatic planes have special symmetric or near symmetric airfoil, To increase critical AOA and fly more extreme without stalling to early. Hope it's clear...
Just in case: critical AOA: is the angle of attack where the wing Stalls
charlieechovictor 1 year ago
@charlieechovictor More recent articles I've read, suggest wings generate most lift by diverting air downwards
i.e Newtons law - equal and opposite forces - so volume of air diverted downwards equals force or lift upwards. The force generated by the reduced pressure above the wing due to increased airflow is apparently very small in comparison - interested tohear thers thoughts on this ?
EFCthebest 8 months ago
@EFCthebest Yep I was actually presenting an exposition on aerodynamics forces, and included this in my expo but I still think that the low pressure area gives a fairly good amount of Lift, especially in level flight... I've spoken to several aerospace engineers, and they all agreed that this downwards force did significantly come into account in the capwings capability to create Lift, but planes and helicopters still fly with Bernoulli's law. :-)
charlieechovictor 6 months ago
Why does the flow move faster along the upper surface? I know it does, but I don't know why.
It isn't true that the molecules have to meet at the end of the surface, because they usually don't. Could someone please give me an explanation?
HipHopUploadz 1 year ago
@HipHopUploadz it's cause the bottom is straight while the top is curved like a dome, so although the horizontal distance is the same, the top side air has to travel across more distance cause there's no straight path available ! so it HAS to speed up ! :D
tell me if it's unclear :)
pashaahsj 1 year ago
@pashaahsj Thanks for your help =) still don't understand why exactly it HAS to speed up and why the air has to meet again at the end of the wing, but I'll just accept it :P thanks for the vid !
HipHopUploadz 1 year ago
@HipHopUploadz well ther's no rule etched in stone that it HAS to meet up... it's just that if the top air doesn't speed up... then there will be a vaccum on the top side where the wing ends.... so this vaccum is what actually pulls the air in faster to fill it up :)
that's why if the air is really thin, the strength of the vaccum is not that high, so it doesn't pull the air on top quickly enough for the aircraft to get any significant lift... so that's why when they are at 40k feet,SPEEDmatters
pashaahsj 1 year ago
@HipHopUploadz here's a video that will explain exactly what i said to you :)
watch?v=1mm8raOaOIU
pashaahsj 1 year ago
@antonmayo89 the magnus effect relies on bernoulli's principle. Maybe you should do YOUR research.
clemsonk 1 year ago 2
thank you so much for this video
imdbstar 1 year ago
This has been flagged as spam show
@blueheat223 for smoke, I recall that the old plans in Scientific American Amateur Scientist use a cluster of cigarettes in a metal pipe.
The pros used to use Titanium Tetrachloride smoke (somewhat toxic), or hydrogen bubbles from a wire electrode in acid or salt fluid flow tank.
wbeaty 1 year ago
This has been flagged as spam show
@blueheat223 for smoke, I recall that the old plans in Scientific American Amateur Scientist use a cluster of cigarettes in a metal pipe.
The pros used to use Titanium Tetrachloride smoke (somewhat toxic), or hydrogen bubbles from a wire electrode in acid or salt fluid flow tank.
wbeaty 1 year ago
@blueheat223 for smoke, I recall that the old plans in Scientific American Amateur Scientist use a cluster of cigarettes in a metal pipe.
The pros used to use Titanium Tetrachloride smoke (somewhat toxic), or hydrogen bubbles from a wire electrode in acid or salt fluid flow tank.
wbeaty 1 year ago
sorry people but it is not this simple, bernoullis equation has been proved not to be the cause, it is caused by the magnus effect, caused by initial starting vortex at the trailing edge, this causes an oposite circulation around the aerofoil known as the bound vortex, due to newtons third law. this is why a pressure difference is created thus causing lift, it can be proven by the use of rotating cylinders in fluids, it also creates lift. research and dont belive what every youtube video tells u
antonmayo89 2 years ago 2
Why isn't it the deflection of the wind moving underneath the wing? I don't understand this. All my life I've thought it was simple, and now I'm clueless.
jaytonbye 1 year ago
the air moving under the wing reaches the trailing edge, this causes a "starting vortex" for a micro second. once this occurs it starts a clockwise circulation of air around the wing ( in principle) this causes an oposing force to the air moving under the wing and therefore its velocity decreases so static pressure increases, causing static pressure bellow the wing to be lower than that above the wing, hence we hav lift, its a rather complex system, if you turn an airofoil upside down it still
antonmayo89 1 year ago 2
@jaytonbye Air flowing on top of the above airfoil has "flow attachment," so it curves downwards while lifting airfoil upwards. If the upper flow should detach from the surface and flow straight back, the wing goes into stall mode. Less lift during stall.
In the above vid, the air going below the airfoil does contribute lots of lift.
wbeaty 1 year ago
Excellente démonstration !
Tilasoldo 2 years ago
Excellent video by the way
electrosnerd 2 years ago
In the slowed down pulsed section I now see how the air flowing over the top goes faster.
electrosnerd 2 years ago
Mauricio tiene mangina!
michelmuffdiver 2 years ago
hello, I am spanish studient and I would like know how is do the video,with a program or with a wind tunnel?
rcanalegre 2 years ago 2
this was done in an actual wind tunnel. The title says "smoke streamlines around an airfoil" - they mean real smoke, so this is a real wind tunnel rather than a computer simulation.
LMF5000 2 years ago
this is a very good video to guide students about aerofoil
tony5309872 3 years ago
needs some sound (besides my cousins blowdryer in the background)...but anyway go future engineers! lets do this!!!!! lol
floevera 3 years ago
future aero engn'r here
kylepatrickribo 3 years ago 2
Hello everybody! My name is Gustav and I am making my senior year of pilotstudies. I am currrently working on a big assignement about Principles of flight (differnet theories like Newton and Bernoullis). I just wonder if there's someone out there who knows any good pages about these theories (A little bit more about Newton is what I really need) I would be very greatful for some help:) Thanks
gesen07 3 years ago
Insane how you can see the rotation or the fluid creating thee lift
hsv6ltr 3 years ago
M. this has all been lots of fun but I'm calling it quits. Number one, it's getting soarable this afternoon. Number two, too much energy for an audience of one. I'd rather put my spare time into my upcoming series of articles on some aspects of flight dynamics for the national hg and pg magazines. Watch for it! S.
stevesei999 3 years ago
Sure the Cd of PG includes a parasitic component from the pilot. So? If all forces and moments are in balance at AoA X (measured from ANY arbitrary reference section), then they will remain in the same balance, w/ the same Cd, Cl, and L/D, when the total weight is increased. OK to be fair the delta weight would need to be evenly distributed but I don't think that's what we're arguing about. PG's are less of a "special" case than you think and the flaws in your theories are fundamental.
stevesei999 3 years ago
M. if you expose yourself to some cross-fertilization by taking a serious look at some of the best, technical sources on sailplane flight, you will be forced to see the core errors in your understanding of gliding flight in general, and will be in a better position to create a correct theory of PG flight.
Sailplane pilots have been using polar curves for decades, exactly as I described.
stevesei999 3 years ago
M. a polar curve can be constructed from actual in-flight tests and DOES include parasitic drag from ALL sources and DOES reflect the real properties of the actual aircraft w/ its finite span, washout, etc. (It could also be created by putting the whole aircraft in a wind tunnel.) The general theory of polar curves and ballasting as I described applies WELL to paragliders, somewhat less well to flex-wing hang gliders due to the way their washout changes under load.
stevesei999 3 years ago
"See how it flys" is riddled with core errors! "
-- I'm not the least bit surprised at your response M. I'll say it again: you have constructed a do-it-yourself theory of gliding flight that is riddled with core errors. The most glaring evidence of this is in your inability to understand the significance of the polar curve and how it is modified to reflect ballasting. No knowledge of the special case of paragliding is required to see the flaws in your general ideas about gliding flight.
stevesei999 3 years ago
M...y at a high power setting an a/c could be climbing at the stall and we have to rotate the movie frame to show that the flight path is rising rather than horizontal. At a low power setting the a/c will be descending at the stall and we have to rotate the movie frame to show that the flight path is descending. No problem, the movie isn't meant to show the pitch attitude of the aircraft at stall, only the aoa at stall. They are not the same, but the movie applies just fine to gliding flight.
stevesei999 3 years ago
Quote 'at a high power setting an a/c....' (or ' a low POWER setting....)
So in other words it's Ok when looking at a POWERED or 'FOUR Force' aircraft (which was my point in the first place!) but of course a gliding PG wing is a 'THREE Force' aircraft...
murrayhay 3 years ago
M...y you are missing the point. The situation at low power (thrust<drag) is essentially the same as the situation at no power (thrust=0). In both cases the wing can be at equilibrium, "held in place" (i.e. no acceleration) by the closed vector triangle of weight, lift, and drag or (drag-thrust). If you don't understand what I'm talking about you need to study up on the basic theory of gliding flight. There is nothing special about the gliding case, be it gliding at low power or without power.
stevesei999 3 years ago
M..y, don't you know that the Wright brothers refined the performance of their airfoils using both wind tunnels and gliders and saw no discontinuity in theory and practice between the two, long before they ever got around building a powered airplane?
stevesei999 3 years ago
NOTE: It is VITAL any paraglider pilots looking at this UNDERSTAND that what is shown is a FOUR force (powered) wing in simulated flight (NB. the wing is held in a fixed position i.e. has thrust, AGAINST the airflow) this MUST NOT, for safety, be considered representive of THREE force soft aerofoil gliding flight... This application of FOUR force flight (good) information to paragliding (3 force) flight is one of the reasons the accident rates still remain so high.
murrayhay 3 years ago
Nonsense--addition or subtractn of thrust doesn't change the basic physics of flight. Even w/ powered a/c, thrust can be less than drag giving a closed vector triangle of gravity, lift, and (drag-thust), just like the GLD closed vector triangle that we have during unpowrd flght. Did you mean that outside the wind tunnel there is no guarantee that the direction of the flight path and airflow will always be horizontal? True but the movie still works; there is no horizon line included in the movie.
stevesei999 3 years ago
No not 'nonsense', I ment EXACTLY what I said.
There are SIGNIFICANT safety issues involved in trying to base 'How PG's fly' on wind tunnel testing which of course adds 'Thrust' to the equation....
The last ten years of RE-training BHPA PG pilots has shown that ONLY sure fire way to avoid 'normal' PG accidents rates is to avoid the common INSTRUCTION errors!
The 'basic physics' of GLIDING (un-powered) flight is NOT the same as POWERED flight..
murrayhay 3 years ago
I disagree. If any powered pilot thinks his flight path will always be horizontal, he has big problems too. The physics of gliding flight are the keystone for the physics of powered flight. I don't know much about what errors PG instructors are making these days but I know the power instructors are telling some whoppers....
stevesei999 3 years ago
And PS I would be willing to bet anything, that powered parachute pilots can get into every problem that unpowered PG pilots can, if they fly the identical wings in the identical conditions. S.
stevesei999 3 years ago
In fact with a PPG (Paramotor) there are an additional set of problems (Example 'Flying at the back end of the power curve'), but at least the wind tunnel test is accurate while there IS a forth force during the powered stages of flight! NOTE: if flying in 'identical conditions' to a GLIDING PG then again my statment holds true (NO '4th force)
murrayhay 3 years ago
My comment has nothing to do with flight being (or not being) 'horizontal', it is all to do with the BASIC physics, in wind tunnel testing (or simulations where the following is true) when the wing section is held in a (left/right) fixed position against the drag of the air you have a 'Four force' type aircraft i.e. Thrust. Another common misconception is due to 'loose' use of english, i.e. refering to 'Pressure' when what is being measured is STATIC Pressure.
murrayhay 3 years ago
Any given frame in the wind tunnel mvie shows how the air flows around the wing at a given angle-of-attack. Any aoa that can be obtained in gliding flight, corresponds to a specific frame in the movie. Nothing about the movie is erroneous when applied to gliding flight. If we want to know our glide ratio at 12 degrees aoa, we can go to the wind tunnel, calculate the lift and drag coefficients from the scale reading, and from there we can calculate our glide ratio.
stevesei999 3 years ago
(Continued) The wind tunnel picture will be an accurate picture of the airflow around our gliding wing, except we have to tilt the wind tunnel picture so the airflow is no longer horizontal. Please don't tell me they are rewriting British paragliding textbooks to obscure these facts. Remember that that the brothers Wright obtained airfoil data from a wind tunnel and then went out and validated it by building gliders and measuring their glide ratios, etc.
stevesei999 3 years ago
You've a number of very basic 'application' errors here when trying to apply this to 'study' THREE force gliding flight, the fact remains that to SUSTAIN gliding i.e. to say study a 'Glide Ratio' (which MUST be a ratio taking DISTANCE [time] vert/horiz moved in the REAL world, the situation MUST match the ballance of forces in 3 force flight, you can NOT do this by ADDING a 4th force where you test the (non-gliding) wing at UN-sustainable (gliding) AoA.. i.e. a real wing won't maintain airspeed!
murrayhay 3 years ago
M...y your fundamental mistake is that you say wind tunnel testing "of course adds thrust into the equation". It does not. Any frame in the movie is a realistic snapshot of how the air flows around the airfoil at a given angle of attack. To apply the snapshot to any real-world steady-state situation where thrust does not exactly equal drag--i.e. where we are climbing or gliding with respect to the airmass--we have to rotate the movie frame so the airflow is no longer horizontal. No problem.
stevesei999 3 years ago
Steve it is VERY simple, if no 'thrust' is added the wing section will NOT remain in place in the tunnel, of course the force comes from the fact the wing section is held in place... A basic look at Newton's 3rd Law will help you correct your error (equal and opposite force).
murrayhay 3 years ago
How simple can we make it? In the tunnel the section stays in place because it is nailed to the wall. In steady-state flight (be it climbing, horizontal, or gliding) the section stays in place (i.e. does no accelerate) because the lift, drag, weight, and thrust (if present) form a closed vector triangle, square, or other polygon. Nothing mysterious happens when thrust happens to go to zero! Steve
stevesei999 3 years ago
The ONLY time 'thrust happens to go to zero' is when the wind tunnel is switched off.. You clearly need, as I've already highlighted, to spend a little time in study of some basic physics, i.e. Newtons 3rd Law, in a WORKING wind tunnel with a 'nailed' wing section and airflow there is always the 'Forth Force' i.e. 'Thrust'. If you still don't understand this just ask yourself what would happen if the 'nails' are removed! IF you never fly a GLIDING aicraft, lack of understanding less important.
murrayhay 3 years ago
M..y study up on the following:
1) An aircraft is in equilibrium whenever acceleration is zero, i.e. whenever net force is zero, i.e. whenever lift, weight, drag, and thrust form a closed vector polygon. If thrust is zero and lift, weight, and drag form a closed vector triangle, the aircraft is in equilibrium. This is the case whenever we glide at a constant airspeed. The competing force vectrs hold the airfoil "still" (no acceleratn)-the net force is zero just as it is in the wind tunnl.
stevesei999 3 years ago
Go back to basics: in the wind tunnel (fan running/wing fixed remain in the flow) you ALWAYS have added 'thrust' into the equation so you do NOT match the physics of gliding flight. In addition as I have already pointed out the win can be HELD at a PITCH that is NOT sustainable in real flight (also often a AoA that can not be maintained!)
murrayhay 3 years ago
Why does fan running = thrust? A glider experiences a relative wind. There are systematic errors in your arguments.
stevesei999 3 years ago
M..y here is the root of your problem. A wind tunnel is not meant to model the pitch attitude of a wing in actual flight. A wind tunnel is also not meant to tell us what AoA's can be sustained by the aircraft's pitch control system. A wind tunnel only tells us the flow, Cl, and Cd for a given AoA. A wind tunnel does NOT model ALL the forces present in free flight. A wind tunnel no more models ALL the forces present in free flight in an airplane, than in a glider.
stevesei999 3 years ago
Rather than saying "Wind tunnel applies only to 4 force aircraft", you should be saying "wind tunnel is valid for any airfoil on any aircraft but wind tunnel is a tool to explore Cl and Cd and is not a simulation of the aircraft's pitch attitude in actual flight."
Which should be obvious--the wing section is held in the tunnel by a mechanical pivot, so the pitch attitude is not determined by the interplay of AoA and flight path as it is in actual flight!
stevesei999 3 years ago
In actual flight the pitch attitude is the sum of the AoA plus the glide or climb angle. The glide/climb angle is determined by the vector sum of weight, lift, drag, and thrust. Of all these forces, only weight acts in a fixed direction and thus constrains the flight path to a single possible trajectory. In the wind tunnel weight is irrelevant and so obviously we are not modeling an actual flight path or pitch attitde. Your problem with the wind tunnel has nothing to do with 3 vs 4 force arcrft.
stevesei999 3 years ago
Quote: 'In actual flight the pitch attitude is the sum of the AoA plus the glide or climb angle'
Your statments is patently flawed: AoA is the RELATIVE angle of the wing/air...
glide/climb angle is the angle of the aircrafts movement to the horizontal...
And if looking at a gliding aircraft in a steady, sustained climb & airspeed the air MUST have a vertical movement..
NB. (absolute) pitch angle is the angle of the wing to the horizon, you also have relative pitch (up or down change)
murrayhay 3 years ago
Example: AoA +5 degrees, glide angle in still air -10 degrees, pitch attitude ends up at -5 degrees. If the airmass happens to be rising or falling or translating sideways this won't affect these angles. I should've stated that I was speaking of the still-air glide or climb angle.
stevesei999 3 years ago
Quote: 'I should've stated that I was speaking of the still-air xxxx or CLIMB angle.' Yep, once again it is perfectly clear you don't understand the basic physics involved in 1) Gliding Aircraft 2) Paragliders... in 'STILL-AIR' a glider does NOT climb!
NOTE: (once again!) due to the wing shape a PG NEVER has a single AoA! In addition as a significant part (parasitic only) of the total drag is 'outside' the aircraft your addition (the values given) is of course incorrect!
murrayhay 3 years ago
M. you are too eager to find fault. We both know that a glider can have a positive climb angle with respect to the airmass as it trades kinetic enery for altitude. And we both know that only a powered aircraft can have a positive climb angle with respect to the airmass in sustained equilibrium flight. Naturally, my comment was inclusive of the powered case. It IS accurate to say that for any aircraft, the pitch attitude equals the AoA plus the glide or climb < with respect to the airmass.
stevesei999 3 years ago
Yes, it can climb in a DYNAMIC (short term) situation... this is NOT under study in the wind tunnel, yet again you are going off at a tangent and ignoring your core errors.
REF: 'It is accurate... ANY aircraft'..
INCORRECT this is NOT true in the case of PG flight where a significant proportion of the drag (PARASITIC only) is situated well 'outside' the aircraft!
Further NOTE: a PG NEVER has a single AoA (nor can it be said to have a single value to its PITCH)
murrayhay 3 years ago
"INCORRECT this is NOT true in the case of PG flight where a significant proportion of the drag (PARASITIC only) is situated well 'outside' the aircraft!"
What are you smoking? Why not include the pilot and lines as part of the aircraft? And why is any of this germain to the idea that pitch attitude = AoA+ glide < ? Pick one particular wing section (rib) to measure if from if it bothers you that the AoA varies across the wing and likewise for pitch.
stevesei999 3 years ago
PG flight your statment is wrong! Pitch will NOT = AoA + glide.
Paraglding: Remember with no 'bodywork' the only reasonable refrence to work from is the PG wing i.e. 'The Aircraft', as such the pilot IS clearly 'outside' the aircraft,
Note: While the same is true of (most) HG's (NB PG's are Class 3 HG's) having the HG pilot (slightly) 'outside' the aircraft is less of an issue as it's helped by 1) higher airspeeds 2) more aerodynamic pilot position 3) less 'draggy' pilot/wing connection.
murrayhay 3 years ago
M . you say "due to the wing shape a PG NEVER has a single AoA!" So? This is true of nearly all aircraft but that shouldn't give us such distractions in figuring out the basic physics. Start with the basics then add the details. We can measure the Cl and Cd of a model in a wind tunnel at various AoA's using some reference line like the mean chord line, without assuming that the whole wing is at the same AoA. These Cl and Cd values allow us to predict the still-air glide angle in free flight.
stevesei999 3 years ago
Quote: 'shouldn't give us such distractions in figuring out the basic physics', I certainly causes me no problems, nor any of the students. YOU can measure a (single) wing section for the said values at various AoA's that are produced in POWERED flight... Try to remember we are intrested in NON-powered (gliding) flight of a paraglider.
The vaules generated in POWERED simulations will NOT gaive valid results for UN-powered flight, NB. most of the AoA's are NOT sustainable in gliding flight!
murrayhay 3 years ago
From a pilots viewpoint, distinction between "3 force aircraft" and "4 force aircraft" is contrived. If we slowly retard the throttle on an airplane to fully closed, at what point does it suddenly turn into a "3 force aircraft"? The fundamental flight dynamics are the same with power as without. Now, if you have paraglider pilots who think the wind tunnel shows them the pitch attitude with respect to the horizon that they can maintain in actual flight, sure they need some re-education.
stevesei999 3 years ago
One reason a pilot might get confused about wind tunnel results: if you think of pitch attitude as being more fundamental than AoA, you might wonder why different pitch attitudes give you different airspeeds depending on power setting. For a given pitch attitude, reducing the power could increase the AoA to the stall AoA. In most aircraft the flight controls determine AoA not pitch attitude and for a given AoA the aircraft's fundamental dynamics are the same regardless of power setting.
stevesei999 3 years ago
Food for thought: is there a distinction between aircraft that trim to a given AoA and aircraft that trim to a given pitch attitude. There would be some fundamental differences in dynamics especially as the flight path becomes steeply descending at the stall, etc. When you hang a pilot below the wing on flexible lines that pull on different parts of the airfoil depending on the aircraft's pitch attitude, this is a pitch-based stability system not an AoA based stab. system at least in part.
stevesei999 3 years ago
The Basics: In PG flight changes in airspeed alter the Parasitic drag (pilot&lines) this directly effects the pitch of the wing, AND ALSO (feedback) alters the airspeed airspeed... hence the AoA.
10 years of study & teaching PG aerodynamics to RE-traning pilots has show me that even the designers of PG's have core errors in their understanding of PG flight! NOTE: As a PG in flight does NOT have a single AoA... fixating on AoA is NOT a good way to gain understanding!
murrayhay 3 years ago
I think you are off track on a lot of basic points. In a glide, drag doesn't control airspeed. Drag controls sink rate. Lift coefficient controls airspeed and that's why AoA will always be very important. If you really want to offer the PG community a more enlightened theory of flight you should collaborate with someone with more training in basic aerodynamics. Still there are some fascinating aspects to a pitch-based trim system.
stevesei999 3 years ago
Wrong, as I've pointed out before you need to sort out your basic, core errors.
Drag, in it's various forms effects the airspeed of PG's, in the case of Parasitic drag of the pilot INCREASING drag results in an INCREASE of airspeed, while increasing parasitic drag at the wing (distorting training edge mainly) results in a REDUCTION of airspeed.
Re 'sink rate', slowly increasing drag at the wing results in first a REDUCTION in sink rate then of course an INCREASE in sink rate.
murrayhay 3 years ago
M..y you said "Drag, in it's various forms effects the airspeed of PG's, in the case of Parasitic drag of the pilot INCREASING drag results in an INCREASE of airspeed, while increasing parasitic drag at the wing (distorting training edge mainly) results in a REDUCTION of airspeed." M...y those effects are 100% due to the effects of the way the drag changes the Cl either by changing the wing shape or changing the wing AoA. Fundamentally Cl is the driving factor in airspeed control, as I posted.
stevesei999 3 years ago
Looking at the big picture that started this discussion: it's a mistake to say a wind tunnel movie doesn't apply to PG flight just because it doesn't represent all the force vectors at play in actual flight. It was never meant to. I still say the interesting difference between airplanes and PG's isn't "3 force" systems vs "4 force" systems. The interesting difference is the contrast between a purely AoA-based trim system, vs a trim system that is to some extent directly based on pitch attitude.
stevesei999 3 years ago
Looking at the big picture some more: your comments suggesting wind tunnels results apply only to "3 force aircraft" and not "4 force aircraft" would have the Wright brothers rolling over in their graves. You should try to get some feedback on your ideas from other knowledgable and qualified folks, and try present your ideas about PG dynamics in ways that don't make the larger glider community scratch their heads and say "Say what?"!
stevesei999 3 years ago
Eh no! My comments STATE that you can not correctly apply these wind tunnel 'results' to PG flight as the forces are NOT the same, specificaly as a FOURTH force is added, NOTE you CAN (to a limited extent) apply it to~ PPG (power on)& Tow Launch. It is clear there is a sever lack of 'knowledgable/qualified' this is abundantly clear from the trully appalling 'normal' PG accident rate.
NOTE: I have been pointing out, for 10 years, EXACTLY how it is EASY to cut PG accident rates by up to 95%!
murrayhay 3 years ago
Congrats on making practical improvements to paragliding technique. But you need to collaborate someone who can help you be less of a loose cannon in your approach to theory. It's completely ridiculous to claim that wind tunnel results only apply to aircraft flying under thrust and not to gliding aircraft. A wind tunnel is a tool for exploring flow, Cl, and Cd at a given AoA. It does not pretend to be a model of stability and control in free unconstrained flight, either for power OR gliding.
stevesei999 3 years ago
The 'practical improvments' MAINLY stem from the fact that 95% of the THEORY I teach DIRECTLY CONTRIDICTS the sort of nonsense you are posting! The THEORY I teach results in SIGNIFICANT changes in the way my students act in controling their paragliders both when flying 'within' and ESP. when flying 'out-with' the normal PG 'Flight Envelope'.
You can NOT have a 'given AoA' in a aircraft that NEVER has a single AoA, due to the wing (3D) shape! You badly need to correct your CORE errors.
murrayhay 3 years ago
M..y I'm not trying to be overly harsh on you buy I do encourage you to try to present your ideas in a way that don't seem so incongruous with established theory of gliding flight (meaning the discoveries of Cayley and the Wrights, etc.) Any sailplane+power pilot with a solid background in basic aerodynamics and a good understanding of the static balance of forces in gliding flight will have multiple problems w/ the idea that a wind tunnel represents only the powered situation.
stevesei999 3 years ago
Also I still say the core difference between PG's and other a/c lies in the difference between a pitch attitude based trim system (PG) and an AoA based trim system (conventional a/c and sailplanes, and hang gliders.) W/ the AoA based trim system, there is no real fundamental differences between dynamics under power and dynamics in a glide. W/ the pitch attitude based trim system, the stab. and control will be substantially different under power-- consider things like the "parachutal" mode etc.
stevesei999 3 years ago
Re. 'Harsh'... Physics when poorly understood/applied and so resulting in numerous needless deaths & injuries is not 'harsh' it simply is a fact of aviation.. as I have ZERO desire for my students to suffer from the 'normal' accident rates I will NEVER teach what you appear to consider to be 'correct', I can only suggest you go back and look at your basic, core errors. Bottom line, a wind tunnel looks at FOUR force (i.e. with thrust) flight.
murrayhay 3 years ago
M. a thought experiment: We place a sailplane model in a wind tunnel. The tunnel is wide compared to the modl's span. We measure the Cl and Cd in the tunnel, at a range of AoA's. We use windspeeds that will give realistic Re's. We now take the model up in a balloon and drop it in still air. We fly it at various AoA's. We note that glide ratio just happens to be equal to the Cl / Cd for each AoA as measured in the tunnel. We discard the idea that a wind tunnel only models "4 force aircraft."
stevesei999 3 years ago
IF you are intrested in science... and want to run the test (sailplane/wind tunnel) you need a TALL tunnel... (not 'wide') you also need the sailplane to be FREE to move i.e. NOT attached to the tunnel... so no FOURTH FORCE (THRUST) applied to the aircraft...
As I've pointed out before you REALLY need to work on correcting your core errors otherwise you will keep making the same mistakes!
murrayhay 3 years ago
M. don't be ridiculous, this isn't a discussion about ground effects and you know it. Finite-span effects are very important (hence the stipulation of the wide tunnel) but that's really not the point either. If you think a model has to be free to move around in a wind tunnel to accurately measure Cl and Cd at a given AoA, you don't understand wind tunnels at all. All this stuff was all well understand by the time of the Wright brothers-- who were glider pilots and wind-tunnel users!
stevesei999 3 years ago
Ground effect, yet another pointless comment from you, remember that with PG line lengths the wing, even with the pilot on the ground, is too high for any (significant) 'ground effect' to take place!
YOU clearly can't grasp the basic issue that in THREE force gliding flight, the wing is NOT subject to 'thrust' by being fixed against the airflow as it is in wind tunnel testing, so until you finaly, (if ever!) correct your core error you will not progress in understanding of gliding flight.
murrayhay 3 years ago
M. if you don't want to make paragliding look like the intellectual backwater of aviation, start by gaining a solid understanding of the general theory of gliding flight as has been well-established by the sailplane community many decades ago and then expand to the special case of paragliding with its unique stability and control issues, rather than trying to re-invent the wheel while making multiple "core errors" (to use your own favorite phrase) along the way.
stevesei999 3 years ago
A starting point: "The Science of Flight : Pilot-oriented Aerodynamics" by W.N. Hubins (1992) is a good place to start understanding how lift and drag coefficients apply to actual flight. So is "Model Aircraft Aerodynamics" by Martin Simons (1987). Google your way to the biblio page of the superb "See How it Flies" website for more. "The Complete Soaring Handbook, revised" by Anne and Lorne Welch and Frank Irving (1968) is one good source for the basic physics of gliding flight.
stevesei999 3 years ago
PS if you want to teach only practical things with no theory that's ok too. I just met a military transport pilot who had no understanding whatsoever of a very basic stability and control issues (dihedral creates a roll torque when pilot applies rudder, due to a difference in AoA between the left and right wings) but I guess he still flew just fine, and was even involved in some level of flight test to create procedures for dealing w/ unusual attitudes.
stevesei999 3 years ago
In 'normal' PG training the lack of any significant (compared to say PPL) amounts of theory training (esp. HP&L training) is part of the reason for the appalling BHPA accident rate, however the single largest factor in the 'normal' PG accident rates IS the theory used to teach.
IF you want to slash PG accident rates 'practical' training MUST be done based on correct application of science to understanding of who PG wings work, any other approach WILL result in avoidable injuries & deaths!
murrayhay 3 years ago
Fine, so find someone you trust with a solid knowledge of the physics of flight to give you an opinion on your wind tunnel theories, they need some help.
stevesei999 3 years ago
Steve, one of us has been teaching paragliding (and you say you don't fly PG's!) for over 10 years and as I do NOT use the nonsense you keep repeating my students do NOT suffer from the accident, injury & death rates that RESULT from 'normal' PG theory/training!
The bottom line is that it is the 'side' that needs help is the side that is teaching dangerous nonsense!
murrayhay 3 years ago
Well, I don't have any horse in this race, I don't know exactly what you are teaching or what the other side is teaching, I just see some mistakes on your comments and most specifcally your comments about wind tunnels, which is what this page is supposed to be about. Just trying to help!
stevesei999 3 years ago
Quote: 'I don't have any horse in this race' ... in other words you DON'T fly paragliders... Yet you keep posting the same old core errors about
1) using wind tunnels to study UN-powered flight in general
2) using wind tunnels specific to the tudy of paragliding flight!
murrayhay 3 years ago
Steve you need to get a grip of some basic science!
The REASON for the appalling accident rates in paragliding is it IS the 'intellectual backwater of aviation', as it is in fact the incompetence of normal training 'terory' (and so practice) that keeps injuring and killing pilots...
Simply REMOVE those errors, as I did over 10 years ago, and you get a 95% DROP in accident rates!
ADDING a 'forth force' means results are NOT the same as UN-powered, THREE force gliding flight.
murrayhay 3 years ago
M. your philosphy that a wind tunnel models a powered situation only, is dead wrong. You don't seem to understand how wind tunnels are used to measure lift and drag coefficients which can be then be taken away from the wind tunnel context and used to determine the complete balance of force vectors in free flight. Only at this point--and not in the tunnel--do the thrust and weight vectors enter the picture.
stevesei999 3 years ago
Steve the bottom line is you still fail to grasp some VERY basic physics, ANY results generated in a wind tunnel REQUIRE:
1) the fans to be running (air forced to move)
2) the wing to be held AGAINST the flow of air.
So the results produced will NOT match that of a THREE force, NON-powered gliding flight.
Another KEY point directly relevant to paragliders is that a PG in flight NEVER EVER has a single Angle of Attack (AoA across the span) even when flying straight!
murrayhay 3 years ago
M. if you want to explore the differences in stability and control in paragliders versus powered paragliders, that's fine and interesting, but you come across a bit silly with this generalized theory of "4 force aircraft versus 3 force aircraft". As a sailplane pilot and power pilot and hang glide pilot and enthusiastic student of the physics of flight, I can tell you that there are no truly fundamental differences in the stability and control of most aircraft in the power on vs off situations.
stevesei999 3 years ago
Your statment makes clear two things..
1) You DON'T fly PARAGLIDERS! (so you have some limited excuse for your errors)
2) You don't understand the core diffrence between UN-powered and POWERED flight (3 & 4 force flight)
3) By DESIGN (most, but not all) powered aircraft will naturaly PITCH down on loss of power due to the vertical offset (corrected by trimers) of the thrust (force 4) and drag lines.
The error you make is not in stability/control but rather in basic flight physics forces.
murrayhay 3 years ago
M. this mantra of "3 forces vs 4 forces" is just plain silly and I'm calling you on it. Tell me in a few words what is fundamentally different about sailplane flight versus powered airplane flight? The response to control inputs is fundamentally the same. To the extent that your theories hold water they apply only to paragliders with their peculiar stability and control attributes. You present them as if they are much more general. Do you think we need to re-educate the sailplane community too?
stevesei999 3 years ago
M. I'll grant that it would be uniquely challenging to test a complete model of a paraglider (including pilot) in a wind tunnel. One problem is that the "dangle angle" (d.a.) of the pilot and suspension lines influences the wing shape, and in free flight the d.a. is strongly influenced by the direction of the weight vector, which in free flight is determined by the glide < with respect to the airmass, which is determined by the wing shape! So we have a recursive problem on our hands.
stevesei999 3 years ago
(Cont.) The solution would appear to be to change the fore/aft tilt of the pilot in a way that is keyed to the Cl and Cd values that we are measuring on the scale, so that the tilt of the pilot's body is keyed to whatever would be the true direction of vertical in free gliding flight. That may be on oversimplification but it touches at the issues involved with the flexible wing/pilot system.
I say the problem is NOT insoluble!
stevesei999 3 years ago
(Continued 2) I can also see that these complications go away when we happen to be modelling the case of precisely horizontal (i.e. powered) flight with respect to the airmass.
Once we begin climbing under power, or descending even slightly, the problems all come back.
So I can see that you are on to something.
I still say all the interesting aspects of the situation arise from the uniquely flexible nature of the aircraft and are NOT generalizable to sailplanes or hang gliders.
stevesei999 3 years ago
MY POINT EXACTLY... Your errors are (fairly) unimportant in teaching most branches of aviation but those errors KILL in paragliding! NB. The issue is NOT the 'flexiable' (soft wing) nature of the aircraft as such but the PHYSICS of PG flight... i.e. if you could fill a PG with [semi-rigid] expanding foam the PHYSICS issues will still remain.
murrayhay 3 years ago
OK thank you for educating me. You fill the wing with foam and nothing changes. The "dangle angle" of the pilot doesn't matter because the suspension lines converge to a single point (or 2 single points). So... I've come full circle. There's no reason whatsoever that you can't model this system in a wind tunnel. Include the pilot and the lines. No problem. The pilot is fixed in place. The wing is free to move. For a given setting of the lines thing will find the right position above the pilot.
stevesei999 3 years ago
(cont) So there's no reason after all that you can't model the PG in a wind tunnel. Include the pilot and the lines. No problem. The pilot is fixed in place. The wing is free to move. For a given setting of the lines the wing will find the right position above the pilot. Measure the Cl and Cd with scales in the usual way. There's your glide ratio in free flight, for that particular setting of the lines. In free flight the pitch attitude of everything will be different but that doesn't matter.
stevesei999 3 years ago
Quote: 'No problem. The pilot is fixed in place. The wing is free to move'
This has to be the CLEAREST evidence you don't have the slightest clue about both basic physics/PG flight
murrayhay 3 years ago
You still don't understand the core science, UNBERSTANDING the 'angle of dangle' in PG flight is VITAL to PG safety (this shows most in PG launch incidents/accidents)
'Pilot fixed in place'... Fine if you are only intrested in FOUR FORCE flight NOT in three force PG gliding flight.
murrayhay 3 years ago
Quote: 'so that the tilt of the pilot's body is keyed to whatever would be the true direction of vertical in free gliding flight.'... (PG) this is Ok PROVIDED you are intrested in study of a PG entering a STALL! The very last place a PG pilot should want to be is vertically below the CoP of the wing!
murrayhay 3 years ago
Quote: 'One problem is that the "dangle angle" (d.a.) of the pilot and suspension lines influences the wing shape'... again you have a problem as you DON'T fly paragliders... as far as the wing x-section is concerned the pilot/suspension lines (excluding B-line stall etc.) do NOT effect the wing shape, rather it is the 'cut' of the wing ribs that do that!
PS. Brake fan (pulled) will of course alter the back of the x-section.
PPS. Some (not all) speed systems will alter the x-section.
murrayhay 3 years ago
Quote: 'plain silly'.... is trying to say that four force POWERED experiments is that same as THREE force UN-powered PG flight!
NOTE: while normal sailplane flight theory (for simplicity) contains a number of core errors these errors, due to the design with drag being 'inside' the aircraft rather than like a PG significant amounts of [parasitic] drag being 'outside' the aircraft... are NOT injuring & killing lots of pilots.... it matters little if the errors (for sailplanes) go uncorrected.
murrayhay 3 years ago
"while normal sailplane flight theory (for simplicity) contains a number of core errors"
That's simply not true. Be more specific about these so-called "errors", especially in the context of the issues we've been discussing.
stevesei999 3 years ago
Well for a start the concept of 'Low pressure above the wing/high pressure below the wing'... this is of course some what simplistic and while it IS handy in teaching pilots in general is 'bad science' (WRONG!)
Note in (steady state) gliding flight the UNDERSIDE of the wing mainly generates 'lift' that is DOWNWARDS (and slighly 'back')
This BASIC physics is VERY important when it comes to teaching 'soft' wing (PG) flight IF you want a 95% REDUCTION in accident rates..
murrayhay 3 years ago
Let's talk about the true state of the science not what you might read in some handbook someone wrote. My specific assertion is that for sailplanes and hang gliders the Cl and Cd across a given spectrum of AoA's can be acdurately measured with a model or full-scale aircraft in a wind tunnel and this can be used to accurately predict the glide angle in free flight.
Your "4 force vs 3 force" theory is way overgeneralized, it applies only to paragliders.
stevesei999 3 years ago
the 'true state of science'... is you KEEP making the same old core errors... NOTE: (as pointed out before (most) HG's will also have the pilot positioned 'outside' (below) the aircraft (wing) so is subject to similar issues to PG flight / wind tunnels.
THREE force for \ UN-powered gliding aircraft
FOUR force = aircraft subject to THRUST
murrayhay 3 years ago
M there is a core error in your approach, you don't want to face the fact that there is no problem with measuring the Cl and Cd of a "normal" (non-paragliding) aircraft in a wind tunnel and then predicting the still-air power-off glide angle from this data, at any desired AoA.
The fact that the hang glider pilot is outside the wing is no problem, he still just part of the total parasitic drag of the aircraft and we can still derive a polar curve just as for a more conventional sailplane.
stevesei999 3 years ago
Where did all this start anyway? The wind tunnel movie shows flow around the wing at various AoA's. Obviously if a paraglider wing happens to be the same shape as a the tunnel model we'll get the same flow for the same AoA. That's not rocket science. Sure I agree that actual paragliders are floppy and difficult to work with in wind tunnels because there is really no easy way to hold them at a given desired AoA. Ok?
stevesei999 3 years ago
Anyway it's interesting to think about the pitch stab and control system of a paraglider which has something in common with any a/c where the CG is well below the wing. Take the wing's aerodynamic center as our pivot point. The airfoil generates a nose-down pitch moment. Pilot's drag adds to this. Thrust (if present) applied at pilot applies nose-up moment. In a glide, the component of the pilot's weight vector that acts parallel to the flight path gives a nose-up pitch moment.(to be continued)
stevesei999 3 years ago
(PG stab and cntrl cont.) Conversely, in a climb the pilot's weight vector contributes a nose-down pitch moment, especially at steep climb angles. That summarizes all the key moments at play. The angle of the flight path plays a key role in the dynamics. In a glide, if we change the wing in a way that steepens the flight path, one consequence of this is that the component of the pilot's weight vector that acts parallel to the flight path gets bigger, which contributes a nose-up pitch moment.
stevesei999 3 years ago
(PG stab and cntrl cont. II) So the glide or climb angle (w/ respect to the airmass) plays a role in determining the angle-of-attack. That makes for an interesting feedback effect: the glider has some tendency to retain a constant pitch attitude and does not freely vary it's pitch attitude to retain a constant AoA in the manner that a sailplane would if we opened the spoilers, for example. Clearly the stab and control system of the PG is partly based on pitch attitude and not purely AoA.
stevesei999 3 years ago
NOTE: In a STEADY state of lift (or sink) i.e. no acceleration, a GLIDING (THREE force) aircraft will fly (ignoring TOTAL pressure/density changes due to altitude etc.) in a similar manner to when in 'still air'.
As I have been pointing out to you for a L O N G time to UNDERSTAND PG flight you need to account for ALL the forces in play.
Quote: 'stab & control systems...' (PG's) has almost NOTHING to do with AoA... if you like AoA is the 'tail' NOT the 'dog'... (Tail wagging the dog)
murrayhay 3 years ago
M. I'm about to give up here. First of all, I don't see how you can get anywhere with your analysis of PG's till you get over your blind spot involving wind tunnels and gliding flight in general. Sure I agree that you can't adequately control the AoA of a flexible PG wing in a wind tunnel but that pertains only to PG's.
Also I understand that PG stability is based on pitch attitude as well as AoA, but to say that AoA plays only a minor role in PG flight dynamics is quite an exaggeration!
stevesei999 3 years ago
FIRST I teach paragliding, hence I comment on PARAGLIDING and PG flight!
Wind tunnel (FOUR force) testing has issues (varying amounts) when looking at all THREE force (gliding) situations, and of course also DYNAMIC situations i.e. most of gliding flight!
Remember as PG wings NEVER EVER have a single AoA 'Fixating' on AoA is one of the BIGGEST errors pilots make! PG stability is mainly related to a combination of pilots MASS and pilots (parasitic) drag... other factors of course exist!
murrayhay 3 years ago
Even in a 'simple' (straight) flight the 'pivot point' is highly unlikely to be 'in' the aircraft, rather generaly it is some (small) distance below the wing, NOTE: it can also easily be ABOVE the wing...
Quote: 'In a glide, the component of the pilot's weight vector that acts parallel to the flight path'... what a load of nonsense, PLEASE learn some basic physics!
murrayhay 3 years ago
"'In a glide, the component of the pilot's weight vector that acts parallel to the flight path'... what a load of nonsense..."
If you prefer you can divide the pilot weight vector into a component that acts through a line drawn to the wing's MAC and a component that acts perpendicular to this line. The results will be nearly the same; only the latter component will contribute a pitch torque about the MAC. At the end of the day it's still a pitch-attitude-based control system, in part.
stevesei999 3 years ago
PLEASE Learn some VERY basic physics before you make yet more core errors!
Quote: 'divide the pilots weight vector......'
This vector ALWAYS has a 'sense' (direction) that is VERTICAL if you are in a 'steady state glide' (no 'G-force' due to turns etc.)
murrayhay 3 years ago
It started because incompetent pilots/instructors use the sort of nonsense you keep spouting to 'teach' / fly paragliders and as a result the spot has a LOT of injuries & deaths.
Re: Tunnel/airflow, to be realistic in modeling PG flight would REQUIRE as a MINIMUM many 100's of thousands of wing sections... for EVERY single (steady state) configuration of PG flight.
In practice I doubt computer/W'tunnel modeling will ever be more than a VERY rough guide regarding PG wing sections/airflows.
murrayhay 3 years ago
Re: 'The fact that (HG)...' YOU are still making the same old errors!
For all (main) types of hang glides (HG & PG) you need to measure and plot at least THREE 'drag curves' to generate a valid 'Polar Curve'...
Of course you can use electronics to measure (in 'still air') a real flight.. with a real aircraft..
BUT the question here is regarding WIND TUNNEL experiments.. and the inherent issues with ADDING the FORTH force, THRUST!
murrayhay 3 years ago
"you need...at least THREE 'drag curves'... "
No, with HG and sailplanes you can just put the whole thing in the tunnel, or a model thereof. Then you measure the Cl and Cd for a given AoA and this gives you the L/D which is the still-air glide ratio for that AoA. Now you are done with the tunnel. Now determine the weight and you can calculate the actual L and D vectors, and the airspeed.
No problems about the "4th force".
Specifically which part of the above don't you follow?
stevesei999 3 years ago
In the wind tunnel you can generate data on how the whole aircraft (including the pilot 'outside' the HG/PG) flys in FOUR force POWERED flight.
In the end you are making the same old core errors, and PLEASE try to stick to the topic which is using a POWERED wind tunnel to 'model' GLIDING i.e. UN-powered PG wings!
murrayhay 3 years ago
"In the wind tunnel you can generate data on how the whole aircraft...flys in FOUR force POWERED flight."--simply not true. In a wind tunnel there is no effort to achieve ANY balance of forces. You just measure L and D on the scales and then compute Cl and Cd. Lift is NOT equal to weight. Looking at the wind tunnel results you have no idea what climb angle will be achieved in actual flight until you know the actual weight and thrust, neither of which plays any role at all in the wind tunnl test.
stevesei999 3 years ago