Direct Downwind Faster Than The Wind #3 (DDWFTTW)

[Jangle2007]

See...now this is where the physics and mechanics behind this concept just LOSES me!

In the downwind vehicle, I understand that the swept area of the propellor rotor generates more "power" than is necessary to move the vehicle at the observed speed. What I don't get is how the propellor generates pwoer when the vehicle has outrun the wind speed?

Can somebody help me? I'm fascinated!

[spork33]

@Jangle2007

That question can be tricky, but I think the best way to think of it is to imagine the prop blades as a couple of sailboats spiraling their way along a long skinny cylindrical earth. Given that a sailboat can tack its way downwind faster than the wind, our prop blades should be able to do so on a long continuous helical downwind tack. Make sense?

[Jangle2007]

@spork33 You refer to "apparent wind", correct? So far so good. But at true wind speed x, how does the prop continue generating energy when the vehicle speed reaches x? I understand that a portion (or all?) of the prop travels faster than the true wind speed, but I'm not seeing how the system continues to capture energy above vehicle speed x directly down wind.

[mebedavis]

I'm trying to duplicate this result with a cart of my own, but I'm finding the gearbox to be a REAL PAIN to build. Can anyone tell me where to buy a _cheap_ right-angle transmission, or offer advice on how to build one. I've shopped out all the local hobby stores, and I can't even find a decent set of 1:1 bevel gears, so I'm trying to cobble the thing together out of a combination of spur and crown gears of thickness ~.5 mm. I just can't seem to keep them stable enough to keep them enmeshed.

[spork33]

@mebedavis

I've posted a series of three build videos to make a cart like this. Look for it under spork33. It gives the part numbers and sources for the gearbox. It's the tail-drive for a radio controlled helicopter.

[t0rc]

There may not be a need for a flywheel but you would think it would be wise to add one so you can store enegry which pushes the vehicle forward during gusts of wind

[t0rc]

when you put the device on the treadmill, enegry from the electic motor is trasnferred via the drive belt to the wheels which in turn produce thust, which if geared correctly will drive the device faster than the rolling speed of the treadmill.

its not magic, you might as well just attach the electirc motor into the propeller because in effect its the same thing driving the propellor

[Mr41432]

@t0rc dude did you not see the beginning of the video? the wind was pushing the car not a treadmill the treadmill was simply there for a test

[Jangle2007]

@t0rc I'm not aware that there is an electric motor. Only a tranmission to exchange power between the wheels and the propellor.

[t0rc]

this is not some sort of perpetual motion machine. I don't know if this is supposed to be a demonstration of such: The 3 factors here are: gear ratio of the large front wheel, vs the prop rotation and the pitch of the prop itself

In this case the thrust produced by the prop is slightly greater than rolling speed of the drive wheels. energy is added to this system by the treadmill, and the drive wheels act as a flywheel.

[spork33]

@t0rc

It is not intended to be a demonstration of a perpetual motion machine. It is a demonstration of a wind powered vehicle going directly downwind faster than the wind.

And there is no need for a flywheel as we make no use of stored energy or momentum.

[00soundwave00]

ahh... I get that it works... i get the concept why it works, with the prop being powered by the wheels + acting like 2 "sailboats" tracking the wind.... but i dont get the math behind it!

[spork33]

@00soundwave00

You're almost there then. You should join us to discuss it on the forums. I don't think I can post the forum name here, but I'll send it to you via PM.

[Abrin19]

A wind-powered car has been clocked in the US traveling down wind faster than the wind. In a recent run at New Jerusalem in Tracy, California, the car reached a top speed of more than 2.85 times faster than the wind blowing at the time (13.5 mph) powered by the wind itself. The run should now settle the DWFTTW (down wind faster than the wind) debate that has been raging for some time on the Internet about whether or not such a feat was possible.

physorg

[spork33]

@Abrin19

Yup - that was us.

[Cloxxki]

Interesting stuff. The treadmill to me is a bit inconclusive, as wheel friction would drag it up.

Could it be that the torque on the propellor due to being connected to the wheels is out of ratio, but ends up working like a sailboat can go much faster than the wind, as long as it is restricted to travel laterally? The long length of the cart suggest that they might fight lateral forces so some extent. Then, the lateral forces might be bi-lateral....

[spork33]

The drag of the wheels would drag it down the treadmill. The thrust from the prop overcomes that and causes the cart to climb the treadmill.

And yes - the blades of the prop are on a continuous spiraling downwind tack that is exactly analogous to a sailboat doing the same.

[Cloxxki]

@spork33 I realized my twisteed thinking, but you beat me to correcting myself, thanks.

I'd going to have to think on another level to understand this thing. But already I'm seeing vehicles that would run on railway tracks. Super fast in side wind, like 4x the wind speed as ice skater inside standing wings manage. Downwind, on technology as DDWFTTW, and against the wind like vehicles well documented for it. My current hunch I need to work out : tailwind diverted to be felt as side wind.

[spork33]

Consider that a sailboat can maintain a 45 degree downwind tack such that it's downwind velocity component is much faster than the wind. From there it's easy to see that the tips of our props are simply two such sails following continuous spiraling downwind tacks. The wheels and transmission simply replace the boats keel in providing the kinematic constraint that keeps the prop blades on the right track.

[reedcalkins75]

the fact that the cart is gaining ground on the treadmill proves nothing except the fact that the wind speed from the fan is faster than the treadmill speed. this allows the cart to have just a little positive traction that it wouldn't have on solid ground.

[spork33]

Wrong. There is no fan. Just a treadmill.

[reedcalkins75]

I get it now. My bad.

[wildblue2]

So, are you guys going to build a full sized ddwfttw land sailor and beat some land sailing speed records?

[spork33]

We're threatening to build a full-scale cart that we can ride in. We hope to definitively beat the wind on a direct downwind heading. I don't expect there are records for such.

[goatpunch]

Really this is as simple and as counter-intuitive (at first) as the yo-yo along a table & cotton reel examples.

Basically the cart is being _pushed_ by the wind, and uses the rotation of the wheels to turn a propeller which pushes against the wind even more and allows it to move a little faster than the wind which is pushing it.

Most people seem to assume that they see a windmill (turbine) which powers the wheels.

[TimpBizkit]

You need less wheel force to spin the propeller at low speeds.

Both the force at the wheel and the thrust generated are proportional to the square of the propeller speeds, so it is a balancing act.

[spork33]

It's really not a "balancing act". There can be a great deal of excess energy which will accelerate the cart well beyond wind speed.

[TimpBizkit]

To get this car to work, you need a propeller that is capable of generating more thrust off the available power than is the wheel friction force required to spin the propeller.

The secret is in choosing an efficient propeller design and a low enough gear ratio so that you don't need a lot of wheel force to turn the propeller.

Then you can spin the running machine up to speed to generate the power required.

[spork33]

You don't "spin the machine up to generate the power". That would imply that it could operate without wind - which it cannot.

[TimpBizkit]

Well in an outside situation, the power would come from the wind pushing the vehicle along the ground. The propeller would rotate and generate extra thrust, which is pushing back on the already moving wind, slinging the craft forward. The art is getting the optimum gear ratio to give the craft the maximum increase in speed relative to the tailwind, without the propeller overspinning and generating so much drag that the craft is slowed down.

[spork33]

The "outside" situation is no different from the "inside" situation. In either case you need a gearing that gives you an advance ratio of less than 1.0 (the distance the prop would advance in a single rotation divided by the distance the wheels would roll in one rotation of the prop). As you get closer to 1.0 the theoretical speed increases - but at the same time the allowable losses decrease.

[TimpBizkit]

That's a good idea, I never thought of it in terms of propeller pitch and development from one rotation

[Twinfire0]

Hmm, very interesting concept. So interesting in fact that me and a few friends are doing a science fair project on it! We're making our own budget cart (about half the size of that one), keeping costs down. If you have any guidance you could give us that would be great! We're definitely fascinated in the physics involved with this project, and hopefully we'll be able to test something which most people have a tough time wrapping their heads around.

[spork33]

I've sent you my contact info. It will be a great project.

[spork33]

I got word that Twinfire0 took 1st place. Congratulations!!!

[lesqual]

The reason it's a demonstrable hoax is that if you have the power ("Extracted from the earth" or "Extracted from the wind-ground system", however you like to obfuscate it) to accelerate to 12mph (limited by increased turbulence) in a 10mph wind, then by your own quotation of the relativity of different frames of motion, you can also accelerate to 2mph (limited by increased turbulence) in a 0mph wind. That, my friends, is perpetual motion.

[spork33]

I'm sorry that your understanding of physics is so limited that you would label this as perpetual motion. Instead of calling me a fool or a liar why don't you build your own cart for $40 and do your own tests. I posted a video series on how to build the cart for exactly this reason.

[Sgrunterundt]

I am sorry spork, the "why don't you build your own" approach to this dicussion does not seem to work, because that is what many of the free energy wackos does too.

lesqual: Don't be so quick to think that you are the only one in the world who understands physics. You are making some very basic mistakes. All available kinetic energy is contained in relative motions. If you have a rock moving alone in space, you can get no energy from it...

[Sgrunterundt]

... (double post for length) but if you have two moving at different speed, the difference could be utilised.

It does work in zero wind - if the treadmill runs underneat.

No differential motion: No energy to be had. So it certainly does not work outside in no wind.Once the wind blows, there is energy, and by clever engineering that energy is still available even ef you are moving fast.

[AcerbicMaelin]

Uh. Unless that extra 2 mph is a function of the RATIO of ground to air, rather than the DIFFERENCE. Perhaps you get an 12 mph in a 10mph because you always get 120% of the windspeed? In which case in a 0mph wind you'd still get 0mph.

[jspade1019]

So with a lever, you can gain force and do more work, but gear ratios don't work because they are perpetual motion?

This is a lot like a bicycle, try riding one.

[1highclassgent]

HB - more like; you could use the mechanism seen here to sail faster in the direction of the wind than the wind is blowing.

[Hankxc]

If the cart can roll forward against the direction of motion of the treadmill Does this mean we can sail a boat up stream in dead wind using a similar mechanism or physics!!!!! I am at a loss.

HB

[Sgrunterundt]

In principle yes. I think those ultra high performance boats you see videos of (->>) could pull it off.