Airfoil Stall (I)

[hihflier]

hi, is it possible to send me the video?  I'd like to use it for a presentation. aprilrbagwell@yahoo.no Thanks!

[nothingIwanted]

And fighters generally have leading edge devices such as slats that deploy at higher AOA to keep the flow attached. Also we aren't comparing apples with apples. A straight wing with no high lift devices compared to one that does and has significant sweep or delta platform. Also low aspect ratios typical of fighters tend to stall at higher AOA then high aspect typical of civilian planes.

[wildblue2]

a symmetrical airfoil will stall at a lower a.o.a than a asymmetrical right?

[SVnerd]

True, that's why civilian planes will have more curvature on the top to create more lift and have a higher stall angle. Drawback is increased drag though. Fighter jets tend have a more symmetrical profile and even though the stall angle is low, they make up for it with engines that generate a lot of thrust.

[JETZcorp]

Really? I always thought that fighters whipped civilian planes in the AoA department. An F-16, for example, will give you about 25 degrees before giving up the goose.  I don't know how this compares to a positive-camber civilian plane like a Cessna or 747. Do you know how it stacks up? I'd be interested to hear this!

[SVnerd]

You make a good point there, I can't say for sure since I don't know the details of an airfoil such as the F-16's wing. However, you should also consider the fact that a fighter jet has a very high thrust to weight ratio so the plane could still climb because of the large thrust even though its airfoil is stalling.

I think the problem here is we're not looking at a 1:1 comparison.

[rocykel]

The cross-sectional characteristics of an airfoil become less and less important the lower the aspect ratio is. For an infinite span, the 2D airfoil performance is the actual performance. For a low-AR, highly-swept wing, airfoil choice has more to do with drag management and internal volume.

A true delta wing will retain lift at very high alphas, but the maximum lift will be a good deal lower.

[33vortex]

@SVnerd

Wing geometry is a significant factor. The F16 fly much like a delta wing and is a low aspect ratio configuration typical for jet fighters. The 747 is a high aspect ratio wing of conventional design. Completely different in design and principles used to achieve lift.

[nakedleader]

Man that's a shallow stall angle. Flow separation takes place at about 10 degrees!

[connors33428]

10 degrees is kind of high for an airfoil.

[nakedleader]

A typical aerofoil stalls clean at around 14-15 degrees

[RafKac]

It's true. This airfoil has quite small thickness ratio, I guess It's a laminar airfoil by the way, and has very small forward radius, it's easy to stall such airfoils, epecially at lower lower Reynolds number

[rlrsk8r1]

@nakedleader More like 19-22 degrees.

[nakedleader]

@rlrsk8r1 I don't think so.

A 'clean wing' i.e. no deployed lift devices will stall long before 19-22 degrees.

[rlrsk8r1]

@nakedleader Ok we're both wrong. I just looked it up in The Pilot's Handbook of Aeronautical Knowledge. The number it gives is 16-20 degrees.

[nakedleader]

@rlrsk8r1 Don't trust what you read in pilot books! The accepted angle for a conventional cambered aerofoil is actually 14.7 degrees. At that point flow separation prevents any increased in lift. I can send you some graphs if you like. I lecture to BSc ATPL students at university and it is amazing how many pilot textbooks have incorrect information. 16-20 degrees it too broad a range and must include a number of variables such as high lift devices and differing aerofoil shapes.for more info.

[rlrsk8r1]

@nakedleader Define "Conventional camber" and give three examples. I'm willing to accept there's an airfoil shape that will stall at 14.7. And 14.8. and 14.9. You also have to define whether you're talking about beginning of separation or full disruption of airflow. The PHoAK is written about airplanes in general, so the 16-20 number takes into account airplanes with low critical angles and airplanes with high critical angles.

[nakedleader]

@rlrsk8r1 Conventional camber i.e. not symmetrical or supercritical which have different characteristics. The standard reference probably refers to a particular NACA aerofoil number but around 15 degrees is the baseline. I would be interested to see an aerofoil that stalled at 20 degrees without lift augmentation. A stall (Vs) is when lift has reached its maximum value typically called the critical angle. Flow sep has occured to the extent that the wing's lift is substantiallty disrupted.