Hey guys, Can anyone tell me WHY the trailing edge of the ailerons is not tapered down to a knife edge? I talked to a friend who has flown for 15 years and he told me that they just work better that way. Is it a strength issue for models. It would seem to me that they would add a ton of drag and reduce the performance to a degree. Thanks Dave...[sm=confused.gif]

Because a knife edge will have some serious stall problems. You can get an efficiency gain at zero angle of attack (less drag), but the aileron needs to be able to modify the flow. A smooth edge will allow the flow to stay attached when the aileron is deflected and let the aileron do what it supposed to do... alter the effective angle of attack of the wing.

Strength has a little to do with it, but more from the above argument.

For very fast flight, tapering is a requirement.
For moderately fast flight, pattern planes for example, it doesn't hurt.
For sport flying, it makes no difference.
The extra mass of a non-tapered trailing edge enhances the opportunity for flutter at speed, as it's furthest from the hinge point.

The main reasin not to taper below appx 1/16 inch thick is the strengnth of the balsa. If you taper balsa to a knife-edge it may as well be tissue paper.

I have on some models laminated 1/64 plywood on the lower side of the control surfaces and then tapered the surfaces down to the plywood. That is for all practical purposes as close to the knife edge as you can get. It works fine. Its a bit heavy... so you get into cutting lightening holes in the control surfaces. The thin ply is fairly expensive. The performance gain is negligible if you are not trying to set a speed record.

If you really want sharp, laminate in a little carbon fiber. I've seen models with TE's you could cut with if you wanted to.

But the hanger rash just sucks. Personally, I'm forever bumping in to things moving my planes around the house and such.

Also, the extra drag on an acrobatic plane isn't bad as it helps with braking on the vertical down lines.

Oh, if you do have a flat TE, be sure to make the corners nice and sharp, don't round the TE. The rounding promotes flutter, a more squared off edge seems to flutter less.

Huh?

I'm sorry Ken but all the above is exactly the opposite. For duration or speed flying the airfoil WILL work best with a proper shape and the trailing edge is part of that shape. Tests have shown that there is significant drag from having the trailing edges even 1% of the chord thick when applied to cases where efficiency counts like is sailplanes and speed models. And as for causing a stalling problem I've seen nothing to support that. A truncated airfoil will still stall at pretty much the same point as a sharp edge one. Selig did some wind tunnel work on this and the results showed a small degradation in lift and gains in drag with truncated trailing edges at all lift coefficients including near the stall.

I've never seen anything to show that a deflected surface will work better with a blunt TE than a sharp one. If there is something on this I'd love to see it. Other than the rounded to blunt comparison there's nothing out there to my knowledge.

The reason most sport models or trainers use flat surfaces with blunt edges is for simplicity of building and strength against twisting. The extra strength helps prevent flutter while at the same time the extra mass encourages it. But for the most part the extra thickness prevents it in most cases. Fun fly models in particular pretty much need the thick and blunt surfaces to provide the wood required to support the structure. But fly these a little too fast and they flutter like the dickens.

Beyond that the other two answers hit all the high points spot on. For gliders the 1/64 ply is quite popular but equally so is using basswood or spruce for the last 3/32 to 1/8 of thickness so it can be shaped down to a fine edge.

Oh, and FH, if you use the ply in short one foot segments with the face grains running chordwise instead of span wise you can sand it down even further to a true knife edge. I have not done it that way yet, only as you described on two models, but I've read about a few souls that do it that way. Then when covering you can run the material about 1/64 to 1/32 past the edge to glue the top cover to the bottom for a nice sharp two covering thickness edge. Now THAT is pretty sharp.... and probably a bit too over the top since I just thought of it...

But all this is more for fast models, electrics or sailplanes. The differences are not that large and when there's a big oily noisemaker up front it doesn't really matter. In fact there are very few models that are as INefficient as modern funfly models. But light weight, lots of wing area and much better than 1:1 power to weight ratios mask all that. In truth the drag of these models is actually designed in to help limit the speed gains during burst of high throttle or when diving.

That's exactly what I was getting at... the difference between a deflected surface and the rest of the wing. Unfortunately i cannot find anything to support what i wrote (i was going for the intuitive explanation). I can't find anything to disagree with it either. Hmmmm... something to ponder.

Perhaps its a non-issue... I'm thinking too hard for a friday

A knife edge trailing edge is a requirement for real high speeds... pylon racers, slope racers, etc.
Nice but not mandatory for something like a pattern plane.
The shape makes no difference on the ordinary sport model.
As long as there's not too much mass hanging off the hinges, that is.
Fun flies with blunt surfaces NEED a tight control system to keep the surfaces on the airplane.
Even tapering won't do much good if the surface is floppy due to loose hinges, pushrod connection too close to the surface at the horn, a thin/whippy pushrod, overly heavy wood for the surface...

Interesting reading:

http://www.aoe.vt.edu/~devenpor/aoe5...20Harrison.pdf

The first time I ever attended a pattern contest, I noticed that all of those sleek pattern ships had squared off trailing edges. I asked several of the pilots why they were not tapered to a sharp edge as are pylon racer wings. I got two answers.

First I was told that blunt trailing edges prevent the control surface from stalling at high defelction angles. (Not a problem on pylon racers since they are usualy set up for smaller control deflection.) Second I was told that in pattern plane design, one of the goals was to make the airframe *clean, yet draggy* so that they do not gain too much speed in vertical dives.

Correct me if I am wrong.

The Pattern planes do want some drag. Thos long verticaldescents in some maneuvers have the Pattern guys doing all kinds of things to try to avoid increased airspeed in the dive. (the newest Pattern designs are going for wide fuselages and fixed landing gear.)

The guys who said anything about "stalling the control surface" were probably talking more about the amount of deflection at full throw rather than the truncated TE... The control surface will stall at about the same angle with the truncated TE as the sharp TE or even the undesireable rounded TE.

If you look at full size Sukhoi, the alierons TE is quite flat and un-tapered, this gives an expotential effect, which means the alieron is quite "damped" when small movements are given, so the aircraft is not jerky for small movements, but gives lots of action when you throw 'em around. "Poor mans expo" is a good explanation...

It is interesting to hear the various theories -
Here is yet another .
On the airfoil used on the fullsize Sukhoi and CAP /EXTRA, Note that it has a large BLUNT LE and then a straight line to the truncated TE.
One theory/guess is that the truncated TE adds a low pressure region, which actually assists in holding the airflow laminar to the flat wing surface-- remember -this is not the old shoe curve airfoil .
Onmodels -- for my money - make the aileron as stiff and light as possible - the rest of it matters very little.
I don't use sealed hinge lines etc. but I am a bug about keeping the cg of the aileron as forward as possible.
On my lil foamies - -these are just flat foam - hinged with surgical paper tape top and bottom - -but the speed is so low - an the weight is so low - it works very well -
When we did pattern designs - we never used thin TE on ailerons - just a waste of time and they got flimsy