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So far I have never come across any testing being done on having the ailerons and/or elevators on the leading edge of a wing, rather than on the trailing edge.
Does anyone know of such tests existing?

It somehow makes sense to me to have the controlling surface in the strongest part of the wing, which is not the trailing edge. Having all the 'heavy stuff' in front of the main carrying component, allows for a fairly light trailing part. Moreover the force applied on controlling the leading edge is less, because it changes the airflow without causing extra drag.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – Federico Apr 26 at 11:02
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They were tested in 1935. Since ailerons are thin (thickness-to-chord ratio), they were placed in a leading edge slot, and it was dubbed slot-lip aileron. The study's conclusion is:

The slot-lip ailerons appear to be usable as a form of lateral control device that shows promise of giving improved lateral control and stability at the high angles of attack through the stall, with negligible lag, light control forces, and relatively simple construction. It is desirable that additional tests be made to find the yawing moments given by these ailerons and the effect of the slot on the drag.

enter image description here

The aforementioned additional tests were carried out in 1937; of note:

  • The more forward the slot-lip is, the higher the lag (control delay after input). It works best when the slot-lip is in the rearmost 20% of the wing.

My understanding of the lag is that it's due to the delay in relieving the low-pressure behind the aileron as it deploys, especially nearer where the lowest pressures exist on the upper surface.

  • Much higher drag in the order of +10% at high speed and +35% in climb attitudes.

Weick, Fred E., and Joseph A. Shortal. "Development of the NACA Slot-lip Aileron." (1935). https://ntrs.nasa.gov/citations/19930081340

Shortal, Joseph A. "Wind-Tunnel and Flight Tests of Slot-Lip Ailerons." (1937). https://ntrs.nasa.gov/citations/19930091678

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Truly leading edge ailerons would not work.

It is mainly the orientation of the trailing edge that determines the direction in which the air flow continues behind the wing and therefore the lift. The forward part of the wing is there mainly to turn the air flow gently so it stays attached to the surface.

If you have a leading edge flap, and deflect it downward, you are increasing camber, which should increase lift, but you are also reducing the effective angle of attack, because the leading edge goes down, but the trailing edge does not, and the overall effect is slight reduction of lift—note that the angle of attack of the whole aircraft does not change much, because most of the lift is generated by the inboard part of the wing that did not change.

Worse, if you deflect it upward, you will increase the lift slightly, but you will also reduce the critical angle of attack, because now the airflow meets the wing at sharper angle. So at higher angle of attack, like during approach, the wing will stall, lift will decrease and now all you have is a decrease of lift on both sides and a lot of extra drag on one. Not what you wanted.

That's why the other answers only mention spoilers—which are quite common for roll control—or that strange variable slot which increases lift in somewhat different way. But a true leading edge flap would not work.

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  • $\begingroup$ The idea was not a leading edge flap, but a change of the shape of the leading edge itself as roll control. My logic tells me that that can not simultaneously result in both too much and too little influence. It does change the AOA as it moves one of the coordinates of that angle up or down, but it does not by far create as much drag as normal ailerons. It can also add to the structural integrity of the wing and can be used as a deicing boot. Its an attempt to get away from the idea that what is the easiest way to control is automatically also the best way to control. $\endgroup$ – Berend Apr 27 at 7:41
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    $\begingroup$ @Berend The "too much or too little" issue is one of nonlinearity. One sees control reversal; a small deflection pushes one way, a larger deflection pushes the other (or, worse, stalls). It generally reduces structural integrity, I don't know why you might think otherwise. $\endgroup$ – Guy Inchbald Apr 27 at 9:34
  • $\begingroup$ @Berend leading edge flap and changing the shape of the leading edge is the same thing implemented using different mechanics. $\endgroup$ – Jan Hudec Apr 27 at 11:14
  • $\begingroup$ @Berend I don't know why you think it would create less drag than trailing-edge flaps (it does not matter whether they are ailerons or elevators or rudder or flaps), I would expect it to create more drag for the change in lift because it creates two effects that cancel each other in lift, but not in drag. $\endgroup$ – Jan Hudec Apr 27 at 11:14
  • $\begingroup$ @Jan Hudec "leading edge flap and changing the shape of the leading edge is the same thing implemented using different mechanics" Theoretically, not really. In practice definitely not. Aerodynamics is a highly conventional science. That is what test pilots make their living on.Leading edge flaps change one flow path, whereas changing the leading edge itself redistributes air over both flow paths, but does so without changing them. $\endgroup$ – Berend Apr 28 at 14:20
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JW Dunne briefly tried leading-edge elevators on the D.3 glider at Blair Atholl in 1908. His aircraft had tailless swept wings. The elevator flaps were triangular shaped and fitted at the nose. They just blew back over the wing in flight, and he gave up on the idea. (The first certified stable aeroplane, the D.5, was of similar design and flew in 1910, and did not have them fitted.)

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I have seen photos of a hang glider that was modified to have spoilers (or "spoilerons")1 on the top surface of the wing for better roll response. The spoilers were controlled by sliders on the control bar, and both could be opened at once for extra drag to increase the descent rate. The spoilers were hinged at their leading edges, like typical spoilers on airliners, not rising upward out of the wing like "blades" like the dive brakes of some sailplanes or like the spoilerons used on the P-61 Black Widow.2 They were fairly narrow in chord, and I believe they deflected up to well over a 45-degree angle.

This hang glider was a traditional "flex wing" design, where the non-fabric parts of the airframe are comprised only of an aluminum leading-edge tube defining the swept leading-edge of each wing, and a spanwise "crossbar" connecting the leading-edge tubes at about mid-span (and forming a strong triangular structure with corners at the apex of the wing and at each of the two junctions between the leading-edge tubes and the "crossbar"), and a fore-and-aft "keel" tube, plus the triangular "control" frame and the kingpost, and steel wires running from the top of the kingpost and the corners of the control frame about to the midspan area of the leading-edge tube. Also there were shaped aluminum "battens" (like the top edge of a traditional wing rib) inserted in pockets on the top surface of the wing. The fabric of the wing was also stiffened by mylar inserts curving around the leading edge and running aft about 12" max on top and less on the bottom.

Anyway, you can see how the fabric trailing edge would be very flexible, and a traditional aileron would only make a servo tab effect that would bend the wing in the opposite direction, negating most of the effect and making some amount of wrong-way effect. So the spoilerons were mounted about midspan, and quite far forward, just about 8-12" (as I recall) aft of the leading edge, where the fabric was not free to flex due to the close proximity to the stiff mylar insert that curved around the leading-edge tube. The spoilers were on the top surface of the wing only.

The pilot reported an effective boost in roll response above that available from pure weight-shift alone.

Obviously in this case the (lack of) stiffness of most of the wing surface played a key role in determining where along the wing chord the spoilers would best be located. Their spanwise location was further inboard than would be optimum in terms of maximizing roll control, but was selected to avoid pitching up the glider when deployed. (The outboard portions of the swept wing of a hang glider are well aft of the C.G., even near the leading edge.)

"Rigid wing" hang gliders constructed with a strong carbon-fiber "D-tube" and fully defined traditional wing ribs have minimal flexibility to the portions of the wing aft of the leading edges, so their spoilerons are located further outboard for better roll control power. They are also located further aft along the wing chord, and deflect upwards only a modest amount, acting somewhat like a cross between an aileron (but moving upwards only) and a spoileron-- presumably this has been found to produce less drag than a pure spoileron design that rises up to a position closer to vertical. The spoilerons do produce some amount of pitch-up tendency due their outboard location behind the CG, but this is minimized by the modest sweep used in these designs. Alternatively, some rigid-wing hang glider designs use actual ailerons attached at the trailing edge itself, while still relying on weight-shift for pitch control. However, a disadvantage of ailerons over spoilerons on a long-spanned, slow-flying, tailless, rudderless "flying wing" design is that ailerons tend to produce "adverse yaw", while spoilerons generate a helpful "proverse" yaw torque.3

Footnotes:

  1. Generally any spoiler that can be used for roll control (by deploying it but not its twin on the other wing) is called a "spoileron". Devices attached to the trailing edge of the wing are usually not called "spoilers". However, in the radio-control modeling community you'll find the word "spoileron" to mean an aileron, mounted at the trailing edge of the wing, that is also used to increase drag by raising it and its twin on the other side of the wing at the same time, in contrast to a "flaperon" which is an aileron, often full-span, that is also used as flap to increase drag as well as lift, by lowering it and its twin on the other side of the wing at the same time. Model airplanes (typically gliders) with surfaces deployed in "crow" mode feature flaps or flaperons inboard, and spoilerons (in the latter sense described above) outboard. So, the word "spoileron" has acquired two rather different usages. The first usage noted above, denoting a spoiler mounted forward of the trailing edge of the wing but used at least in part for roll control, is arguably more correct and more broadly accepted in the aviation world, and it's the one that is used in this answer.

  2. Photo of "circular arc" spoilerons used as the primary roll control surfaces on the P-61 Black Widow-- they were located aft of the wing spar, and basically rose out of the wing like vertical blades, but also curving forward slightly-- http://www.maam.org/p61/images/ntwo%20new%20spoiler%20panels.jpg , from http://www.maam.org/p61/p61_rest.htm . "Circular arc" is mentioned here-- https://en.wikipedia.org/wiki/Northrop_P-61_Black_Widow#Design .

  3. Rigid-wing hang gliders controlled by spoilerons are designed with dihedral as well as sweep, to help harness "proverse yaw" to create a helpful roll torque. Those controlled by ailerons never have dihedral and often have some anhedral to minimize the unhelpful roll torque generated by "adverse yaw"-- as do modern "flex wing" hang gliders controlled by weight shift, which also experience some degree of "adverse yaw".

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  • $\begingroup$ Somewhere I have a photo, but won't be able to find it today-- $\endgroup$ – quiet flyer Apr 26 at 13:15

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