Glenn Curtiss's 1912-1918 Model F biplane flying boat has large ailerons on the struts between the wings, not contiguous with any other flying surface. Did this aileron position have any advantage that either he knew of or that we now know?

[Per SupBruh's comment, contrast this position to conventional upper-wing position. Lower wing was rare even then, and particularly for a seaplane where an aileron this big would hit the water when deflected.]

He knew about wing warping, so he knew the effectiveness of camber (hinging an aileron at a wing's trailing edge) as compared to just changing an airfoil's angle of attack. He knew how to make a hinge: this aircraft uses them at rudder and elevator.

He also knew about modern ailerons: in 1908 he flew an aircraft with them.

Maybe he chose such a large chord for the ailerons to try to increase their authority. Photos suggest that the airfoil was just a flat plate, which stalls at a deflection angle much less than that of a conventional aileron. But the drag from a stalled aileron that big would produce scary adverse yaw.


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    $\begingroup$ At times, patent battles with the Wright brothers were an issue in where Curtis chose to place his ailerons. I don't have time to type up an authoritative answer right now so maybe someone else can run with it-- but I see that you are not primarily asking about motivation. But that being the case, you might consider changing the title to match what you really most want to know? $\endgroup$ May 20, 2021 at 17:01

2 Answers 2


Disclaimer: I can only speculate, having no direct involvement with the development of the Curtiss Model F.

Stick forces were not really an issue with the small and slow early aeroplanes. As both size and speed went up, they would become a design consideration first because the pilot strength is lowest for lateral motion of the stick or control horn. With more aerodynamic experience, elaborated ways of reducing stick forces were employed.

My explanation, therefore, is that this is an early attempt at reducing stick forces. Hinged at the trailing edge, the complete chord length of the aileron would be behind the hinge line and no offsetting moment could be produced. Also, being part of the wing, ailerons hinged at the trailing edge will float up once lift is created. This requires both ailerons to be interlinked with stiff control cables which again raises stick forces, now from friction. Also, this preload from lift limits the effectiveness of a downward deflection of a wing-mounted aileron.

The free-flying aileron, on the other hand, has the advantage of starting with a clean sheet, if you will:

  • No preload exists, so it will be equally effective in up- and downward deflection.
  • It is not encumbered by the boundary layer of a wing flying before it, so its neutral axis will be easy to find and be constant over angle of attack, unlike that of a wing-mounted aileron. This was certainly not formulated this way at the time of the design of the Model F, but experience would bring the designers to exactly the same conclusion. The same considerations brought Junkers (more precisely: Otto Mader) to the offset flaps and ailerons in their designs from the late 1920s to early '30s.
  • With the aileron position between wings, the hinge line can be freely chosen at any chord position. The picture below (source) shows that a small part of the aileron was ahead of the strut, suggesting that the hinge line was not at its leading edge, but set back a bit. This enables the forward part to act as a balance, thus reducing stick forces.

Curtiss Model F on the water

I would expect that the Curtiss Aerorplane Company tried out different hinge line positions, which is easily done with the inter-wing arrangement, and settled on the one shown above, because it reduces stick forces considerably but still produces a clear centering force, which is important for agreeable handling qualities.

I do not think that the inter-wing position would reduce wing flex: Lift forces from the aileron are introduced into the wing by the rear strut, so both torsion and bending are very similar to a wing with ailerons on its trailing edge. To reduce torsion, the aileron would have to be mounted on the forward struts. Then they would even warp the wing in a way supporting the intended rolling moment. I speculate, however, that Curtiss and his engineers maybe considered that option but were afraid of the resulting aerodynamic interference between wings and ailerons.

That the inter-wing position lost against the balanced, wing-mounted aileron is certainly due to the much lower drag of the latter. Note that the inter-wing aileron needs its own rigging. With the increased experience of balancing ailerons, the wing-mounted version lost its main disadvantage and became the overall better solution.

  • $\begingroup$ "Disclaimer: I am not over 130 years old." :) $\endgroup$ May 21, 2021 at 20:02

Interplane-strut mountings would offer multiple benefits:

  • Aerodynamic stability (it would reduce the chances of structural failure)
  • Structural stability (to put it simply, it would make the plane stronger)
  • reduce wing flex (it would greatly reduce the flexing of the wing caused by strong forces of, well, air)
  • render ailerons more useful (Interplane-strut mountings would keep the wing straight, allowing the ailerons to do their job properly. without interplane strut mountings, the wing would flex due to the upward force that the ailerons produce when you roll. When the wings flex, (specifically to top wings of this biplane) it will reduce airflow to the ailerons, rendering them much less useful. with interplane strut mountings, the wing will no longer be able to flex as much, allowing maximum airflow to the ailerons.
  • $\begingroup$ By stability I refer to both more aerodynamic stability and less wing flexing. interplane strut-mounting would also increase the planes critical velocity (enabling you to go faster without completely destroying your sweet ride) and with the wing warping, my mistake. with all the wires between the wings I had though for a moment that the airplane's primary form of control was wing-warping. Now that I remember what plane this is and after zooming in much closer, I can see that this plane is fully aileron-controlled and not partially aileron-controlled. $\endgroup$
    – SupBruh
    May 20, 2021 at 21:14
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    $\begingroup$ and it also depends on specifically which wing(s) you're seeking to apply (or seeking to find the results of what improvements it would make if you applied) the mountings onto. $\endgroup$
    – SupBruh
    May 20, 2021 at 21:57
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    $\begingroup$ I disagree with your assertion that wing flex is reduced. After all, the aileron lift force is introduced into the wing by means of the strut. Bending and torsion will be the same as for a wing-mounted aileron. Besides, this wrong reasoning is used to support all four bullet points, so this answer is completely wrong. $\endgroup$ May 21, 2021 at 1:51
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    $\begingroup$ Re " interplane strut mounting will form a triangle between the strut, the wing, and the fuselage" -- I fail to see how the specific items named here form a triangle. (But read here if you want to learn how other parts of the rigging of a biplane typically do form a triangle -- aviation.stackexchange.com/a/66468/34686 ). Maybe what you are trying to say is that having an interplane strut allows the diagonal wire to complete a triangle whose other sides are defined by the strut and the wing, and whose apex is at the fuselage> $\endgroup$ May 21, 2021 at 5:59
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    $\begingroup$ In fact the first comment states that when he originally conceived the answer, he didn't realize the plane had ailerons at all. And it's still not clear that he has yet realized that the ailerons are actually mounted on the interplane struts, and that that's what the question is actually about. Again, maybe a good answer-- but not to the present question. Hope something in these comments helps the answer to be edited to be improved. $\endgroup$ May 21, 2021 at 6:57

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