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Could this be a viable way to control roll on an aerobatic airplane, using some sort of tab to control a monobloc variable incidence wing? Both wings are mechanically linked together in order to only allow roll ( like conventional ailerons are )

Advantages could be a very high roll rate (deg/sec) and low effort on stick (get rid of ailerons aerodynamic compensators)

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  • $\begingroup$ On the update: low effort on stick might not be desirable, force feedback is actually useful. FAR 25 and 23 have requirements on the stick forces, they may neither be too high or too low. $\endgroup$ – Koyovis Aug 18 '17 at 14:02
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When changing wing incidence for roll control, it is important to have equal but opposite deflections on both sides. Otherwise you will add load factors to the results of your roll inputs. If you have some gearing which will couple the movements of the two wing halves, the idea works. But try to avoid a stall; stalling has a high potential to result in an uncommanded roll.

Next, make sure that the lengthwise center of gravity location of each wing is as near to the hinge of the wing as possible. If you have some distance between the two, any roll input will add inertial moments around the hinge. This will make your stick come alive in a rolling maneuver, in unpleasant ways. If the center of gravity is behind the hinge, roll accelerations will cause amplifying moments. Your stick will move into the roll if you accelerate fast enough! Conversely, if the center of gravity is ahead of the hinge, every roll acceleration will be damped by inertial moments around the hinge. Much safer, but will cause more stick forces than necessary to achieve a good roll acceleration.

If the hinge is close to the aerodynamic center of the wing, control forces should be low (except for friction, which might be immense in case of poor design). To move the wing by a servo tab, however, adds one more degree of freedom and is an invitation for flutter. It might work, but is best tried in a second step. Only make sure that the hinge is ahead of the aerodynamic center, or the wing will not return to neutral with zero force on the stick.

Unfortunately, a stalled wing has a much lower suction peak at its nose and less pressure recovery in the region of separated flow, so its center of pressure moves back. In an unsymmetrical stall, the stalled wing will pitch down and will pull the unstalled wing into its positive stops, stalling it in the process and resulting in an uncommanded roll input.

Structurally, there is no problem if you use this on a small plane. Many designs have used tubular spars, so use one smaller one which sticks out of the center wing and two bigger ones in the outer wings which will slide over the center tube. Adding two needle bearings on each side with enough spacing should lower friction enough to make the wings move even under load.

Historically, the Akaflieg Berlin 02 "Teufelchen" used a similar way of roll control. The inner wing was conventional, using two box spars and a stiff nose covered with plywood. The outer wing was fully fabric covered and had its ribs movably fitted to a tubular spar. By twisting the outermost rib, the outer wing could be warped for roll control.

EDIT:

Three other aircraft with a fixed center wing and all-moving wingtips were the Granger Archaeopteryx, the Short SB.1 glider and the Short SB.4 Sherpa research aircraft. All were built once only. Please follow the last link for pictures of the control surfaces in action.

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    $\begingroup$ Were any of the light aircraft with tubular spars fully aerobatic? $\endgroup$ – Koyovis Aug 15 '17 at 21:55
  • $\begingroup$ Thank you Peter for your inputs. Let's assume we have equal but opposite deflections on both sides (mechanically), adequate centers of lift and gravity for every moving parts (close to zero stick effort, but natural tendency to go back to neutral) and structural solutions (tubular spars + bearings) : How would everything behave when entering (for instance) a flat spin? Would it even be possible to start and quit a flat spin, or a snap roll? By generalization, how about very high AoA behavior? $\endgroup$ – qq jkztd Aug 15 '17 at 23:39
  • $\begingroup$ (And asymmetrical AoA?) $\endgroup$ – qq jkztd Aug 16 '17 at 0:06
  • $\begingroup$ @Koyovis: There was even a dive bomber with a tubular spar. $\endgroup$ – Peter Kämpf Aug 16 '17 at 22:46
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    $\begingroup$ @qqjkztd: Ah, excellent question! With separated flow the center of pressure moves back, so at least one wing will try to move to the stop for smallest incidence. If there are coupled, I would expect that the wing with more separation (in a spin that is the one moving back) will overpower the other wing, so you will end up with a massive roll input into the spin. This makes the spin steeper, so ending it will be easier than with neutral ailerons. However, you need to force the wings to neutral for a clean recovery. Symmetric flight at high AoA should not be unusual, however. $\endgroup$ – Peter Kämpf Aug 17 '17 at 20:38
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Yes it is a viable way for roll control - aerodynamically that is. The mechanism you describe is a proper servo tab, where the aeroforces on the tab are fed back, and the tab acts like a lever on the main control surface. This was the way the post WW2 airliners were controlled, before they got too big and fast and only hydraulics could do it.

However, as usual, an aerodynamically beneficial solution has some serious structural drawbacks. An aerobatics plane can do 6g and the wings must be seriously strong and resistant to bending and warping. Normally, a wing has two spars which form a torsion box with the upper and lower skin.

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If this torsion box can extend from wing tip to root, it is a strong and light construction. But if it has to be cut somewhere and the two wing halves are to be connected by a hinging rod, that spot will be a weak spot in the wing. It can be made strong enough, but the real problem is wing stiffness.

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The full-wing control is already invented... In a well-known ultralight design of the 30s, the 'Pou du Ciel', the pitch was controlled by tilting the entire front wing (directly, not with a servo-tab. The 'Pou' had also a rear wing...) thus changing its AoA... https://en.wikipedia.org/wiki/Mignet_Pou-du-Ciel

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This design isn't too far off from the wing warping used in the Wright Flyer and some other early airplanes. However I think this design has the same problem as swing wings - weight. The rotation system would have to be strong enough to handle the weight of the fuselage and you would have to run fuel lines, hydraulic lines (flaps, etc.) as well.

We do see this idea used in all-flying tails, but there you are dealing with smaller surfaces.

So in summary, I would call it feasible but not economic.

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