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When taxiing, taking off and rolling out we are told to use cross-wind correction inputs. Assuming a headwind, you bank into the wind by raising the aileron on the windward side and lowering the other aileron.

However, it appears that just before you take off, even with the wings level, holding the ailerons as above helps the plane avoid skidding sideways on its wheels. I understand the physics once you are airborne and the wings are no longer level (you will slip into the wind). But what I don’t understand is why holding up aileron on the windward side helps prevent skidding sideways, even with the wings level.

How do the physics work to prevent sideways force on the gear? Or am I imagining that the wings are level and in fact the windward wing is lowered?

I’ve felt this more strongly in a Citabria 7ECA or similar light tailwheel, high-wing aircraft, but I think it also applies to a tricycle-gear C172 or similar.

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  • $\begingroup$ Perhaps the raised aileron provides more torque (given the aircraft’s forward speed) than the lowered aileron? That would give a net into-the-wind torque which could correct the weathervaning tendency. (But would be in the opposite direction to your theory.) $\endgroup$ – Peter Nov 19 '14 at 7:15
  • $\begingroup$ Some good points here @falstro. My question is in fact precisely whether the correction you do does prevent skidding without increasing the downward force on the wheels. It feels like it helps prevent skidding (even when the aircraft is about to lift off) but I could be wrong. $\endgroup$ – Peter Nov 19 '14 at 7:21
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When taxiing in a crosswind, the fuselage blocks the airflow over the downwind wing (just as if it were in a slip), so the upwind wing has that much more lift than the downwind wing.

Also, if the wind gets under the upwind wing, it will just push the upwind wing further upwards, just as if you were carrying a 4x8 piece of plywood.

These two factors can contribute to tipping the airplane over.

With gliders, this is much more pronounced because they don't have wheels under the wings, so the ground crew always keeps the upwind wing lower in a crosswind. In a glider crosswind takeoff, full upwind aileron is applied before takeoff, and as the aircraft starts to roll, the aileron is continually reduced to keep the wings level, with downwind rudder to keep the fuselage aligned with the runway.

And now to answer your question: up aileron on the upwind wing will reduce the lift on the upwind side and increase the lift on the downwind side, to help the aircraft from sliding sideways.

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  • $\begingroup$ The question is about side force, not tipping over. @Peter mentions a skidding motion shortly before takeoff, and that can only be explained by a side force, not a rolling moment. $\endgroup$ – Peter Kämpf Nov 19 '14 at 14:53
  • $\begingroup$ good catch re slideing versus tipping. corrected. I don't have "rolling moment" anywhere in my answer. you must mean the other one. $\endgroup$ – rbp Nov 19 '14 at 14:57
  • $\begingroup$ I understand your mentioning of a lift difference as a rolling moment. Pushing one side of the wing upwards sounds awfully like a rolling motion. $\endgroup$ – Peter Kämpf Nov 19 '14 at 15:50
  • $\begingroup$ there is both a horizontal and vertical component. you need enough horizontal component to counteract the wheel skidding. $\endgroup$ – rbp Nov 19 '14 at 15:53
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Your assumption is that the wings are level when both main wheels are on the ground. But this is not necessarily true. Even when both wheels are on the ground, the wings can still have a small tilt. The suspension of each gear works independent of the other, and the weight on the wheel determines how much the suspension compresses or the gear strut will bend. By deflecting the ailerons, you re-distribute the lift between the wings, and control gear compression.

Please note that with the ailerons level, your wing will create a sizable rolling moment in a crosswind, which means that the load distribution between the two main wheels is uneven without aileron correction. Also, the fuselage will create a side force of its own, and this force increases with speed as more of the flow around the fuselage will be attached.

The slight tilt angle of the wing also slightly tilts the lift vector, and this, together with the side force of the fuselage, creates the side force which causes the skidding when this side force is large in comparison with the low load on the wheels shortly before takeoff. You need to tilt the wing into the crosswind to create a counter-force which can compensate the side force of the fuselage, only then the whole aircraft will not experience a side force.

I know, the picture below shows a glider and not a Citabria, but the principle is the same. The high wing arrangement creates a sizable rolling moment even without dihedral.

enter image description here

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  • $\begingroup$ how can it have the same local alpha distribution and opposite rolling moment? $\endgroup$ – Federico Nov 19 '14 at 8:48
  • $\begingroup$ @Federico According to whether the fuselage is masking the wind above or below the wing. In the top diagram, the high wings mean the air pressure is lower below the downwind wing; in the bottom diagram, the low wings make the air pressure lower above the downward wing. $\endgroup$ – Dan Hulme Nov 19 '14 at 9:23
  • $\begingroup$ @Federico: You are right, the diagram labels were wrong. Thanks for finding it! $\endgroup$ – Peter Kämpf Nov 19 '14 at 9:53
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    $\begingroup$ the part that this answer is missing is that even without the lift of an airfoil, allowing the wind to get under the upwind wing is enough to flip the plane over. Try carrying a 4x8 piece of plywood in a crosswind. also gear compression has little if any to do with it: citabrias have unsprung gear, and gliders have none, but they are both still susceptible to flipping over in a crosswind. $\endgroup$ – rbp Nov 19 '14 at 14:34
  • $\begingroup$ @rbp: Then I ask you to walk up to a Citabria 7ECA and rock the wings. That thing will rock like a swing! The cantilevered gear struts have plenty of elasticity. $\endgroup$ – Peter Kämpf Nov 19 '14 at 14:45
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I think the biggest component of the difference is the surmised answer, that is the wings are not level in the described situation. If wing is strong enough, you will have to transition to rolling on one, upwind wheel, or you will not be able to keep on the centerline.

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