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Can aileron differential completely eliminate adverse yaw?

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    $\begingroup$ Do you intend a fly-by-wire computerized system that can apply different amounts of differential aileron at different airspeeds etc to be within the scope of your question? If not, consider expanding the question to something like-- "can a simple mechanical system of aileron differential completely eliminate adverse yaw at all airspeeds?" $\endgroup$ – quiet flyer Jan 14 at 19:38
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    $\begingroup$ @quietflyer I mean at real plane $\endgroup$ – user53913 Jan 14 at 20:01
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    $\begingroup$ That didn't answer the question! $\endgroup$ – quiet flyer Jan 15 at 1:34
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That depends on flight speed.

Adverse yaw is strongest at low speed. A high-aspect ratio configuration will not be able to avoid adverse yaw, regardless of the degree of aileron differential.

At high speed the same airplane might roll fine without rudder corrections, because then aileron differential will be able to compensate for the then low adverse yaw if enough differential is provided.

Depending on airfoil rear camber, adverse yaw is also good for reducing aileron control forces. Again, the effect varies with speed and will result in a high degree of force reduction at low speed while forces are still high at high speed. The need to keep aileron forces growing with deflection angle is normally the limit for aileron differential, so it is impossible to tailor differential to overcome adverse yaw at low speed even for wings of moderate aspect ratio.

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  • $\begingroup$ I've yet to fly a non-yaw dampered airplane with zero adverse yaw even at higher speed. Even a flies-itself plane like a 172 will displace about half a ball if you roll into a turn with no rudder input and I seem to remember that half a ball maximum displacement is considered to be within the range of "no rudder required" so to speak. $\endgroup$ – John K Jan 14 at 22:57
  • $\begingroup$ "Adverse yaw is strongest at low speed. A high-aspect ratio configuration will not be able to avoid adverse yaw, regardless of the degree of aileron differential." -- even if the ratio of motion of the rising aileron to the descending aileron is so high that the descending aileron stays essentially fixed? $\endgroup$ – quiet flyer Jan 15 at 2:11
  • $\begingroup$ @quietflyer Yes. For rolling you need a lift difference which will translate into different directions of the lift vector on the outer wing. Even if one aileron doesn't move, the other does and produces plenty of yaw. With a deflection of >30° you can create a lot of drag and reduce the lift vector tilt, so if you operate the ailerons as drag rudders, you might be able to avoid adverse yaw initially. But then the rolling motion starts and the angles of attack on both wings diverge, again creating yaw. It will be awfully hard to completely cancel this yaw, too. $\endgroup$ – Peter Kämpf Jan 15 at 11:36
  • $\begingroup$ @JohnK: I can confirm your observation. But differential on the C-172 isn't very pronounced (20° up / 15° down). I made the second paragraph more weasel-worded, because you are right, not every airplane avoids adverse yaw only by flying fast. $\endgroup$ – Peter Kämpf Jan 15 at 11:40
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Based on my experience (seeing, not flying) with radio control models, no, not completely. I have seen "drag buckets" added to ailerons, as well as coupled spoilers, which can come close, but there are large efficiency penalties for drag buckets, and added complexity and weight for coupled spoilers. Coupled split aileron "drag rudder" function (as in the B-2 Spirit bomber) is probably the closest you can come to eliminating adverse yaw with only "aileron" surfaces.

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  • $\begingroup$ I mean at real plane.Does this exsit at real planes? $\endgroup$ – user53913 Jan 14 at 20:00
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    $\begingroup$ Aerodynamics works essentially the same whether your wingspan is two meters or twenty. As noted, the B-2 Spirit uses split ailerons for yaw control, including computer control to cancel oscillations and presumably adverse yaw, and I've seen coupled spoilers mounted on full size sailplanes. If you have a fin and rudder, however, you don't really need this in aircraft that don't make very tight, underbanked circles at low speed the way sailplanes sometimes do -- you just use a little rudder, and watch turn-and-bank or just a yaw string to keep the airplane in coordinated flight. $\endgroup$ – Zeiss Ikon Jan 14 at 20:20
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Can aileron differential completely eliminate adverse yaw?

Absolutely.

All you need to do is have the amount of aileron differential be controlled by a computerized fly-by-wire system that continually tailors the amount of differential to whatever part of the flight envelope (airspeed, G-loading, angle-of-attack) the aircraft is operating it, so that the nose of the aircraft keeps pointing directly into the airflow / relative wind.

There's no need to actually measure all these parameters-- all you have to do is sense the sideways G-load on the aircraft and tailor the aileron differential to drive the sideways G-load to zero. Exactly as is more typically done with an automatic rudder coordination system.

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    $\begingroup$ At the time of this answer being posted, Peter Kämpf had already good reasons for believing that aileron differential alone cannot completely cancel adverse yaw. While fly-by-wire can modify the degree of differential according to the circumstances, it is not magic. What this answer lacks is an argument for the proposition that this degree of control over the differential is sufficient to cancel adverse yaw in all circumstances (or at least those that are likely to be met in practice - and one such case is on the point of a stall, given the role of yawing in spin initiation.) $\endgroup$ – sdenham Jan 15 at 13:11

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