# What is the axis of rotation in a high alpha roll?

When doing an aileron roll at a high alpha (say 25 degs), which axis will the aircraft rotate about?

Is it the incoming air-stream vector or is it more the body X axis? And does the pitch inertial coupling factor into this or is that an unrelated phenomenon?

I am thinking in terms of high agility aircraft like for example the F-16.

That depends on the control system of the aircraft, in case of fighter jets with fly by wire technology and modern fly by wire airliners the yaw control law usually tries to rotate around the velocity vector, which in calm air is the same as the incoming air-stream vector. (according to my university lectures)

The difference in the target yaw rate (r) is just the body roll rate around the x-axis (p) times the tangent of the angle of attack (with the sign flipped, depending on which coordinate system you use).

Conservation of momentum demands that the aileron roll is around the body axis (more precisely: the longitudinal axis of inertia). If only an aileron input is made, the result is a rolling moment (plus a bit of yawing from adverse yaw) but no change in pitch.

Proof: Do an aileron roll once without a bit of pitching up before you start aileron input and then with. In an airplane with a regular cambered wing, you need to increase pitch to compensate for the wing's incidence and zero-lift-angle, or your loss of lift will be substantial when flying inverted.

Of course, in an F-16 all pilot commands are interpreted by a computer and will result in movements of all control surfaces, so a pure aileron roll is probably impossible.

• "so a pure aileron roll is probably impossible" at slower speed .But at the maneuver speed @440knits it is not a problem .The F16 CAD will do it very fast (@320degrees per second)The problem with F16 is it will deflect the horizontal tails also to do the rolling. The GA aircraft-designed use only rudder to keep the ball centred and ailerons to do it. Jan 16, 2021 at 21:57

To my understanding if the F16 is at 25 degree's alpha for more than a second it is very slow(@ 110_120knots in level flight ) with no G's available (one G only) and rolling (is prohibited anyway - to assault two limiters at once, rolling /pitching, but let's do it anyway )now it is prone to get you into the inertia coupling, anti-yaw rudder kickin now - no pilot input with rudder pedals- so before that pro adverse yaw rudder but if inertia coupling begin to get into the play anti yaw - opposite rudder is used by Ailerons Rudder Interconnect system ). But let's see if we can arrange for that.The rate of the rolling movements is half from the maximum amount. So it is a small helix (barrel roll) but is no longer around the lift center but more likely to the CG center (so it is a little behind) due Cg position only to the F16(behind the pressure center). In sim the application will not allow any further action (more coupled roll if you have central tank you can get the upright spin)

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The video show slow speed maneuvering