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Aeronautical Engineering/Control surface/yoke movement in turbulent air


QUESTION: Do you think there would be so much effect on control surface movement in moderate turbulence/updrafts/gusts that the controls might move considerably on their own? I thought even because of inertia, when a gust hits the main surface where the control surface is attached i.e. stabilizer - elevator, wing - aileron? Or it will be insignificant considering the laminar flow makes harder to deflect them? Talking about small aircraft, mechanical reversible controls, not hyd actuated or others on complex aircraft. THANKS!

ANSWER: I believe control surfaces of small aircraft can vibrate and move small amounts due to turbulence or gust encounter primarily because, over time, cables can stretch and allow flexibility in the system. Large movements of control surfaces do not normally occur because a) the forces would have to feed back to the actuator by overcoming a lot of friction and inertia, and b) the control surface such as aileron or elevator usually has a small moment arm compared to the pilots yoke. Flaps are mechanically fixed in position. Laminar flow has nothing to do with control surface motion. The amount of motion depends on the aircraft design and age. It is hard to generalize more than this because of the many different control system designs in operation.

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QUESTION: And you think if the surface would be disconnected from the ctl system, do you think it would move considerable? Usually in flutter books it is said that if the CG of the surface is not on the hinge line, the surface would react when the wing (or stabilizer) is hitted by a gust, do you think that it's told so due to the fact that to induce flutter is enough even a small movement or the movement in case of a disconnected surface is large?

ANSWER: I am having a little trouble understanding your question. I guess we are back on the flutter issue again. Yes, if a control surface were disconnected it would react to a gust with large movement. Whether or not it flutters or damps out and floats to a new position depends on the design of the hinge moments and related stability. I think we already discussed hinge moments.

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QUESTION: Yeah, it is a little related to the flutter issue, actually I don't get it why it would float to a new position or it would react differently when it it disconnected because I think the hinge moment is the same in both conditions, disconnected or not, and the friction in the system is neglijable. For example, on ground, there is no noticeable difference between moving the elevator with your hand and moving wht the yoke and even if there is a difference, usually it is easier to move the elevator with the yoke, not otherwise. Sorry if I'm wrong.

OK - now I am a little confused as to which problem you are trying to solve. If an aileron is set to +20 deg it will have aerodynamic loading that creates a hinge moment that tries to raise the aileron tail, but is prevented by the actuator forces. The aileron loading is of course causing the increased wing lift to roll the airplane. If the aileron were disconnected at that point, it would float up. As soon as the aileron floats up, the new pressure distribution will tend to prevent the aileron from floating up. At some aileron angle, the floating tendency will be equal to the restoring tendency and the aileron will be floating at some new angle. Whether or not it flutters in the process is a whole different problem that is hard to predict.

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Paul Soderman


Aeronautics, Aerodynamics, Fluid Mechanics, Aeroacoustics, Noise Control, Muffler Design, Wind Tunnel Research.... I know nothing about India - do not ask about schools, jobs, application requirements, career choices, etc. for India. Please, no text message verbiage; I prefer full words in full sentences. Thanks.


38 years as research engineer at NASA


B.S. and M.S. Aeronautical Engineering - U. of Washington, Graduate work Standford U.

Awards and Honors
AIAA Associate Fellow (American Institute of Aeronautics and Astronautics)

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