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Aeronautical Engineering/Deflection direction change at low/high speeds


QUESTION: Would a disconnected surface, especially aileron, deflect upwards as you slow down due to increased alpha? I figure out it is more likely to deflect upwards as you increase your airspeed, thus having more airflow pressure below it and pushing it upwards.

ANSWER: Control surface deflection depends on hinge moment which has a component due to control surface angle and a component due to aircraft angle of attack. As airspeed decreases, you would have to increase aircraft angle of attack. That would cause the aileron to float up more in most cases. See Aircraft Performance Stability and Control by Perkins and Hage, if you can still find it.

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QUESTION: What I don't understand is why the flaps, which are usually affected by the same conditions, seem to behave contrary. For example, if you have a manual actuated lever to deploy flaps, you'll find that it is easier to deploy flaps at low speeds (high alpha) and harder at high speeds. Being so, I figure out they are not sucked up at high alpha and I don't know why.

It's all in the hinge moment design. As you slow the airplane and increase angle of attack the wing suction pressure has to go up to provide the same lift at lower dynamic pressure. That causes the absolute value of hinge moment due to angle of attack to increase for a simply hinged flap. The lower the speed, the greater the moment trying to float up the flap.

However, depending on hinge line location aft, the moments can be small or even in the opposite direction from that of a simply hinged flap. This all feeds into the actuator forces you feel.

The other possibility is that at low speeds and high angles of attack, the flow over the flap is becoming partially separated so the suction force is less near the trailing edge of the flap.

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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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