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Thread: Elevator Stall
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Old 09-30-2002, 04:16 PM
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Ollie
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Default Elevator Stall

As the CG is moved forward, more and more down load on the horizontal tail is required to achieve trim. If, for example, the CG was within 5 or 10 percent of the MAC from the leading edge, the down load on the tail would be excessive and the tail might stall in an attempt to achieve trim. This never happens on a plane that is properly set up for aerobatics with neutral stability. The horizontal tails of such planes are low aspect ratio, of ample area and very lightly loaded with very ample stall margin until well after the main wing has stalled.

When a control surface is deflected more than about 10 or 15 degrees, the flow seperates at the hinge line and that might be thought of as a partially stalled condition. However, if the lift of the tail is not seriously reduced, then control isn't lost.

Most horizontal tails have low aspect ratios and the induced angle of attack goes up as the aspect ratio is reduced and the squared coefficient of lift increases. So, these low aspect ratio "wings" with variable camber airfoils can still function at very high (extreme) angles of attack.

Wide control surfaces at high deflections put huge loads on the servo. A stalled servo can give symtoms of lost control that could easily be mistaken for horizontal tail surface stall. See:
http://www.multiplexrc.com/calcservo.htm
to calculate the required servo torque for the application.