Wide Chord Elevators - Stability & Stalls

April 1979

Many builders have been confused about the various aerodynamic effects of shortening the canard and widening the elevator. The reason for the confusion is that the effects are not what one would immediately think. Hopefully the following discussion will clear the air:

Question - the wide chord elevator is bigger and thus gives more effect per degree - why doesn’t this make the controls more sensitive? Answer - sensitivity is more a function of effect per stick force, not displacement. True, the wide chord increases effect per degree by about 12%, but the distance from the elevators center-of-pressure to its pivot is twice as far, thus stick force per effect is nearly doubled.

Question - I understand why shortening the canard moved my airplane center of pressure (and cg range) back 1.2 inches due to less area on front. But, why didn’t the cg move back forward, when I installed the wide-chord elevator, thus increasing canard area? Answer - The wide chord elevator has a negligible effect on aircraft static stability for two reasons, (1) the elevator "floats" when angle of attack changes. Thus its area has much less effect on stability than the fixed canard area. (2) The wide chord elevator results in less, not more, aspect ratio, as compared to the area added by increasing canard span.

Question - Since the wide-chord elevator has more aerodynamic power, it should be able to drive the airplane into a deeper stall, correct? Answer - No, the airplane’s maximum angle-of-attack is limited by the stall angle of canard airfoil. Adding more elevator can get it to stall with less elevator position, but it will still stall at the same angle, and still limit the aircraft to a safe attitude without stalling the aft wing. Note that even though a large elevator can increase the maximum lift coefficient of the canard, larger elevators actually decrease the stall angle and thus cannot increase the angle seen by the aft wing. The aft wing angle is the main factor for stall characteristics.

Copyright 1999-2017 ez.org All Rights Reserved