Mark Drela wrote:
> If changing the decalage on the glider DOES produce a noticable change in
> handling, then one can conclude that the elevator response is nonlinear,
> which indicates something bad and draggy is happening.
Two instances of "bad and draggy behavior" were the same error in decalage
on a couple very different ships. One was a foamy and the other was a
stab-ele T-tail glass slipper.
Both ships had too much decalage. Down elevator was needed to compensate
for the excess decalage. This makes a terrible airfoil for creating the
downward force needed especially at stable CG locations. Both ships
howed terrible pitch sensitivity especially at low speeds. They would
tend to dive for the ground or pitch up into a stall after applying
elevator correction. What may have been happening is that the downward
deflected elevator would stall under the pitching moment loads and let
the nose drop sharply. Then when up elevator was applied the stab was
suddenly able to apply enough force to raise the nose once the airflow
reattached. The reattachment though was not prdictable due to hysteresis.
So the poor pilot was left with a terrible handful to fly on landing approach.
In both cases reducing the decalage helped tremendously. The lesson here
may be that too much decalage is worse than not enough. The nutral trim
airfoil resulting from too little decalage is much better for exerting
downward force.
John Hazel
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