It seems to me that decalage is greatly oversold. Changing decalage simply
biases the elevator position, and can be entirely compensated via the elevator
trim
on the TX as long as the elevator deflection remains modest. There should be
no effect on handling.
This is obvious on an all-flying tail... Increase the wing incidence
1 degree, and add 1 degree down elevator trim on the TX. The wing and tail
are at the same relative orientation as before, and the glider should fly as
before.
In a conventional hinged elevator, you'll have to add about 2 degrees of down
trim
for 1 degree of wing incidence, but again, the glider should fly as before,
even though 1 degree of "decalage" has been added.
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 possible
causes for tail nonlinearity are:
1) The elevator deflection in trimmed flight is excessive ( > 5 degrees, say)
before or after the decalage change. Changing decalage to reduce this
deflection
will surely reduce the tail's profile drag.
2) The horizontal tail has a crummy airfoil which suffers from hysteresis
or other flow pathologies. Such airfoils are usually draggy. A proper airfoil
will eliminate this behavior and will reduce the profile drag as a bonus.
If the tail response is linear and the handling doesn't change, then a minor
effect
of adding the 1 degree of wing incidence in the example above is that it lowers
the
angle of the fuselage by 1 degree. This may increase or decrease the fuselage
drag.
Another minor effect af adding 1 degree of wing incidence is to raise the tail
relative to the wing's wake. This is not an issue on most gliders, which have
the tail flying well above the wing's wake to begin with.
- Mark Drela
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