In theory this is correct. In reality the theory leaves a little to be
desired - when working with very small static margins.
Decalage is the last thing I tweak up when I'm trimming out a new fixed
stab airplane. (Although sometimes if the decalage is way off it will need
initial tweaking before the sweet CG spot can be found.) The final
decalage tweak produces better turning and landing pitch stability.
The hardest plane I ever tried to set up was a BOB hlg. The tail must have
been 3 degrees off and when I finally got the decalage set the stab turned
out to be too small. By the time I got it flying well it was pretty beat
up so I trashed it. Very innovative construction tho. Guess its out of
production now.
Have you ever noticed that airplanes with all flying stabs tend to be the
ones that are "classics" in terms of performance? (Sagitta, Bird of Time,
Falcon 880....) One of the prime features of each of these is the fact
that decalage is always "right". Move the cg, adjust the trim and the
plane still flies great. Fixed stabs require a bit more work when you get
to aft cg's and small static margins.
Rick (Voodoo Aerodynamicist) Eckel
At 01:46 AM 3/10/00 -0500, Mark Drela wrote:
>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
>
>
>RCSE-List facilities provided by Model Airplane News. Send "subscribe"
and "unsubscribe" requests to [EMAIL PROTECTED]
>
RCSE-List facilities provided by Model Airplane News. Send "subscribe" and
"unsubscribe" requests to [EMAIL PROTECTED]
