Decalage Question

[Don Cooper]

If a plane trims in level flight at normal cruising speed with zero elevator deflection, does that indicate that the decalage is in the right ball park, or are there other factors to consider?

[Dave Robelen]

Hello Don,
You can achieve that trim condition over a pretty wide range of CG locations. If you move the CG aft, reduce the decalage, the elevator will trim at neutral, but there will be a point where the model is too pitch sensitive to fly smoothly. Conversely, a forward CG with greater decalage could also trim with the elevator neutral, but the model will get very speed sensitive. Slow down, and the nose drops requiring large amounts of up elevator to trim. Speed up, and the model will require some down trim to fly level.
The forward CG case is often useful in moderation with gentle indoor models, where the throttle can be used to change altitude and pitch trim. Outdoor glider pilots often prefer a very aft CG to get the best L/D from the model when cruising between thermals.
For many models, the simple dive test will be a good check point. Starting from level flight at moderate power, and ample altitude, nose over about 30 deg. and release the stick. If the model starts recovering back to level fairly quickly, moving the CG aft and reducing the decalage would be indicated. For the all-out aerobatic model, it would be desireable for the model to continue the diving path hands off, and still not be excessively pitch sensitive.
The amount of decalage is coupled into the size of the surfaces, the vertical location of the wing relative to the stab, and the C.G. I hope this did not muddy the water too badly.
Regards, Dave

[Don Cooper]

Dave,
Thanks for the help. I had adjusted the C.G. using the dive test you mentioned. Then when I started playing with decalage and elevator trim, the decalage for zero elevator deflection at cruise was considerably smaller than the 3 or 4 degrees I have seen stated several places as the norm. So I wandered if I had missed something.

[Dave Robelen]

Hello Don,
Sometimes it is well to remember that the reference line of the wing airfoil passes through the LE center. If the airfoil is flat bottomed, this may give you several degrees if you are referencing the bottom of the wing.
Regards, Dave