Washout

[Dopler]

I hope this is the most appropriate forum for the question:

How critical is the need to construct the wing with a washout (smaller incidence angle at the tip relative to the root)?

Without it, am I expecting a possible uncontrolled stall?

[Dave Robelen]

Hello Dopler,
Several things need to be considered. If the wing in question is rectangular (the tips may be rounded) and does not have ailerons, then washout is not necessary. A tapered wing, especially one with ailerons and narrow tips will benefit from washout. About 3 deg. difference between the root and tip will do the job. It is equally important to seal the gap along the hingeline when ailerons are used.
Regards, Dave

[Dopler]

Thanks Dave. The model does not have ailerons, so zero washout will do.

But in general: Is there any method, other than washout to prevent early tip stall in model airplanes? Did anybody try using some kind of rough surface sticker (or an array of short pins) just ahead of the maximum thickness line on the upper surface of a wing in order to postpone stall (Turbulators work well in full size airplanes)?

[Dave Robelen]

Hello Dopler,
As far as I am aware, surface devices are not particularly effective in delaying the "tip stall". I have seen upper surface vortex generators on certain models of the early Learjets, but that was more of a treatment to enhance aileron effectiveness. when the stall pattern begins, there is so much seperated flow that surface devices are generally not useful.
Many full scale airplanes add "stall strips" on the inboard LE as a result of flight testing. This is sort of a "season to taste" procedure for each vehicle to insure that the wing stalls inboard first with good buffet.. Part of my work at NASA involved research into this issue. Probably the most desireable appraoch was to otate the tip leading edge downward, and use a large radius on the upper surface to blend it in. This treatment can be seen on the variants of the Piper Cherokee that have the longer wing with tapered tips. Cessna has also implemented this approach on various models as well.
Regards, Dave