RCU Forums - The science behind the Infamous tip stall...

OOOHH! Deceptively simple question...:eek:

This topic has been discussed many many times on RCU, and you can find all you want and more by using the forum's search engine on the phrase "tip stall".

There is some debate as to whether this is a meaningful term, but leaving that aside, it is bad news if the outboard end of a wing panel stalls before the inboard. The stall will usually rapidly progress inboard, so it doesn't remain entirely localized at the tip (hence the debate), but the bottom line is that you don't want it to happen.

Basically, any wing can tip stall if flown roughly. However, as I understand it there are two common situations:

1) highly tapered wings - the short chord of the wingtip means a lower Reynolds number. This in turn means that the tip will reach its stall point before the wider chord sections of the wing, for a given airspeed.

2) very tight, highly banked turns - the inboard tip is moving slower and may drop below stall speed. Also, the aileron deflection can lead to an effective increase in angle of attack, and hence it may progress beyond stall at the tip if the aileron is out at the end of the wing.

I don't think you need to start out with unequal wing loading - but as soon as the tip of one wing panel stalls (and hence is not generating lift over a portion of its area) that panel instantly is more heavily loaded over what remains working. If the load exceeds the lift, the wing panel will drop - and you flip over.

What can be done to avoid tip stall? When you fly, keep your airspeed up, avoid extreme maneuvers. If you design your own planes, avoid highly tapered wing planforms (unless you know how to modify your airfoil progressively out to the tip), and build in a degree or so of "washout" - that is, build a downward twist into the wing so that the angle of attack at the tip is always less than the angle of attack at the root.