Washout

[from the AMA National Newsletter, November 2001]

by Bob Mabli

Washout may be confusing to the novice glider builder and flier, but it can make the difference between a gentle, easy-flying and forgiving floater and an untamed one.

Most all polyhedral glider kits like the Wonderer and the Gentle Lady instruct the builder to add “washout” to the wing tips. We may assume that the ARFs provide for washout but it is a good thing to check before the first flight.

Washout is a term that refers to the twisting of the outboard portion of a wing (trailing edge up) to reduce the angle of attack of the outboard wing section relative to the inboard portion. Doing this assures that the outboard section will not stall before the inboard section. At stall speed, the center will stall first and the tips will follow. This prevents the onset of uncontrolled roll during slow speed flight, especially during the landing approach and touch down where you don’t have altitude to correct the condition. The roll will occur when one wing tip stalls before the other or before the rest of the wing.

Washout is easy to add and adjust in a built up wing (ribs and spars) with heat shrink covering. All it takes is blocking up the trailing edge of the outboard section and heating the covering to set the twist. This usually requires no more than 3/4 inch at each tip. Be sure that both sides are equal or the result will be a tendency to roll off onto the wing tip having less twist. While you’re at it, check for warps in the rest of the wing and correct those by heating and twisting.

Fome-Cor® wings require either that the washout be cut into the foam when made or that different airfoil sections with different stall characteristics are used at the root and tip.

A high performance full house thermal ship will have different airfoil sections blended along the span for optimized lift and drag. The Crow configuration (flaps down and ailerons up) used in landing and high descent rates provides effective washout by virtue of the aileron deflection.

High speed slope airplanes do not usually use any washout because of the drag impact. Powered aerobatic airplanes have to have absolutely symmetrical wing sections for precise flying at any attitude-so washout is out. Landing approaches have to be “flown” in. Too often you will see an airplane snap roll turning on final because the pilot allowed the airspeed to drop too low and tip stall resulted on the wing on the inside of the turn.

from The Peninsula Silent Flyer
Peninsula Silent Flyers
Torrance CA