Flying Wing - Design Issues - Directional Stability

Directional Stability

For any aircraft to fly without constant correction it must have directional stability in yaw.

Flying wings lack the long fuselage which provides a convenient attachment point for an efficient vertical stabilizer or fin.

Any fin must attach directly on to the rear part of the wing, giving a small moment arm from the aerodynamic center, which in turn means that to be effective the fin area must be large. This large fin has weight and drag penalties, and can negate the advantages of the flying wing. The problem can be minimized by increasing the leading edge sweepback and placing twin fins outboard near the tips, as for example in a low-aspect-ratio delta wing, but many flying wings have gentler sweepback and consequently have, at best, marginal stability.

Another solution is to angle or crank the wing tip sections downwards with significant (anhedral), increasing the area at the rear of the aircraft when viewed from the side.

Yet another approach uses differential twist or wash out, together with a swept back wing planform and a suitable airfoil section. Prandtl, Pankonin and others discovered this and it was fundamental to the yaw stability of the Horten Bros. flying wings of the 1930s and 40s. The Hortens described a "Bell Shaped Lift Distribution" across the span of the wing, with more lift in the center section and less at the tips due to their reduced angle of incidence, or washing out. This creates a slightly forward-pointing lift vector for the rear (outer) section of the wing. When displaced, this vector essentially "pulls" the trailing wing forward to re-align the aircraft along its flight path.