Stability of tailless aircraft

The inherent destabilising effect of a cambered wing section comes from the fact that the centre of lift moves forward with increasing angle of attack. For a tailless aircraft, we could overcome this problem by means of a negatively cam­bered wing section, or a reflex cambered section, as shown in Fig. 11.10. Such sections are rather poor in terms of their lift and drag characteristics, however, and the approach normally used for tailless aircraft is to sweep the wings backward. The wing tips, which are then aft of (behind) the inboard section, are twisted to give a smaller incidence (washout), and take the place of the tail. This principle is employed on many hang-glider designs, and on the powered microlight aircraft derivative shown in Fig. 11.11.

Fig. 11.11 Wing sweep helps to provide both longitudinal and lateral stability on this tailless microlight

The main advantage of a tailless design is that drag-producing junctions between components are reduced. In the Northrop design shown in Fig. 4.19, even the fuselage has been eliminated. For low-speed aircraft, however, the swept tailless type shows little or no significant advantage in terms of drag reduction, as the swept-wing configuration has an inherently lower lift-to-drag ratio than a straight wing. As explained in Chapter 2, for a given wing area, the lift decreases with increasing sweep angle, whereas the drag remains more or less constant, or may even increase. For a transonic aircraft, though, where wing sweep is necessary to reduce compressibility effects, the advantages of the swept tailless configuration might be exploited.

An incidental important feature of all-wing designs such as the Northrop is that the absence of junctions between surfaces makes them relatively poor radar reflectors, thus making them suitable as the basis for development of stealth technology (low detectability).