Large Airplanes with Reduced-Static Longitudinal Stability

In a paper on stability augmentation for a large, flexible airplane that appears to be the Boeing 777, Greta Ward (1996) makes an interesting observation on the limitations of reduced-static longitudinal stability when applied to large airplanes. Reduced-static lon­gitudinal stability is an attractive feature for long-range airplanes, reducing cruise flight trim drag and fuel consumption. In any such application, a designer must retain a suitable margin of longitudinal control power to recover from inadvertent upsets, upsets that would be opposed by static stability in airplanes of normal longitudinal stability.

Pitch moment of inertia varies as the fifth power of fuselage length, while the maximum available pitching moment produced by a horizontal tail surface varies only as the third power of fuselage length. This implies an upper limit to fuselage length and airplane size if reduced-static longitudinal stability is to be used.

23.2 Large Supersonic Airplanes

A successor to the supersonic Concorde is likely to be a large airplane, in the sense considered here. The stability and control problems of supersonic flight and low-speed flight of a low-aspect-ratio design would be added to those of large airplanes. A general review of the combined problems (Steer and Cook, 1999) reflects experience with the Concorde. The authors conclude that a foreplane, or canard surface, would be needed in place of the tailplane used in the Boeing/NASA High-Speed Commercial Transport (HCST) design.