Stability and Control of Hypersonic Airplanes
Hypersonic flight is generally understood to mean flight at Mach numbers above 5.0. The only experience with manned airplanes at hypersonic speeds has come from the North American/NASA X-15 and space shuttle Orbiter programs. Stability and control phenomena at hypersonic speeds are qualitatively no different than at moderate supersonic speeds. There is the same relative loss in the effectiveness of lifting stabilizing surfaces relative to fuselage-destabilizing moments. The high altitudes at which hypersonic flights are carried out lengthen the periods of uncontrolled motions, always a piloting problem.
The influence of the propulsion system on aerodynamic forces and moments is expected to be more extreme in powered hypersonic flight than at lower speeds. The reverse is also true in that slight sideslip angles could cause severe inlet problems, depending on details of the design. Sufficient control surface authority may be required to overcome yawing, pitching, and rolling moments caused by engine inlet unstarts precisely at altitudes where control moments are low because of low air density.
However, the most pressing stability and control problems of hypersonic airplanes are probably encountered at low speeds, as a result of the unique design features that go along with hypersonic flight. Wing slats or drooped leading edges that could improve low-speed longitudinal and directional stability are apparently ruled out because of aerodynamic heating problems at seams in the forward lower wing surface. A hypersonic passenger airplane for long over-ocean flights remains an interesting, but probably distant, goal for aviation planners. The aerodynamic research that has gone into this concept so far has quite properly dealt mostly with performance and aerodynamic heating. Conceptual designs that have been published show configurations that look like stretched-out space shuttle Orbiters.