Supersonic Directional Instability
A rather simple static directional instability problem first appeared in a test flight of the North American F-100 Super Sabre. It is simple because the problem has one well-known cause, the loss in lifting surface effectiveness as Mach number increases beyond 1. The instability of bodies of revolution, on the other hand, remains essentially invariant with Mach number. Static directional stability is to a first order the balance between the unstable fuselage and the stabilizing vertical tail. The vertical tail is supposed to
Figure 11.15 A North American XB-70 airplane in flight. The wing tips are deflected downward for increased directional stability at supersonic speeds. (From Bilstein, Orders of Magnitude, 1989)
dominate, but as its effectiveness, or lift curve slope, drops off neutral stability is eventually reached.
The point of neutral directional stability on any supersonic airplane evidently should be beyond the attainable flight envelope. However, supersonic directional instability actually occurred in a dive on an early F-100 before an enlarged vertical tail was adopted, leading to a tragic accident. On the F-100 vertical tail, bending contributed to the loss in effectiveness. Modern stability augmentation techniques can provide artificial directional stability at supersonic speeds, if it is impractical or economically undesirable to have a large enough vertical tail.
The North American XB-70 bomber used a configuration change to return directional stability to acceptable levels at high supersonic Mach numbers. The wing outer panels folded down 65 degrees for flight at a Mach number of 2 and a larger angle above (Figure 11.15). Unfortunately, this made the dihedral effect negative, resulting in poor flying qualities. This was corrected on the second XB-70 prototype by a triangular wedge welded between the fuselage and wing, producing 5 degrees of geometric dihedral.
There was concern that if the XB-70’s wing tips ever stuck down in the folded position, the airplane could not be landed because of lack of ground clearance. Fortunately, this never happened. An additional benefit of the folded-down wing tips was reduction in excess static longitudinal stability at supersonic speeds, due to the change in planform. Also, compression lift was generated at supersonic speeds by shock waves from the folded tips producing positive pressures on the bottom of the wing and fuselage.
The British Aircraft Corporation’s TSR-2, designed for a Mach number of 2.0, had neutral directional stability at a Mach number of 1.7. The vertical fin was made small to
reduce tail loads in high-speed flight at low altitudes. The airplane was canceled for other reasons before a directional stability augmenter could be installed for flight faster than a Mach number of 1.7.