Part of the Charles A. Lindbergh Lecture at the National Air and Space Museum, Smithsonian Institution by lifting-body pilot Bill Dana.
The M2-F2 had fifteen successful glide flights, each following a launch from a B-52 mother ship at an altitude of 45,000 feet (slide). Four pilots were checked out in the aircraft, and data were obtained on vehicle stability, gliding performance, structural loads, and approach maneuvering.
On its sixteenth flight, in May 1967, the M2-F2 experienced lateral pilot-induced oscillations while on its landing approach. It is believed that the roll motions occurring during this time period disoriented the pilot. In any case, he touched down before the landing gear were extended, and rolled over several times, damaging the vehicle and injuring the pilot.
M2-F2 under B-52 wing
An extended wind-tunnel analysis of the problems encountered by the M2-F2 was conducted in the months following the crash. It was ultimately determined that the addition of a third vertical fin would eliminate the roll instability. The M2-F2 was rebuilt into a three-finned M2-F3 (slide).
If it appears that use of a third vertical fin to prevent roll upsets was "black magic," a closer look at the geometry of the airplane indicates that there is a more scientific explanation for the beneficial effect of the center fin. The M2 shape had an extremely high dihedral effect. In other words, a little bit of left yaw caused a very large left roll. (Slide.) When the M2-F2 pilot raised the left aileron, as shown here, to roll left, the high pressure in the vicinity of the aileron impinged on the inboard side of the left fin, yawing the M2-F2 to the right. This right yaw then coupled with the dihedral effect to roll the M2-F2 right, opposite of the roll commanded by the pilot. This adverse yaw due to aileron deflection caused roll upsets at low angle of attack on several flights, and finally contributed to the accident on flight 16. The addition of the center fin (slide) end-plated both ends of each aileron, so that when an aileron was raised, the high pressure impinged equally on one side fin and the center fin. There was no yaw due to aileron deflection, and roll control became straightforward. No lateral instabilities were encountered in the M2-F3.
M2-F3 test firing its 8,000-pound
thrust rocket engine
The M2-F3 was equipped with an 8,000-pound thrust rocket engine (slide) which powered it to 1.6 times the speed of sound for investigation of stability, control, and performance at transonic and supersonic speeds. The M2-F3 flew 27 times.