Sky High

My climb to the top in the F-104.

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(Continued from page 5)

The T-tail on the McDonnell F-101 Voodoo and the Lockheed F-104 Starfighter could create major problems. At high angles of attack, the outer wing sections stalled before the inner wing sections did, and that tended to move the center of lift forward. At the same time, the downwash from the wing began to impinge upon the horizontal tail, changing the angle of airflow over it and reducing its effectiveness. The combined effect caused the aircraft to pitch up.
Most aircraft pitch down when they stall. The nose drops, the aircraft picks up speed, and it returns to controlled flight. In a pitch-up, the angle of attack increases even with the control stick full forward. The aircraft goes out of control and may end up in a spin from which, in both the F-101 and the F-104, it was sometimes impossible to recover. If a pilot could recover by deploying a small drag chute attached to the tail, the ensuing dive recovery could take up to 10,000 feet. A pitch-up below 10,000 feet resulted in an automatic ejection. Pilots were directed never to intentionally pitch up or spin either the F-101 or the F-104; the pilot’s flight manual called those prohibited maneuvers.
Lurking in the background was another serious phenomenon. Beginning with the North American F-100 Super Sabre, “Century Series” fighters (those with numerical designations from -100 up) like the F-101 and F-104 were designed with a high concentration of mass along the fuselage. This led to a dynamic characteristic known as inertial coupling, a phenomenon that can best be explained by considering a rapid rolling maneuver. Picture the aircraft at a positive angle of attack. It begins a rapid roll around its longitudinal axis, which is displaced from the direction the aircraft is moving in by the angle of attack. As the high mass along the fuselage begins rotating at an angle to the flight path, it tends to diverge from that path, increasing its displacement in pitch and yaw the longer the roll continues. So in addition to needing a big vertical tail for directional stability, an even larger tail was needed to prevent inertial divergence.

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