Faceted Airframe Issues

The Lockheed F-117A’s faceted airframe flies in the face of conventional aero­dynamic wisdom, which requires smooth surfaces to maintain attached flow under the widest possible ranges of angles of attack, sideslip, and angular velocities (Figure 22.1). On the other hand, the aerodynamic forces and moments of faceted airframes are reasonably linear functions of these variables for sufficiently small ranges.

Large-wing sweepback, 67 1/2 degrees in the case of the F-117A, extends the linear ranges somewhat, making facet edges into side edges instead of breaks normal to the flow direction. Still, the stability and control engineer who is faced with a faceted airframe such as the F-117A must expect to restrict flight parameters in order to avoid nonlinear and unstable aerodynamic moments that exceed available control power. The F-117A was originally called “The Hopeless Diamond” by Lockheed aerodynamicists.

On the F-117A, the angle of attack is hard-limited, but sideslip angles are unlimited with the landing gear down for cross-wind landings. With landing gear up, the sideslip angle is nulled by closed-loop control, a normal loop closure. F-117A longitudinal static margins are low or negative within the angle-of-attack limit range, but air combat maneuvers can be made within that range. Severe pitchups and pitchdowns occur outside of the angle-of-attack limit range (Farley and Abrams, 1990). Without augmentation, the airplane is directionally unstable over large parts of its operational envelope.

The four F-117A elevons have relatively large travels of 60 degrees up and down, which are necessary to deal with nonlinear and unstable moments within the angle-of-attack limit range. The two vertical tails are all-moving, for the same reason. The F-117A has quadruple fail-safe fly-by-wire controls, using F-16 technology. An 18-foot-diameter brak­ing parachute doubles as a spin chute, an unusual feature for a service airplane. Nominal landing speed is 160 knots, at an angle of attack of 9 1/2 degrees.