Supermaneuverability, High Angles of Attack
Until the 1970s, fighter air-to-air combat followed the pattern set during World War I. Fighter pilots maneuver behind opposing fighters to bring fixed guns to bear long enough for a burst. The tactics are much the same for narrow-field-of-view guided missiles, such as the AIM-9 Sidewinder. In the missile case, a tail position is held long enough for an acquisition tone; then the missile is launched.
Hawker-Siddeley in Britain came up with the thrust-vector-controlled Taildog missile concept in the late 1960s, making an off-boresight launch a possibility. Combined with a helmet-mounted sight, a Taildog-type missile can be launched at target airplanes at almost any position where the pilot can follow the target with his eyes. However, even with off – boresight missile lockons and launches now possible, there is still interest in gunnery for air-to-air combat. Furthermore, there is interest in gun bearing at high angles of attack, increasing firing opportunities in a dogfight.
Supermaneuverability is defined as controlled, or partially controlled, flight in the stalled regime. It takes two forms: first, a dynamic maneuver to a high angle of attack, beyond any equilibrium or trim point. Pitching angular momentum carries the airplane to a momentary peak angle of attack. The second form of supermaneuverability is flight to a sustainable trim equilibrium beyond the stall. Supermaneuverability is seen as a way to get into the tail chase position, by a feint, tricking a pursuing airplane into overrunning one’s position. Supermaneuverability adds to a dogfighting airplane’s options.
The Cobra maneuver, demonstrated with a Sukhoi Su-27 airplane by the Russian pilot Viktor Pugatchov at Le Bourget in 1989, is in the first category. After Pugatchov’s demonstration in the Su-27, the same maneuver was performed in a MiG-29. The Cobra is started from unstalled flight with a rapid application of full nose-up control, which is held up to the maximum angle of attack point, about 90 degrees. Control is neutralized for the recovery, assuming that the airplane has a negative or nose-down pitching moment at that point.
The entire maneuver takes about 5 seconds. There is a small altitude gain but a huge loss in airspeed and kinetic energy. Ordinarily, during air combat, one tries to maximize airspeed and total (potential plus kinetic) energy as a reserve for further maneuvers. Thus, U. S. Major Michael A. Gerzanics, project test pilot for a vectored-thrust F-16, has stated that supermaneuverability is not beneficial in all tactical situations, but is rather something that he would like to have available for close combat with a strong adversary. Clearly, any uncontrolled yawing and rolling moments that develop in the 5-second period beyond the stall must be small. The Cobra maneuver has been elaborated with a sidewise variant, called the Hook.
10.2 Unsteady Aerodynamics in the Supermaneuverability Regime
Mathematical modeling in the supermaneuverability regime has to account for unsteady aerodynamic effects above the stall (Zagainov, 1993). Zagainov describes a state variable mathematical model, developed by M. G. Goman and A. N. Khrabrov, for coefficients such as Cz and Cm. The model has a first-order state equation that defines time dependence (Figure 10.11). The typical hysteresis loop found in forced oscillation tests into the stalled regime can be modeled in this way. Zagainov also discusses the strong rolling and yawing moments that appear in the angle of attack range where vortices are shed from inboard strakes and extended forebodies. These vortex-generated rolling and yawing moments not only appear to exceed values measured in steady wind-tunnel tests, but they are also time-dependent, exhibiting hysteresis loops.
Additional light on the complex, unsteady air flows in the supermaneuverability regime has been shed by a combined wind-tunnel test and flow visualization program (Ericsson and Byers, 1997). A major factor is a coupling between vehicle motion and asymmetric cross-flow separation on a slender forebody. Wing leading-edge extensions or LEX, such as found on the F-16 and F-18 airplanes, change the nature of the cross-flow separation, apparently in a beneficial direction.