Tactical Airplane Maneuverability
Tactical airplanes have always had special stability and control problems because of the extreme maneuvers required of them. The rapid aileron roll, the sharp pullup, and the rapid turn entry all present special problems. Some examples are the level of rolling velocity actually required, overcontrol in pullups, and badly coordinated turn entries. Finally, controlled flight at angles of attack beyond the stall is a new field of required maneuvers for tactical airplanes.
10.1 How Fast Should Fighter Airplanes Roll?
Fighter roll capability became a crucial question during the early days of World War II. Many allied fighter airplanes carried gun cameras into combat. Gun cameras are movie cameras pointed in the direction of the ship’s fixed-wing guns. Movies are taken as long as the firing trigger was pressed, witnessing hits (or misses) on enemy airplanes or missiles. Gun cameras carried on Curtiss P-40s and North American P-51 s witnessed interesting moments in dogfights and bore out pilots’ accounts of loss in combat advantages due to relatively low rates of roll on the U. S. aircraft.
Some Axis aircraft, particularly the Mitsubishi Zero at low airspeeds, would feint a roll in one direction and then roll rapidly in the other direction. The horizon or cloud background in the gun camera pictures would show the Allied airplane following the feint, a bit slower perhaps, then be left behind as the Zero did a rapid roll in the opposite direction and disappeared from the gun camera’s view.
Clearly, high-rolling velocity performance was needed at dogfight airspeeds in order not to lose firing opportunities when in the favorable trailing position. At the low end of the fighter airspeed range the Gilruth/NACA criterion pb/2V = 0.07 was a reasonable guide, although higher levels, up to 0.10, were considered. Higher pb/2V levels could be attained with extra-large ailerons. But in early World War II days, before hydraulically powered controls, the wide-chord, long-span ailerons that provided high pb/2V values meant high stick forces, restricting rolling velocities at high airspeeds.
In other words, an airplane could be designed for fast rolling performance at either low airspeeds, say below 200 knots, or high airspeeds, but not both. The Curtiss P-40 was typical in that its maximum rolling velocity of 95 degrees per second occurred at an airspeed of 270 miles per hour. At 400 miles per hour (not shown in Fig. 10.1) maximum available rolling velocity dropped to 65 degrees per second, limited by a nominal 30-pound stick force.
Restricted maneuverability due to high stick forces started an intense research program on both sides of the Atlantic. The British seemed to have had the innovative edge, coming up with two significant stick force reduction schemes: the spring tab, ultimately used on the Hawker Tempest, and the beveled-edge control surface. The history of these devices is given in Chapter 5, “Managing Control Forces.” Beveled-edge ailerons worked quite well for the P-51 Mustang, almost doubling the available rate of roll.
Figure 10.1 Rolling velocities obtainable with 50 pounds of stick force for a number of World War II fighter airplanes, all at an altitude of 10,000 feet. These data were heavily classified during the war. (From Toll, NACA Rept. 868, 1947)
Hydraulic power assistance came into the picture for fighter-type airplanes only at the very end of World War II, on the ailerons of the late version Lockheed P-38J Lightning. However, once power controls became common, in about 1950, stick force limitations to rate of roll were overcome. Now the only limits were hydraulic system capacity, control system and wing strength, wing torsional stiffness, and the inertial coupling phenomenon discussed in Chapter 8. The military specification version of that period reflected these new capabilities. Fighter roll rates up to 360 degrees per second were required. A limiting factor in fighter roll maneuverability at high airspeeds and low altitudes is wing twist, treated in Chapter 19, “The Elastic Airplane.”