Sweptback Wings Are Tamed at Low Speeds
The successful, routine use of wings swept back 30 to 45 degrees is a source of wonder to stability and control engineers who were active in the 1940s. Then, a wing that was tapered by sweeping back the leading edge, while keeping a straight or slightly swept trailing edge, giving no more than about 5 degrees of sweepback, was deplored. One could expect early wing tip stall with increasing angle of attack, wing drop, and roll damping reversal. Airplanes with sweptback leading edges and straight or nearly straight trailing edges included the Douglas DC-3 and the North American SNJ Texan.
11.7.1 Wing Leading-Edge Devices
When really large amounts of wing sweepback became a necessity for high Mach number airplanes, sweptback wings had to be designed that would have decent low-speed stalling chacteristics. It was soon found that large amounts of wing sweepback combined with moderate aspect ratios could be made practical by wing leading-edge devices such as slots, slats, leading-edge flaps, cambers, and blunt-nose radii (Figure 11.10). Fixed-wing leading-edge slots were used before the advent of sweptback wings, to correct wing tip stall on heavily tapered straight wings, such as the Lockheed PV-1 Ventura. But fixed slots obviously added to drag in normal flight. The modern slat or leading-edge flap extends at low airspeeds but retracts fully for cruising flight. When opened, the leading-edge slat or flap delays separation by increasing the local camber.
High suction pressure on the slat’s upper surface as the wing nears the stall was used successfully to open wing slats on the North American F-86 and the Douglas A3D and A4D airplanes, avoiding hydraulic opening and closing systems. These self-opening slats could prove nerve-racking to pilots. When angle of attack is increased one wing slat tends to bang open a bit earlier than the other. However, at the angles of attack considerably below the wing stall point where air loads open the slats the opened slats have little effect on wing lift. While unsymmetrical slats look dangerous, at angles of attack well below the stall the airplane has little or no tendency to roll.
With the coming of better hydraulic systems wing slats are now universally powered. Wing slats are undesirable in some applications, as in stealth airplanes, because of the special treatments needed to avoid radar returns from the slat-wing seams. Where slats are impractical designers have learned to use wing camber and nose radius changes as substitutes. However, it may be doubted whether these measures can be effective as slats in preventing early wing tip stall. Other wing devices to improve stalling characteristics are considered next.