V/STOL, standing for vertical or short takeoff and landing, is a term used to describe the ability of a special class of aircraft to ascend or descend vertically, so permitting them to operate from very confined spaces on the ground. The description now specifically applies to aircraft having a more or less fixed-wing layout but possessing special features that permit them to hover or land or takeoff without significant forward motion. The term VTOL, for vertical takeoff and landing, is synonymous with V/STOL and was current some years ago. However, designers now recognize that significant increases in payload or range may be secured where circumstances permit a short ground run, and the newer term more accurately reflects this benefit. The term STOL in isolation denotes a type of aircraft that, though capable of operating from very short runways, cannot land or takeoff vertically.
V/STOL airplanes share some of the benefits and drawbacks of both fixed-wing types and helicopters. They may be regarded as extending the low-speed capability of the former and improving the high-speed performance of the latter.
Although of outstanding versatility, helicopters have the fundamental disadvantage of being slow. While a few can attain 250 mph (400 km/h), most can cruise at only half that speed. They are essentially short-range vehicles, and their value as a means of transport has been largely overshadowed by their usefulness for tasks demanding an ability to hover or fly slowly for long periods or to land and takeoff from restricted spaces or unprepared surfaces. In general, their special abilities are obtained at the expense of generally low structural, aerodynamic, and propulsive efficiency.
Since the 1950s, many attempts have been made to improve the speed of helicopters by adding engines and propellers to give forward thrust and small wings to relieve the load on the rotor so that at high speed it provides no lift but simply freewheels. By these means, some of the more undesirable effects of compelling the rotor to generate lift and thrust simultaneously at speed are postponed. Although a number of compound helicopters, as they are called, have been built and flown experimentally, their performance has not justified the extra complexity and cost.
The convertiplane
The next step was the convertiplane, the simplest form of which has a conventional fuselage and tailplane and a small wing having two rotors at its tips, similar to those of a helicopter but of smaller diameter. For takeoff and landing, they rotate horizontally, as on a helicopter, but are turned to spin in a vertical plane for forward flight, when they act as outsize propellers. Convertiplanes have been investigated in more detail than probably any other form of V/STOL configuration, because in the opinion of many designers, they offer a more acceptable set of compromises. Wide variations of the basic idea are possible; the LTV-Hiller-Ryan XC-142, designed in the United States, was a four-engine transport in which the entire wing, carrying the engines and large propellers, rotated about a horizontal axis so that the propellers could be moved in unison. Germany’s VFW-designed VC-400, though it never flew, had two rotors on the tips of each of its tandem wings—four in all. An alternative approach uses ducted fans instead of the propellers, with the fan assemblies being tilted to the vertical position for takeoff and horizontally for cruising. This design was used in the Bell X-22A research aircraft, which had tail-mounted General Electric T58 turbine engines, driving four ducted fans through a transmission system that maintained drive to all the fans, even when one of the engines failed. The ducted propellers were mounted on the wings and tailplane and directed downward for takeoff and hovering. During hovering, control of the aircraft pitch and roll movements was achieved by appropriate adjustment of the propeller pitch. For horizontal flight, the ducted fans were moved to the horizontal position. Exhaust from the engine turbines was directed at 65 degrees to the horizontal to give additional lift.
The V-22 Osprey, developed by Bell Helicopters and Boeing Vertol, uses a pair of tilting engines mounted at the ends of the wings. This aircraft can travel at twice the speed of a helicopter and is used by the U.S. Department of Defense for purposes such as transport of troops and cargo and search and rescue operations.
