vertical takeoff and landing
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vertical takeoff and landing[′vərd·ə·kəl ′tāk‚ȯf ən ′land·iŋ]
Vertical takeoff and landing (VTOL)
A flight technique in which an aircraft rises directly into the air and settles vertically onto the ground. Such aircraft do not need runways but can operate from a small pad or, in some cases, from an unprepared site. The helicopter was the first aircraft that could hover and take off and land vertically, and is now the most widely used VTOL concept. See Helicopter
The helicopter is ideally suited for hovering flight, but in cruise its rotor must move essentially edgewise through the air, causing vibration, high drag, and large power losses. The aerodynamic efficiency of the helicopter in cruising flight is only about one-quarter of that of a good conventional airplane. The success of the helicopter in spite of these deficiencies started a wide-ranging study of aircraft concepts that could take off like a helicopter and cruise like an airplane. The term VTOL is usually used to designate the aircraft other than the helicopter that can take off and land vertically. The term V/STOL indicates an aircraft that can take off vertically when necessary, but can also use a short running takeoff, when space is available, to lift a greater load. See Short takeoff and landing (STOL)
The tiltrotor is closest to the conventional helicopter and relies heavily on helicopter technology. The rotor disks are horizontal in VTOL operation and are tilted 90° to act as propellers in cruising flight. Such aircraft can have cruise efficiencies at least twice those of the helicopter, making them especially useful for helicopter missions where greater range, speed, and time on station are desired.
Numerous advanced rotorcraft concepts are being investigated in research programs. These include designs that take off and land as a rotor system and fix the blades to act as wings for cruise flight. Other concepts feature tiltrotor technology for vertical flight, but fold the blades and employ other propulsion modes for cruise.
The vectored-thrust concept features an engine with four rotating nozzles that deflect the thrust from horizontal for conventional flight to vertical for VTOL operation. This activity led to the very successful British Harrier aircraft, which is considered to be a V/STOL aircraft because its normal mode of operation is to use a short takeoff run, when space is available, to greatly increase its payload and range capability.
With the success of the Harrier, design studies and technology development programs have been directed at expanding the flight envelope of this class of aircraft to include supersonic capability. The term STOVL (short takeoff vertical landing) is used to define this supersonic fighter-attack aircraft since the large benefits in payload and range of a short takeoff will be factored into the basic design.