Convertiplane

(aerospace engineering) A hybrid form of heavier-than-air craft capable, because of one or more horizontal rotors or units acting as rotors, of taking off, hovering, and landing in a fashion similar to a helicopter; and once aloft and moving forward, capable, by means of a mechanical conversion, of flying purely as a fixed-wing aircraft, especially in higher speed ranges.

An aircraft combining vertical takeoff and landing capabilities as in the helicopter with forward-flight effectiveness and high-speed potentials of the airplane. In forward flight a convertiplane relies, at least partially, on the fixed wing, while for vertical takeoffs, landings, and hovering, a separate vertical thrust generator is provided. Between vertical and forward-flight regimes, the aircraft goes through a conversion. See also Airplane; Helicopter; Vertical takeoff and landing (VTOL).

Of many systems suitable as vertical-thrust generators, those applied to practical convertiplanes will probably be either of the direct type, in which combustion products are used directly for thrust generation (turbojets and rockets), or of the indirect type, in which all or a part of the available combustion energy is used to drive a mechanical system, functioning as an actuator disk to accelerate the ambient air. The actuator disks may be either free (helicopter rotors and propellers) or shrouded (shrouded propellers and ducted fans).

The principle of the convertiplane can be applied to many types of aircraft. The first practical attempts were directed toward rotary-wing aircraft. In the so-called compounds as much lift as possible is transferred in forward flight from the rotor (which is either put into autorotation or slowed down) to more efficient fixed wings, while a propeller (or propellers) provides the forward thrust.

To achieve high speeds the helicopter-type rotor must be eliminated from the forward-flight configuration, which should be as close as possible to that of conventional aircraft. Configurations have been studied wherein rotors are stopped, folded, and retracted into the fuselage or into nacelles.

In the nonhelicopter-type convertiplane, the same device can be used both as a vertical-thrust generator and as a means of forward propulsion. Two basic solutions are technically feasible: (1) the wing remains fixed, while either the entire thrust generator or only the thrust vector tilts from the vertical to the horizontal position, and (2) both the wing and the thrust generator tilt as a unit.

In the fixed-wing, free-airscrew group, the NASA-Army-Bell XV-15, a flight research aircraft with 375 mi/h (167 m/s) speed capabilities, went through complete conversion in 1979 (Fig. 1). In the ensuing years of extensive flight testing, the XV-15 proved the feasibility of the tilt-rotor concept, thus paving the way for the development of an operational aircraft, the Bell-Boeing V-22 Osprey (Fig. 2), for the U.S. military forces, especially the Marine corps.

NASA-Army-Bell XV-15 in airplane configuration after conversion from helicopter mode. (Bell Helicopter Textron)

Bell-Boeing V-22 Osprey tilt-rotor aircraft. (Bell Helicopter Textron)

The Harrier (manufactured by Hawker Siddeley, now British Aerospace, and McDonnell Douglas), with Pegasus vectored-thrust turbofan engines (manufactured by British Siddeley, now Rolls Royce), is a fighter that is produced for the military forces (including the U.S. Marines) of several Western countries and was an important factor for the British in the 1982 Falkland Islands conflict (Fig. 3). The Yakovlev, a Russian single-seat, carrier-based combat aircraft, is similar in concept.

Harrier fighter aircraft, manufactured by Hawker Siddeley, now British Aerospace, and McDonnell Douglas. Close ground support and reconnaissance are provided through use of vectored-thrust turbofans.