VERTICAL-TAKEOFF-AND-LANDING AIRCRAFT (VTOL)
Aircraft capable of vertical takeoff and landing (usually abbreviated as VTOL) have important military and civil potentialities and are under active development by aircraft firms in a number of countries. One general advantage claimed for these aircraft is their greater safety in that the hazards associated with conventional takeoff and landing are eliminated. A further advantage is that they do not require airfields with long and expensive runways, so that despite the higher direct operating costs of VTOL aircraft; an overall saving is effected in comparison with conventional aircraft.
Of course, the helicopter has essentially solved the problem of vertical takeoff and landing, but its applications are limited by its relatively low speed. In recent years a large number of new VTOL aircraft types have emerged. Vertical takeoff without the use of large rotors became practicable with the advent of the gas-turbine engine because it could generate much more thrust for a given weight than the piston engine. Many new problems have had to be solved: e.g., in connection with stability and control during hovering and the transition from vertical to forward flight, and vice-verse, since conventional control surfaces are ineffective at low forward speeds.
Depending on the position of the aircraft during the takeoff, a distinction is made between “tail sitters” and “flat risers.” In the first-mentioned category the whole aircraft rests with its tail on the ground and its nose pointing vertically upwards. After takeoff, it is gradually brought into the normal flying position by operation of the controls. The “flat riser” takes off in the normal position, i.e., with the fuselage parallel to the ground. In this last-mentioned category of VTOL aircraft, the propulsion engines may be swiveled from the vertical position for takeoff and landing to horizontal for forward propulsion. With turbojet propulsion, the propulsion engines can be used for takeoff and landing by suitably directing the jets downwards. In addition to the propeller VTOL aircraft and the turbojet VTOL aircraft, a third type is based on the ducted fan, this being a propeller or fan within a duct or shroud, which in some types can be tilted in the same manner as the propeller engine. It is a combination of a ducted fan and a jet engine. Each of the two wings of the aircraft may be provided with such a fan, “buried” in the thickness of the wing. The jet engine provides the propulsion in the normal way when the aircraft is in forward flight. For takeoff and landing, the jet exhaust is deflected to drive the fan, which thus develops a powerful vertical thrust.
The present trend of development is toward the direct utilization of the thrust developed by turbojet. In a case where separate lift engines are provided in addition to the propulsion engine there is of course the problem of extra weight due to having two sets of engines, only one of which is in use at any particular time. In this respect the arrangement where only one set of engines is provided, which can be swiveled from vertical to horizontal, and vice-verse, or where the jets themselves can be deflected to produce a thrust in the desired direction is advantageous. This is especially true in high-speed fighter aircraft, whose engines produce a large thrust which can be utilized for vertical takeoff. On the other hand, separate lift engines or a combination of swiveling jet engines and a set of auxiliary lift engines may be more advantageous for other types of aircraft, such as civil aircraft, with lower cruising speeds.