Vertical and Short Takeoff and Landing Aircraft V/STOL

To date, there have only been a few operational jet VTOL designs, the British Harrier and the Russian YAK-36, both of which are subsonic aircraft. While at least one supersonic VTOL design has flown, the Mach 2 Mirage III-V back in 1966, there has yet to be an operational supersonic VTOL aircraft.

This is largely due to the need for increased internal volume for the vertical lift apparatus and fuel capacity. Also, most concepts for vertical lift tend to increase the aircraft's cross-sectional area near the aircraft's center of gravity, increasing the supersonic wave drag. Finally, the state of the art in engine thrust-to-weight ratio has imposed an excessive weight penalty on VTOL designs. It has simply been impossible, up to now, to provide an operational aircraft having both vertical and supersonic forward flight with any practical range.

Modern supersonic jet fighters have a thrust-to-weight ratio exceeding one, so it would seem fairly easy to point the jet exhaust downward and, therefore, attain vertical flight at "no cost". Unfortunately, this is complicated by the balance problem. Many subsonic jets and virtually all supersonic jets are designed with the engine at the rear, the cockpit and avionics at the nose and the payload and fuel near the center of the aircraft. This traditional layout places the expendables at the center of gravity, co-locates the parts of the aircraft requiring cooling (crew and avionics), and keeps the avionics away from the hot and vibrating engine.

If the thrust exceeds the weight, vertical flight could be obtained simply by deflecting the thrust downwards. However, some vertical upward force is required forward of the aircraft's center of gravity. In order to balance the vertical thrust force at the tail. This balance problem is possibly the single most important problem encountered in the design of a VTOL jet fighter.

There are two conceptual approaches to solving the balance problem. Either the thrust location must be moved to the center of gravity, or an additional thrust force has to be applied near the nose. Both of these approaches will tend to move the design of the aircraft away from the optimal layout. Generally, jet VTOL concepts can be divided into those which utilize fairly conventional engines and those which use modified engines so that the fan and core air are split, with the fan air ducted and exhausted from some location separate from the core air.

The conventional engine VTOL concepts which do not use additional lift engines for vertical flight must have a net takeoff thrust-to-weight ratio in excess of one. If the jet exhaust is not diverted to some other location for vertical flight, the aircraft must either be a tail sitter (VATOL), or it must have the engine exhaust located at the aircraft center of gravity and capable of vectoring downward for vertical flight. This can be accomplished by using a vectoring nozzle or nacelles which tilt.

The X-14 research aircraft had vectoring nozzles at the center of gravity, with the engines out in front. This is probably not a good arrangement for most applications because the cockpit winds up in the rear for balance, which does not provide acceptable visibility for the pilot. Also, in forward flight, the jet exhaust scrubs alongside the fuselage which causes thermal and acoustic problems. Tilt nacelles are heavy, but may be the best compromise for some applications. Grumman Aircraft Corporation has been pursuing a tilt-nacelle concept for Naval applications for a number of years.

Some VTOL concepts provide a means of diverting the exhaust flow to provide vertical lift. This is generally done by a retracting blocker device in the engine which shuts off the flow through the rearward-facing nozzle. The flow is then diverted forward through internal ducting. All of these VSTOL approaches, however, exact significant penalties in weight, cost, and completely when compared to a conventional jet aircraft design.

In the area of propeller-powered aircraft, the tilt-rotor concept, as tested in the Bell XV-15, seems to offer the best compromise between helicopter-like vertical flight and efficient wing-borne cruise. The tilt-rotor concept is the basis of the V-22 Osprey.