Assisted take-off

Any system for helping aircraft into the air (as opposed to purely under its own power), is known as assisted take off. The reason it might be needed is due to the aircraft's weight exceeding the normal maximum take off weight, insufficient power, or the available runway length may be insufficient, or a combination of all three factors. Assisted take off is also required for gliders, which do not have an engine and are unable to take off by themselves.

The most well-known type of assisted take off is the steam catapult found on aircraft carriers. In this system, a piston, known as the shuttle, is propelled down a long cylinder under steam pressure. The aircraft is attached to the shuttle using steel cables, and is flung off the deck at above the mimimum flying speed.

The United States is replacing carrier steam catapults with linear induction motors. The system is called the Electromagnetic Aircraft Launch System (EMALS). An electromagnetic wave travelling through the motor drags the armature along its length, pulling the plne with it. It will be possible to make launch power to aircraft weight more closely with the electrical system than with the steam system, causing less wear on the aircraft.

The catapult approach is also used for towing gliders into the air. This can be accomplished using an elastic bungee arrangement, or more commonly using a cable wound onto a winch, powered by a large diesel engine. The bungee approach is rarely used for man-carrying gliders, as the acceleration is largely uncontrolled and can yield very high G-forces. It is commonly used to launch model gliders however. Gliders are also commonly launched simply by towing them aloft behind a powered aircraft.

A system for assisted take off known as JATO for Jet-assisted take off (and the similar RATO, Rocket-assisted take off) was popular during the 1950s, when heavy bombers started to require two or more miles of runways to take off fully laden. This was exacerbated by the relatively low powers available from jet engines at the time - for example the B-52 Stratofortress required 8 turbojet engines to yield the required performance, and still required RATO for very heavy payloads. A recent update of the B-52 replaces these with half the number of much more powerful engines. In the JATO and RATO system, additional engines are mounted on the airframe which are used only during take off. After that the engines are either jettisoned, or else just add to the parasitic weight and drag of the aircraft.

An unusual assisted take off scheme was developed during the 1950s which consisted of a jet powered truck or dolly which ran either on rubber tyres or rails, used to push a heavy aircraft into the air. Once airborne, the dolly would instantly detach. Because the dolly did not need to fly itself, it was not contrained by the need for low weight, and so could be fitted with very large and powerful engines. The system only reached the early stages of development. The same company was also drawing up plans for a flyable version of the dolly launch system, which it called "jet donkeys". The idea was that a small powerful secondary aircraft could push the heavy main aircraft into the air, detach in flight and return to the airfield to be reused. Sketches of the proposed system show a strange canard-layout aircraft with its cockpit in the tail, pushing the main aircraft via a long extended nose. The system was never developed - presumably common sense and safety considerations prevailed - in any case it was not long before the further development of the jet engine meant that most of these assisted take off schemes became unnecessary.