Assisted take off

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

A well-known type of assisted takeoff is an aircraft catapult. In modern systems fitted on aircraft carriers, a piston, known as a shuttle, is propelled down a long cylinder under steam pressure. The aircraft is attached to the shuttle using a tow bar or launch bar mounted to the nose landing gear (an older system used a steel cable called a catapult bridle; the forward ramps on older carrier bows were used to catch these cables), and is flung off the deck at about 15 knots above minimum flying speed, achieved by the catapult in a four-second run.

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 traveling through the motor propels the armature along its length, pulling the plane with it. With this system, it will be possible to match launch power and aircraft weight more closely 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 cord 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 uncontrolled and can yield very high G-forces. It is commonly used to launch model gliders however. Manned gliders are commonly launched simply by towing them aloft behind a powered aircraft.

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