BRIEFING FILE

摘要

Stopping a jet aircraft on landing isn't easy. They usually have a large mass and, even with the various high-lift devices to lower approach speeds, still touch down at a significant velocity. The ground contact area of the wheels is normally small, meaning the use of wheel braking alone is not an efficient means of slowing the aircraft on the runway - the tyres and brakes are at risk of significant heat spikes due to the energy transfer. While jet aircraft tyres, wheels and brakes are designed to be able to slow and stop the aeroplane without other assistance under normal conditions, lessening the demand on the landing gear has significant advantages in terms of wear and re-use rates, and reliability. Directing the engine's thrust part-forwards (anywhere past 90°) uses the aircraft's main power source to decelerate efficiently instead. The design of thrust reversers is usually relatively simple, of low weight penalty and economical, particularly in contrast to the requirements otherwise loaded onto the landing gear. By directing the energy from thrust to braking, it is an efficient use of the power immediately available Jet aircraft thrust reversers come in three types: the target, clamshell, and cold stream systems. The target type is essentially a pair of targets, or 'buckets,' rotated from around the body of the engine area to redirect the engine thrust forward to a desired degree, as illustrated. This is most efficient for low-bypass or turbojet engines. Patents were filed by Boeing engineers for versions of this concept in 1954 and 1968.