Earlier lower bypass engines use shroud type reversers due to the small area of airflow behind the engine therefore not too much weight is added by the reversing mechanism. Cascade or sleeve reverses work by only reversing the direction of the cold stream, which is about 80% of the thrust on a modern high bypass turbofan. Reversers are most effective at high airspeed, however many aircraft can be reversed using thrust reverse. The critical aspect in using reverse thrust is the risk of FOD (foreign object damage) by blowing loose objects from the ground into the engine or onto the aircraft surface. The Boeing 747-400 actually retracts the leading edge flaps on selection of reverse to avoid the risk of FOD damaging the flaps.
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