Thursday, March 8, 2012

Thrust reversal - INTRODUCTION

Fig. 15-1 Comparative landing runs with and without thrust reversal.
1. Modern aircraft brakes are very efficient but on wet, icy or snow covered runways this efficiency may  be reduced by the loss of adhesion between the aircraft tyre and the runway thus creating a need for an additional method of bringing the aircraft to rest within the required distance.
2. A simple and effective way to reduce the aircraft landing run on both dry and slippery runways is to reverse the direction of the exhaust gas stream, thus using engine power as a deceleration force. Thrust reversal has been used to reduce airspeed in flight but it is not commonly used on modern aircraft. The difference in landing distances between an aircraft without reverse thrust and one using reverse thrust is illustrated in fig. 15-1.
3. On high by-pass ratio (fan) engines, reverse thrust action is achieved by reversing the fan (cold stream) airflow. It is not necessary to reverse the exhaust gas flow (hot stream) as the majority of the engine thrust is derived from the fan.

4. On  propeller-powered  aircraft,  reverse  thrust action is obtained by changing the pitch of the propeller blades. This is usually achieved by a hydro- mechanical system, which changes the blade angle to give the braking action under the response of the power or throttle lever in the aircraft.
5. Ideally,  the  gas  should  be  directed  in  a completely  forward  direction.  It  is  not  possible, however, to achieve this, mainly for aerodynamic reasons, and a discharge angle of approximately 45 degrees is chosen. Therefore, the effective power in reverse thrust is proportionately less than the power in forward thrust for the same throttle angle.

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