The Runway Magazine Jan/Feb 2017 | Page 11

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To overcome the problems of low drag on large aircraft with high momentum , the designers have introduced a variety of drag inducing devices such as :
• Spoilers
• Lift dumpers
• Speed brakes
• Thrust reversers
When landing the aircraft , for the different landing distances required are based on variables such as ;
1 )
Headwinds .
2 )
Aircraft Weight
3 )
Runway length
4 )
Runway Condition ( Dry , Wet , Snow , Ice )
5 )
Reverse thrust ( Engaged or not )
“ Thrust Reversers ” are used as the braking system of the aircraft . In modern aircraft , the braking system in the wheel are sufficient during normal conditions . Thrust reversers change the direction of the aircraft ’ s exhaust gases so that they are directed forwards , rather than backwards . The method of redirecting the flow varies with the size , configuration and the manufacturer of the engine . When the runway becomes icy or snow covered , stopping the aircraft requires an additional procedure . This is where “ Thrust reversers ” are used . In this way , the pilot is able to reduce the landing distance of an aircraft . In high by-pass ratio engines the majority of the thrust is derived from the fan airflow ( bypass air from the fan ), the core air from the engine will exit normally . So changing the direction of the fan airflow would create a net reverse thrust . Thrust reversers are used to change the direction of the fan airflow . In Turboprop aircraft reverse thrust action is used by changing the pitch of the propeller blades . The pitch in which the blade provides reverse thrust is called the “ beta range ”. Usually , a hydro-mechanical system is used to change the blade angle , giving a braking response when activated . Ideally , the airflow should be directed in a completely forward direction to gain the complete drag to stop the aircraft ; however , this is not possible , mainly due to aerodynamic reasons . A discharge angle near 45 degrees is usually chosen , resulting in a proportionally effective reverse thrust than the thrust of the same engine in its normal direction .
There are several methods of obtaining reverse thrust on “ turbo-jet engines ”: 1 . CLAMSHELL TYPE deflector doors to reverse the exhaust gas stream .
2 . BUCKET TARGET SYSTEM with external type doors ( Bucket doors ) to reverse the exhaust . 3 . Fan engines utilize BLOCKER DOORS to reverse the COLD STREAM AIRFLOW SYSTEM .

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