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Region of reverse command: Control Surfaces

  • Cranenium
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Cranenium created the topic: Region of reverse command: Control Surfaces

In region of reverse command what happens to the control surfaces? I was told that slipstream would be present however because of straight and level flying with high attitude the slipstream effect will not affect the rudder and elevator. Another instructor told me otherwise. Help please


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  • John.Heddles
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John.Heddles replied the topic: Region of reverse command: Control Surfaces

I presume you are referring to operation below min drag ?

I don't have anything definitive re slipstream at high alpha on light aircraft but I suspect that it will still impact on the tailfeathers in unstalled flight. Certainly this is exploited on high alpha capable fighters, such as the F18 where the vortex shed from the LEX (leading edge extension .. the slender delta bit at the LE root of the main wing which extends up alongside the fuselage) is used to provide airflow for high alpha tailplane control. It also is responsible for a lot of the tail end fatigue problems on the aircraft but that is another story. A good picture of the vortex can be seen in . You can get an idea of the alpha from the trailing vortex at the back of the aircraft.

The main control surface concern at high alpha of which I am aware is in respect of stability. Especially for piston to turboprop conversions - where the engine is stuck out quite a bit to the front due CG considerations - a low speed missed approach in the landing configuration (which occurs for a high alpha) can see a very large destabilising prop plane force well forward of the aircraft CG which, for some Types, requires a SAS (stability augmentation system) to counteract the stick load variations when the power levers are advanced for the miss. For some aircraft, if the SAS is U/S, hamfisted power lever movement can see the stick forces reverse which is altogether not a good thing. It is for this reason that one sees restrictions on maximum power (ie thrust) for the miss while in this low speed regime for SAS inop flight.

Engineering specialist in aircraft performance and weight control.

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