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Old 26th Apr 2005, 18:02
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Dick Whittingham
 
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Location: Bristol
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A basic swept wing setup, without any fixes, will pitch up with increasing alpha. No civil airliner with these characteristics would pass certification. Therefore, fixes are incorporated, mainly to induce root stall and delay tip stall, so the aircraft shows an increasing nose down tendency as alpha increases to the stall.

The problem arises at alpha above the stall, above critical alpha, when these fixes no longer work, and the setup reverts to a nose up tendency. Even then, all is not lost if you have sufficient tailplane authority to overcome this and lower the nose.

The attitude of the aircraft in a superstall, above critical alpha, is nose level or nose up, but it is sinking like a brick s...house, so alpha is 20 - 30 deg, and the "downwash" is going high over the tail. If you have a T- tail, this makes the tail ineffective, and delays or prevents recovery.

So the superstall itself is a factor of swept wing setups. Delayed or impossible recovery is a factor of T-tails.

Aircraft that are going to have difficulty recovering from superstall will have stall avoidance systems - not just stall warning. These will be stick pushers and possibly auto-throttle to full power. In fact, Airbus, in its various forms, has auto-protection of this sort.

Basic recovery is to select TOGA, flap to the optimum for nose down trim change, stick forward, wait, and pray.

Dick W
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