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What is the super-stall ?

Swept wings naturally stall at the tips first. This is due the air approaching the leading edge of the wing deflecting upwards (called upwash), thereby increasing the effective angle of attack. Also, with the tips being of quite thin section, they tend to stall first. The stalled air moves forwards and inwards as A of A is increased toward the stall speed. This is undesirable, as the outboard ailerons quickly become within the stalled air and lose their control effectiveness.

The swept wing aircraft will continue to nose-up despite a large percentage of the wing having stalled, as the centre of lift moves forward, creating a runaway pitch-up couple with the C of G. We are approaching a condition called the “super-stall”, or deep-stall”. This is definitely a case of negative pitch (longitudinal) stability. Refer fig Lift 1.

A flat spin is possible, and it may be un-recoverable due to the lack of elevator effectiveness, as this control surface can be blanketed and buffeted in the aerodynamically dead turbulent air streaming back and upward from the stalled wing. This is especially true in the case of aircraft that feature a high mounted tailplane (“T” tails). Refer fig Lift 2.

Fig Lift 1. Centre of lift moves forward as stall advances forward and inward from tips. An adverse nose-up couple is created. Fig Lift 2. “T” tail aircraft with stalled turbulent air blanketing the tailplane.

The above briefing is a sample from the ATPL Aerodynamics and Systems course, which will soon be available via Internet. I hope you found this of value. It is a typical airline technical interview question. Solving wing tip stalling will be the topic of a future free ATPL training editorial.

Best wishes

Rob Avery

ATPL Lecturer

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Marty says ... "Goodbye to GA".

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