What is the
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
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.
Lift 1. Centre of lift moves forward as stall advances forward and
inward from tips. An adverse nose-up couple is created.
Lift 2. “T” tail aircraft with stalled turbulent air blanketing
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.
Marty says ... "Goodbye to GA".