User Guide

66
Flight Instruction
TOO MUCH OF A GOOD THING:
Unfortunately, we can’t just go on
increasing angle of attack forever as
speed bleeds off to zero; if we
could, we’d have no need for hel-
icopters. Instead, once the angle
of attach reaches a certain point
(called critical angle of attack), the
air can no longer make the curve
around the leading edge and over
the top of the wing. Instead, the flow
separates, becoming turbulent over the
top of the wing (fig. 5). Notice how simi-
lar this is to the flat plate in figure 1? That’s
right: when this happens, most of the lift disappears, and the wing
is stalled. At this point the wing has, for all practical purposes, “quit
flying;” gravity reasserts itself, and the airplane begins to drop.
Sounds serious, doesn’t it? It is, of course...but hardly fatal.
All that’s necessary to recover from the stall is to reduce the angle
of attack below the critical level by easing forward on the controls.
The airflow promptly reattaches itself, and the wing resumes its job
of producing lift. You’ll practice stalls, and stall recoveries, in all
the airplanes in FLY! A stall isn’t even a particularly dangerous or
unusual situation. Until the last years of World War II, almost all
airplanes were “taildraggers,” with two large main landing gear
and a small caster under the tail. These airplanes sit on the ground
right at the critical angle of attack, and thus have to be fully stalled
for a “three point” landing. In fact, a perfect landing in a taildrag-
ger is actually a complete stall followed by an uncontrolled
crash...from an altitude of, say, half an inch!
Figure 5
Axes of Movement in an Airplane
Roll Pitch Yaw
67
IT’S ALL IN THE ANGLE:
There are two vital things to remember about stalls:
The most important one is that, while we may often talk
about an airplane’s “stalling speed,” that can be misleading.
Whether or not a wing will stall depends only on its angle of at-
tack - and the stall can and will occur at any speed if the critical
angle of attack is exceeded. Wrap an airplane up into a steep turn,
so that centrifugal force adds to its apparent weight, and you’ll
have to increase the angle of attack to compensate. At some point,
you’ll have pulled all the way to the critical angle of attack, and
the wing will stall even though you’re flying well above the pub-
lished stall speed. Don’t worry - these high-speed, or accelerated,
stalls are no more fearsome than the regular kind, and we’ll prac-
tice them together.
The “stall speed” published in an airplane’s specs only ap-
plies to a stall entered gently from straight and level flight. Most
airplane handbooks include a table that shows vividly how the
stall speed goes up at increasing angles of bank.
The other thing to remember is that, in aviation, the word
“stall” means only one thing: the condition in which the airflow
over the wing has separated, and lift has been impaired. It has
nothing to do with the engine quitting (nor, for that matter, with the
small enclosures used to confine farm animals). After all, even
gliders can stall with no engines at all!
Flight Instruction
Angles of Attack