User Guide

Flight Instruction
238
Flight Instruction
239
SWEPT-WING STALLS
Because of their geometry, swept wings tend to stall first at
the tips, with the stalled area progressing inboard.
This is unfortunate for at least two reasons. One is that since
the ailerons are out near the tips, there’s a tendency for roll con-
trol to be lost in the stall. Worse, however, is that the wing’s sweep
puts the tips well aft of the airplane’s center of gravity - so as the
tips stall, and lose lift, the airplane pitches up, making the stall
much worse. Moreover, once a T-tailed jet like the Hawker pitch-
es up into a stall, the horizontal tail will be immersed in the tur-
bulent, separated wake of the wing and engine nacelles. This is
called a “deep stall” in this country (the ever-colorful Brits call it
a “superstall.”) What’s particularly un-super about it is that with
the horizontal tail blanked out, you have no elevator control. In
other words, it’s unrecoverable. Repeat after me, class: “Gravity
never sleeps...
With this in mind, my advice about stalls in swept-wing jets
can be summed up in one word:
DON’T!
Fortunately, the airplane itself has a strong sense of self-
preservation in the form of an active stall prevention system. In the
Hawker, it’s operated by a pair of angle of attack vanes on the
sides of the fuselage and constantly evaluates factors including
airspeed, angle of attack, flap position, and pitch rate. If it senses
the airplane getting unpleasantly close to a stall, it activates its first
“stall warning” phase. Since there’s virtually no aerodynamic buf-
fet to warn the crew, it turns on an electric control column shak-
er, as well as warning lights on the instrument panel. If you have
a force feedback joystick or yoke, you’ll be able to feel the stick
shaker - but it’s pretty noisy in any case!
Let’s say a swept-wing jet, flying along, yaws slightly to its
right. Now the right wing acts as if it were swept even more, thus
having less effective span, while the left one acts as if it were
swept less, with more effective span. Result? The airplane begins
to roll off to the right.
As it does, however, the “tailfeather effect” of the vertical fin,
as well as the increased drag of the left wing, try to slew it back
around to the left. The right wing starts to come back up, while the
yaw switches around the other way. Unfortunately, it’s out of phase
with the rolling motion, so the airplane starts to wallow back and
forth. Depending on the airplane, this reaction can range from
mildly uncomfortable, through nauseating, to “divergent” - mean-
ing that each successive swing gets larger until the airplane
becomes uncontrollable. Moreover, again depending on the air-
plane, it will range from counterintuitive, through extremely diffi-
cult, to completely impossible for the pilot to regain control.
Most jets are equipped with a device called a yaw damper,
which automatically actuates the rudder to eliminate Dutch Roll.
The Hawker is actually quite well-behaved in this regard, and can
be flown - if you stay on top of it! - without its yaw damper
engaged. Normally, however, you’ll engage the YD right after
takeoff, and disengage it just before landing. If you want to sam-
ple the Dutch Roll, disengage the YD at high altitude, make a
decisive rudder input, and release all controls; the airplane will
start a definite left-right rocking, with the skid ball sliding back
and forth. If you’re really sharp, you may be able to damp out the
motion with ailerons and rudder. You may find it easier, however,
to wait until you near the extreme of a swing, then apply a dab of
aileron into it to put the airplane into a steady turn, then recover
from there.