Stall break is the behavior of an aircraft when it reaches
stall speed. Wings with varying
airfoil profile stall progressively, that is to say, parts of the wings stall before others, which keep
flying. For a
tapered,
delta wing aircraft such as a
hang glider, the
root of the wing stalls first, progressing
outboard to the tips. Because the wingtips are
aft of the nose of the hang glider, the lift they produce when the root stalls causes the aircraft to try to rotate more nose-down and resume flying properly. This action is called the stall break.
A stall break can be sharp and distinct or it can be a gradual increase in nose-down force. Higher
performance gliders generally are allowed to have sharper stall breaks than are
acceptable on
beginner wings, since the pilots
flying high performance wings are expected to be ready and able to deal with such behavior. The severity of a stall break is somewhat dependent on
angle of attack and the rate at which one slows from flying to stall. One can get
even the most
docile glider to do a sharp stall break by picking up speed and then rapidly pushing the nose up and holding it until stall. This is known as a "
whipstall" and is a common way to initiate a high-speed
dive in preparation for
aerobatic maneuvers, such as
loops and
wingovers. A whipstall can easily lead to loss of control of the glider and is generally best
avoided.
Since hang gliders are usually flown as slowly as possible in
lift (i.e.
minimum sink speed) to get the best
climb rate, a sharp stall break can mean that small errors in speed control are
costly in terms of
altitude and
efficiency. In rough air, one's
airspeed and
angle of attack frequently change
abruptly and by several miles per hour in either direction, so a sharp stall break is a major
liability, and even dangerous if one is flying
near others or close to the
terrain.