Stall speed is the
airspeed at which a given
aircraft stops producing
lift and stops responding to
aerodynamic control inputs. The stall speed varies inversely with
angle of attack in most cases, and is clearly defined for wings with the same airfoil profile from
root to
wingtip. For aircraft with non-constant airfoil wings, or flexible wings, such as
hang gliders, stall speed is more difficult to define.
A hang glider wing stalls first at the root, since that part of the wing has a higher angle of attack than the wingtips. As the glider goes slower and slower, or the nose is raised higher and higher, the stall
progresses from the root out to the tips. This is what makes a hang glider stable, or
self-correcting in
pitch (see
hang glider). As more and more of the wing stalls the glider becomes harder to turn, and responds more slowly. This is known as "
mush" mode. There is a point at which the aircraft will not respond to any
roll (turn) input, and this speed is generally accepted as the stall speed. The range from
flying to
mush to full
stall is 2-4 mph for most hang gliders.
The size of the speed range of "mush" mode determines a very important
characteristic of a hang glider or other similar aircarft: the
stall break. A narrow mush range equals a
sharp stall break.
For a typical
hang glider,
stall speed is 18-20 mph,
minimum sink is about 21-22,
trim speed is set to about 22-24, and
best glide is somewhere in the 25-30 mph range. On
specification tables, stall speed is often
symbolized by "
Vs", with "V" standing for
velocity.