Trim Speed is the speed to which a vehicle, such as a
glider or
boat will
settle in the absence of
control inputs. "
Trim" alone refers to the left/right
balance of a vehicle (see
trim tab).
Trim Speed is one of the
cardinal speeds which it is important to know when flying a
glider of any kind. Trim, as it is also known, is the speed at which the
aircraft will fly without
pitch inputs (in still air).
Trim speed depends partly on the location of the aircraft's
center of gravity relative to the
center of lift of the lift producing surfaces, and partly on the
load the aircraft is carrying. In
hang gliders, trim speed is adjusted by setting the location of the
hang strap (hang point) from which the pilot's weight is
suspended (from the
keel). Forward equals
faster,
aft slower. A lighter pilot will have
a faster trim speed than a heavier pilot on the same hang glider, all else being equal. This is because the heavier load causes the
airframe to
flex more, so the
wingtips produce less lift, raising the nose and slowing the aircraft.
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. When the trim speed is set in this range it takes minimal effort to maintain either minimum sink or best glide, and flying
hands off will lead to neither a
dive nor a
stall. Pilots who race hang gliders sometimes set their time speed at or near best glide, to reduce fatigue flying at that speed (as they are wont), but their gliders then require more effort to fly slowly in
lift and are harder to
land because they trim so far above stall speed. Devices to allow trim speed adjustment have had sproadic interest over the years in hang glider racing, but have never
caught on.