Transient trim is a common side effect
of using a variable geometry
(VG) system on a hang glider
Hang gliders are usually designed and tuned such that they will settle
to an airspeed
between minimum sink
and best glide
, somewhere in the low 20 MPH
vicinity, without input from the pilot
". This is known as trim speed
or simply trim
, and is a very desirable characteristic, since the glider
can be flown at optimal
or near-optimal speeds with little physical effort on the pilot's part.
is applied to the wing
via a VG system, trim speed usually changes, typically increasing at least a few MPH. The rate of trim speed increase may increase as the VG is applied, or the rate may reduce and even reverse - many variables
are at work in this situation
. If the trim speed increases beyond best glide speed then the glider is not flying efficiently, and it takes a constant and fatiguing
push out on the control bar
(by the pilot) to fly at lower speeds. Other characteristics of the glider, including pitch
stability, tend to change as VG is applied as well.
The hang glider certification
requirements (such as those of the U.S. Hang Glider Manufacturer's Assn. or HGMA) at first required VG-equipped gliders to be tested and documented VG-loose and VG-tight, but were amended to require VG-middle as well after there were a few incidents with gliders flown at intermediate VG settings. It is not uncommon for a prototype
to have passing data loose and tight, but fail at VG middle settings, requiring adjustment such that loose and tight values far exceed what is necessary - which usually costs in performance
or handling feel. Most hang glider manufacturers have made great efforts over the years to reduce or eliminate transient trim when working on new glider designs.
In the mid 1990s, as higher performance levels were achieved and previously minor variables began having more and more effect, transient trim became harder to combat, though pitch stability issues remained at previous levels or improved. One manufacturer gave up
battling transient trim on its new (late 90s) glider and instead touted
it as a feature
pilots. The reasoning was that comp pilots tend to fly very fast from thermal
to thermal and make up for the inefficiency by spending more time in lift
(see speed to fly
), so having the glider trim at higher speeds means less fatigue while racing, and thus better pilot performance. Over the course of several years this approach was embraced by all of the manufacturers in the (small) industry.
The cost of this change to the recreational
pilots, who far outnumber
the competition pilots, is that VG is a bit less useful to them. Most rec pilots use intermediate VG to squeeze a little better glide
from their wings when the lift gets light, at the cost of increased effort for turning. The move toward high transient trim gliders means that significantly more effort is now required, since pitch force has to be applied as well.