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GLIDER INDUCED OSCILLATIONS
PITCH INFLUENCE OF THE
GLIDER TOWHOOK POSITION
The location of the glider’s aerotow towhook influences
pitch attitude control of the glider during aerotow
operations. During these operations, the towline is
under considerable tension. If the towline is connected
8-6
to a glider towhook located more or less directly on the
longitudinal axis of the glider, the towline tension has
little effect on the pitch attitude of the glider. This is
the case when the towhook is located in the most forward
part of the gliders nose, such as in the air vent
intake hole. This is the ideal location for the aerotow
hook for most gliders.
The towhook on many gliders is located below or aft
of this ideal location. In particular, many European
gliders have the towhook located on the belly of the
glider, just forward of the main landing gear, far below
the longitudinal axis of the glider. The glider’s center
of mass is well above the location of the towhook in
this position. In fact, virtually all of the glider’s mass
is above the towhook. The mass of the glider has inertia
and resists acceleration when the towline tension
increases. In these bellyhook-equipped gliders, an
increase in tension on the towline causes an uncommanded
pitch-up of the glider nose as shown in
Figure 8-4. Decrease in towline tension results in an
uncommanded pitch-down.
Rapid changes in towline tension, most likely to occur
during aerotow in turbulent air, cause these effects in
alternation. Naturally, on days when good lift is available,
the aerotow will be conducted in turbulent air.
The potential for inducing pitch oscillations is obvious,
as rapid alternations in towline tension induce
rapid changes in the pitch attitude of the glider. To
maintain a steady pitch attitude during aerotow in a
bellyhook-equipped glider, the pilot must be alert to
variations in towline tension and adjust pressure on the
control stick to counteract the pitch effect of variations
in towline tension.
SELF-LAUNCH GLIDER OSCILLATIONS
DURING POWERED FLIGHT
Gliders equipped with an extended pod engine and
propeller located high above the glider’s longitudinal
axis exhibit a complex relationship between power
setting and pitch attitude. When power changes are
made, the location of the thrust line of the propeller in
this location has a noticeable effect on pitch attitude.
The changing speed of the propwash over the elevator
causes considerable variation in elevator effectiveness,
modifying pitch attitude still further. Prior to flight,
study the GFM/POH carefully to discover what these
undesired effects are and how to counteract them.
When throttle settings must be changed, it is good
practice to move the throttle control smoothly and
gradually. This gives the pilot time to recognize and
counteract the effect the power setting change has on
pitch attitude. In most self-launched gliders, the effect
is greatest when flying at or near minimum controllable
airspeed (MCA). Self-launch glider pilots avoid
slow flight when flying at low altitude under power.
[Figure 8-5]
Self-launch gliders may also be susceptible to PIOs
during takeoff roll, particularly those with a pylon
engine mounted high above the longitudinal axis. The
high thrust line and the propeller wash influence on
the air flow over the self-launch glider’s elevator may
tend to cause considerable change in the pitch attitude
of the glider when power changes are made.
NOSEWHEEL GLIDER OSCILLATIONS
DURING LAUNCHES AND LANDINGS
Many tandem two-seat fiberglass gliders, and some
single-seat fiberglass gliders as well, feature a threewheel
landing gear configuration. The main wheel is
equipped with a traditional large pneumatic tire; the
tailwheel and the nosewheel are equipped with smaller
pneumatic tires. During taxi operations, if the pneumatic
nosewheel remains in contact with the ground,
any bump will compress the nosewheel tire. When the
pneumatic nosewheel tire rebounds, an uncommanded
pitch-up occurs. If the pitch-up is sufficient, as is likely
to be the case after hitting a bump at fast taxi speeds,
the tailwheel will contact the runway, compress, and
rebound. This can result in porpoising, as the nosewheel
and tailwheel alternate in hitting the runway,
compressing, and rebounding. In extreme cases, the
fuselage of the glider may be heavily damaged.
Figure 8-4. Effect of increased towline on pitch altitude of bellyhook-equipped glider during aerotow.
8-7
During takeoff roll, the best way to avoid porpoising
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本文链接地址：
Glider Flying Handbook(91)
