larger input control made
to correct
Figure 21-1. Pilot-induced oscillation can result if the gyroplane’s reactions to control inputs are not anticipated and become
out of phase.
21-3
BUNTOVER (POWER PUSHOVER)
As you learned in Chapter 16—Gyroplane
Aerodynamics, the stability of a gyroplane is greatly
influenced by rotor force. If rotor force is rapidly
removed, some gyroplanes have a tendency to pitch
forward abruptly. This is often referred to as a forward
tumble, buntover, or power pushover. Removing the
rotor force is often referred to as unloading the rotor,
and can occur if pilot-induced oscillations become
excessive, if extremely turbulent conditions are
encountered, or the nose of the gyroplane is pushed forward
rapidly after a steep climb.
A power pushover can occur on some gyroplanes that
have the propeller thrust line above the center of gravity
and do not have an adequate horizontal stabilizer. In
this case, when the rotor is unloaded, the propeller
thrust magnifies the pitching moment around the center
of gravity. Unless a correction is made, this nose
pitching action could become self-sustaining and
irreversible. An adequate horizontal stabilizer slows the
pitching rate and allows time for recovery.
Since there is some disagreement between manufacturers
as to the proper recovery procedure for this
situation, you must check with the manufacturer of
your gyroplane. In most cases, you need to remove
power and load the rotor blades. Some manufacturers,
especially those with gyroplanes where the propeller
thrust line is above the center of gravity, recommend that
you need to immediately remove power in order to prevent
a power pushover situation. Other manufacturers
recommend that you first try to load the rotor blades. For
the proper positioning of the cyclic when loading up the
rotor blades, check with the manufacturer.
When compared to other aircraft, the gyroplane is just
as safe and very reliable. The most important factor, as
in all aircraft, is pilot proficiency. Proper training and
flight experience helps prevent the risks associated
with pilot-induced oscillation or buntover.
GROUND RESONANCE
Ground resonance is a potentially damaging aerodynamic
phenomenon associated with articulated rotor
systems. It develops when the rotor blades move out of
phase with each other and cause the rotor disc to
become unbalanced. If not corrected, ground resonance
can cause serious damage in a matter of seconds.
Ground resonance can only occur while the gyroplane
is on the ground. If a shock is transmitted to the rotor
system, such as with a hard landing on one gear or
when operating on rough terrain, one or more of the
blades could lag or lead and allow the rotor system’s
center of gravity to be displaced from the center of rotation.
Subsequent shocks to the other gear aggravate the
imbalance causing the rotor center of gravity to rotate
around the hub. This phenomenon is not unlike an outof-
balance washing machine. [Figure 21-2]
To reduce the chance of experiencing ground resonance,
every preflight should include a check for
proper strut inflation, tire pressure, and lag-lead
damper operation. Improper strut or tire inflation can
change the vibration frequency of the airframe, while
improper damper settings change the vibration frequency
of the rotor.
If you experience ground resonance, and the rotor
r.p.m. is not yet sufficient for flight, apply the rotor
brake to maximum and stop the rotor as soon as possible.
If ground resonance occurs during takeoff, when
rotor r.p.m. is sufficient for flight, lift off immediately.
Ground resonance cannot occur in flight, and the rotor
blades will automatically realign themselves once the
gyroplane is airborne. When prerotating the rotor system
prior to takeoff, a slight vibration may be felt that
is a very mild form of ground resonance. Should this
oscillation amplify, discontinue the prerotation and
apply maximum rotor brake.
EMERGENCY APPROACH AND
LANDING
The modern engines used for powering gyroplanes are
generally very reliable, and an actual mechanical malfunction
forcing a landing is not a common occurrence.
Failures are possible, which necessitates planning for
and practicing emergency approaches and landings.
The best way to ensure that important items are not
overlooked during an emergency procedure is to use a
checklist, if one is available and time permits. Most
gyroplanes do not have complex electrical, hydraulic,
or pneumatic systems that require lengthy checklists.
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