AGARD R-800 - FTP Directory Listing - Nato

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2nd (5 1 .d Hz) vertical bending are not involved in this
oscillation problem.
C) Tyre Pressure
Under the circumstances that the original tyre was not
available and the used tyre has a different characteristic,
an increase of pressure could solve the problem due to
shorten the relaxation line.
d) Installation of shimmy damper
The shimmy damper is a hydraulic unit which reduces
the tendency of the wheel to oscillate from one side to
side. This dampers are usually constructed in one of two
general designs, piston type and vane type, both of which
might be modified to provide power steering as well as
shimmy damper actions.
e) Mounting of massbalance
Tuning the eigenfrequencies of the landing gear
massbalance weights have to be mounted on an arm
forwards or rearwards on the strut. Massbalance is a
useful tool if there is enough space to mount the weights.
f) Landing gear torsion stiffness
Possible effects of torsional backlash is a reduced
effective torsional stiffness of the strut. Generally there is
an undue sensitivity by reduction in torsional stiffness.
Taxi tests after increasing the tyre pressure reveals no
shimmy. Fig. 2-6 shows this sufficient improvement
- because the nose landing gear is almost free of
oscillation.
No further changes were investigated.
It should be noticed that worn and cold tyres made
undercarriages less stable than it was with new tyres.
This was reported by pilots during taxi tests and is likely
due to the change in tyre stiffness as the tread wears
away. There is also a possibility that out of balance
forces in the worn tyre induces high frequency
oscillations which nullifies the effect of friction in the
landing gear.
3. BRAKE TORQUE OSCILLATIONS AT
BRAKE INITIATION
The case following demonstrates that not only unsuitable
combinations of structural stiffness, damping, and
pneumatic tyre characteristics may lead to unexpected
vibration problems on landing gears. Rather, an unlucky
combination of brake system design with the peculiarities
of circumferential force development by a tyre can also
produce a serious vibration problem.
3.1 THE SITUATION
In a series of development rig tests for a fighter aircraft
brake system. some test, were scheduled to demonstrate
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fastest possible brake force rise at high speed. A fast
brake force rise was considered to yield. at least in
theory. shortest possible landing ground run distance.
The test set-up "dynamometer"consisted of one main
wheel with brake running within a heavy drum. where the
rotational inertia of the drum was to represent respective
mass per braked wheel of the aircraft. This arrangement
not only gives a representative picture of aircraft
deceleration by the brake but also provides correct
kinetic energy to be dissipated by the brake. The brake
was actuated by representative hydraulic components.
Brake control was performed by one lane of the brake
control computer, pilot's pedal input being replaced by a
synthetic signal. Anti-skid functions of the brake control
computer were implemented and active.
3.2 DESCRIPTION OF THE PHENOMENON
At brake initiation ("pilot" quasi "jumping" onto brake
pedal) there was not the expected crisp yet steady rise of
brake force with eventual subsequent anti-skid system
activity. Rather there was a sequence of rapid on/off
switching of the brake which lasted for about 1.5 seconds
at a frequency of about 6 Hz. Thereafter the system
stabilised to a normal behaviour.
Fig. 3-1 shows a principal sketch of those three test
measurement traces which are considered essential for
description and explanation of the phenomenon. From
top to bottom, these are Wheel Speed. Brake Pressure
(i.e. pressure on brake piston face), and Brake Torque.
The very first increase of brake pressure is needed to
overcome the piston lifting spring. To arrive at the net
pressure acting on the brake pad, that "spring pressure" is
to be subtracted from the value measured: thus, at the
first three troughs in Brake Pressure trace, brake pads are
practically unloaded. This becomes apparent in Brake
Torque trace, where for all practical purposes torque
reduces. to zero at the corresponding non-zero troughs of
Brake Pressure.
The initial phase of Brake Pressure deserves a further
comment: While brake servo valve output pressure (not
shown) follows solenoid current with negligible delay.
this is not true for the brake pressure proper as long as
brake pistons are moving to close the gaps between rotor
and stator disks of the brake package. The flow of
hydraulic fluid from servo valve to brake is restricted by
a quite narrow Restrictor orifice which limits loss of
hydraulic fluid in case of e.g. rupture of a flexible hose.
The Restrictor leads to a pressure drop between servo
valve output and brake piston as long as the piston is
moving. This pressure drop is the larger the faster the
pilot tries to actuate the brake. However, when fluid flow
stops, pressure drop across the Restrictor vanishes and
full servo valve brake pressure gets through to brake