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In comparing these graphs it is noted that as the
taxi speed increases from 80 to 90 knots a
progressively longer period of time is required for the
vibration to damp out. At 95 knots the gear has the
onset of limit cycle shimmy. At 100 hots the limit
cycle shimmy is definitely established. From these
time histories it is seen that the shimmy oscillation
frequency is about 27.06 HZ at 95 knots. When the
actual aircraft was tested, it was found that the nose
gear went into a sudden onset of shimmy at 92 knots
and had a frequency of 27.5 HZ. Repeated testing of
this landing gear, yielded reasonably consistent
repeatability at this shimmy speed and frequency.
In the course of the shimmy investigation for
Fighter C, one very important point came to
light concerning the measurement of landing gear
shimmy parameters. The vendor's original analysis
consistently indicated that this landing gear was stable
at all taxi speeds. The analysis provided by the
author indicated the 95 knot shimmy event at 27.06
HZ. The question arose as to what was different
between the two analyses. After review of the input
data a primary cause of this difference was
established. The stability critical parameters KT, K,,
K,, and K, used in the vendor's analysis were
obtained by rigidly mounting the landing gear in a
fixture and measuring the resulting parameters.
Whereas, the author's analysis took into consideration
the softening effects of the landing gear mounted in a
flexible fuselage and reduced the stiffness values
supplied by the vendor. The amount of this
reduction was largely based on experience with other
landing gear.
The reason for taking fuselage flexibility effects
into consideration was based on some data obtained
in another landing gear shimmy investigation.
UTLRALSCRINO RATE
FIGURE 15
SRUT ONLY IYEAWREDI
2-9
Referring to Figure 15, it is seen that the lateral
stiffness values obtained for a landing gear rigidly
mounted in a fixture (in the chart, strut only
(measured)) vary from one and a half to three times
the values of the lateral stiffness actually measured on
a landing gear mounted on the aircraft. This
comparison establishes the fact that backup structural
flexibilities of the airframe must be taken into
consideration when the four stability critical stiffness
values are determined.
In addition to this primary cause of the difference
between analysis predictions, a secondary cause was
also established. The vendor's analysis assumed a
value of 6% of critical damping as the magnitude of
the structural damping available. Whereas, based upon
personal experience with the variablity of available
structural damping, the author decided to assume that
the maximum amount of available damping was 1%
of critical. At the time, the main reason for this
selection was a desire to select a damping value that
would expose any potential sensitivities the gear may
have toward shimmying. Further support for this
position was established after the fact when Denis J.
Feld published his results on an analytical
investigation of landing gear shimmy damping (8).
The essential conclusion of the analytical
investigation was that the amount of available
damping at any instant during a taxi run could vary
from 1% to 10% of critical viscous damping.
Therefore, at the beginning of any landing gear
shimmy investigation an assumption of 1% critical
damping is a reasonable starting point.
Before departing from Figure 15, another subject
is in need of mention. In the discussion of landing
gear stability critical parameters above in section 4, it
was pointed out that caution should be exercised
when obtaining stability critical stiffness values
because the finite element analysis predictions for
these values tend to error on the high side. The proof
of this error is established in the above figure for the
strut only (calculated) case where it is seen that the
largest error in stiffness values occurs between the
calculated values and the strut mounted on an actual
airframe.
6. CONCLUSIONS
In conclusion, a detailed analysis of the specific
problem of landing gear shimmy was presented in
which the two mathematical models described have
been consistently successful in the prediction of
landing gear stability characteristics to date. One of