HEADQUARTERS OH-58A, Technical Manual

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TM 55-1520-228-10
8-12 Change 1
4
d. During landing, starting, and rotor coastdown,
spike knock could also occur, especially if there are high
winds and/or the elastomeric damper is deteriorated.
This type of spike knock is not considered damaging to
the aircraft.
8-30. PYLON WHIRL.
Pylon whirl is a condition which occurs after blade flap-
ping and mast bumping. The resultant motion of the
pylon is elliptical, and spike knock is apt to occur. If the
frequency of motion coincides with a particular natural
frequency of the helicopter, and the amplitude and direc-
tion of the force is large enough, damaging vibrations
can occur in the aft section tailboom of the helicopter.
Motion of this type could occur during touchdown auto-
rotations, if operational limits are exceeded.
8-31. CRITICAL TAILBOOM DYNAMIC
MODES.
Two critical tailboom dynamic modes exist. One of these
may occur during an improperly executed touchdown
autorotational landing, and corresponds to a frequency
of less than 64 percent
rotor RPM
. The second may occur during
a high speed autorotational entry, or any maneuver in
which application of collective allows a significant decay
in
rotor
RPM down to a critical frequency corresponding to
approximately 68-73 percent
RPM
. At high blade angles of
attack (increased collective), there may be a point
where the blade does not produce more lift. When there
is this condition of low rotor speeds and high collective
blade angles, there will be excessive flapping of the
main rotor. The cycle will be as follows: rotor blade flap,
mast bumping and spike knock, which ultimately results
in main rotor inertia/energy transfer to the airframe.
These conditions generate a resonance and the tail-
boom will rapidly respond to these frequencies. The
tailboom will then have up and down movements as it
responds to the resonant condition and at some point,
a structural failure will occur. Typically, there will be
wrinkles in the tailboom just aft of the boom attaching
points. After the tailboom has buckled and/or been dam-
aged, the vibrations may (and usually will) cease; pre-
dominantly, because the failure unloads the condition or
the landing has stopped or main rotor flapping has
ceased. These could be aggravated by high winds and
abrupt cyclic inputs while in the condition. High forward
speed relative to the maneuver may provide the driving
force for excessive blade flapping, mast bumping, and
as a result, damaging vibration. Likelihood of encounter-
ing the second mode is remote and is avoidable. If oper-
ating limitations of the helicopter are observed.
8-32. LOSS OF TAIL ROTOR
EFFECTIVENESS.
a. Loss of tail rotor effectiveness (LTE) is the occur-
rence of an uncommanded and rapid right yaw rate
which does not subside of its own accord and which, if
not quickly reacted to, can result in loss of aircraft con-
trol. However, the term “loss of tail rotor effectiveness”
is misleading. The tail rotor on this aircraft has exhibited
the capability to produce thrust during all flight regimes.
Under varying combinations of wind azimuth and veloc-
ity, tail rotor thrust variations can occur. When this oc-
curs, the helicopter will attempt to yaw to the right. This
yaw is usually correctable if immediate additional left
pedal is applied. Correct and timely pilot response to an
uncommanded right yaw is critical. If the response is
incorrect or slow, the yaw rate may rapidly increase to
a point where recovery may not be possible in the terrain
flight regime.
NOTE
The pilot must anticipate these varia-
tions, concentrate on flying the aircraft,
and not allow a yaw rate to build.
b. Extensive flight testing and wind-tunnel tests
have identified three relative wind azimuth and velocity
regions as capable of adversely affecting aircraft con-
trollability and dramatically increasing pilot workload.
For illustration, specific wind azimuths and velocities are
identified for each region (see figure 8-2). However, the
pilot must realize the boundaries of these regions may
shift in azimuth or velocity depending on the ambient
conditions.
(1) Weathercock stability (120-240 degrees).
Winds within this region will attempt to weathervane the
aircraft into the relative wind. The helicopter exhibits a
tendency to make a slow uncommanded yaw to either
the left or right, depending upon the exact wind direc-
tion. Due to the inherent yaw characteristics of this heli-
copter, the right yaw rate will increase unless arrested
by the pilot. A right yaw can develop into an LTE condi-
tion and requires immediate correction.
(2) Vortex ring state (210-330 degrees). Winds
within this region will cause a vortex ring state to develop
around the tail rotor, which, in turn, causes tail rotor
thrust variations. The helicopter exhibits a tendency to
make uncommanded pitch, roll, and yaw excursions.
The subsequent aircraft reactions require multiple ped-
al, cyclic, and collective inputs by the pilot. Maintaining
a precise heading in this region will be impossible. Pilot
workload in this region will be high; therefore, the pilot
must concentrate fully on flying the aircraft and not allow
a right yaw rate to build.