Vortex Interference
315°
11-13
WEATHERCOCK STABILITY
(120-240°)
In this region, the helicopter attempts to weathervane
its nose into the relative wind. [Figure 11-11] Unless a
resisting pedal input is made, the helicopter starts a
slow, uncommanded turn either to the right or left
depending upon the wind direction. If the pilot allows a
right yaw rate to develop and the tail of the helicopter
moves into this region, the yaw rate can accelerate
rapidly. In order to avoid the onset of LTE in this
downwind condition, it is imperative to maintain positive
control of the yaw rate and devote full attention to
flying the helicopter.
Figure 11-11. Weathercock stability.
TAIL ROTOR VORTEX RING STATE
(210-330°)
Winds within this region cause a tail rotor vortex ring
state to develop. [Figure 11-12] The result is a non-uniform,
unsteady flow into the tail rotor. The vortex ring
state causes tail rotor thrust variations, which result in
yaw deviations. The net effect of the unsteady flow is
an oscillation of tail rotor thrust. Rapid and continuous
pedal movements are necessary to compensate for the
rapid changes in tail rotor thrust when hovering in a left
crosswind. Maintaining a precise heading in this region
is difficult, but this characteristic presents no significant
problem unless corrective action is delayed.
However, high pedal workload, lack of concentration
and overcontrolling can all lead to LTE.
When the tail rotor thrust being generated is less than
the thrust required, the helicopter yaws to the right.
When hovering in left crosswinds, you must concentrated
on smooth pedal coordination and not allow an
uncontrolled right yaw to develop. If a right yaw rate
is allowed to build, the helicopter can rotate into the
wind azimuth region where weathercock stability then
accelerates the right turn rate. Pilot workload during a
tail rotor vortex ring state is high. Do not allow a right
yaw rate to increase.
Figure 11-12. Tail rotor vortex ring state.
LTE AT ALTITUDE
At higher altitudes, where the air is thinner, tail rotor
thrust and efficiency is reduced. When operating at
high altitudes and high gross weights, especially while
hovering, the tail rotor thrust may not be sufficient to
maintain directional control and LTE can occur. In this
case, the hovering ceiling is limited by tail rotor thrust
and not necessarily power available. In these conditions
gross weights need to be reduced and/or
operations need to be limited to lower density altitudes.
REDUCING THE ONSET OF LTE
To help reduce the onset of loss of tail rotor effectiveness,
there are some steps you can follow.
1. Maintain maximum power-on rotor r.p.m. If the
main rotor r.p.m. is allowed to decrease, the antitorque
thrust available is decreased proportionally.
2. Avoid tailwinds below an airspeed of 30 knots. If
loss of translational lift occurs, it results in an
increased power demand and additional antitorque
pressures.
3. Avoid out of ground effect (OGE) operations and
high power demand situations below an airspeed
of 30 knots.
4. Be especially aware of wind direction and velocity
when hovering in winds of about 8-12 knots. There
are no strong indicators that translational lift has
been reduced. A loss of translational lift results in
an unexpected high power demand and an
increased antitorque requirement.
Region Where Weathercock
Stability Can Introduce Yaw Rates
360°
0°
15 Knots
10 Knots
5 Knots
17 Knots
30°
60°
90°
120°
150°
180°
210°
240°
270°
300°
330°
17 Knots
15 Knots
10 Knots
5 Knots
0°
180°
150°
30°
120°
60°
90°
210°
240°
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