AOPA Turbine Pilot Magazine - June 2020 - 95

speed is not stable, select a stable airspeed
well above 10 knots; and when airspeed is
not stable, select a stable vertical speed less
than 300 fpm. This is to prevent at least one
VRS condition from existing at any one time.
Another aspect to be considered is loss
of tail rotor effectiveness. Many helicopters
have a critical wind azimuth chart in the
rotorcraft flight manual depicting the range
of wind direction that can cause the tail rotor
to lose its ability to control yaw. Just as the
main rotor can be susceptible to VRS, the tail
rotor can as well. The critical wind azimuth
chart usually has a correlation of density altitude to gross weight, where a specific wind
direction can result in tail rotor VRS and subsequent loss of tail rotor effectiveness. This
condition is more susceptible at higher density altitudes and gross weights because the
helicopter already requires more tail rotor
pitch to control yaw. With an already high
pitch setting across the tail rotor blade airfoil, it is closer to the critical angle of attack
and more susceptible to loss of tail rotor

effectiveness. When the helicopter is not in
coordinated forward flight, the pilot must
be aware of changes in wind direction and
speed, and adjust the aircraft heading so the
tail rotor is never within the danger zone on
the critical wind azimuth chart.
Flying helicopters at slow speed safely
requires a pilot to anticipate varying airflow
conditions; to recognize when those conditions are beginning to affect the aircraft; and
then to provide quick corrective action.
CRUISE FLIGHT

Helicopters do not have a published
maneuvering airspeed (VA) or turbulence
penetration speed (VB), and while helicopters tend to be less susceptible than
airplanes to the effects of turbulence in
cruise flight, there remain aspects the
pilot should consider.
Despite best efforts in avoidance, turbulence can be encountered in flight and the
pilot must mitigate the situation by controlling the helicopter, minimizing the effect of

turbulence, and possibly exiting the condition. When encountering turbulence in a
helicopter, most pilots will slow down. While
decreasing airspeed helps avoid VNE during a gust with a headwind component, it is
equally important to decrease the pitch and
angle of attack of the main rotor blades. A sudden updraft will increase the angle of attack of
the relative wind over the blade airfoil, and if
it's enough to reach the critical angle of attack,
that part of the main rotor blade will stall.
This is often seen during mountain flying
at relatively high altitudes. As the helicopter
flies higher, the air is less dense and therefore
the helicopter needs more collective pitch
to maintain the same amount of lift. This is
why a helicopter's VNE will decrease with altitude, effectively limiting the angle of attack
of the main rotor blades so the critical angle
of attack is not exceeded. However, a sudden
updraft or gust can change all that by causing
an area of the rotor disc to stall.
Many years ago, traversing New
Mexico's Black Range mountains in an

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AOPA Turbine Pilot Magazine - June 2020

Table of Contents for the Digital Edition of AOPA Turbine Pilot Magazine - June 2020