Hélicoptères Guimbal
SERVICE LETTER
CABRI G2
SL 12-001 A
4 / 10
Original issue
5. Typical situations
Shallow approach (see sketches on following pages)
A very common situation is a rather shallow approach to a landing spot. Say
indicated power is 30 - 40% with an airspeed of 30 - 60 kt, and the string is
centered. The left pedal is forward by about 2 - 4 cm. The vertical fin produces
most of the antitorque force.
When getting closer to the ground, if the pilot does not control sideslip, the
reduction of airspeed causes the tail fin to increase its angle of attack to
maintain the torque / antitorque balance : the helicopter nose moves to the left
progressively as the airspeed is reduced.
A significant aggravating factor is crosswind coming from the right : the
described yawing to the left and corresponding right-hand sideslip can be
hidden to the pilot, because the helicopter line of flight is centered on the
approach axis.
Then during the flare, the pilot faces in a few seconds, and simultaneously :
•
A rapid increase in main rotor power, thus in the requirement for anti-
torque thrust,
•
A rapid decrease in the tail fin lift, the angle of attack of which is at its
maximum and cannot compensate anymore the loss of airspeed.
They tend to provoke a departure in yaw to the left, requiring a quick and large
right pedal input.
Moreover, three additional factors may increase this effect :
•
If the pilot is surprised by the departure in yaw, he will instinctively raise
the collective, thus accelerating the yaw to the left.
•
If the wind comes from the right, and the pilot is slow to react, allowing
the helicopter to depart in yaw to the left, the yaw will accelerate as the
helicopter’s tail passes through the wind due to the windvane effect (can be
compared to a jibe).
•
If the helicopter is landing close to IGE hover ceiling, over the transition
altitude (3800 ft ISA), full power will be obtained with full throttle (100% FLO
on MLI). If collective is increased too much, the rotor speed will decrease,
reducing tail rotor thrust and increasing yaw to the left.
The combination of all these factors can provoke a mild to severe departure in
yaw to the left if the pilot fails to apply enough right pedal to prevent it.
This departure
can always be controlled by full right pedal
.