The Art of the Helicopter John Watkinson - Karatunov.net

centre of mass. The solution was to cant the tail rotor so that it produces some lift
as well as side thrust. Canted tail rotors solve some problems but they do introduce
interaction between controls. For example, pedal inputs will also cause some pitching.
This is usually cancelled out to some extent by interconnections in the control system.
5.5 Tail rotor performance
In a still air hover, the slightest imbalance in the two couples will allow a slow yaw. If
the pilot applies pedal input, the helicopter will accelerate about the yaw axis, but as
it does so the tail rotor will then move through the air at a velocity determined by the
yaw rate and the boom length. This velocity will change the inflow conditions and alter
the RAF seen by the blades.
Figure 5.8 shows that the hardest yaw direction is against main rotor torque. The tail
rotor has to produce a higher than normal side thrust and the pitch of the blades will
increase to provide it. However, as the yaw proceeds, the tail rotor accelerates towards
the inflow, so that the inflow velocity increases. This has the effect of reducing the angle
of attack of the tail blades. This result is not surprising because in section 4.2 it was seen
that the collective control determines the vertical velocity in the main rotor because of
the same inflow effect. The yaw rate will come to equilibrium when the change in RAF
reduces the angle of attack to the point where the tail rotor thrust imbalance is equal
to the drag of the yawing tail boom.
If the yaw is required to be with the main rotor torque, the tail rotor needs to produce
less thrust and the blades will be set to a reduced pitch. As the tail accelerates, the inflow
velocity falls and this has the effect of increasing the angle of attack, again bringing
the yaw rate to a constant value. If a greater yaw rate is required, the blades will
need to be set to a negative pitch. This will also be a requirement of yaw control in
autorotation where the main rotor torque reaction becomes very much smaller and
changes direction.
When in a fast power-on yaw with the direction of main rotor torque, the tail
rotor may be in negative pitch and the inflow may reverse. The tail rotor can enter
a windmilling state where it is actually being driven by the airflow. At a critical yaw
rate the inflow becomes zero. As was seen in section 3.16, zero inflow is an undesirable
condition because it causes the rotor to enter a vortex ring or recirculation condition.
Fig. 5.8 When yawing against main rotor torque the tail rotor needs a large pitch angle in order to maintain
the angle of attack with the increased inflow velocity.
The tail 177