The Art of the Helicopter John Watkinson - Karatunov.net

Other types of rotorcraft 363
Fig. 9.14 In powered flight, with equal pitch on both rotors there is no net torque. Increasing the pitch of the
upper rotor causes its blade reaction to rotate back whilst that of the other rotor rotates forward and a yaw
results.
Figure 9.14 shows how the yaw control works. The machine is in a power-on condition
so the blade reactions will be tilted back with respect to the shaft axis. The vector sum
of these two reactions is vertical so there is no net torque and no yaw effect. Figure 9.14
shows that increasing the pitch of the upper blade will increase the thrust and tilt it
back against the direction of rotation. At the same time the pitch of the lower blade is
reduced. This will reduce the thrust and tilt it forwards, with the direction of rotation.
The vector sum will now be non-vertical so that there is a net torque turning the hull
the same way as the rotor whose pitch has been reduced.
In autorotation, the yaw control of a coaxial machine using differential collective is
problematic as it may have no effect, or a reverse effect. This was initially not appreciated,
and caused some hair-raising moments. Figure 3.14 introduced the autorotation
diagram and Figure 9.15 shows an autorotation diagram appropriate for coaxial helicopters.
Here the horizontal axis is the average angle of attack of the two rotors. In
autorotation, the blade pitch will be chosen for the best rotor L/D at the bottom of the
drag bucket. In most helicopters, the lower collective stop is set at the intersection of
the α–θ line and the drag bucket. However, when differential collective is used for yaw,
the pitch of one of the rotors can be even less and so operation can be to the left of the
intersection. The problem is that when operating at L/Dmax., an increase in pitch or a
decrease in pitch will have the same effect: the rotor experiences more drag and slows
down. At some collective settings there is no yaw control at all. To make matters worse,
helicopter designers often select blade sections having a relatively flat bottomed drag
bucket. If the collective stop is slightly lower than the setting for L/Dmax., increasing
the pitch will reduce drag and reducing pitch will increase it. As a result the yaw control
reverses.
To overcome the problem, coaxial helicopters need a significant amount of vertical
tail area to give yaw stability in autorotation. The fins are often slatted to prevent
stalling at high angles of attack. In some machines a mechanism is fitted so that the
action of the pedals is automatically reversed when the collective is fully lowered.
An alternative yaw control mechanism for coaxial helicopters was seen on the
Gyrodyne helicopter. This consisted of drag brakes in the blade tips that could be