The Art of the Helicopter John Watkinson - Karatunov.net

Other types of rotorcraft 371
the rotors are mounted so that they remain substantially in the same plane in translation.
This allows the rear rotor to operate in relatively clean air. In order to keep the
cabin level and to provide blade clearance, the rear rotor is mounted on a pylon. It is a
characteristic of this approach that ground clearance of the front rotor disc is relatively
small. However, the poor clearance exists directly in the pilot’s field of view. With no
tail rotor to weathercock and a short moment arm, a large amount of fin area is usually
needed and even then tandem rotor machines are aerodynamically marginal in yaw.
This is not all bad because many operations require the machine to hover at an
arbitrary orientation to the prevailing wind and the tandem does that very well. The
rear pylon is always shaped to act as a substantial fin and the front pylon may have
aerodynamic treatment to stop it acting as a fin. This may consist of irregularities
designed to cause early flow separation when the relative airflow is off the fore-and-aft
axis. Figure 9.23(b) shows that the resulting drag causes a smaller yaw component than
if the pylon caused lift.
Figure 9.23(c) shows that the rear rotor is affected by the front rotor vortices in
translational flight. Effectively the rear rotor is operating with a greater inflow velocity
than the front rotor. In a machine with a central CM, both rotors will need to produce
the same amount of lift. To develop the same thrust the rear rotor will need to use
a higher blade pitch and will absorb more torque because of the higher inflow. The
swirl of the forward rotor also acts asymmetrically on the hull. For these reasons the
torque is not precisely balanced in translation. One prototype tandem helicopter could
not initially fly in a straight line in translational flight because these effects were more
powerful than the yaw control. Once the nature of the problem was understood a
solution was not far behind.
There are ways to mitigate the problem. An obvious suggestion would be to move
the CM forward. Whilst this would work, it somewhat negates the advantage of CM
position freedom. In practice, the two rotors can be set at a different angle. Figure 9.24
shows that the front rotor shaft may be inclined forward with respect to the rear. This
is done on the CH-47 in which there is a 5 ◦ difference between the angles of the shafts.
This increases the inflow seen by the front rotor and decreases the lift component of
its thrust, whereas the inflow of the rear rotor is reduced and the lift component of its
thrust is greater. Another possibility is that the fin can be built with an integral camber
so that in translational flight it produces a side thrust. The cambered fin of the CH-46
is obvious in a rear view.
The interference between the rotors can be minimized by flying the machine sideways
as this increases the diameter of the stream tube and improves the aspect ratio,
Fig. 9.24 The front and rear rotor shafts may be inclined at different angles.