The Art of the Helicopter John Watkinson - Karatunov.net

72 The Art of the Helicopter
Fig. 3.9 The blades must be accelerated inwards to make them rotate. At (a) the blades cone upwards until
the resultant of the lift and the blade tension is perfectly horizontal. At (b) the force balance at the rotor head
is shown. The tension in the upwardly coned blades cancels in the horizontal direction leaving only a vertical
component to balance the weight of the machine.
are not balanced or not tracking, vibration will result. Accurate blade balancing and
tracking is important and the necessary techniques will be considered in section 4.19.
3.8 Torque and thrust in rotors
Whilst resolving the airfoil reaction into lift and drag is useful for fixed wings, it is less
useful for helicopters because of the feathering action of the blades. Figure 3.10 shows
how the reaction on a rotor blade can be resolved in a more useful fashion. As the
only real force on the airfoil is the reaction, resolving the force into components is only
taking place in our imagination, so we can resolve into whatever directions we find
useful. In helicopters, whatever the mechanical pitch angle of the blade, and whatever
angle of attack results, it is more convenient if the reaction is resolved into the rotor
thrust acting substantially at right angles to the tip path plane or rotor disc, and the
rotor drag acting in the tip path plane.
The angle of attack is the angle by which the pitch angle exceeds the angle of the
RAF. The reaction of the airfoil is tilted further back in the presence of inflow, so that
a larger component of the reaction is opposing the thrust delivered by the engine, and
power is consumed simply driving against it. The rotor thrust is also slightly reduced
by the tilt of the reaction. The hover out of ground effect (HOGE) is a particularly
power consuming exercise. Inflow tilts back the blade reaction to oppose the engine, so
that more torque is required to drive the rotor and more tail rotor power will be needed
to balance the torque. HOGE is also one of the worst situations in which to lose power
as the high rotor drag will reduce RRPM more before the pilot reacts and lowers the
collective lever.
Clearly increasing the collective pitch will increase the induced drag, and so more
power will be required to maintain rotor speed. In simple piston engine helicopters,
the end of the collective lever is fitted with a twist-grip connected to the throttle. Maintaining
RRPM is then the responsibility of the pilot who will need to monitor the rev.
counter. In more sophisticated piston engine machines and in all turbine machines
automatic rotor speed governing systems or throttle correlators are used. The pilot