The Art of the Helicopter John Watkinson - Karatunov.net

68 The Art of the Helicopter
A rotor hovering at L/Dmax. has a load factor of unity and would stall in any real
manoeuvre. It is thus a characteristic of helicopters that the rotor must operate below
L/Dmax. of the blade section in the hover in order to allow a reasonable load factor for
forward flight.
Turning at a given rate (number of degrees per second) requires a lateral acceleration
proportional to the square of the airspeed. As aeroplanes tend to fly faster than helicopters,
they will need high load factors to obtain reasonable rates of turn. Aeroplanes
automatically have high load factors at speed because available lift increases as the
square of the airspeed. In helicopters lift is dominated by rotor speed, not airspeed
and so load factors in helicopters tend to be small although helicopters can still easily
out-turn most aeroplanes.
As the relative airflow in helicopters is dominated by the rotor speed, the gust response
of helicopters is much reduced in comparison to that of aeroplanes. This is one reason
why helicopters can operate in bad weather. A further small luxury is that the flexibility
of the rotor gives in gusts a decoupling effect similar to that given by the suspension
of a car.
3.4 Collective control
In a hovering helicopter, the only source of lift is the rotor. Obtaining sufficient lift is
only a matter of providing a suitable combination of power and efficiency, and is much
easier than controlling lift. As the lift is proportional to the square of the speed, it is
in principle possible to control lift by changing the rotor speed. Some early machines
did just that. Unfortunately, the inertia of the rotor means that speed changes cannot
rapidly be accomplished. For practical reasons a constant rotor speed is much to be
preferred.
The proportionality between the coefficient of lift and the angle of attack is the
solution. As was shown in Figure 3.1, the rotor blades are mounted on feathering
bearings. Figure 1.21 showed that the pilot holds in his left hand a collective pitch
lever pivoted near the back of his seat. The lever gets its name because by lifting it,
the pitch angle of all of the rotor blades is increased by the same amount, and the
rotor lift increases immediately. This feature of the helicopter contributes enormously
to its safety. When a fixed-wing aircraft flies slowly, sudden loss of lift can only be
countered by raising the airspeed and this takes time. In the helicopter the airspeed is
always present due to the rotating blades and as this speed is many times higher than
any normal windspeed, the airspeed seen by the blades is always adequate.
In addition to instantaneous response to changing lift conditions, the helicopter rotor
stores in kinetic energy the equivalent of full engine power applied for several seconds.
The pilot can transiently increase the available power by using so much collective pitch
that the rotor slows down and converts its kinetic energy into lift.
3.5 In the hover
Figure 3.6(a) shows a helicopter with flight RRPM, but with the collective lever lowered.
The thrust vector is small, so gravity keeps the machine firmly on the ground. If the
collective lever is raised, eventually the rotor thrust vector will exceed the weight, and
the machine will rise. By adjusting the collective lever, the machine can be made to
hover with the thrust exactly balancing the weight as in (b). If the thrust is directly