The Art of the Helicopter John Watkinson - Karatunov.net

76 The Art of the Helicopter
from the available power. Some of that lift will be lost because the larger rotor will be
heavier.
In a turbine helicopter, the engine and fuel supply weigh less for the power the
engine produces and so a better result may be obtained by using a smaller rotor and
transmission that also weighs less to obtain a greater useful payload. A comparison of,
for example, an Enstrom F-28 and a Hughes 500 illustrates this point.
This cannot be taken to extremes, as there is a practical limit to the downwash
velocity. This is reached when objects on the ground are blown about and become
a hazard. The CH-53E Stallion represents about the limit of acceptable downwash
velocity as it is almost impossible to stand in the downwash. A further consideration
is that in autorotation a machine with a high disc loading may have a rapid rate of
descent, leaving the pilot little margin for error in judging the flare-out in the case of
an engine failure. On the other hand a machine with a low disc loading may have very
good autorotation performance, the Enstrom being a particularly good glider.
Once a disc area and loading has been decided, some consideration has to be given to
the tip speed. Profile power is proportional to the cube of the tip speed, whereas thrust
is proportional to the square of the tip speed. The minimum profile power is where the
minimum possible tip speed is used to operate the blades at L/Dmax., which will be just
below the stall. Reducing the tip speed will require the blade area to be increased to
maintain thrust. Consequently the disc area is chosen to give the desired disc loading,
whereas the blade area is chosen to produce sufficient rotor thrust (with an adequate
load factor) at the chosen RPM. The ratio of the total blade area to the disc area is
known as the solidity. Although low tip speeds and high solidity reduce profile drag
and improve hovering performance, a machine built to these criteria would not have
an adequate load factor for forward flight. It will be seen in section 3.24 that a low tip
speed is also detrimental to forward flight as well as being a liability in the case of a
power failure as the rotor stores little energy, giving the pilot little time to react and
establish autorotation.
In a conventional helicopter there will always be a compromise between hover and
forward flight performance. Possible technical solutions include variable rotor speed
and variable diameter rotors.
In the hover a low rotor speed would reduce profile drag by allowing the blades to
operate closer to L/Dmax., whereas in cruise a high rotor speed would improve the
load factor. To obtain maximum power from a piston engine at both speeds, a twospeed
transmission would be needed, and there would be some weight and cost penalty.
Changing the speed would be easy with a free turbine engine. There would also be a
problem in detuning the rotor to minimize vibration at two different speeds. These
problems are not insuperable.
A variable diameter rotor could have a low disc loading for efficient hover, but a
raised disc loading in cruising flight. The technical problems here are enormous but
may one day be economically viable.
3.12 Figure of merit
Whatever the disc loading, in a constant height hover, the potential and kinetic energy
of the helicopter remain constant, and so no work is being done on the helicopter. Thus
the mechanical efficiency of all helicopters in the hover is zero. Clearly mechanical
efficiency is not a useful metric, because it doesn’t allow comparison. A better metric is
to compare the power theoretically needed by an ideal actuator with the actual power