The Art of the Helicopter John Watkinson - Karatunov.net

192 The Art of the Helicopter
Statically compressing air with a piston in a cylinder suffers much less power loss
than dynamic means such as a turbine. This makes piston engines more efficient at
partial throttle settings, hence their popularity in automotive applications. The energy
released on each power stroke is finite and so a good way of increasing the power is to
increase the RPM. The piston engine must handle the stresses raised by reciprocating
that increase with the square of the RPM.
This limits the RPM available as a function of piston size. Very small piston engines
used in models can run at over 25 000 RPM and produce an astonishing power to
weight ratio, but this cannot be scaled up because of the forces involved. Another
fundamental of piston engines is that the power available is a function of the charge
that can be admitted to the cylinder. The cylinder volume is proportional to the cube of
the bore whereas the cylinder head area, where the valves are, is only proportional to the
square of the bore. As a result the small cylinder has the advantage because not only is
the piston lighter, but more valve area is available in relation to the displacement. Thus
in piston engines there is a power to weight advantage in using a lot of small cylinders
rather than a few big ones. There is an optimum cylinder capacity for a given RPM.
The difficulty is that in order to produce a lot of power, a lot of cylinders are needed
and this causes practical difficulties in induction, exhaust, cooling and reliability, to say
nothing of cost. During World War II aircraft engines having as many as 28 cylinders
were built and these were immensely complicated.
At 400–500 kW the turbine and piston engine are about equal. For higher powers
the turbine is to be preferred, whereas for lower powers the piston engine would be
chosen. The power to weight ratio may not be as high, but if the fuel efficiency is good,
the saving in fuel load may more than compensate. As a result, helicopter design has
diverged into two camps. Large machines use two or three turbine engines and tend to
use high disc loading because plenty of power is available. Small machines use piston
engines and lower disc loadings.
In between the extremes there is some variability. The small single-engine turbine
helicopter cannot use high disc loading because it must be designed to autorotate well.
As a result it will not need much power and so the turbine will be heavily derated and
have poor fuel efficiency. Unfortunately derating an engine does not lower the cost.
Well-engineered piston machines can compete in this market simply by costing less to
purchase and less to run. Frank Robinson has demonstrated this amply with the R-44,
despite its elderly engine technology.
Unfortunately the aviation piston engine went into a period of stagnation for several
decades after the arrival of the turbine, with the result that progress in piston engine
development moved to the automotive sphere. This situation is now being remedied
and a new generation of aviation piston engines will give helicopter designers some
better solutions in light machines. In the automotive sphere fuel economy does not
drastically improve performance because the fuel load is a relatively small proportion
of the vehicle weight. In helicopters the fuel weight at take-off is limited by available
rotor thrust. Thus an improvement in fuel efficiency in the helicopter will translate
directly into either an increase in range for the same payload, or an increase in payload
for the same range.
The power of the gasoline engine is controlled by an induction throttle, and when
this is used, the effective compression ratio of the engine falls and with it the efficiency.
The induction throttle reduces the manifold pressure and this opposes the motion
of the piston during the induction stroke causing pumping loss. The power of the
Diesel engine is not controlled with a throttle. There is negligible pumping loss and
the compression ratio is always at its highest value. As only air is compressed, there is
no risk of detonation and the compression ratio can be very high indeed. This makes