The Art of the Helicopter John Watkinson - Karatunov.net

32 The Art of the Helicopter
In the case of an engine, the input power is the heat released by burning the fuel.
In practical engines, this exceeds the output shaft power considerably. The percentage
of the thermal energy in the fuel that emerges as shaft power is the thermal efficiency
of the engine. The waste heat has to be removed by a cooling system to prevent the
temperature of the engine rising to the point where components are damaged. Cooling
systems waste further power in driving fans and pumps and usually increase the drag
of the airframe.
Given the necessary high power to weight ratio in helicopters, the power plant and
fuel form a significant part of the all-up weight. It is beneficial to explore means to
improve the thermal efficiency of the engine. Not only will this reduce the weight of
the fuel to be carried for a given range, but it may also allow the cooling system to be
lighter, to consume less power and to cause less intake drag. Thus a small improvement
in thermal efficiency may result in a significant increase in performance.
Passenger aircraft may be compared using specific air range (the mass of fuel used per
unit of distance), but in a hovering helicopter this figure is meaningless. In helicopters,
it may be better to compare the power actually used to hover with the theoretical power
needed by an ideal rotor under the same conditions.
2.9 Gases and the atmosphere
The atmosphere is the medium in which helicopters fly but it is also one of the fuels
for the engine and the occupants breathe it. It is a highly variable medium that is
constantly being forced out of equilibrium by heat from the sun and in which the
pressure, temperature, and humidity can vary with height and with time and in which
winds blow in complex time- and height-variant patterns. The effect of atmospheric
conditions on flight is so significant that no pilot can obtain qualifications without
demonstrating a working knowledge of these effects.
The atmosphere is a mixture of gases. About 78% is nitrogen–arelativelyunreactive
element – whereas about 21% is oxygen, which is highly reactive. The remainder is a
mixture of water in the gaseous state and various other traces. The reactive nature of
oxygen is both good and bad. The good part is that it provides a source of energy
for life and helicopters alike because hydrocarbons can react with oxygen to release
energy. The bad part is that many materials will react with oxygen when we would
rather they didn’t. Chemically, combustion and corrosion are one and the same thing.
The difference is based on the human reaction to the chemical reaction.
Gases are the highest energy state of matter, for example the application of energy
to ice produces water and the application of more energy produces water vapour. The
reason that a gas takes up so much more room than a liquid is that the molecules
contain so much energy that they break free from their neighbours and rush around at
a high speed, which is a function of absolute temperature. As Figure 2.12(a) shows, the
innumerable elastic collisions of these high speed molecules produce pressure on the
walls of any gas container. In fact the distance a molecule can go without a collision,
the mean free path, is quite short at atmospheric pressure. Consequently gas molecules
also collide with each other elastically, so that if left undisturbed, in a container at a
constant temperature, every molecule would end up with essentially the same energy
and the pressure throughout would be constant and uniform.
Pressure is measured by physicists and by engineers in units of force per unit of area
using imperial units of pounds per square inch or SI units of Newtons per square metre.
At sea level, the atmosphere exerts a pressure of about 15 pounds per square inch
and has a density of about 0.075 pounds per cubic foot or in metric units about