The Art of the Helicopter John Watkinson - Karatunov.net

324 The Art of the Helicopter
for underslung delivery of stores by the military may be well suited to logging or
construction of ski lifts.
Performance and safety are closely related. Operating an aircraft outside its designed
performance envelope may result in excessive stresses that can cause damage or even
failure. Consequently during the development of a machine, extensive tests are needed
to establish just what these limits are. These limits are published in the flight manual
in order that pilots are aware of them. A helicopter’s performance can be measured
in terms of factors that contribute to safety. The degree of stability may affect pilot
workload and compromise safety on long flights in poor conditions. If it is intended
to operate a helicopter in IFR, the stability performance may have to be improved by
passive or active means. Performance is important not just under normal conditions,
but also under abnormal conditions. Autorotation performance becomes important
in the case of power loss. In twin-engine machines the performance with one engine
inoperative (OEI) is crucial if the safety benefit of two engines is to be more than an
illusion. The ability of a helicopter to continue flight under icing conditions is also
a performance factor.
8.2 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: a relatively unreactive
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 form the highest energy state of matter in which the molecules are no longer
bound together strongly as in solids or weakly as in liquids but instead are free to
rush around at a high speed that is a function of absolute temperature. The countless
collisions between gas molecules and any non-gaseous object result in pressure at the
interface. 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/0.002378 slugs per cubic foot,
or in metric units about 100 000 Newtons per square metre with a density of 1.225 kg
per cubic metre. Over the years, many other units of pressure have evolved, some from
meteorology. One of these is the bar (after barometry) where one bar is the average
atmospheric pressure at the place where the bar was defined. In practical use, the bar is
divided into 1000 millibars. The bar is slowly being replaced by a numerically identical
unit known as the hectoPascal (hPa). The bar and hPa are commonly used in aviation
altimetry. The principle of the mercury barometer is that atmospheric pressure supports
a column of mercury exposed to a vacuum at the top. Consequently the length of the