The Art of the Helicopter John Watkinson - Karatunov.net

8
Helicopter performance
8.1 Introduction
Helicopter performance may be assessed in a large number of areas and as these interact
to a considerable extent it is difficult to categorize. As helicopters are expensive to
operate in comparison with aeroplanes, economic performance is important. Factors
such as the fuel consumed, parts worn out and maintenance needed determine the
operating cost that is generally calculated on an hourly basis. Operating cost will be
assessed by commercial users in proportion to payload. Clearly such economy will only
be realized if the machine is fully loaded. Operators will consider machines that offer
a payload close to their typical requirement in order to avoid the poor economics of
flying partly loaded. In the case of privately owned machines, the payload may be little
more than the pilot and passengers, but here different economics apply as the cost of
flying may be offset against time saved. In the case of military operators, economics
may not be the overriding concern.
Operational performance may be taken to include factors such as the range, airspeed
and altitude that can be obtained with various payloads. As the advantage of the
helicopter is its ability to hover, the greatest altitude that can be reached in forward
flight is somewhat academic and less important that the altitudes at which the machine
can hover in and out of ground effect at various weights. Hover out of ground effect
(HOGE) altitude is the highest altitude at which the machine can take off vertically,
and will be lower than the hover in ground effect (HIGE) altitude. This is the highest
altitude at which the machine can get into the hover, which would allow a running
take-off if a sufficiently unobstructed area is available. These altitudes, and many other
performance factors, are functions of air temperature. If valid comparisons are to be
made, test results must be presented in a way that eliminates atmospheric variables.
The necessary techniques will be discussed shortly.
Tactical performance may include the manoeuvrability of the machine. Many aspects
of this are determined by the load factor, although the power of the tail rotor controls
the yawing ability and the ability to hover in crosswinds. Associated factors are the
range of positions the CM may take and the weight of underslung load that can be
lifted. Military and coastguard users find these aspects of performance more important
than commercial users. Achieving high load factor and high tail rotor power may
raise weight and operating costs that a commercial operator cannot tolerate. In some
cases there are variants of a basic machine each of which is optimized for a particular
market. There will be exceptions in the case of specialist operators. Helicopters designed