The Art of the Helicopter John Watkinson - Karatunov.net

Helicopter performance 337
In the case of a light piston-engine helicopter, the payload is small and the altitude
performance modest. The flight manual may be quite sparse in the power curve department
and safety is primarily assured by piloting technique. In the case of a heavy
military transport helicopter, the payload may exceed the empty weight of the machine
and flight in a wide combination of AUWs and density altitudes is possible. The
flight manual may contain many pages of fuel flow curves for various AUWs, pressure
altitudes and air temperatures. In the case of the CH-47D there are over 100 such
curves.
When plenty of power is available, the steepest climb will be vertical. However,
under certain combinations of payload and density altitude, vertical climb may not
be possible. In this case obstacle clearance becomes an issue and finding the airspeed
at which the steepest climb can be obtained is important. Even if there is
a power margin, knowing the speed at which the steepest climb occurs is useful. The
climb gradient is the ratio of climb speed to forward airspeed. Finding the optimum
airspeed requires finding a point on the power curve where the surplus power is proportional
to airspeed. Figure 8.6 shows that this can be done graphically by finding
the point on the power curve where a line from the available power at zero airspeed is
tangent.
In most cases the steepest climb performance will be required for obstacle clearance
on take-off where there is insufficient power margin for HOGE. In this case the correct
procedure is to make a running take-off by accelerating as hard as possible in ground
effect and making no attempt to gain height until the best climb gradient airspeed is
obtained. At this airspeed the attitude of the machine is adjusted so that no further
increase in airspeed occurs. The machine will then climb at constant airspeed so that
power margin goes entirely towards climbing. The flight manual may contain charts
that allow obstacle clearance ability to be predicted for various AUWs and density
altitudes.
8.8 Power management in multiple-engine
machines
It is not generally the case that multiple engines are fitted to obtain more power. Turboshaft
engines are available with phenomenal power output and a single engine could
lift any but the most extreme helicopter. Instead the goal of multiple engines is to
provide some degree of resilience to engine failure. In the case of passenger carrying
civil helicopters, multiple engines and suitable operational procedures together allow
safe flight to be maintained in the case of a single engine failure at any time. In such
cases a landing would be made as soon as practicable after the failure. In military
machines, engine failure may result from hostile acts and has to be considered more
probable than natural failure. Frequently it will be a goal that the mission shall be
completed despite the loss of one engine. A prompt landing is not viable if the machine
is over enemy territory or water and flight may have to be sustained.
Turbine engines are light in weight for the power they produce, so there is no technical
difficulty in providing further engines, although this will reflect in purchase and running
costs. If the remaining engine(s) is to be able to provide enough power for flight should
one fail, then clearly under normal conditions all engines will be operating at a fraction
of their maximum power and will be relatively inefficient because of the profile drag in
the compressors. Turbine engines are very reliable and failures are relatively uncommon.
Consequently instead of providing excessive continuous power, it is more sensible to
design engines that can be overrated for short periods of time. The continuous power