The Art of the Helicopter John Watkinson - Karatunov.net

336 The Art of the Helicopter
Fig. 8.10 Minimum and maximum airspeed as a function of density altitude. Note that at the operational
ceiling there is only one airspeed available.
can increase rotor efficiency by reducing RRPM. This will reduce profile drag and
require a larger angle of attack so that the lift to drag ratio will improve. Whilst very
low RRPM is dangerous because it may not be possible to recover, a reduction of 10%
is quite safe and has a dramatic effect on autorotation range. Prior to landing RRPM
can be recovered by gradual application of back cyclic. In some machines the use of
low RRPM may result in increased vibration if the rotor detuning has been optimized
for a narrow RRPM range.
Figure 8.9 applies at one density altitude only and obviously climb or descent will
change that. Figure 8.10 shows an alternative way of depicting the performance. This is
a chart of minimum and maximum airspeed as a function of density altitude. Minimum
power and maximum range speeds may also be shown as a function of density altitude.
At sea level minimum airspeed is zero showing that vertical climb is possible. At some
density altitude the HOGE power will exceed available power and then the minimum
speed becomes non-zero. At sea level the maximum airspeed is power limited but may
peak at some density altitude where reduced air density is reducing profile drag and
improving rotor L/D. At higher density altitude maximum airspeed will fall again. As
density altitude increases, the available power falls down into the power-required curve
and the differences between minimum power speed, minimum airspeed and maximum
airspeed become smaller. Where these speeds coincide, the absolute ceiling has been
reached. As the absolute ceiling can only be reached asymptotically, the service ceiling
is often taken to be a slightly lower density altitude at which a rate of climb of 100 fpm
remains.
Given the existence of a minimum airspeed, potentially a helicopter could take off
vertically at sea level with a full load and fly to some high altitude destination at best
rate of climb speed where an attempted vertical landing would result in a crash. The
fact that this does not happen in practice is due to a combination of the availability
of power curves in the flight manual and piloting techniques that establish what the
power margin is before attempting a landing.