The Art of the Helicopter John Watkinson - Karatunov.net

340 The Art of the Helicopter
Fig. 8.12 (a) Allowed locations for CM with zero-offset head lie in a cone below the rotor head. Distance
behind the rotor may be less than in front. (b) Cone apex is higher with offset or hingeless head. (c) Tandem
rotor CM limits lie between two vertical lines which move closer as AUW increases.
When lightly loaded, the CM of a tandem is subject to remarkable freedom, but as
AUW approaches the maximum value the CM position is more constrained to prevent
an excessive load being carried by one rotor. Note that a tandem rotor helicopter will
also have lateral CM position limits and heavy payloads must be positioned on the
hull centreline. In general the certification authorities are not concerned with how the
position of the CM is to be calculated, only where it is. The manufacturer’s data will
usually contain details on how the CM position is established.
In helicopters having zero-offset rotor heads, further limitations may be applied. The
carrying of loads attached to the skids may be prohibited. There may also be a section
describing the consequences of low-g or pushover manoeuvres designed to avoid mast
bumping or boom strikes that are a characteristic hazard with this class of machine.
A windspeed limit for rotor starting may be found. Above that limit, blade sailing
may be excessive, leading to droop stop pounding, blade damage or boom strikes.
In addition to the approved section, the flight manual will also contain a section
provided by the manufacturer to assist in the operation of the helicopter. Civil flight
manuals tend to be relatively sparse because aviation manufacturers have become
targets for litigation in recent years. In contrast military flight manuals can be exceedingly
comprehensive, particularly in areas such as fuel flow with respect to AUW and
density altitude so that the best performance can be obtained from the machine.