The Art of the Helicopter John Watkinson - Karatunov.net

Helicopter performance 339
of its continuous torque rating during contingency. This means that the helicopter has
70% of its usual torque available to deal with the failure and for landing. In the case
of a heavily loaded helicopter, 70% of torque may not be enough under some conditions
and as a result the helicopter will have a form of avoid curve with one engine
inoperative within which it cannot maintain height. This condition can be dealt with
by appropriate piloting. Pilots of twin-engine machines prefer to stay out of the OEI
avoid region so that an unexpected engine failure can be dealt with readily.
8.9 The flight manual
Elementary safety considerations suggest that the operation of aircraft must be subject
to regulations and procedures many of which are ultimately part of the law of the
land. The flight manual forms part of that framework as in most countries it is a legal
requirement to carry the flight manual in the aircraft to which it relates. All qualified
pilots are aware of the general procedures and regulations regarding aviation, but the
flight manual lists those facts specific to a certain model of aircraft.
Neglecting prototypes and experimental machines, aircraft in general operation must
be certified. This means that, independently of the designer and manufacturer, the relevant
authorities have examined the design of the machine and carried out flight tests.
The examination is intended to ensure that all relevant regulations regarding the construction
have been met and that as far as reasonably possible any areas of weakness
or mechanical unreliability have been eliminated. The flight tests are intended to show
that the aircraft has no handling peculiarities that might be beyond the skill of the
average pilot and to establish the limitations of flight conditions. The compilation of
the flight manual and the accuracy of its contents will thus be seen to be part of the
certification process.
Part of the flight manual is known as the approved section. This part contains the
information that is mandatory for safe operation. It will contain a table of contents
and a mechanism for logging the incorporation of any subsequent updated or amended
sections so that it can readily be established that the manual is complete and up to date.
The approved section contains four basic types of information: limitations, normal
procedures, emergency procedures and performance data.
Limitations set out in the flight manual are intended to prevent the machine being
flown in inappropriate circumstances, or, when it is flown, to prevent it being subject
to excessive stresses. Many helicopters are not certified for flight in icing conditions.
Many are not certified for IFR. Some can only safely be operated with a pilot and
a co-pilot. Few are certified for aerobatics.
Vne (airspeed not to be exceeded) primarily protects the blades and will often be
specified as a function of altitude. The torque limit primarily protects the transmission.
Turbine temperature limits protect the engine. The AUW of the machine must
not exceed the figure in the flight manual. All-up weight can remain the same if the
relative proportions of fuel and payload are changed. The distribution of weight is very
important in a helicopter having a single main rotor. If the CM is some distance from
the rotor shaft, cyclic control authority may be diminished in some directions and large
alternating stresses are set up in the rotor. Figure 8.12(a) shows that the CM must be
maintained within a conical region surrounding the shaft axis. In the case of a teetering
rotor head, the apex of the cone coincides with the teetering bearing. In rotor heads with
inbuilt flapping stiffness, the apex of the cone (b) will be above the rotor. Figure 8.12(c)
shows that in a tandem rotor helicopter the fore-and-aft CM position must lie between
two vertical limit lines. However, the position of these lines is a function of AUW.