The Art of the Helicopter John Watkinson - Karatunov.net

16 The Art of the Helicopter
Canopy
Instruments
Undercarriage
Controls
Rotorhead
Engine
Mast
Tanks
Gearbox
Main rotor blade
Tail rotor drive
Tailplane
Tail rotor
gearbox
Tail boom Tail
rotor
blade
Fig. 1.19 The major components of a conventional helicopter.Note the location of the fuel tanks close to
the mast to minimize CM (centre of mass) shifts.
instrumentation and avionics. These subjects will briefly be introduced in the remaining
sections of this chapter, and references will be made to more detailed treatments
elsewhere in the book.
1.6 The airframe
The fuselage or hull has a number of jobs to perform. One obvious task is to hold all of
the components in the correct position and to transfer forces from the rotors, the tail
surfaces, the landing gear and any internal or underslung payload. It also protects the
occupants and the mechanisms from the elements whilst still allowing good visibility
for the pilot. Space has to be found for fuel tanks close to the centre of mass (CM)
as single rotor machines are sensitive to trim shifts as fuel burns off. It will also have
a more or less aerodynamic shape, although the other requirements often combine to
prevent this. In some cases the hull will also be designed to float in case the machine is
forced down over water, whereas in others, amphibious operation is planned.
Fuselage construction varies considerably, but the materials and techniques are not
much different from those used in any aircraft. Early machines such as the Bell 47
were no more than a steel tube lattice frame with a blown acrylic canopy for the crew.
Aerodynamic improvements came when the hull was faired in. The tail cone is often a
stressed skin structure, but the centre section has too many doors, windows and access
hatches for the skin to carry all the loads, and alloy frames or steel tubes are often
used beneath the skin. The main landing gear often shares the same framework as the
engine and transmission so that the skin is not unduly stressed on landing.
Figure 1.20 shows the structure of an Enstrom F-28. The landing skids, engine, fuel
tanks and transmission are all attached to a welded tube frame known as a pylon. A
stressed skin tail cone is attached to the rear of the pylon structure, and the aluminium
seat and cabin floor is attached to the front. The cabin is glass fibre with plastics
glazing. The cabin lines are faired into the tail cone by unstressed panelling attached to
the pylon, so the machine has a sleek outline. Much of the unstressed centre panelling
can be removed for servicing.
Composite materials are ideal for airframe construction and are becoming increasingly
important since they are less dense than metals and are inherently well damped,
which helps to control the inevitable vibration that characterizes helicopters. They can
also have an indefinite fatigue life.