The Art of the Helicopter John Watkinson - Karatunov.net

26 The Art of the Helicopter
Fig. 2.5 Elongation is initially proportional to stress, followed by differing characteristics according to whether
the material is ductile or brittle.
such as rotor heads may need extremely fine finishing and surface treatment to minimize
any irregularities from which a crack might propagate. Periodic inspection of highly
stressed parts may be needed. Cracks can be detected by X-rays, ultrasonic testing or
by penetrating dyes. At high temperatures, materials working within their elastic limit
may creep if the load is sustained. This phenomenon limits the life of turbine blades.
In many cases it is not the ability of the component to resist the load that matters,
but the amount of deflection the load is allowed to cause. Unwanted deflections are
generally unwelcome in aircraft as they may result in flutter or imprecise control.
Consequently stiff materials are advantageous. All aircraft have to lift themselves as well
as any payload, so it is an advantage if the weight of the structure can be minimized. It is
often thought that this will be achieved using low density materials, but this is not always
the case. In practice the lowest weight will be achieved by using materials with a high
stiffness to relative density ratio. This ratio also controls the speed of sound in the material,
which is generally high in aerospace materials as can easily be established by tapping
them. The result is a sound of a higher pitch than that found in everyday materials.
Such materials also make good loudspeaker cones, where the same criteria apply.
In practice the designer would like to have a range of materials all having high stiffness
to relative density ratio, but with a range of relative densities and strengths. Where
stresses are concentrated, a high strength, high density material is used. Where stresses
are reduced, a lower density material will be superior. For example, if a component were
made from an excessively strong material, the thickness required to carry the design
load might be so small that the component might suffer handling damage. Honeycomb
and foam cored materials are one approach to reducing density. In a helicopter, the
density of materials used tends to fall with distance from the rotor axis.
2.5 Resultants and components offorces
Earlier in this chapter, the forces were either up or down in order to introduce one
concept at a time. In the real world forces can act in arbitrary directions, except for the