The Art of the Helicopter John Watkinson - Karatunov.net

162 The Art of the Helicopter
4.21 Blade construction
The rotor blade has an arduous life being subject to considerable and time-variant
forces which in the early days made that life all too short. The construction of rotor
blades has been the subject of numerous advances directed towards increasing blade life.
Early blades were built up rather like the wings of aircraft. Blades must have a certain
weight both to store energy in the case of engine failure and to prevent an excessive
coning angle. High torsional stiffness is also required. These requirements were initially
met by building the blade around a steel tube. Collars were attached to the tube and
these supported wooden ribs. A wooden leading edge was attached to the ribs and this
would contain a metal insert to bring the mass centroid of the blade forward. The
leading edge of the blade as far back as the spar might be covered with thin plywood.
The whole was then covered with canvas and doped. Such construction lent itself readily
to the incorporation of blade taper and twist. Machines having such blades had to be
hangared because the blades were porous and moisture absorption could cause them
to go out of balance. Such blades were of no use in tropical countries where the glues
used could not withstand heat, humidity and bacterial/insect attack.
The solution was to make all-metal blades. Typically the leading third of the chord
was a heavy D-shaped extrusion with the remainder of the chord made up of thin
sheet metal, perhaps supported by aluminium honeycomb. Initially riveting was used;
later advances in adhesives allowed bonded structures. Metal structures are subject to
fatigue, particularly given the alternating loads experienced by the helicopter blade. In
some blades a warning of cracks was given by pressurization of the blade and fitting
a pressure indicator. In other cases the blade or spar was evacuated and a vacuum
indicator was used instead. If the indicator reading was incorrect this might indicate
an incipient crack or bonding failure. The use of extrusions made it difficult to
incorporate blade taper.
As the military helicopter developed, consideration was given to resisting battle damage
by designing redundant structures. In civilian applications a redundant structure
can have a longer service life because a failure is not catastrophic. Redundant structures
work by providing parallel paths for forces. In a rotor blade the single spar can
be replaced by a series of smaller parallel box spars. The box sections may be of glass
fibre which give support against buckling to steel spars between them. In some cases the
spars can be made entirely of glass fibre, but it will usually be necessary to incorporate
a metal strip bonded into the leading edge to give the correct mass centroid.
The trailing edge of the blade is relatively lightly loaded and it is advantageous if it is
not too stiff in bending in comparison with the leading edge because this combination
results in the blade washing out as it bends up; a stable condition. Generally a thin
outer skin is used which is supported against buckling by a low density filling material
such as alloy or composite honeycomb, foam plastic or end grain balsawood. In some
machines the trailing edge skin is deliberately weakened by chord-wise slits that allow
the blade stiffness to be defined by the spar. The trailing edge of the blade is a relatively
non-critical structure and many machines have returned safely following substantial
trailing edge damage.
As the conditions in which machines can operate are extended, it becomes necessary
to protect against lightning strikes. The result of a strike is massive energy dissipation
in any material presenting electrical resistance to the current path. The means of protection
is to make the outside surface of the blades electrically conductive and to bond
the blades together electrically. In this way potential differences across the rotor are
minimized along with the damage.