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Fiberglass
The main load-carrying member of a fiberglass blade
is a hollow, extruded steel spar (Figure 3-7). The
fairing or pockets are fiberglass covers bonded over
either aluminum ribs or aluminum foil honeycomb.
The fairing assembly is then bonded to the trailing
edge of the spar. The trailing edge of the fairing is
bonded to a stainless steel strip forming the blade
trailing edge. Rubber chafing strips are bonded between
the fairings to prevent fairing chafing and
provide a weather seal for the blade fairings. A steel
socket threaded to the blade spar shank provides an
attaching point to the rotor head. A stainless steel
tip cap is fastened by screws to the blade spar and
blade tip pocket.
Blade Nomenclature
Planform
The blade planform is the shape of the rotor blade
when viewed from above (Figure 3-8). It can be
uniform (parallel) or tapered. Uniform planforms
are most often selected by the manufacturer because,
with all the ribs and other internal blade
parts the same size, they are easier to make. The
uniform blade requires only one stamping die for
all ribs, which reduces blade cost. This design has
a large blade surface area at the tip; it must therefore
incorporate negative tip twists to make a more
uniform lift along the blade span. If the blade
angle is the same for the length of the blade, the
blade will produce more lift toward the tip because
it moves at a higher speed than the blade root. This
unequal lift will cause the blade to cone too much
or bend up on the end. The tapered planform
blade makes a more uniform lift throughout its
length. Few blade manufacturers use it, however,
because the manufacturing cost is too high due to
the many different-shaped parts required to fit the
tapered airfoil interior.
Twist
The blade-element theory applies to a rotor blade
as well as to a propeller. Therefore, most rotor
blades are twisted negatively from root to tip to get
more even distribution of lift.
Skin
The skin may be fiberglass or aluminum and may
consist of single or multiple layers. The thin skin
can easily be damaged by careless handling on the
ground. Three types of blade coverings are
used: one-piece wraparound aluminum alloy,
single pocket (or fairing), and multiple pocket
3-6
FM 1-514
(or fairing). Most main rotor blades are of singlepocket
or multiple-pocket construction.
Root
The blade root is the section nearest the center of
rotation that provides a means of attachment to the
rotor head (Figure 3-9). It is heavier and thicker
than the rest of the blade to resist centrifugal forces.
Tip
The tip is located furthest from the center of rotation
and travels at the highest speed during operation
(Figure 3-10). The blade tip cap also has a means for
attaching balance weights.
3-7
FM 1-514
Leading Edge
The part of the blade that meets the air first is the
leading edge (Figure 3-11). For the edge to work
efficiently, airfoils must have a leading edge that is
thicker than the trailing edge. The leading edge of
all blades is covered with a hard, abrasion-resistant
cap or coating to protect against erosion caused by
sand and dust.
Trailing Edge
Trailing edge is that part of the blade that follows or
trails the leading edge and is the thinnest section of
the airfoil (Figure 3-12). The trailing edge is
strengthened to resist damage, which most often happens
during ground handling.
Span and Span Line
The span of a rotor blade is its length from root to tip
(Figure 3-13). The span line is an imaginary line
running parallel to the leading edge from the root of
the blade to the tip. Span line is important to the
blade repairer because damages are often located
and classified according to their relation to it.
Defects paralleling the span line are usually less
3-8
serious because stress lines move parallel to the span
line and would therefore pass the damage without
interruption. Chordwise damage interrupts lines of
stress.
Chord and Chord Line
The chord of a rotor blade is its width measured at
the widest point (Figure 3-14). The chord line of a
rotor blade is an imaginary line from the leading edge
to the trailing edge, perpendicular to the span line.
Blade chord line is used as a reference line to make
angular measurements.
Spar
The main supporting part of a rotor blade is the spar
(Figure 3-15). Spars are usually made of aluminum,
steel, or fiberglass; they always extend along the span
line of the blade. Often the spar is D-shaped and
　
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