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forms the leading edge of the airfoil. Spars are of
FM 1-514
Bottom
The high-pressure side of the blade is the bottom.
The bottom is the blade surface which is viewed from
the ground. It is always painted a lusterless black to
prevent glare from reflecting off the blade and into
crew compartments during flight.
Blade Stations
Rotor blade stations are numbered in inches and are
measured from one of two starting points. Some
rotor blades are numbered from the center of
rotation (center of the mast), which is designated
different shapes, depending on the blade material station zero, and outward to the blade tip. Others
and on how they fit into the blade airfoil. are numbered from the root end of the blade, station
Doublers zero, and outward to the blade tip (Figure 3-16).
Doublers are flat plates that are bonded to both sides Blade Construction
of the root end of some rotor blades to provide more Single Pocket or Fairing
ssptraernsg athre. mNaodet athlli cbkla edneosu ugshe tdoo purbolveirdse s tihnec en eseodmede The single-pocket or fairing blade is made with a
strength at the root end. one-piece skin on top and bottom (Figure 3-17).
Top Each skin extends across the entire span and chord,
The low-pressure side of the blade is the top. The behind the spar. This style is simple and easy to make
top is the blade surface which is viewed from above because of the minimum number of pockets or fairthe
helicopter. It is usually painted olive drab when ings that need positioning and clamping during the
bonding process. However, minor damage to the
the blade skin is plastic or metal.
3-9
FM 1-514
skin often results in the blade being thrown away
since replacing the skin costs more than replacing the
blade.
Multiple Pockets or Fairings
Most large rotor blades built with the multiplepocket
or fairing shape behind the spar are costly
(Figure 3-18). This type of blade is selected since
damage to the skin cover requires that only the pocket
(or fairing) be replaced. The high-cost blade can
then be used over and over. This type of blade is
more flexible across the span, which cuts down on
blade vibrations.
Internal Structural Components
Rotor blades have internal structural parts that help
to support the blade skin – ribs, I-beams, spanwise
channels, and aluminum honeycomb foil.
Bonds and Bonding
Bonding is a method of putting two or more parts
together with an adhesive compound. Bonding helps
reduce the use of hardware like bolts, rivets, and
screws that need holes and therefore weaken the
strength of the bond. To ensure full strength,
manufacturers never drill holes in load-carrying
parts of the blade except at the inboard and outboard
ends. However, bonds react to the chemical action
of paint thinners and many cleaning solvents.
Careless use of these solvents will dissolve bonded
joints. The surface area where two objects are
bonded together is known as the faying surface
(Figure 3-19).
Blade Balance
Three types of weights to balance the blade are mass
chordwise, spanwise, and tracking (Figure 3-20).
Mass balance weights (bars) are placed into the leading
edge of a blade while the blade is being made
(Figure 3-21). This is to ensure that correct
chordwise balance is about 25 percent of chord. The
type of metal and its shape and location vary with the
3-10
FM 1-514
manufacturer. The repairer is not allowed to move
the weights in most Army helicopter blades. When
moving of weights is allowed, however, the repairer
must remember that changing weights will move the
center of gravity forward or backward.
•
•
•
Spanwise balance weights are at the tip of the blade,
usually where they can be attached securely to the
spar (Figure 3-22). They are normally installed in the
blade during manufacture. The repairer is not
always permitted to move these weights. When
movement is necessary, the repairer should always
remember that adding spanwise weight moves the
center of gravity outward. Subtracting weight moves
the center of gravity inward. When moving the spanwise
weight is permitted, the weight change is computed
by the repairer mathematically after the blade
has been weighed.
To be efficient and vibration-free, all rotating blades
should track on about the same level or plane of
rotation. Failure of blades to track correctly causes
vibrations which can —
Damage parts of the helicopter.
Reduce riding comfort.
Cause a loss in blade performance due to air
turbulence made by the rotating blades.
　
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