FM 1-514
DISTRIBUTION RESTRICTION: Approved for public release; distribution is unlimited.
HEADQUARTERS, DEPARTMENT OF THE ARMY
FUNDAMENTALS OF ROTOR AND
POWER TRAIN MAINTENANCE—
TECHNIQUES AND PROCEDURES
FM 1-514
CHAPTER 1
PRINCIPLES OF HELICOPTER FLIGHT
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Basic flight theory and aerodynamics are considered
in full detail when an aircraft is designed. The rotor
repairer must understand these principles in order to
maintain aircraft safely and to make repairs that are
structurally sound and aerodynamically smooth.
AERODYNAMICS
Aerodynamics deals with the motion of air and with
the forces acting on objects moving through air or
remaining stationary in a current of air. The same
principles of aerodynamics apply to both rotary-wing
and fixed-wing aircraft. Four forces that affect an
aircraft at all times are weight, lift, thrust, and drag:
Weight is the force exerted on an aircraft by
gravity. The pull of gravity acts through the
aircraft’s center of gravity, which is the point
at which an aircraft would balance if
suspended. The magnitude of this force
changes only with a change in aircraft weight.
Lift is produced by air passing over the wing
of an airplane or over the rotor blades of a
helicopter. Lift is the force that overcomes
the weight of an aircraft so that it can rise in
the air.
Thrust is the force that moves an aircraft
through the air. In a conventional fixed-wing
aircraft, thrust provided by the propeller
moves the plane forward while the wings
supply the lift. In a helicopter both thrust and
lift are supplied by the main rotor blades.
Drag is the force of resistance by the air to the
passage of an aircraft through it. Thrust force
sets an aircraft in motion and keeps it in motion
against drag force.
Any device designed to produce lift or thrust when
passed through air is an airfoil. Airplane wings,
propeller blades, and helicopter main and tail rotor
blades are all airfoils (Figure 1-1).
Chord is the distance, or imaginary line, between the
leading and the trailing edge of an airfoil. The
amount of curve, or departure of the airfoil surface
from the chord line, is known as the camber. Upper
camber refers to the upper surface; lower camber
refers to the lower surface. If the surface is flat, the
camber is zero. The camber is positive if the surface
is convex (curves outward from the chord line). The
camber is negative if the surface is concave (curves
inward toward the chord line). The upper surface of
an airfoil always has positive camber, but the lower
surface may have positive, negative, or zero camber
(Figure 1-2).
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1-1
FM 1-514
BERNOULLI’S PRINCIPLE •
Bernoulli, an eighteenth century physicist, discovered
that air moving over a surface decreases air
pressure on the surface (Figure 1-3). As air speed
increases, surface air pressure decreases accordingly.
This is directly related to the flight of an aircraft.
As an airfoil starts moving through the air, it divides
the mass of air molecules at its leading edge. The
distance across the curved top surface is greater than
that across the relatively flat bottom surface. Air
molecules that pass over the top must therefore move
faster than those passing under the bottom in order
to meet at the same time along the trailing edge. The
faster airflow across the top surface creates a lowpressure
area above the airfoil. Air pressure below
the airfoil is greater than the pressure above it and
tends to push the airfoil up into the area of lower
pressure. As long as air passes over the airfoil, this
condition will exist. It is the difference in pressure
that causes lift. When air movement is fast enough
over a wing or rotor blade, the lift produced matches
the weight of the airfoil and its attached parts. This
lift is able to support the entire aircraft. As airspeed
across the wing or rotor increases further, the lift
exceeds the weight of the aircraft and the aircraft
rises. Not all of the air met by an airfoil is used in lift.
Some of it creates resistance, or drag, that hinders
forward motion. Lift and drag increase and decrease
together. They are therefore affected by the airfoil’s
angle of attack into the air, the speed of airflow, the
air density, and the shape of the airfoil or wing.
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LIFT AND THRUST
The amount of lift that an airfoil can develop depends
on five major factors:
Area (size or surface area of the airfoil).
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