1-1 Purpose. This volume provides general informa-
tion pertaining to aircraft propeller, rotor, and powertrain
maintenance practices and procedures. The application
of materials and techniques used on specific aircraft is
not covered in this volume. Specific aircraft application,
usage, and substitution are found in the individual aircraft
maintenance manuals. This volume is of maximum
benefit to the mechanic who desires information about
propellers, rotors, and powertrains. This volume fur-
nishes the mechanic a source of information about how
to perform various mechanical functions which are used
on all aircraft. This volume is not a requisitioning author-
ity, and applicable repair parts and special tools list
should be consulted to obtain the unit of issue and Na-
tional Stock Number of the items required for mainte-
1-2 Scope. General information to guide aircraft main-
tenance personnel is covered within this volume; howev-
er, no attempt has been made to include special parts or
equipment which are applicable only to individual or spe-
cific aircraft. Propeller maintenance practices and proce-
dures are covered in Chapter NO TAG. Rotor mainte-
nance practices and procedures are discussed in
Chapter NO TAG. Powertrain maintenance practices
and procedures are presented in Chapter NO TAG.
1--1500--204--23--6 for consumable materials in this vol-
1-4 Principles of Helicopter 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
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:
D Weight is the force exerted on an aircraft by
gravity. The pull of gravity acts through the air-
crafts 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.
D Lift is produced by air passing over the wing 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.
D 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 heli-
copter both thrust and lift are supplied by the
main rotor blades.
D 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).
1-1. Example of an airfoil.
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 con-
cave (curves inward toward the chord line). The upper
surface of an airfoil always has positive camber, but the
lowcr surface may have positive, negative, or zero cam-
ber (Figure 1-2).