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mechanisms. Currently, all helicopters in the Army
inventory use a hub drive system (Figure 2-l). In the
hub drive, blades are attached to a rotor hub that is
splined to the mast, which, in turn, rotates the rotor
hub and blades.
FORCES ACTING ON ROTORS
Since the rotor system of a helicopter provides both
lift and thrust, it is exposed to all of the forces that act
on aircraft wings and propellers. When applied to
rotor blades, the thrust-bending force that acts on
propellers is called coning. Because of the large
mass and weight of the rotating heads, the amount of
centrifugal force (Figure 2-2) that acts on the rotor
blades must be considered.
TERMINOLOGY
Angle of Incidence
The angular connection between a reference line on
a rotor blade cuff, socket, or attachment point and
the blade chord line at a specific blade station is
called the angle of incidence (Figure 2-3). On most
blades, this angle is determined during design and is
not adjustable.
2-1
FM 1-514
Plane of Rotation Area equals 3.14159 multiplied by the radius, then
A plane formed by the average tip path of the blades squared (multiplied by itself). The span length of one
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Axis of Rotation disc area:
An imaginary line that passes through a point on
which a body rotates is called the axis of rotation
(Figure 2-4). Its rotation is at a right angle to the
plane of rotation.
Disc loading = gross weight of aircraft
disc area
Disc Area and Loading Symmetry and Dissymmetry of Lift
The disc area (Figure 2-5) is the total space in the Lift varies according to the square of the velocity
area of the circle formed by the rotating rotor blades. (speed of blade and forward airspeed of aircraft).
The following formula is used to figure disc area: Symmetry and dissymmetry of lift are shown in
A = TTR2 Figure 2-6.
A = area The example in the figure uses a blade tip speed of
300 MPH. The blade speed varies from 300 MPH at
TT = total the tip station to 0 at the center of blade rotation on
R = radius the hub. When a helicopter is hovering in a no-wind
condition, there is symmetry of lift. The lift is equal
on advancing and retreating halves of the rotor disc
2-2
FM 1-514
area because speed is the same on both halves. Dissymmetry
of lift is the difference in lift that exists
between the advancing half of a rotor disc and the
retreating half. Dissymmetry is created by forward
movement of the helicopter. When the helicopter is
moving forward, the speed of the advancing blade is
the sum of the indicated airspeed of the helicopter
plus the rotational speed of the blade. The speed of
the retreating blade is the rotational speed of the
blade minus the forward speed of the helicopter. The
advancing half of the disc area has a blade tip speed
of 300 MPH plus the indicated helicopter speed of
100 MPH – a total blade tip speed of 400 MPH. The
total speed squared is 160,000. The retreating half of
the disc has a blade tip speed of 300 MPH minus the
100 MPH indicated forward speed of 200 MPH, and
velocity squared is 40,000. In this example the advancing
blade creates four times as much lift as the
retreating blade.
Horsepower Loading
Also called power loading, horsepower loading is the
ratio of aircraft gross weight to maximum horsepower
(gross weight divided by available horsepower).
The horsepower loading factor is used in
determining rotor system design and testing.
Flapping
The up-and-down movement of rotor blades positioned
at a right angle to the plane of rotation is
referred to as flapping (Figures 2-7 and 2-8). This
permits the rotor disc to tilt, providing directional
control in flight. It also controls the required lift on
each blade when in contact with dissymmetry of lift.
Up-and-down flapping is limited by the centrifugal
force acting against a smaller lifting force. Some
hubs have droop stops tO limit downward movement
at low rotor speed.
Coning and Preconing
The upward flexing of a rotor blade due to lift forces
acting on it is called coning (Figure 2-9). Coning is
the result of lift and centrifugal force acting on a
blade in flight. The lift force is almost 7 percent as
great as the centrifugal force, which causes the blade
to deflect upward about 3° to 4°. Coning is often
expressed as an angle. Helicopter manufacturers
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