Of all airfoils the rotor blade on a helicopter is unique. Like most airfoils it provides lift, but it also provides thrust and directional control. The rotor system produces the lift, thrust, and directional control needed for helicopter flight.
ROTOR SYSTEM
The rotor system includes a rotor head, rotor blades, and control systems that drive and control the pitch angles of the blade. The rotor head is the main assembly of the rotor system; it contains the rotor hub, blade attachment fittings, and blade controlling mechanisms. Currently, all helicopters in the Army inventory use a hub drive system (Figure 2-1). 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.
Plane of Rotation
A plane formed by the average tip path of the blades is known as the plane of rotation. The plane of rotation is at a right angle to the axis of rotation.
Axis of Rotation
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.
Area equals 3.14159 multiplied by the radius, then squared (multiplied by itself). The span length of one blade is used as the radius. The area of the hub in the disc area is not included since it doesn't make any lift. Disc loading is the ratio of aircraft gross weight to the disc area:
Disc Area and Loading
The disc area (Figure 2-5) is the total space in the area of the circle formed by the rotating rotor blades. The following formula is used to figure disc area:
Symmetry and Dissymmetry of Lift
Lift varies according to the square of the velocity (speed of blade and forward airspeed of aircraft). Symmetry and dissymmetry of lift are shown in Figure 2-6.
The example in the figure uses a blade tip speed of 300 MPH. The blade speed varies from 300 MPH at the tip station to 0 at the center of blade rotation on 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 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 determine the coning angle mathematically and build a precone angle into the rotor hub that is similar to the coning effect in normal flight. The preconed hub lets the blades operate at normal coning angles without bending, which reduces stress. It is not necessary to precone the articulated rotor hub because the blade can flap up on horizontal hinges to the correct coning angle.
Lead and Lag of Blades
The horizontal movement of the blades around a vertical pin is called leading and lagging (hunting) (Figure 2-10). This is found only on fully articulated rotor heads. During starting the blades will resist rotational movement and will lag behind their (true radial) position. As centrifugal force reacts on the blade, the blade will gain momentum and find its own position of rotation. The blade will hunt about the vertical hinge close to a 5° range during normal operation. The movement of the blades about the vertical hinge is restricted by a hydraulic damper.
Feathering Axis
The spanwise axis about which a rotor blade rotates to change pitch is known as the feathering axis (Figure 2-11). Feathering action varies according to the position of the cyclic control in forward flight, the dissymmetry of lift, and the collective pitch control when the helicopter hovers.
Hover
The versatility of a helicopter is due to its ability to hover at a point above the ground. This lets the helicopter vertically rise from and descend to small, unimproved landing areas. When main rotor angle of attack and engine power are adjusted so that lift equals weight, the helicopter will hover. Hover is considered an element of vertical flight. Assuming a no-wind condition exists during hover, the tip path plane of the rotor will remain horizontal with the earth. When the angle of attack of both blades is increased equally while blade speed remains constant, more thrust will result and the helicopter will rise. By upsetting the lift-gravity balance, the helicopter will rise or come down depending on which force is greater. Hovering takes a great deal of power because a large mass of air must be drawn through the rotor blades at high speeds.
Ground Effect
When hovering near ground or water surfaces at a height no more than one-half of the rotor diameter, the helicopter encounters a condition referred to as ground effect. This condition is more pronounced nearer the ground. Helicopter operations within ground effect are more efficient due to reduction of the rotor tip vortex and the flattening out of the rotor downwash. The benefit of ground effect is lower blade angle of attack, which results in a reduction of power requirements for a given load.
