by Richard Sheffield
When the helicopter moves forward, a number of interesting things happen. As the forward speed increases, the aircraft suddenly starts to climb without a change in the collective control. This is due to translational lift.
This lift is caused by the rotor blades moving forward into undisturbed air. When this occurs, the blades become more efficient and an increase in lift results. This can be compensated for by decreasing the collective and total lift or by tilting the nose of the aircraft further forward. Thus, you increase the forward speed and decrease the amount of lift being generated straight up. Once a compromise is reached, straight and level flight will result.
Figure 3-7. Rotor Flapping
When a helicopter is moving forward, the rotor blade varies in airspeed during its spin. If the helicopter is moving forward at 100 mph and the blades are spinning at 400 mph, then the 100 mph forward motion of the aircraft is added to the 400 mph speed of the spinning blade, giving the blade an airspeed of 500 mph when it's spinning toward the front of the aircraft. This speed difference causes an increase in lift during this part of the cycle and is compensated for by allowing the blades to flap, or tilt up.
Another result of forward flight is known as dissymmetry of lift. One of the things that affects how much lift is generated by wings or rotor blades is the speed at which it is moving through the air. When a helicopter is moving forward, each rotor blade is moving in the direction of flight half of the time and away from the direction of flight half of the time.
When the blade is rotating forward, its speed is the sum of its rotational speed and the aircraft's forward speed. When the blade is rotating rearwards its speed is the rotational speed minus the forward speed of the aircraft. As a result, the blade generates more lift while rotating towards the front of the aircraft than it does while rotating towards the rear of the aircraft. This difference in lift causes the helicopter to want to flip over onto its back.
To prevent this from happening, each blade is attached to the mast, or hub, with a hinge. This hinge allows the blade to flap up during the faster portion of its cycle, thus dissipating the extra lift, and down during the slower part of the rotation. By allowing the rotors to flap in this manner, the overall lift of the blade can be equalized over the entire rotation. The centrifugal force of the spinning blades keeps them from moving too high or low.