During hovering flight, a helicopter tends to move in the direction of tail rotor thrust. This lateral (or sideward) movement is called translating tendency.
The spinning main rotor of a helicopter acts like a gyroscope. Gyroscopic precession is a phenomenon occurring in rotating bodies in which an applied force is manifested 90 degrees later in the direction of rotation from where the force was applied.
In order for a helicopter to generate lift, the rotor blades must be turning. The faster the rotation is, the greater the centrifugal force; and the slower the rotation is, the smaller the centrifugal force.
As lift on the blades is increased (in a takeoff, for example), two major forces are acting at the same time—centrifugal force acting outward, and lift acting upward. The result of these two forces is that the blades assume a conical path instead of remaining in the plane perpendicular to the mast. This can be seen in any helicopter when it takes off; the rotor disk changes from flat to a slight cone shape.
Conservation of Angular Momentum
An excellent example of this principle in action is a figure skater performing a spin on ice skates. The skater begins rotation on one foot, with the other leg and both arms extended. The rotation of the skater’s body is relatively slow. When a skater draws both arms and one leg inward, the moment of inertia (mass times radius squared) becomes much smaller and the body is rotating almost faster than the eye can follow. Because the angular momentum must, by law of nature, remain the same (no external force applied), the angular velocity must increase.
The rotor blade rotating about the rotor hub possesses angular momentum. As the rotor begins to cone due to G-loading maneuvers, the diameter or the rotor disk shrinks. Due to conservation of angular momentum, the blade rotation rate tends to increase.