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higher pitch angle, and more power is needed to move the air down through the rotor. 

Ground effect is at its maximum in a no-wind condition over a firm, smooth surface. Tall grass, rough terrain, revetments, and water surfaces alter the airflow pattern, causing an increase in rotor tip vortices. 

## GYROSCOPIC PRECESSION 

The spinning main rotor of a helicopter acts like a gyro- scope. As such, it has the properties of gyroscopic action, one of which is precession. Gyroscopic preces- sion is the resultant action or deflection of a spinning object when a force is applied to this object. This action occurs approximately 90° in the direction of rotation from the point where the force is applied. [Figure 3-8] 

Let us look at a two-bladed rotor system to see how gyroscopic precession affects the movement of the tip- path plane. Moving the cyclic pitch control increases the angle of attack of one rotor blade with the result that a greater lifting force is applied at that point in the plane of rotation. This same control movement simul- taneously decreases the angle of attack of the other blade the same amount, thus decreasing the lifting force applied at that point in the plane of rotation. The blade with the increased angle of attack tends to flap up; the blade with the decreased angle of attack tends to flap down. Because the rotor disk acts like a gyro, the blades reach maximum deflection at a point approxi- mately 90° later in the plane of rotation. As shown in figure 3-9, the retreating blade angle of attack is increased and the advancing blade angle of attack is decreased resulting in a tipping forward of the tip-path plane, since maximum deflection takes place 90° later when the blades are at the rear and front, respectively. 

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In a rotor system using three or more blades, the move- ment of the cyclic pitch control changes the angle of attack of each blade an appropriate amount so that the end result is the same. 

## VERTICAL FLIGHT 

Hovering is actually an element of vertical flight. Increasing the angle of attack of the rotor blades (pitch) while their velocity remains constant generates addi- tional vertical lift and thrust and the helicopter ascends. Decreasing the pitch causes the helicopter to descend. In a no wind condition when lift and thrust are less than weight and drag, the helicopter descends vertically. If 

