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3.13.  GROUND EFFECT

Figure 3.9 illustrates air flow in and out of ground effect. Notice the information in A and B that is printed close to the sketches. Ground effect is a condition of improved performance found when hovering near the ground. The best height is approximately one-half the main rotor diameter.

Figure 3.9. Airflow In and Out of Ground Effect.

The improved lift and airfoil efficiency while operating in ground effect are due to the following effects. First and most important, the main rotor-tip vortex is reduced. When operating in ground effect, the downward and outward airflow reduces the vortex. A vortex is a flow involving rotation about an axis or center. This makes the outward portion of the main rotor blade more efficient. Reducing the vortex also reduces the turbulence caused by recirculating the vortex swirl, as shown in figure 3.9A. Second the angle of the air is reduced as it leaves the airfoil, as illustrated in figure 3.9B. When the airfoil angle is reduced the resultant lift is rotated slightly forward, making it more vertical. Reduction of induced drag permits lower angles of attack for the same amount of lift and reduces the power required to drive the blades.

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