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3.14.  TRANSLATIONAL LIFT

The efficiency of the hovering rotor system is improved by each knot of incoming wind gained by forward motion of the helicopter or by a surface headwind. As the helicopter moves forward, as shown in figure 3.10, fresh air enters in an amount sufficient to relieve the hovering air-supply problem and improve performance. At approximately 18 knots, the rotor system receives enough free, undisturbed air to eliminate the air supply problem. At this time, lift noticeably improves; this distinct change is referred to as translational lift. At the instant of translational lift, and as the hovering air supply pattern is broken, dissymmetry of lift is created. As airspeed increases, translational lift continues to improve up to the speed that is used for best climb.

Figure 3.10. Airflow with Translational Lift in Forward Flight.

In forward flight, air passing through the rear portion of the rotor disc has a higher downwash velocity than the air passing through the forward portion. This is known as transverse flow effect and is illustrated in figure 3.11. This effect, in combination with gyroscopic precession, causes the rotor to tilt sideward and results in vibration that is most noticeable on entry into effective translation.

Figure 3.11. Transverse Flow Effect.

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