RETREATING BLADE FLOW REVERSAL

The forward speed of the helicopter at any time may be compared with the rearward speed of any segment of the retreating blade. The innermost part of the blade moves aft relatively slowly compared to the forward velocity of the entire rotor system. Even at slow flight speeds, some part of the rotor on the retreating side will therefore experience reversed flow. Since the first few percent of the blade is invariably occupied with hub mechanism, pitch control pushrods and hinges or flexible support, the effect is of little importance when the forward speed is slow.

As the helicopter accelerates towards its maximum flight speed, even though the rotor speed as a whole may increase, the area of blade which comes under reversedlflow extends further outwards (Figure 15.12). When this begins to affect the aerofoil shaped part of the blade, not only does this segment produce no supporting lift but, because the blade is now meeting the air backwards at a negative angle of attack, a downforce results. Further out still there is a point on the blade where there is no relative movement of the air (other than some outward dragging because of centrifugal forces). Beyond this, the blade begins again to produce lift.

The outer parts of the retreating blade at high speed are required to produce as much lift, at their lower relative airspeed, as the entire advancing rotor on the other side, plus an additional quantity to compensate for the downforce in the reversed flow region. To do this, blade flap increases die angle of attack on the retreating side considerably.