Steady Rate of Descent in Autorotation

The isolated rotor charts of Chapter 3 can be used with the method outlined there to calculate the steady rate of descent in autorotation. The results for the example helicopter are plotted in Figure 5.5 as a function of forward speed (the value at zero forward speed is from the example calculation in Chapter 2). The speed for minimum rate of descent is at the bottom of the curve, but the speed for minimum angle of descent—or maximum glide distance—is at a higher speed where a ray from the origin is tangent to the curve.

The figure shows that the example helicopter can make approaches down paths as steep as 9° at any speed; but if it is required to come down a steeper path, the allowable speed range is limited by the inability to dissipate fast enough the potential energy above that required just to maintain autorotation. The figure also shows that very steep approaches at intermediate rates of descent would be complicated by the possibility of power settling in the vortex/ring state, as discussed in Chapter 2.

As a practical matter, there are two requirements for safely doing steep approaches, especially with limited visibility: an airspeed (or possibly ground speed) indicator, which is accurate at low speeds, and an adequate field of view from the cockpit to the intended touchdown point.

In airplane studies, the ratio of forward speed to ntyof descent in a glide is used to define the lift-to-drag ratio, since:

G. W. = 20,000 lb

FIGURE 5.5 Rate of Descent in Autorotation, Example Helicopter

L _ VM.

^ ^descent

For the example helicopter the maximum lift-to-drag ratio based on this definition is 6.3. This is lower than would be expected for a comparable airplane. The discrepancy can be charged to the drag of the rotor hubs and to the fact that, over a large portion of the rotor disc, the blade elements are not operating at their angles of attack for maximum airfoil lift-to-drag ratio, whereas on an airplane each wing element can be operating near its optimum angle of attack.

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