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Estimating Nonideal Effects from Rotor Measurements
It is often necessary to estimate values for the induced power factor, к, and the average profile drag coefficients of the blades, Cdo, from rotor thrust and power measurements. These values are then used in various performance estimations. This can be done given measurements of CT and C pin sufficient quality and quantity, these normally being made by testing an isolated rotor (i. e., without the airframe) on a hover tower when the rotor is high enough off the ground to be free of ground interference effects. It has been shown that the result in Eq. 2.43 gives a good representation of hover performance, that is,
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C, |
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(2.50) |
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when using the result for the profile power in Eq. 2.42. Strictly, о must be viewed the equivalent or weighted rotor solidity – see Section 3.4. By plotting 8C/>meas/a versus C’j2 Jyj2 as shown in Fig. 2.10, it is apparent that the results lie almost on a perfect straight line. The results can then be fitted in a least squares sense to find the slope (proportional to к) and the intercept on the у axis (Q0). From the results in this case for the rotor with rectangular blades and of solidity a = 0.098, then к = 1.233 and Q, = 0.0075. This latter result is quite reasonable in light of the results for the drag coefficients found for two-dimensional airfoil sections, as shown in Chapter 3 (Fig. 3.14) and also in Chapter 7. |
