CORRELATION WITH TEST RESULTS

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CORRELATION WITH TEST RESULTS

The chart method has been used to produce results that can be compared with a set of wind tunnel test results for a full-scale rotor reported in reference 3.27 and the correlation is shown in Figure 3.62. (The test results have been corrected to free – air conditions for wind tunnel wall effects by the method of reference 3.47.)

During the correlation process, it was found that the input collective pitch had to be adjusted somewhat from that given in the test report. The difficulty appears to be connected with the test conditions rather than with the theory. During the tests of reference 3.27, a rotor with no twist was operated at zero shaft angle of attack. At a collective pitch of zero, it produced positive thrust. The data indicated that at a collective pitch of —1°, the thrust would be zero. The senior author of reference 3.27 believed that the effect was the result of local flow distortions around the body used to fair the rotor support system. Similar discrepancies have been noted in other wind tunnel tests. For this reason, the values of collective pitch selected for correlation in figure 3.62 are 1° higher than those stated in the test report.

A more extensive correlation study of the results of reference 3.27 is reported in reference 3.48. In that study, the effects of including blade flexibility, vortex-caused variable-induced velocity distribution, and unsteady aerodynamics in the calculations were investigated. The conclusions reached were that with respect to performance, the incorporation of unsteady aerodynamics is of primary importance, but that blade flexibility and variable-induced velocity distribution have little significance. It thus appears that the methods outlined in this chapter, which do not account for flexibility or variable inflow but do account for unsteady aerodynamics, are adequate for performance calculations.

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