The success of Sikorsky's X2 technology demonstrator program has revitalized research interest in coaxial rotor performance. Generally, a counter-rotating coaxial rotor is more efficient than the corresponding single rotor with twice the number of blades. Some of the performance gains are attributed to the unequal thrust sharing between the two rotors with the upper rotor, which operates more efficiently without direct interference of the lower rotor wake, carrying a larger share of the thrust. Furthermore, for counter-rotating coaxial rotors the swirl in the wake of the upper rotor has a beneficial effect in mitigating at least some of the swirl losses for the lower rotor. Experimental measurements are somewhat inconclusive. One experiment with a three-bladed coaxial rotor system shows that there is indeed some swirl recovery, and a counter-rotating coaxial rotor gives better performance than a co-rotating coaxial rotor. Another, more recent, experiment with a two-bladed coaxial rotor system showed that the performance of the co-rotating coaxial rotor varied significantly with the blade separation angle (or the index angle) between the two rotors. At some separation angles, the co-rotating rotor actually performed better than the counter-rotating rotor. This behavior contradicts the swirl recovery hypothesis. Perhaps, some of these differences stem from different induced inflow effects with blade separation angles. This will be explored in the present work using performance computations of both counter-and co-rotating coaxial rotors using a free vortex wake (FVW) model. Such a model has lower fidelity than more accurate computational fluid dynamics (CFD) models, but it allows easy separation of different constituents of the rotor performance, such as induced inflow/power and interactional effects, and provides physical insights into the resulting rotor performance behavior.
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