Abstract
This paper numerically examines possible alleviation of parallel blade-vortex interaction (BVI), experienced by helicopters in low-speed descent, through the introduction of blade-to-blade dissimilarity. A four-bladed rotor with two sets of opposite blades is considered, and the radius of one set is reduced to 80% of the baseline radius. A free-wake analysis is developed for calculating the distorted wake geometry for such a rotor with dissimilar blades. Results indicate that with dissimilarity in blade length, the parallel BVI appear to be weaker due to factors such as increased miss-distance and increased angle (less parallel); so that the blade loading is generally less impulsive in nature. Increased miss-distances for the dissimilar rotor were attributed to the significantly different coning angle of the short and long blades, causing the tip vortices to be released from two different planes. The vortices from the longer blade are released higher, and tend to sail above the shorter blades with a low coning angle, and vice-versa. The acoustic code WOPWOP was used to calculate the BVI sound pressure level in a plane below the rotor disk. For the low-speed descent (severe BVI) flight condition examined, it was found that the peak BVI noise level was reduced by 4dB.
Published Version
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