No abstract

A suite of computational tools capable of predicting in-plane low-frequency rotorcraft noise and its control using blade-tip geometry modifications is developed. The combined code, consisting of AVINOR, a comprehensive rotorcraft analysis code, and an acoustic code called HELINOIR, is first validated against wind-tunnel tests and subsequently verified by comparing with computational results. Three rotor configurations resembling the Messerschmitt-Bölkow-Blohm BO105 with a tip sweep, dihedral, and anhedral were simulated for level flight at a moderate advance ratio. The impact of passive blade geometry modification on in-plane noise and vibration is studied and compared to the in-plane noise reduction obtained using a single 20% chord active plain trailing-edge flap with a feedback microphone on the left boom. Active control, which is implemented using an adaptive higher harmonic control algorithm, reduces in-plane low-frequency sound pressure levels below the horizon by up to 6 dB, but there is an increase in vibratory loads. The tip dihedral reduces low-frequency sound pressure level by up to 2 dB without a vibratory load penalty, but there is an increase in the midfrequency sound pressure levels. The tip sweep and tip anhedral increase in-plane low-frequency sound pressure level below the horizon. There is a general tradeoff associated with in-plane low-frequency sound pressure level reduction, vibration performance, and midfrequency sound pressure level.

Active Gurney flaps, or microflaps, are studied to determine their effectiveness in reducing noise and vibration in rotorcraft as well as improving rotor performance. The effectiveness of the microflap is examined using a comprehensive rotorcraft simulation code. The aerodynamic properties of the microflap are modeled using a nonlinear computational-fluid-dynamics-based reduced-order aerodynamic model that takes into account unsteadiness, compressibility, and time-varying freestream effects. Active control studies are conducted on a hingeless rotor configuration resembling the Messerschmitt-Bölkow-Blohm BO-105, using various spanwise microflap configurations, including single, dual, and segmented five-microflap configurations. Results indicate that the microflap is capable of substantial reductions in blade-vortex interaction noise ranging from 3 to 6 dB. Vibration reduction ranging from 70 to 90% is also demonstrated. The effect of vibration reduction on noise and vice versa is also examined, and it was found that reduction in one objective is accompanied by an increase in the other, a trend also observed when using other active control approaches. Finally, the microflap is considered for combined vibration reduction and performance enhancement at a high-speed cruise flight condition. The results clearly indicate that the microflaps are very effective for both noise and vibration reduction in helicopters, and they also have potential for rotor performance enhancement.

A suite of computational tools capable of predicting in-plane low frequency rotorcraft noise and its control using blade tip geometry modifications is developed. The combined code, consisting of AVINOR, a comprehensive rotorcraft analysis code, and an acoustic code called HELINOIR, is first validated aginst wind tunnel tests, and subsequently verified against computational results. A rotor configuration resembling the MBB BO105 with a swept tip was simulated for level flight at a moderate advance ratio. The impact of passive blade geometry modification on in-plane noise and vibration was studied and compared to the in-plane noise reduction obtained using a single 20% chord active plain trailing edge flap with a feedback microphone on the left boom. In-plane noise below the horizon was reduced using active control whereas it was amplified using passive control. There is a vibration performance tradeoff associated with in-plane noise reduction.