Speaker: Dr. Daniel Patt

Boeing Phantom Works

Abstract:

Modern rotorcraft are plagued by the persistent problems of excessive noise and vibration.  High approach noise precludes the use of helicopters for transportation in many dense urban areas and affects detectability, while large vibration levels cause passenger discomfort, and prevent the use of many sensitive surveillance and radar payloads.  The mitigation of these phenomena has been repeatedly identified as a high priority both for civil transport and military applications.

This work presents the development and application of an active control approach for reduction of both vibration and noise induced by helicopter rotor blade vortex interaction (BVI). Control is implemented through single or dual actively controlled flaps (ACFs) on each blade. Low-speed helicopter flight is prone to severe BVI, resulting in elevated vibration and noise levels. Existing research has suggested that when some form of active control is used to reduce vibration, noise will increase and vice versa. The present research achieves simultaneous reduction of noise and vibration, and also investigates the physical sources of the observed reduction. The initial portion of this work focuses on developing a tool for simulating helicopter noise and vibrations in the BVI flight regime, including predicting compressible unsteady blade surface pressure distribution, as well as an enhanced free-wake model and an acoustic prediction tool with provisions for blade flexibility. These elements were incorporated within an aeroelastic analysis featuring fully coupled flap-lag-torsional blade dynamics. Subsequently, control algorithms were developed that were effective for reducing noise and vibration even in the nonlinear BVI flight regime; saturation limits are incorporated constraining flap deflections to specified limits. The resulting simulation was validated with a wide range of experimental data, achieving excellent correlation.

A number of active control studies were also performed. First, multi-component vibration reductions of 40-80% could be achieved, while incurring a small noise penalty.  Second, noise was reduced using an onboard feedback microphone; reductions of 4-10dB on the advancing side were observed when using dual flaps.  Third, simultaneous noise and vibration reduction was studied; a reduction of about 5dB in noise on the advancing side combined with a 60% reduction in vibration was achieved. The physical changes in the rotor wake and aerodynamics associated with these reductions were also examined. It was found that using ACFs for active control does not significantly affect rotor trim. It was also observed that all flap deflection schedules were accompanied by a small rotor power penalty. In pioneering the computational study of the simultaneous rotor noise and vibration problem, this work serves as an important precursor to future rotorcraft research.

In closing, some attention is given to innovative new directions in rotorcraft research, including unmanned vehicles, and the enabling technologies which make this possible.