12/03/2013
Noise-free rotor chopper:
Current technology for noise reduction employed in new rotorcraft designs, must ensure that the cost, performance, and other impositions on the design are met in concert with reduced noise.
Control of main rotor noise has traditionally been accomplished by the judicious selection of rotor blade configurations and rotational tip speed. Airfoils, blade planforms, and tip shapes are chosen which mitigate the effects of HSI noise and BVI noise. For a given design gross weight, increasing the blade chord and changing the number of rotor blades are means of reaching an acoustically desirable rotational tip speed. The blade number change also alters the frequency distribution of the sound generated.
The most direct method of controlling BVI noise is by reducing or diffusing the tip vortex. Tip shapes such as the sub-wing, Ogee tip, and others have been shown to cause measurable reductions in BVI noise by modifying the vortex structure.
Conventional means of noise reduction, e.g., tip speed reduction, tip shapes and airfoil tailoring, are inferior to several innovative design concepts: modulated blade spacing and x-force control when used to significantly reduce noise with minimal performance degradation and no vibration increase.
Helicopter main rotors have historically been designed with equally spaced blades. This equal spacing from one blade to the next translates to a main rotor acoustic spectrum characterized by a single fundamental blade-passage frequency and its harmonics. As many as 20 or 30 harmonics are commonly present in a main rotor's acoustic spectrum, each of which is a multiple of the fundamental blade-passage frequency. In a typical spectral plot, these frequencies appear as pronounced, ordered "peaks" spread evenly across the acoustic spectrum.
Since the acoustic frequencies associated with the rotating blades are directly related to the blade spacing, intuitively the use of unevenly spaced blades holds the potential of lower sound levels and less perceptibility. The acoustic effect of uneven or modulated blade spacing is to generate several blade-passage frequencies, one for each unique angle between blades. Each blade passage frequency, in turn, generates its own set of harmonics. The total acoustic energy is thereby spread over a broader range of frequencies, rather than being concentrated at one blade-passage frequency and a single set of harmonics.
Main rotor designs that incorporate modulated blade spacing have reduced peak noise levels in most flight operations. X-force control alters the helicopter's force balance whereby the miss distance between main rotor blades and shed vortices can be controlled. This control provides a high potential to mitigate BVI noise radiation. A main rotor design, incorporating the modulated blade spacing concept, offers significantly reduced noise levels and the potential of a break-through in how a helicopter's sound is perceived and judged.
The advantages of the modulated blade spacing concept are many: it has minimal impact on performance and potentially reduces vibration; it reduces sound levels and improves sound quality when incorporated on tail rotors; it lessens perceptibility; and it potentially has aural detection benefits. It is believed that the lower source frequencies associated with a main rotor can be altered similarly to those of a tail rotor.
One configuration studied had five blades, a radius of 19.5 feet, a thrust weighted chord of 12 inches, and a rotational tip speed of 665 feet per second. This rotor incorporated modulated blade spacing with angles between blades of 72, 68.5, 79, 65, and 76.5 degrees. If incorporated on the baseline helicopter, the rotor results in a 16 percent payload penalty for the full fuel case. The cruise airspeed would be reduced by 6.2 percent and the maximum airspeed by 17.2 percent. The reduction in peak noise levels is predicted to be 4, 8 and 4 dBA during takeoff, flyover and approach, respectively. The noise reductions up-range (15-20 seconds before overhead) are even greater: 16, 16 and 9 dBA during takeoff, flyover and approach, respectively.
