As part of our celebration of R&WI's 50th anniversary this year, we are reprinting selected columns by perhaps our most popular writer, rotorcraft aerodynamicist Ray Prouty.
Compared to a quarter-horsepower vacuum cleaner, a 1,000-shp helicopter is quiet — but not quiet enough. Designers have the prime responsibility for improving the situation — but pilots also have some opportunities to hold the noise down. To control noise, one must understand where it is coming from. There are not only several instinct sources of helicopter noise, but what you hear depends upon where you are.
Rotor noise in hover
For example, if you were to sit on the hub of a r [sic] turning at flat pitch on an electrically powered whirl tower, you would hear only a steady “swoosh” as the blades swept through the air. This noise is made primarily by air molecules that have been brought up to speed in the boundary layer of the blades, and then dumped off the trailing edge with much bouncing and jouncing. This is similar to the air noise you hear in a high-speed car. As blade pitch and bade lift re increased, tip cortices are formed that spin like a whirlpool and can change the noise characteristics from a “swoosh” to a “roar”. Part of this change is caused by the gusty inflow generated by a discrete frequency. For this reason, they are called broadband or “white” noise to compare them to white light, which contains all frequencies — or colors — of the spectrum.
Getting off the hub and standing somewhere near the rotor would give a different impression of the noise characteristics. Now you would be aware of “rotational noise,” in which you could hear each blade passing by and also multiples — or higher harmonica — of this frequency.
The higher the rotor rpm and the more blades the rotor has, the higher the blade-passage frequency will be Our ears are more sensitive to high than low frequencies. Thus, a small fast-turning tail rotor with four blades maybe perceived as making a louder noise than a slower-turning main rotor with two blades — even though the tail rotor is disturbing less air and radiating small pressure pulses. Carried to an extreme, a siren can produce a high level of “perceived noise” with relatively little power.
This is a good place to discuss how sound is measured. Sound travels through the air as a series of expansion and compression waves, much like ripples on a pond. The change in pressure from peak-to-trough of these waves causes a deflection of your eardrums and you hear a sound. The number of waves per second is the frequency, or “pitch,” of the sound. Unless you are listening to a tuning fork, most of what you hear is coming at many different frequencies with amplitudes continuously changing.
Noise scientists (who call themselves “acousticians”) use calibrated microphones and recording equipment to measure the sound pressure produce by the oscillating waves passing a given point Instead of reporting the pressure in absolute units of pounds per square foot, the acousticians prefer to compare the ratio fo their measurements with a standard low level that moat [sic] people can just barely hear. The units of this ratio are called decibels or, simply dBs. Normal speech is about 70 dB and noises that get our attention go up from there — with about 140 dB starting to cause physical pain.
To be able to plot both soft and loud noises on the same piece of graph paper, the dB scale is “logarithmic”. That is, doubling the measurement from 70dB to 140 dB does not represent doubling the loudness of normal speech, but something much higher. Each 6 B increase doubles the loudness from the previous level, so that by the time it gets to 140 dB, it is about 130 times as loud as normal conversation. (A good cocktail party gets up to 110 dB). Because of the ear’s sensitivity to high frequencies, the acousticians have developed a compensating correction to the sound levels measured by their equipment. The result is a “perceived sound level” or “weighted noise” that decreases the measured values coming from low-frequency sources, such as piccolos. (Remember how clearly you can hear the piccolo above the rest of the band in “Stars and Stripes Forever,” even when it is being played by a 90-pound high school girl?)
Noise in forward flight
Now, getting back to helicopters, we have already seen how you would hear rotational noise at blade-passage frequency while standing near a rotor whirl tower or a hovering helicopter. If, instead, the helicopter is flying over, the rotational noise is still a factor. But you might also hear another characteristic called “blade slap” or “impulsive noise”. There are two possible sources for this. In one case, the advancing tip is going fast enough to appreciably compress the air ahead of it. This causes chock waves that project strong pressure waves ahead of the tip like stones fro David’s sling. At short distances, the “crispness” of theses impulses is especially annoying. Farther out, the noise degenerates to a series of thuds that can be heard for miles.
The other type of impulsive noise is “lade-vortex interaction” which, as the name implies, is due to a blade running into — or at least near — the trailing tip cortex left by a preceding blade. In most flight conditions, the tip vortices and the blades keep away from each other. But if the helicopter has a slight rate of descent or is rolling into a turn, this separation may not be maintained. The flow pattern around the vortex induces sudden changes in the angle of attack and velocity on the blade, causing local stall and possible shock waves. What counts here is the rate of pressure change. Slowly pressing your palm against a tabletop is silent, but slapping the table is noisy — even if the final “slap” pressure is the same as the “press” pressure. This type of blade slap is generally projected down and forward. To the untrained ear, it sounds the same as that caused by high-speed compressibility.
All of this forward noise projection results in a sound pattern on the ground in which lines of constant sound levels form a “footprint” as in Figure 78-1. This is important in military operations in terms of detectability, and in commercial operations in terms of good neighborliness. Of course, both of these situations are influenced subjectively by the “ambient” noise level existing in the vicinity of the listener. Flying a helicopter over a freeway during rush hour will attract much less attention than over a residential neighborhood Sunday at 8 p.m.
If you are riding in the helicopter, you get a somewhat different impression of the noise characters. The rotor rotational noise is much reduced, since you are closer to the eye of the sound pattern.
You may, however, hear a similar noise due to each blade’s pressure pulse impinging on the fuselage. In some helicopters, this takes the form of “canopy drumming” and can be not only heard but also seen as rattling of thin window and skin panels. Impulsive noise due to advancing blade compressibility can never be heard in the cockpit, and impulsive noise due to blade-vortex interaction may not always be heard. Thus, a quiet cockpit is no assurance that you are not causing a problem on the ground.
Quieting the helicopter
For the designer who wants to minimize external noise, the moat [sic] promising method is to choose low tip speeds for both the main and tail rotors. This choice will reduce the perceived rotational noise and minimize compressibility noise, especially if the blades have thing and/or swept blade tips. The dilemma, however, is that lowering the tip speed requires more blade area and, thus, a heavier rotor to develop the same performance. Weights of the transmission and drive shaft also increase, due to lower rpm and higher torque. The resulting design is always a compromise between high performance and low noise, with customer requirements and competitive pressure playing significant roles in arriving at the final design decisions.
There has been some research done recently on modifying the characteristics of the tip cortex by special tip shapes or by injecting jets of air. The objectives of the modifications are to either spread out the vortex and make it less intense, or to move it out of the way of the following blades. At this writing, vortex modification is promising but has not yet had an effect on production rotor designs.
The pilot also has some control over external noise, especially with respect to blade slap due to blade-vortex interaction. Figure 78-2 shows the result of a test program that measured bade slap in various flight conditions. For this program, microphones were mounted on the outside of a Bell Helicopter 212. The measured noise characteristics may not be exactly the same as would be heard on the ground but can still be used for guidance. It may be seen that the worse flight condition for blade slap on this helicopter is 75 kts with a rate of descent of 300 fpm. Avoiding this condition when making a landing approach will minimize the blade slap both for ears on the ground and in the helicopter.
Internal noise can be controlled by the designer through the use of a wide range of techniques, including quieting such noise sources as gearing and pumps; avoiding metallic paths between rotating parts and the passenger compartments; changing the “rattling” characteristics of sheet-metal panels and equipment by stiffening, softening or damping, and by adding sound insulation as a last resort.
In all regards, helicopter noise in an important factor in the general public’s acceptance of these otherwise quite neighborly aircraft. The study is complicated enough that it promises to provide job security for a number of people into the foreseeable future.
This article has been edited to comply with our latest style and grammar rules.