Unquiet Helicopters

I wrote about helicopter noise in The God Machine and last night was reminded that this problem is still with us, despite some research and many assurances from helicopter users. This while sitting through Game One of the Twins-Yankees playoff series.

After the Twins' early lead got knocked out of the park in the sixth inning, I needed some kind of distraction. I found it by watching a helicopter that was orbiting the stadium, presumably to gather video for the TBS broadcast. It was flying wide circles in a clockwise direction, staying outside the footprint of the ballpark.

Why clockwise? The pilot-in-command of a helicopter sits on the right side, so his visibility of the ground is best on the right side when the ship (as pilots say) is orbiting.

The helicopter was noisy enough to attract the attention of some fans in our section.While details were hard to discern in the dark, from the profile I'd guess it was a turbine-powered, four-bladed model from Bell called the 407.

The Bell 407 uses the popular single-main rotor design for lift, with a tail rotor to counteract the torque from the main rotor. The tail rotor is essential to keeping the helicopter under control and pointed in the right direction.

The blades of the main rotor and tail rotor were making most of the noise that Twins fans were noticing, along with minor contributions from the turbine exhaust.

A helicopter's noise changes greatly as it approaches, passes overhead, then recedes. If you have the opportunity on some sidewalk or rooftop, stop and listen the next time one's coming your way. You'll notice that the helicopter makes a pounding noise as it approaches. But keep listening as the helicopter is almost overhead. As abruptly as if someone flipped a switch, the pounding noise fades and is replaced by a penetrating whine from the tail rotor. After the aircraft is moving away, the thrumming picks up again but at a lower noise level. The larger the helicopter, the louder the noise, because the blades have to be bigger to support them in flight. The Black Hawk and Chinook are among the loudest in the US fleet.

The pounding noise that shakes the windows is from the main rotor. The reason is the noisy and fuel-wasting phenomenon called blade vortex interaction, or BVI.

As the rotor turns, tiny tornados called vortices tear loose from each blade tip. The next blade to arrive smashes through those whirlwinds. This result is blade slap that with larger helicopters, can be heard for a mile or more. With a rudimentary sound gathering device, Vietcong fighters could hear BVI noise from American UH-1 Huey helicopters a full five miles off.

Most BVI noise from a moving helicopter is blasted out in the forward direction, as if from a bullhorn. The strongest effect is on the right side because the advancing blades (which in US models, are on the right side) in striking the tip vortices throw the energy forward and somewhat to the right. Unfortunately for residents who live near heliports, BVI noise is particularly loud along the flight path when a helicopter is on final approach.

Was there ever a totally quiet helicopter? We wish! An air-ambulance pilot told me about attending a neighborhood meeting near his hospital, in which residents pressed their desire to reduce noise levels. A resident asked him, "What about that Whisper Mode, why don’t you use that?" The pilot replied, “That’s only available in Hollywood.” The resident replied with some indignation: “I don’t care where you have to go to get it, just get it!”

Notwithstanding the compelling special effects of plot churners like Endangered Species, there is no Whisper Mode that can silence any full-sized helicopter at short range. That is, a Bell, Sikorsky, Aerospatiale, or Agusta can't hover a hundred feet from an office window as in Blue Thunder and take pictures without attracting a lot of attention.

But, that said, some surprising advances have been made in learning how to cut back on noise generation. But commonly these modifications add expense while also reducing the payload of the ships, so the most effective methods are not yet in general use.

In another post I'll describe Hughes Tool's work on their one-off “500P” model during the Vietnam War, along with more recent experiments in quietizing the machines.