Energy & Sustainability Science
Eurocopter’s low-noise “Blue Edge” rotor blade
blue edge rotor blade.jpg

Maybe I’m venturing into tinfoil hat country, here, but I’m pretty sure I once experienced a flyover by a stealth helicopter. I was camping at a lake in central Texas, during the Fall of 2003. Everyone else had gone to bed, but I was unable to sleep and was sitting up by the remains of the campfire, around 2 AM, just listening to the sounds of the forest, when I very clearly heard a distinctly unnatural sound pass across the dark sky overhead. It was very quiet, and very slow (rhythmically), but unmistakably a helicopter: whup whup whup whup whup. It was a clear night, and the speed at which the sound passed overhead meant it had to be flying at low altitude. There were no lights, just the sound, and I had a very eerie mental image of the glowing silhouette of my body, sitting beside the bright star of the cooling campfire, on a thermal imager cruising somewhere through the blackness above.

This wicked-looking rotor design is called “blue edge.” You can read more about it, and hear a comparison to normal rotor noise, over at Wired’s Autopia.

[via Geekologie]

12 thoughts on “Eurocopter’s low-noise “Blue Edge” rotor blade

    1. From the OP’s description, the only thing “stealth” or unusual was the lack of lights. They described it as a pretty familiar slow whup-whup-whup, which says to me it was a traditional helo with a two-blade rotor.

      Multiblade rotor aircraft have a sound that can’t really be described as “whup-whup-whup”.

      Eurocopter has a pretty long track record of noise reduction technologies, most of their helos have “fenestrons”, which are effectively a ducted fan that replaces the normal tail rotor. A lot of the noise in normal helicopters results in the interaction of the main rotor blades with the tail rotor, so shrouding the tail rotor reduces noise quite a bit.

      Kaman K-Max helicopters are quite quiet since they two have counterrotating intermeshing main rotors instead of a tail rotor. It’s more of a “swish-swish” sound than “whup-whup”.

      1. You’re right, I’m abusing the term “stealth,” since it tends to imply low-radar cross-section aircraft in aviation circles. And of course I don’t know anything about the radar signature of the aircraft I heard.

        But I would emphasize that, while it was clearly a helicopter sound, it was both extremely quiet and extremely fast-moving through the sky. This was a quiet night in the country, and it was only slightly louder than the crickets. It seemed to pass directly overhead, and the sound moved so quickly it was either flying low or, if it was at high altitude, moving faster than I’ve ever observed any type of airplane to move, ever.

        I suppose it might’ve been some kind of strange acoustical phenomenon, like it was really some long distance away and just sounded like it was directly overhead. And it’s just a silly anecdote, anyway. But I like to believe it was something mysterious and vaguely sinister. =]

        1. Fun practice exercises.. when you hear a jet or helicopter, immediately close your eyes. Now point to where you think it is and open your eyes. It is very very difficult to acoustically locate a jet or heli, the sound bounces off of everything and get’s stretched/squashed based on speed and direction. Most military heli like oh1/uh60/ah64 can do 150-200 mph, about the same speed as a Cessna.

          I’m not saying a silent helicopter doesn’t exist. But i personally have not seen one (or heard, hah) and they have to land and be stored somewhere. There is only so much range a helicopter can have, which means they must land to refuel and occasionally get repairs. The only thing i can think of is the silent helis can visually look like a normal helicopter and engage the sound dampening during flight. Exotic blades would be a big tip off and photos would get out quick.

  1. The Coast Guard around here uses Aerospatiale Dauphin helicopters. They are some of the most silent I’ve heard in operation. Under certain conditions, the only notice of approach is the turbine whine, more of a whisper and the tail fan completely removes the soundwave interaction between it and the main rotor.

    We have Kaman K-Max’s come in for fire duty at our airport. They have a distinctive low intensity sound. One was used to log the hill above us where I live, the operation took two months and was surprisingly quiet. You could tell when he was picking up a particularly large load as the turbine whine increased and a certain popping would occur during turns.

    The worst for noise intensity and pressure wave propagation are the National Guard Hueys. Three of them took off in formation once. The large windows in the front of our office were literally deflecting almost an inch. I saw the diaphragming action, immediately turned my head away and fled across the room. Everyone laughed at me, but I always stay away from the windows when any major Guard operations occur. I don’t want to deal with glass failure up close and personal.

    The neatest are the Ericksons (Sikorsky Skycrane). Giant flying grasshoppers operated by Superior, Evergreen and test flights from Erickson pass through here. They sometimes come in and do autorotation tests. Watching the main rotor cone is something to be experienced. Their 2700 gallon tank development was pretty neat to watch. They went from doing unwieldy bucket dumps to sucking water into a tank in 30 seconds and being able to control water dispersal from a mist to a complete dump.

  2. So, a few off the cuff thoughts on how you could make a more quiet helicopter (from someone who knows very, very little about helicopters in the first place). Thinking in terms of how actual stealth aircraft hide themselves from radar, they basically try to minimize their cross section, absorb what radar they can, and deflect the rest until they just blend into the background. So, thinking in terms of sound equivalents:

    Reduce the noise – From a really brief reading around of where the noise comes from, it’s seems like it’s mostly from each blade on the main rotor hitting the turbulence left by the blade in front of it, and from the air currents from the tail rotor interacting with the ones from the main rotor. So, I would think you could make the main rotor quieter by effectively moving the blades out – have the first 10 or 15 feet of “blade” just be an extremely flat, thin bar that will disrupt the air around it a minimal amount, with the actual blade further out. That would allow you to generate lift over a larger area, which would let you rotate the blades slower, and let the air in between settle more. Likewise, you could make the tail much longer to keep the tail rotor farther from the main one, and reduce the noise more that way. Granted you’ll need some very, very strong materials to do that, and you’ll have one very funny looking craft.

    For reducing the noise you made anyway – Since the pattern and frequency of the noise they make is probably fairly regular and predictable (at least on a frequency level), you could put big speakers on the bottom of it that would broadcast complementary sound waves to cancel them out. It would effectively function like big noise canceling headphones. Not really sure if that would work in open air like that instead of a closed headphone, but I would imagine you could at least quiet things a bit. The benefit would be that it could still look just like a normal helicopter.

    And, to deflect and dampen the noise more, we get out the bailing wire and duct tape solution – fly with a big cone or disk hanging underneath you. If you made it out of a fairly light weight material (mattress toppers and rebar hanging on a chain?) you could hang it low enough to keep it out of the air currents and not have to worry about it getting tangled in the blades, but still make it wide enough to dampen the sound for a rather large area under it.

    Put them all together and you’ve either got a quiet helicopter or an mega-expensive death trap. I’m not sure which.

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I am descended from 5,000 generations of tool-using primates. Also, I went to college and stuff. I am a long-time contributor to MAKE magazine and makezine.com. My work has also appeared in ReadyMade, c't – Magazin für Computertechnik, and The Wall Street Journal.

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