20
$\begingroup$

Living closer to a Sikorsky factory, I can attest that modern civilian (and military) helicopters are quite noisy. In any case, they are considerably louder than light aircraft, turboprops, and even some jetliners.

This has resulted in helicopters being banned from approach and landing in some jurisdictions.

It seems though, that the techniques for sound mitigation of helicopters have been known since at least the 1970s (The Quiet One) let alone the stealth helicopters used in the Bin Laden raid.

That is, part of the solution appears to be to use more rotor blades while decreasing the speed of the rotors. There are also modifications to the tips of the rotor blades that while they may be classified, could be deduced and reverse engineered.

Why are helicopters so loud and why aren't contemporary helicopters quieter given what we know and how far we've come in materials sciences and aviation design?

Or is a question of economics (quieter == less economic) or politics (national security prevents the use of sound-mitigating technology) or something else?

$\endgroup$
  • 2
    $\begingroup$ I may be wrong, but I thought that the forward moving tip breaking the sound barrier was one of the limiting factors in both a helicopter's forward speed as well as the rotational speed of the disk. i.e. having a tip break the sound barrier was a Bad Thing™ in a helicopter. $\endgroup$ – FreeMan Jul 16 '15 at 15:38
  • 2
    $\begingroup$ @Freeman - That's why lowering the speed of the rotors is (generally imagined to be) one of the stealth technologies used in the OH-6 as well as Bin Laden copters. But why this isn't commonly used is part of my question. Seems a no-brainer, especially when the folks living near Bin Laden's compound said they didn't hear the copters until they were overhead. If only that were true for general aviation! :-) $\endgroup$ – RoboKaren Jul 16 '15 at 15:53
  • 2
    $\begingroup$ I was thinking that supersonic blade tips creating shock waves that would hit the fuselage and other blades was a more serious issue than just creating noise (i.e. vibration, loss of lift, etc). Maybe I need to ask another question... $\endgroup$ – FreeMan Jul 16 '15 at 16:08
  • 2
    $\begingroup$ One aspect: More blades => smaller disc radius => higher disc loading => less efficiency => reduced range => higher fuel consumption => higher weight => reduced autorotation performance. I'm guessing that all of the above are a design compromise trade-off against market demand and the military don't care much except for the stealth bit. $\endgroup$ – Simon Jul 16 '15 at 16:28
  • 3
    $\begingroup$ For what it's worth, MD Helicopters claims their elimination of the traditional tail rotor results in a significant noise reduction. See mdhelicopters.com/v2/notar.php. $\endgroup$ – Terry Jul 17 '15 at 3:42
16
$\begingroup$

It's an R&D and therefore ultimately a cost-benefits decision. Quieting techniques for helicopters aren't as simple as "hush kits" for fixed-wings, because the engines and rotors are a much more integral part of the entire airframe. So, to incorporate these features into a civilian helicopter is a much more involved process than swapping blades on a turboprop or putting hush rings on the exhausts of older airliner engines. Changing the physical properties of the rotors (lift, induced drag, moment of inertia) requires compensating changes in the engine; shorter blades reduces lift requiring higher RPM, more blades increases MOI requiring more torque, etc. Those engine changes will in turn require changes to tail rotor characteristics, and may also require frame and body tweaks, etc etc.

Most helicopters in civilian use today, like the Bell 200-series, Sikorsky S-70 family, Dauphin etc are Vietnam-era designs, almost unchanged save for a few minor tweaks along the way like incremental engine improvements or new avionics systems. They're still being built and flown because they work; take the rigorous proving tests required of any aviation system and the maintenance and inspection requirements of fixed wing aircraft, and add an additional level of caution for the inherent instability and lack of fail-safes in the design of a rotary-wing aircraft, and you'll have some idea of what it takes to get a new helicopter vetted for GA use. If the helicopter's engine cuts out or throws a prop blade, you can't just glide down to the ground the same way as a fixed wing. So, any changes you make to a basic, proven design would be subject to extreme skepticism and testing by both aviation regulators and owners, to say nothing of starting from scratch. This decreases initial sales of a new model in the civilian market, increasing the payback period of your R&D to develop this new design.

Second, in part due to this hesitance by GA to adopt new chopper designs, the funding availability to research and develop any new civilian helicopter design is dwarfed by military interests. As mentioned, practically every design you see in the skies today has a military pedigree and was designed primarily for that purpose, then civilianized for GA use (a notable exception being the Eurocopter Dauphin 2 which was established in the private and law enforcement sectors before militaries took a look). As a result, a lot of the newest stealth technologies available for helicopters, including noise reduction, are heavily classified. For instance, the modifications to the UH-60s used in the Bin Laden raid; civilians really only know that stealth Blackhawks exist, as what they look like and their performance/noise characteristics are all still top secret. That hasn't stopped the human imagination; artist impressions including the concept art for Zero Dark Thirty abound.

In addition, the civilian quiet-copter technologies that aren't classified, like Eurocopter's "Blue Edge" blade design, are mostly protected by international patent. Bell or Sikorsky can't just reverse-engineer the Blue Edge blade and stick it on their choppers. An independently-developed design must be different enough to be patentable, which probably means it will be more complex, and complexity is a bad word in aviation because it means increased probability of system failure or operator error. The other avenues would be to either license Blue Edge from Eurocopter (increasing the cost of a Bell or Sikorsky Blue Edge-equipped model) or wait 15-20 years until the patent expires (further delaying widespread adoption of the technology beyond one brand).

$\endgroup$
  • 2
    $\begingroup$ "Bell or Sikorsky can't just reverse-engineer the Blue Edge blade" They don't need to: to patent something, you have to fully disclose the design. $\endgroup$ – David Richerby Jul 16 '15 at 19:54
  • 6
    $\begingroup$ Your be surprised how much you can obfuscate how a patented mechanism really works, including various dynamic variables like how the design scales for optimal performance at different blade lengths, which would be necessary to put the blade on different helicopters. $\endgroup$ – KeithS Jul 17 '15 at 3:33
  • $\begingroup$ Helicopters glide a bit worse than fixed wing aircraft (and call it autorotation), but compensate by still being able to land with no or very low forward speed at the end, so engine failure is not a fatal problem. They do have a single point of fatal failure though, the rotor hub. $\endgroup$ – Jan Hudec May 13 '18 at 21:31
  • $\begingroup$ Not everything is classified: Scissor-blade tail rotors is one technology to reduce noise: aviation.stackexchange.com/questions/43279/… $\endgroup$ – asmaier Sep 15 at 20:51
  • $\begingroup$ Wikipedia mentions even more technologies: "In order to reduce their acoustic signature, the helicopters (N351X and N352X) received a four-blade 'scissors' style tail rotor (later incorporated into the Hughes-designed AH-64 Apache), a fifth rotor blade and reshaped rotor tips, a modified exhaust system and various performance-boosts." (en.wikipedia.org/wiki/Hughes_OH-6_Cayuse#Vietnam_War) All these technologies are known for a long time, but not used in civilian helicopters. $\endgroup$ – asmaier Sep 15 at 20:53
3
$\begingroup$

A large part of the noise produced by rotor wing aircraft comes from the drive system. The engines themselves make quite a bit of noise and it is technically infeasible to make them quieter.

$\endgroup$
  • 1
    $\begingroup$ Presumably the same hush kits on turbofans could be used -- or other muffling technologies. $\endgroup$ – RoboKaren Oct 3 '16 at 2:00
0
$\begingroup$

The pilots joined Vietnam War say that UH-1 helicopters turn fuel and oil into "Vibration and Noise" :) The answer you have chosen is the best answer to your question. I have nothing to say over it. I just want to add that designing a helicopter is not an easy thing. There are lots of things you have to think and calculate before building a prototype.

While you can fly a plane made of paper, you can not fly a helicopter made of paper.
I admire all the work behind designing a helicopter. The first helicopter was built and flown in 1936 : https://en.wikipedia.org/wiki/Focke-Wulf_Fw_61 And one of the first men in history who has accomplished the unpowered-manned-flight is Hezarfen Ahmed Celebi. He has flown with the wings he has designed and flown about 2 miles. When? Around 1630! : https://en.wikipedia.org/wiki/Hezârfen_Ahmed_Çelebi

So, helicopters and airplanes are different types of aircrafts. While helicopters have hundreds of moving or rotating parts, some airplanes don't have any rotating parts other than gyros :)

As technology is improving, helicopter noise will get lower. I am sure of it. Just like old type of Hard Disk Drives vs. Solid State Disks. While former has lots of moving parts and has lots of noise, latter has no moving parts and no noise.

$\endgroup$
  • 3
    $\begingroup$ So, solid-state helicopters with no moving parts then? $\endgroup$ – Transistor May 10 '18 at 21:28
  • 1
    $\begingroup$ I tried to explain that its all about technology. But remembering that fastest train uses magnetic levitation line, helicopters may use technology like this. Like NOTAR for example. $\endgroup$ – tekin beyaz May 11 '18 at 16:37

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.