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I was wondering about the ability of owls to fly so silently. Is it because they flap gently, they flap less often or do is their structure responsible? I was thinking their principle of stalking can be applied to stealth aircraft. Is it possible?

kevin
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Pratham Shah
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5 Answers5

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Their primary wing feathers have an unusual structure incorporating a fringed, comblike leading-edge, which reduces wind noise. The wing feathers also have an overall softness or flexibility. The trailing-edge of the wing is also dominated by soft, fringed edges. Even the underwing lining (covert) feathers have an unusual softness that plays a role in sound suppression.

See for example -- https://www.owlpages.com/owls/articles.php?a=7n , https://animals.howstuffworks.com/birds/owl-fly-silently1.htm . Google "owl feather structure" for more.

I'll leave it mostly unsaid, as to whether any of these features are worth incorporating into a jet-powered aircraft. Maybe some of these features might be applied to the intake or exhaust areas, in a more rigid metallic form?

quiet flyer
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    I vote for fur covered wings on aircraft then :-) – PerlDuck Oct 14 '19 at 17:32
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    +1 for the combination of this answer and your user name :-) – DeltaLima Oct 14 '19 at 18:42
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    ha didn't even notice that, is a reference to flying gliders! – quiet flyer Oct 14 '19 at 18:44
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    There are actually fans and such that are inspired by old feathers, e.g. https://cosmosmagazine.com/physics/turbine-blades-inspired-owls and cooling fans for computers. But I think part of the problem is that owl wings are low-speed aerodynamics, turbine blades are high-speed. And even with GA planes, few if any actually have adequate mufflers, so the engine noise far outweighs noise from the wings &c. – jamesqf Oct 15 '19 at 05:04
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    Intake and exhaust areas of jet engines are already designed to be "acoustically soft" to reduce noise. – fooot Oct 15 '19 at 19:10
  • I once read a paper and watched some videos on dynamic stability from 'feathers' on the trailing edge of the wing. The videos compared model airplanes with feathered wings and without. I remember the reduction in turbulence was impressive. –  Oct 17 '19 at 06:46
  • Modern jet engine exhausts already do make use of this - they have a shape that is reminiscent of a castle turret, but rounded off, precisely so that it merges the exhaust with the general wake in a blended manner. – MikeB Oct 17 '19 at 13:18
  • OP didn't specify that they were thinking of jet-powered aircraft. They could be considering quiet helicopters or prop-planes. – EldritchWarlord Oct 17 '19 at 14:43
  • @EldritchWarlord It doesn't get any easier with piston-powered aircraft. Feathers on the wings are a bit less likely to reach auto-ignition temperatures with piston-powered aircraft, but they're about as unlikely to survive flight at 120 knots as they are flight at 500 knots. Fortunately for owls, they typically don't need to fly at 120 knots. – reirab Oct 17 '19 at 17:44
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    @DeltaLima haha - That reminds me of this answer. – reirab Oct 17 '19 at 17:48
  • The engineering to allow feathers to be grown on aircraft flight surfaces would be...fascinating. :-) – Bob Jarvis - Слава Україні Oct 18 '19 at 00:16
  • As soon as airflow starts to reach Mach 1 (the speed of sound) you'll get noise, so owls do it by flying very slow and suppressing big vortices, like some flying wings keeping flying for days without recharging.

    The US tried silent aircraft during the Vietnam War Era, by combining slow-rotating propeller with extremely big and long exhaust systems. Worked quite well, so the aircraft was inaudible when flying at 200 ft. The aircraft were modified Schweizer gliders, with increased inboard wing chord, and trike landing gear.

    One was still in use by NASA a few years back.

     – Tord55 Oct 25 '19 at 06:28
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In addition to quiet flyer's excellent answer:

Owls have large wings in relation to their body size and weight. One might think that no, their bodies are quite large, but actually owls are kind of fluffy flying feather balls: what you percieve as their bodies, is mostly air.

This leads to two things:

  1. Low wing loading. Their large wings do not need to create as much lift per area unit as, say, with pigeons. This leads to less turbulence, which in turn means less sound. Because of low wing loading they also can fly with very little wing movement, also leading to less sound generation.

  2. The fluffyness (for lack of a better word) suppresses turbulence in wing - body attachment area, and any other turbulence around owl body. This means, of course, less sound.

When combined with wing structure described in q f's answer, all these factors create a pretty much absolutely silent flight.

Check out this excellent BBC Earth clip comparing the sound different birds make when they fly, and why so.

As for the aplicability of these features in stealth aircraft, not likely, athough I strongly second PerlDuck's vote for fur covered wings on aircraft. The fact is, however, that there is no sense in making the wings or the airframe more silent, when the sound of the propulsion is orders of magnitude louder than any other part of the aircraft.

P.S. It's actually quite eery to encounter an owl in flight when it's dark. You can hear other birds when they fly in your vicinity, but owls, no, they just blast right by you totally silently, without a warning. They've scared me sh__less a couple of times out in the wild, and I'm pretty sure they enjoy it...

Kat
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Jpe61
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    There's a tree-lined avenue near where I used to live where owls do swoops on people from behind, coming within inches of the top of your head and back up to the next tree. I'm never sure if they're doing it for fun or for practice or just to see what happens! – Dan Oct 14 '19 at 19:12
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    +1 for the BBC video link. It was the first thing that came to mind the moment I saw the question title. – aerobot Oct 15 '19 at 02:10
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    Not just owls - you can't hear a hawk coming either. Having a hawk fly over the audience's heads is a favourite stunt for raptor demos. – Graham Oct 16 '19 at 12:28
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    But you can hear a hawk fly past you. Owls, nope... – Jpe61 Oct 16 '19 at 12:39
  • Search for Owl X-ray pictures and you'll understand – Antzi Oct 17 '19 at 07:50
  • @jpe61 Not if they don't flap. Which is kind of the point of the BBC clip - the noise birds make is from flapping, and owls avoid that by being ambush predators and only gliding. – Graham Oct 17 '19 at 08:22
  • @Graham this might be subspecies and/or speed dependend, as the one that flew by me in a birds of prey -show definitely made an audible "swishhh" :) – Jpe61 Oct 17 '19 at 09:21
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    Fun fact: If an owl could turn its head by additional 90°, nobody would notice it can turn its head at all. – PerlDuck Oct 17 '19 at 18:03
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Stealth aircraft are built to reduce their observability in 3 main areas, with the goal of reducing the warning time an enemy has:

  1. radar
  2. optical and IR
  3. sound

This is in order of detection range: Radar can find an aircraft potentially at hundreds of km, optical systems go to a few tens of km, and sound becomes a factor only when the aircraft is very near (at low altitude and high speed, you can get within 100 m of the target before you become audible).

The detection range also gives you the order of importance. Radar takes priority.

The sound produced by an aircraft is dominated by its engine noise. The air rushing over the skin makes some noise, but that's generally only audible when the engines are shut off. This means there's no benefit to reducing skin noise.

In addition:

a fringed, comblike leading-edge

This is a nightmare for radar stealth. You want the structure to be as simple and smooth as possible, because when you have a large number of surfaces in many directions, you get many radar returns. Improving audio stealth would compromise radar stealth.

Hobbes
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    “because every edge produces a strong radar return” – really? I think edges per se are pretty irrelevant to radar return, it's surface alignments that matter. Thus the very different approaches in the faceted F117 and smooth B2 or F-22/F-35 both make sense. Granted, the latter are probably a lot better not only in flight characteristics but also in stealth, but this has more to do with better materials and computer power than with absence of edges. – leftaroundabout Oct 16 '19 at 10:07
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    What you probably mean really are concave edges, in particular corner reflectors. But concavities can also help surpressing echo, c.f. RF anechoic chambers. The trick is to “trap” the signals between the opposing surfaces, and only let them escape greatly attenuated and scattered. – leftaroundabout Oct 16 '19 at 10:07
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    yeah, that was a brainfart. – Hobbes Oct 16 '19 at 13:25
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    This is the correct answer, since it actually addresses the applicability to aircraft that OP asked about instead of getting sidetracked with owl geometry. – AEhere supports Monica Oct 16 '19 at 16:13
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    a fringed, comblike leading-edge could in theory be made of a radar-transparent material, not metal, which could preserve radar stealth. In supersonic aircraft leading edges get a lot of heating and aerodynamic forces, so this might only be viable for low-speed stealth drones (disguised as owls, like abligh suggests :) – Peter Cordes Oct 16 '19 at 16:51
  • https://en.wikipedia.org/wiki/Lockheed_YO-3 is an example of design when first two are not a concern (still not fluffy). Fun fact - in Museum of Flight (Seattle) it stands next to completely opposite approach - SR-71 - "so you can detect me - what are you going to do". – Alexei Levenkov Oct 16 '19 at 19:20
  • whereas with high altitude and/or low speed it's possible to hear a jet engine over 5 miles away, or a helicopter at 2 miles. – Michael Oct 17 '19 at 02:26
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Owls' bodies are optimised for silent unpowered flight; even when owl flight is powered flight, it's powered by wing flapping (rather than jet engines or turboprops). At most I suspect you'll be able to help silence the noise from a body passing through the air; as gliders are rather quieter than light aircraft, I suspect this is the minority of the sound, at least until speeds get very fast where the aerodynamic and acoustic models will be very different anyway.

However, as "aircraft" is widely defined, perhaps they would be useful for a drone that for portions of its flight mostly glides silently at low-ish (owl-like) speeds, for covert photography, covert small payload delivery, or some other likely nefarious purpose. Heck, it could actually be disguised visually as an owl.

abligh
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    Maybe all those "owls" really are... Nah. – Jeff Y Oct 16 '19 at 14:27
  • Expanding on the impact of audio levels on some mission profiles may be useful. [I remember reading about owl wings influencing prop blade design, but I'm drawing a blank on the specific project.] 'silent' glider designs also have some utility in an instance where you might want to pick up audio, and I've seen some draft ideas on the use of powered glider drones with loud speaker and microphone arrays for use in search and rescue applications. – TheLuckless Oct 16 '19 at 23:07
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If the other excellent answers roll off you like water off a duck's back, this may convince you:

What owls had to give up in exchange for quieter (not silent!) flight, is fast flying and flying on rainy days. Their feathers miss (most of) the oils that make other flying birds able to take quite a drenching without much trouble (and water birds even more so).

Planes crashing because of wet wings would be frowned upon, of course --- but the fixed wings would not have that problem; however any moisture would defeat this noise reduction. Only during landing & take-off noise reduction is really wanted (cruising altitude is too high to care); the added drag throughout the flight is (and added weight+drag when wet during takeoff+landing) will surely rule it out.

However, you can consider applications where it would be justified: If there's a large premium for silence, no need for high speed? Sounds like e.g. a specialized drone for observations to me. Then: If it's raining, take out the regular spy-drone (as the rain covers the drone noise); if it's dry, your special 'owl-drone' goes out... This more or less doubles your cost-of-ownership as you need two not one drone.

user3445853
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  • If your owl-drone could ride the thermals there wouldn't even be any need for engine sounds most of the time. Trouble is that during the day your owl-drone can be, er, seen (unless it can also be made see-through :) ). At night I presume there are fewer thermals generally to take advantage of (I'm not an aviation person). – mike rodent Oct 17 '19 at 13:11
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    @mikerodent based on your user name, I'd think you're not much of an owl person, either! ;) – FreeMan Oct 18 '19 at 19:55