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?
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?
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:
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.
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...
Stealth aircraft are built to reduce their observability in 3 main areas, with the goal of reducing the warning time an enemy has:
- optical and IR
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.
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.
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.
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.