Eagles fly slowly for a long time.

Many other species fly faster and move their wings faster. But eagles keep their wings steady, and move only their tail.

  • How do they move slowly in the air, without falling down?

  • Can this eagle flying technique be used in aviation?

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    $\begingroup$ The second subquestion is unrelated to the first and seems to be engineering. $\endgroup$
    – ACuriousMind
    May 14 '15 at 12:39
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    $\begingroup$ Anyone who's read Animorphs should know the answer to this question $\endgroup$
    – Nacht
    May 14 '15 at 23:37
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    $\begingroup$ It's what sailplanes do. $\endgroup$
    – Hot Licks
    May 15 '15 at 1:16
  • $\begingroup$ Note that, since eagles fly at a relatively high altitude, they appear to move rather slowly because their angular position changes slowly. $\endgroup$ May 15 '15 at 8:49
  • $\begingroup$ @DavidRicherby - Though a soaring bird, positioned appropriately over a thermal or the updraft along a line of hills can maintain a relatively static position for a considerable period of time. $\endgroup$
    – Hot Licks
    May 21 '15 at 0:16

How does they move slowly in air, without falling down?

One possibility is soaring using a ridge lift - typically a situation when the wind is approx. perpendicular to a mountain ridge. The air is lifted at the front side of the ridge and an eagle can soar in the lifting air stream. This can also work without the wind,

Which is a situation of thermal flying. Typically, the ground is heated by the Sun, the air layer just above the ground is heated by conduction and at some moment it forms a kind of bubble that starts to rise. This bubble is usually long, resembling a column and lasts until the warm air is depleted. The situation can repeat (this behavior is called an interval). If a ridge is oriented south, then the Sun can create a thermal wind (intervals) that enables a bird to soar.

Can we use eagle's flying technic for flights? Yes, however, man will never be that good.

soaring: https://www.youtube.com/watch?v=63qJn9HrB7E

thermal flight: https://www.youtube.com/watch?v=KXqTCM0-zXQ

Edit: Just for completness - there exists also a wave soaring, that is reachable for gliders and maybe for hangliders, probably not possible for birds and paragliders - see pictures here : http://www.ssa.org/GliderLiftSources

  • $\begingroup$ Ssmall birds can do this also, but 1/ they dont have usually a reason as they consume an insect 2/ it could display them to predators. Sometimes, however, one can see them $\endgroup$
    – jaromrax
    May 14 '15 at 8:11
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    $\begingroup$ Gliders use the same technique to stay in the air for hours. No engine, no flapping wings. I knew a guy in the 70's who was an expert glider pilot who would look for birds of prey - they would help him find the best updrafts/thermals... Modern LIDAR techniques make it possible to find these air currents without the help of birds. $\endgroup$
    – Floris
    May 14 '15 at 12:28
  • $\begingroup$ @Floris: And more often (at least hereabouts) you find thermals by knowing the terrain and where they're likely to form, or looking at the bases of cumulus clouds. $\endgroup$
    – jamesqf
    May 14 '15 at 18:41
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    $\begingroup$ I believe the precise term is "sailplane" which means a kind of glider specialized for soaring. Current glider records are over 56h for duration and 2200km for distance. So I'd say we're pretty damn good at eagle flying technique. $\endgroup$
    – Agent_L
    May 14 '15 at 21:30
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    $\begingroup$ @jaromrax - yes, it is astonishing that you should be able to detect these things - but it is astonishing how much signal can be extracted from the scatter off aerosol particles. I was once involved in a smoke detector program that used time resolved LIDAR. When it was first tested, it "failed" because it detected smoke before the fire was lit. But it turns out it detected the trace of smoke left in the air from the previous test... we just needed to tweak the sensitivity (down)! Your magic glasses may be closer than you think. Until then, use eagles. $\endgroup$
    – Floris
    May 15 '15 at 17:08

Eagles (and most large birds) fly by soaring; it's much more energy efficient than flapping their wings. We do use the technique for our own flights.

The reason eagles and other soaring birds do this rather than flap is that they generally hunt from the air and so spend a lot of time waiting for prey. They don't need to go anywhere specific and don't need to move fast until they spot prey. Until then they want to use as little energy as possible.

  • $\begingroup$ Ok, but an explanation in evolutionary terms would be preferable. The reason eagles soar is not because "they hunt from the air", etc. but because soaring presents an evolutionary advantage over flapping, in this case. $\endgroup$
    – jub0bs
    May 14 '15 at 14:29
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    $\begingroup$ This doesn't answer the question, and indeed doesn't seem to contain any physics. $\endgroup$
    – Kyle Oman
    May 14 '15 at 14:46
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    $\begingroup$ @Jubobs: It's not that soaring is different from flapping. All birds flap their wings to put energy into their flight, and all birds glide when they don't need any additional height or speed. All birds are able to soar, which is just gliding in updrafts. Some soar more than others, depending on what they need to do. $\endgroup$ May 14 '15 at 17:24

Every fixed-wing aircraft has a (fairly slow, just above stall) speed at which it minimizes its descent rate while gliding, so that speed is good for loitering. If it can also find an updraft, it can stay up indefinitely without any power.

But if the aircraft needs high speed, that also means high drag, so it needs to expend power. In an aircraft, that means it needs to generate thrust. In a bird, it means flapping wings.

You can try it yourself. Make a paper airplane with extra-wide wings. (Put a paper clip near the nose so it flies stably, not in a scallop-pattern.) Then turn up the back edge so it flies slowly at a high angle of attack. You should be able to get it to float along lazily while coming down quite slowly. Then create an updraft by having a fan blow up underneath it, and it should go up.


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