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Ornithopters are vehicles that fly by flapping their wings, rather than trough use of a propeller or jet engine. They've been proposed and built many times throughout history, and are a common element of science fiction and fantasy.

Da Vinci's Ornithopter

Leonardo DaVinci's design for an ornithopter

There's at least one good reason why ornithopters have captivated so many: they are incredibly cool. However, while bio-mimicry can be useful for micro UAVs that are attempting to disguise themselves as birds, I'm not sure what advantages they would have over conventional aircraft. Is there any compelling reason, aside from a cool factor, why a civilization would develop and use ornithopters over fixed-wing aircraft?

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    $\begingroup$ Fixed wing aircraft need to generate lift over a fixed wing with linear speed. Birds can flap to generate lift and thus can take off much more vertically. If the huge mechanical issues of ornithopters are somehow solved, this advantage might pay off. $\endgroup$ – Oldcat Jan 30 '15 at 0:37
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    $\begingroup$ One of the big reasons that people nowadays are even looking at flapping-wing machines is their ability to perch. Watch youtube.com/watch?v=2QqTcQ1BxIs to see what might be possible. Imagine a world where people live on the side of steep cliffs, and their flying machines need to be able to "land" on tiny outcroppings of rock. $\endgroup$ – Richiban Jan 30 '15 at 10:39
  • $\begingroup$ Relevant: ted.com/talks/a_robot_that_flies_like_a_bird?language=en $\endgroup$ – Graph Theory Jan 30 '15 at 19:39
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You are so right about the cool factor. :-)

I can think of one reason. OK, two.

First, if you posit a technology whose main mechanical motion is biological, or at least has a gross design not unlike animal muscle/bone arrangements, the reason for using an ornithopter is straightforward: same as birds. Propellers and fixed wings work beautifully for systems engineered with bearings for rotary motion; but when fuel is converted to motion via a flex action, wings make a lot more sense.

Second, and somewhat more subtle: the shaft/bearings/propeller anatomy favors high-speed rotational inputs, while stretching/shrinking muscles are much more efficient at lower speeds of motion. The speed of the original motion generated is roughly proportional to the intensity of the energy source.

My muscles do not burn as hot as gasoline, but I can ride my bike further on a given quantity of food calories than an equivalent amount of gasoline calories, fed into a little bolt-on internal-combustion engine, could take it...

So anyway, if you have low-intensity energy inputs, your technology would probably want to fly an ornithopter.

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  • $\begingroup$ Real-world bio-mimicry doesn't compete with the power of internal combustion and rotors, but maybe your world has a different balance. For example, dune.wikia.com/wiki/Heart_Scallop $\endgroup$ – Foo Bar Jan 30 '15 at 16:29
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Stealth

Flapping does make noise, but significantly less than a propeller or jet. If it's important to you for any reason that you make less noise, you might go with a flapping-wing solution. That could be anything from a species with incredibly sensitive hearing, to large airborne predators that are attracted to sound, or being sneaky during warfare.

Maneuverability

I'm not sure there's any real-world data for this, but presumably a flapping-wing aircraft would be able to go at significantly lower speeds than a fixed-wing craft without stalling. That would give it a smaller turn radius when it needs it, and the ability to land or take off from smaller runways.

Any coptor design though will beat a flapping-wing design for this though, so it's not sufficient by itself to justify a flapping-wing craft.

So where do you use them...?

The most obvious answer to me, based on the above two criteria, would be if you needed to fly in something like mega-jungles (huge trees with wide separations and a canopy overhead). A fixed-wing craft needs decent runways - a flapping wing craft could possibly take off by being pushed off the right tree. And the need for a low speed to avoid hitting things favors flapping-wing as well.

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  • $\begingroup$ Beside the above, wouldn't it be possible to make an onithopter VTOL ? $\endgroup$ – Magic-Mouse Jan 30 '15 at 14:05
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Flapping wing designs are what came first in our history but they didn't work because too high gravity and too thin atmosphere for the weight of the craft.

So in a world with thicker atmosphere and/or lighter gravity and perhaps lighter materials to build them out of, an ornithopter will work and be used until people discover the efficiency of the fixed wing+propeller.

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  • $\begingroup$ Flapping wings seem to work just fine for billions of organisms on this planet! $\endgroup$ – Brian Drummond Jan 30 '15 at 10:47
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    $\begingroup$ @BrianDrummond except we humans don't have the circumstances to imitate them and have them carry significant cargo. a thicker atmosphere and/or lighter gravity will allow that without first making the fixed wing variants $\endgroup$ – ratchet freak Jan 30 '15 at 10:49
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    $\begingroup$ So they would be developed at an earlier stage and become popular. When the fixed wings are designed for speed they are used to make hybrids which take off from perches and lock in place in air. That is what is seen as normal. Note: this atmosphere would better support the megajungles of other answers. $\endgroup$ – kaine Jan 30 '15 at 13:16
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    $\begingroup$ Another thought: if people were significantly smaller, (say, 40 lbs), they would have a much easier time getting an ornithopter to work, thanks to being on the favorable side of the square-cube law. $\endgroup$ – ckersch Jan 30 '15 at 14:44
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Hazardous environments where bearing seals would not last well enough to allow for practical propellers or jets.

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We're looking for long answers that provide some explanation and context. Don't just give a one-line answer; explain why your answer is right, ideally with citations. Answers that don't include explanations may be removed.

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Flapping as the first attempt at mechanical flight makes sense, since that's what people see birds do. If we're talking a society such as Victorian England, if someone had managed to get a flapping flying machine off the ground, that may have been the focus of inventers for a few decades... at least until the principles behind fixed wing lift were fully understood.

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An orni-like technology could make sense for a kind of vehicle which is more a ground vehicle than an aircraft.

I'm thinking about a grasshopper.

You could have a vehicle capable of making great jumps. Then, once airborne, you could use your flapping wings for three purposes : to make your leap longer, to adjust the landing point with extreme precision and to have a soft landing.

Such a thing could be very useful to move fast in an all mountain and jungle uncivilized environment: with one single jump you can go from a mountain top to another, avoiding days of walking in a dangerous valley.

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