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Many airplanes have parts that are designed to mimic some features of animals. I have also read that birds are incredibly more efficient than most of the airplanes we see today. I wonder, will the future passenger airplanes resemble a big flapping bird? Considering that in the future we might have a lighter and more durable materials for wings.

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  • $\begingroup$ 3 people voted to close this! Amazing. Maybe this site should split into an aeronautical engineering section and those who wish to fix everything with software patches. $\endgroup$ – Robert DiGiovanni Jul 25 at 11:56
  • $\begingroup$ @RobertDiGiovanni as you can see, the reason for closing is that another question already have answers that can answer this question. If you look at the linked question, answers speak in a concise way of ornithopters, square-cube-law, already attempted flapping wing designs, speed consideration... I think this is a dupe because element in answers are already tackled in the other question. $\endgroup$ – Manu H Jul 25 at 14:10
  • $\begingroup$ Yet Aaron's picture is priceless. $\endgroup$ – Robert DiGiovanni Jul 25 at 14:20
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Several things come to mind:

Probably not. A flapping wing is only producing thrust during half its operating cycle (except hummingbirds and insects, who's wings generate thrust in both directions when hovering) so there is energy wasted in the wing's recovery cycle in normal forward flight. It might be better for the bird to keep its wings "fixed" and use another oscillating surface to provide the thrust, like a fish tail, but nature has apparently decided this is not a viable engineering solution for making horizontal thrust in air and in balance, it's better to oscillate the lifting surface.

Efficiency aside, the big show stopper is that a mechanical flapping wing machine, to the extent it can be perfected, will always suffer from the the problem of the body moving vertically in an inertial response to the wings being driven up and down. You would not want to ride is such a machine vary long.

The main difference between the natural world and the man-made one is the man-made one's ability to assemble separate parts and make them operate together to convert energy potential to force without touching (either elements separated by an oil film, or a rolling interface) to produce power in various forms (crankshafts, connecting rods, pistons, turbine rotors, etc). Living beings must produce their thrust from elements that are physically interconnected in a single organism grown from a single egg. This is a huge engineering limitation for mother nature and is why nature couldn't invent the wheel.

So I would say, as an engineering exercise, ornithopters are actually sub-optimal, and nature and evolution produces ornithopters because that's the only design solution available when the "design toolbox" so to speak is limited to what can be done with a single interconnected organism, and to the extent that a man made version can be replicated, nobody would want to spend more than 2 minutes in the thing anyway.

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  • $\begingroup$ The tail is evolutionary lost in modern birds, but ancient birds (Archaeopteryx) had the feathered tail. For some reason it did not evolve into the main thruster like for a fish. Probably flapping wing has some advantages. $\endgroup$ – h22 Jul 25 at 13:57
  • $\begingroup$ Yeah i expect that to work with any efficiency the oscillating tail would need to be nearly as large as the wing, defeating the purpose. $\endgroup$ – John K Jul 25 at 14:01
  • $\begingroup$ Note that the most efficient birds mainly flap to take off, otherwise they use air currents to soar. The albatross even has a locking mechanism so that its wings stay extended without using any energy: blogs.bu.edu/bioaerial2012/2012/11/17/flying-without-flapping $\endgroup$ – jamesqf Jul 25 at 17:45
  • $\begingroup$ Get the book called The Miracle of Flight by Stephen Dalton. He takes you on a journey through the Reynolds number scale from insects to birds to aircraft. Written for the layperson, with terrific photography. $\endgroup$ – John K Jul 25 at 19:29
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It is amazing how ingrained into the psyche of many aviation enthusiasts, even today, is the issue of lifting the plane "as birds" when the issue of propulsion against drag is predominant as speed increases.

So, reduction in drag becomes the key issue. Indeed, a flapping wing is perfect for a relatively slow, lightly wing loaded bird as it "moves a lot of air a little at a time" much more efficiently than a duck holding its wings straight out and paddling franticly with its webbed feet (which would work fine in higher density water). But a bird can flap its wings due to much lower wing loading, which allows to lift and thrust with the same device, and lower airspeed, which allows for a much lighter structure from relatively lower aerodynamic stresses. Early biplanes with very large propellers took a similar approach until...

Increasing speed placed emphasis on the need for increased thrust. The higher wing loading of larger aircraft require higher speed to fly. So, at the expense of efficiency and weight savings, aircraft trend towards greater but less efficient thrust, partially offset by the ability to fly higher, giving a higher True Air Speed.

But recently, with the incorporation of high by-pass fans (and higher fuel prices), designers are returning to the old wisdom that birds employ so well.

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In short, the answer is No. For many centuries, many proto-aeronautical engineers believed that the secret to manned flight would be found in complex flying machines that mimicked the flapping techniques of birds. These ornithopter designs were doomed from the start. Ornithopter

Aside from the obvious issues involving strong, rigid, durable, and light materials, there are tremendously complex psycho-muscular factors involved in biological flight mechanics that are impossible to duplicate with machinery. A bird has the benefit of nerve endings all over its body which provide immediate and intimate feedback of every minute shift and change. Additionally, the bird has fine motor control over its flight surfaces to a much greater degree than any aircraft can realistically replicate, and all at the speed of thought.

The good news is, efficiency can be relative. While the deft flight of a bird can be graceful, beautiful, and evolutionarily effective for its avian lifestyle, it will certainly never accomplish a trans-Pacific trip at 500 knots. Mankind has learned a lot from watching our feathered neighbors, but ultimately, our journey among the clouds lies along a different path from theirs.

That said, some small drone applications show promising possibilities for mimicking insect wings, so mother nature may still have a few more tricks to teach us.

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  • $\begingroup$ I strongly disagree with the second paragraph. If we assume lighter more durable materials, we can also safely assume use of multiple integrated sensors and neural network computing to control the aircraft. No biggie with current thechnology. I do, however, agree that it is not, and will not be more efficient to flap the wings of an aircraft. It is best to separate lift, propulsion and control into separate entities. In future, control may be integrated into wings (no separate control surfaces), but thats still decades away from being commercially viable. $\endgroup$ – Jpe61 Jul 25 at 8:18
  • $\begingroup$ @Jpe61: I’m sorry that you disagree, but every square inch of skin can contain over 1,000 separate nerve endings, each sending entirely unique information which is then processed and responded to in a fraction of a second. The idea of mechanically replicating the complexity of a biological creature to this degree is pure science fiction speculation at best. We can barely get robots to achieve bipedal ambulation on a flat surface due to the complexity involved - a task which many biological creatures handle with ease. Introduce uneven terrain, and suddenly most robots fall flat. $\endgroup$ – Aaron Holmes Jul 25 at 12:40
  • $\begingroup$ Our brains do not continuously process data from every nerv ending in our body. There are special filters, so to say, that block most of the data from ever reaching even the reflex level of our nervous system. Imagine the processing power needed to fully monitor everything our senses are subjected to; simply impossible. As for bipedal ambulation, or, as we common ppl say, walking, pls check out Boston Dynamics, namely their two-legged (or bipedal, as the more civilized would say🤭) Atlas. Pretty fancy stuff he does for a robot. $\endgroup$ – Jpe61 Jul 25 at 16:12
  • $\begingroup$ @Jpe61: I never used the word brain. I said that the information from the nerves was processed and responded to. There is not even an argument to be made here. I know of Boston Dynamics work, I’ve watched videos of Atlas wipe out on simple obstacles dozens of times, even as his own creators explain the tremendous difficulty of getting a machine to accomplish what animals can easily do. There are no humanoid robots doing parkour. No humanoid robots climbing El Cap. How much more difficult for avian flight? Biology excels at adaptation. Programming doesn’t even come close. $\endgroup$ – Aaron Holmes Jul 25 at 17:26
  • $\begingroup$ @Jpe61: Snicker at the scientific terminology all you like. Walking is unspecific. Cats walk. Ants and spiders and centipedes walk. “People-style-walking”, or bipedal ambulation, immediately let’s you know what category of locomotion we are talking about. I’m not trying to rain on your sci fi parade - just pointing out that billions of years of development has led to extremely sophisticated methods of movement that far outshine our best attempts to replicate them. $\endgroup$ – Aaron Holmes Jul 25 at 17:39
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Likely no but let's see

The relevant research study can be found here. In this study, it was shown that for some insects, the induced power can be twice the ideal value in hover, and can be even higher than this in forward flight. This is worse than possible with the fixed wing.

From the other side, the same study says that flapping wings do not show classical abrupt stall characteristics at high angles of attack. This is due to the leading-edge vortex on the top surface of the flapping wing. Thus, the flapping wing achieves significantly higher lift coefficient at high angles of attack and may be more effective, even if less efficient.

Flapping wings can provide hovering as well when fixed wing cannot.

Also, propeller powered insects do not exist so it is not clear how the control experiment is done. If versus aircraft, the size difference may matter. A 3 mm propeller or jet engine for a mosquito insect may not be a more efficient solution.

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