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Birds use their strong breast muscles to flap their wings and give them the thrust to move throught the air and fly. In a way, birds use a swimming motion to get the lift needed to fly. Plane wings have a similar shape as bird wings, but instead of flapping their wings, we use engines to thrust them into the air and create the lift needed to fly. (Source)

Can we design an aircraft which could also "flap" its wings to fly? Yes, we would need fuel to anyways do that. But, from a design point of view, is it possible to do that?

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    $\begingroup$ They are called ornithopters $\endgroup$
    – ratchet freak
    Commented Jul 22, 2015 at 15:32
  • $\begingroup$ Does not seem to be much point? $\endgroup$
    – Martin James
    Commented Jul 22, 2015 at 15:33
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    $\begingroup$ It's a good evolutionary solution, needing only, er, evolution rather than revolution to go from limbs to flapping feathered limbs. The main problem with fixed wings would be a source of propulsion. A bird would have to eat a whole lot of chilli beans to generate that kind of thrust. $\endgroup$
    – Simon
    Commented Jul 22, 2015 at 16:41
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    $\begingroup$ IIRC, there is no biomechanical equivalent of a rotating shaft and bearing, (so far). $\endgroup$
    – Martin James
    Commented Jul 22, 2015 at 17:47
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    $\begingroup$ @MartinJames I was thinking more along the lines of jet propulsion. No moving parts required - just a good supply of beans. $\endgroup$
    – Simon
    Commented Jul 22, 2015 at 17:56

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From a biological point of view, flapping wings is a viable means of flight. However, this presents some problems when scaled up for human flight.

One issue is the square-cube law: as the wings are scaled up, the area scales as a square (relating to lift), but the volume scales as a cube (relating to weight). This means the wings increase in weight faster than they increase in lift, resulting in less effective wings. The higher weight presents issues in making them flap.

ratchet freak pointed out that we do have such machines, and they are called ornithopters. Although there is some potential at smaller sizes (for small UAV's), the weight and force issue prevents them from being very useful at larger scales. Ornithopters were some of the first unsuccessful designs for heavier than air flight.

The best solution we have found is propellers. This doesn't work as well as biological propulsion, but it's much easier to spin a prop than flap the whole wing, and works much better for the larger scales we need to move humans. Flapping is also not effective for reaching higher speeds.

You can also see from the biological side that there is a size limitation. The largest birds are much smaller than our airplanes.

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There can be several reasons:

  • Airplanes' wings are huge and heavy. Birds' wings are light.
  • A bird's wings are flexible but an airplane's are not.
  • A mechanism to flap an airplane's wings will be a very complex process.
  • A lot more power will be required to flap airplanes' wings (even if flapping wings are designed successfully) than the current jet engines.

  • The flight will not be as smooth as it is now in airplanes, as a bird constantly moves up and down when it is flapping its wings. This can be seen in this picture of an ornithopte:

    Flapping wings

Did you spot this airplane flying in the evening sky?

An interesting article about this: Why don't airplanes have flapping wings?

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    $\begingroup$ I don't think it should take a lot more power. When you increase the diameter of a propeller you get a greater thrust yield for the same power. If you think of a wing as huge propeller, then you can see how it would be able to generate highly efficient thrust. $\endgroup$
    – Daniel Caoili
    Commented Jun 16, 2019 at 12:53
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Yeah! It is possible to build an aircraft that flaps its wings to fly, but the fuel efficiency, complexity of design and cost required for that flight would be a burden to implement. This has been attempted in the past, but virtually all designs have failed, mostly for mechanical reasons.

The commercial aircraft and their working principles are a far more satisfactory design to carry payload in a cost-effective manner. An ornithopter design, were it successful, would require more energy and mechanical complexity to carry even a small payload.

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