RC Ornithopters are a thing, some are small and lightweight, others have a 3 foot or more wingspan. [https://www.youtube.com/watch?v=Pn5pPy9BX3w](https://www.youtube.com/watch?v=Pn5pPy9BX3w) the video is 10 years old, but a good example of a RC ornithopter. Here is a small newer one [https://www.youtube.com/watch?v=ScLVUCHaYwQ](https://www.youtube.com/watch?v=ScLVUCHaYwQ)
They are both fun to fly, but the smaller one is easier, no set up
The shortest and most succinct answer to your question is because birds are light and humans/cargo are heavy. Flapping bird wings generate much less lift than a fixed wing, enough for a bird but not enough for people or cargo (if you’ve ever held a bird, you know that they are shockingly light). This is completely ignoring efficiency – in which case the fixed wing aircraft wins as well. Basically, fixed wing aircraft are actually better than flapping bird wings for lifting the kinds of things that we want to be able to fly.
Birds have an incredible amount of control over their wing shape. They can clap their wings, they can (partially) close their wings but they can also make minute alterations to the shape of their wrings. Do you know how the hair on your arm stands up when you get goosebumps? It’s kinda like that, each feather is an individual surface and together they make up a wing.
That’s why birds have an incredible amount of control over their flight. Airplanes simply have wings that create lift if they’re moving forward with sufficient thrust and some control surfaces to steer. A bird can flap, close, fold and alter the surface of each individual wing constantly mid flight.
It’s also very energy-consuming. For the most part, we just want our airplanes to move from point A to point B. Even if we had the systems and control technology to do that with birdlike wings, and we don’t, it would be very energy inefficient. We have no need to dodge tree branches or predators. We don’t need to hover our airplane over flowers while we feed.
[We do have simple bird like drones](https://www.youtube.com/watch?v=w6VLzKACnS8) these days that by flapping their wings and they’re already much more agile than fixed-wing drones. But they don’t nearly have the wing control birds have and they consume much more energy than fixed-wing drones.
The main reason birds flap instead of using jets or propellers isn’t because flapping is more efficient (it’s not). It’s because body parts need to be connected to the circulatory system to stay alive, making it very hard for an animal to build an “axle”. An axle is necessary to make a wheel, propeller, or jet turbine. To build such a thing, an animal would have to extrude a living bit of itself into the shape of the spinny bit, then “pinch it off” and allow it to die in place there. (For example, grow a propeller-shaped bone, then disconnect it and let it become dead bone in a propeller shape). It’s hypothetically do-able, but very hard for evolution to “land on” such a solution. There are a few bacteria that can sort of do this – making a flagellum that spins to “whip around” and push the bacterium forward, but it’s never been seen on a larger scale, as far as I know.
As to why we don’t make flapping planes, well it’s not as efficient, and it’s super hard to control the complex mechanism (something an instinctual brain is better at than our computer software, but maybe the day will come when it’s do-able with some kind of machine learning.) About the only advantage it would confer would be maneuverability. Being able to fold a wing any which way would let you do all kinds of fancy stunts. But it would also be a *very uncomfortable ride*. Imagine bobbing up and down on a 3 hour flight.
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