Why is it so difficult for us to just replicate a bird instead of the kind of fixed wing aircraft we have?
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The answer is because we can’t. It’s somewhere between incredibly difficult and impossible. If we just built a plane-sized bird it would never get up off the ground, or it would shatter itself into pieces.
There is a general principle in mechanics and bio-mechanics called the square-cubed law. It applies to a lot of things, but in this case we’re referring to muscles and bones. The strength of a rope or the power of a muscle are proportional to their cross-sectional area. Making them thicker makes them stronger, but making them longer doesn’t. Same thing goes for bone.
When you double a 3D object in size, you’re doubling each dimension: height, width, *and* length. That will make each part of the object 4x as thick, but 8x as voluminous… which also means 8x as heavy. The strength scales up with the square of the multiplier, while the weight scales up with the cube of the multiplier.
So if you scale something up in size, the relationship between an object’s mass and its strength-to-weight ratio will go down at roughly a linear rate. Scaling up a bird to something large enough to carry a human would be 10x too heavy to lift itself. You’d have to replace the bones with much thinner, lighter materials, and the ‘muscles’ with much more power-dense actuators. And while we have developed materials and engines that can do that are better than bone and muscle, we haven’t developed ones that are *ten times times* stronger than bone and *ten times times* more powerful than muscles. And that’s just for carrying one person – forget about a vehicle large enough to carry hundreds of people.
So, it’s just mechanically not possible with the materials we have and, above a certain size, not possible with materials likely to exist. We could make toy ornithopters similar in size to birds, but that wouldn’t carry people. And that’s easier said than done because biological muscles are actually very impressive and power dense, and have a very convenient form-factor. We haven’t really developed an artificial muscle that can compete, and electric motors of similar power density are going to be bulky blocks with pullys and cabels rather than a directly embedded elastic muscle.
The closest we have are gliders, which are somewhat similar to birds in flight. These are massive, fragile vehicles that only weigh a few hundred pounds despite having wingspans of over 40 feet. The pilot is easily a third of the weight, and these craft are little more than fiberglass shells and a few cables to control a few aluminum flaps, rudders, and elevators on single hinges. The complexity and weight of including the hinges and larger, sturdier wings for complex wing movements and motors to move them would easily double or triple the weight, and render them unable to fly at all. If we want to make gliders fly, we need to use very power-dense petrochemical fuel and small jet engines, and even then that’s just enough to sustain flight, not to let the glider take off under its own power.
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