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The physics of dragon flight is a very interesting topic. There are a
number of problems with the notion of such large creatures taking
flight, with one being the amount of sustainable power required to
maintain flight. Some sample lists of power requirements can be found
here
but the basic idea still remains that even without a rider a dragon
would not be able to sustain itself. For example, a common horse can
barely sustain a power of a horsepower (0.745 kW).
Another issue is the strength of the muscle associated with the wings.
Such issues are not applicable to airplanes because the material used
is quite strong and designed for large loads. However, muscles isn't
very strong compared to it's mass. To determine the strength needed in
the wing, we simply check the torque imposed by flight divided by the
distance between the wing joint and the wing tendon. Given a torque of
50000 ft-lb and muscle strength of 50 psi, we see that the area of the
wing needed must be about 1000 square inches. Given the general size
and shape of the dragon, this does not seem reasonable.