[Spaceships] Ornithopter Wings no longer TL5+2!

[Cheomesh]

Originally Posted by Anthony

To give an example of why we don't see large fliers, let's look at the performance requirements on a real fairly small plane: a Cessna 172. It has a weight of 2,450 lb and a fueled (empty) weight of 2,028 lb, making it somewhat more massive than a draft horse; figure an animal in that weight range could manage somewhere on the order of 1.2 horsepower.

The Cessna's cruising power requirement is 88 horsepower and its max is 160. Doing some aerodynamics calculations (1,110 kg, 11m wingspan, 16.2 m^2 wing area, 0.027 Cd, cruising at 63m/s at a nominal altitude of 12,000' (using a standard atmosphere, air density 0.85 m^2) it requires at least 59 kW power, or 61 kW power using stream tube analysis as if it were a helicopter. Thus, we're pretty close to the efficiency limits.

Now, on average the wings are supporting 11,000N weight, meaning each one supports half that; however, what we actually care about is torque, meaning we multiply by the distance from the wing attachment point to the support point. For wings with low taper, this averages to slightly under half the wing length, and thus something like 14,000N*m per wing. Mammalian skeletal muscle has a maximum contraction of about 20%, and we can't really have an attachment point for the main muscle that's more than around half way out the wing, so for wings that can be folded with muscle and can flex somewhat above straight, that's an attachment point not more than about 0.4m below the main wing body, meaning we need 35,000N muscle per wing. That's about 0.1m^2, and the attachment point can be a meter long, so it's only about 0.1m^2 thickness, which isn't too bad. However, that's a total of 0.25m^3 of muscle per wing, and at typical muscle density would amount to 530 kg of muscle. With a reasonable margin (at least 2) of extra power for turning and flapping, that's 1,060 kg of muscle, or basically the entire craft mass. That's really not practical. Moving the attachment point up or down the body doesn't really matter, you wind up moving the attachment point on the wing as well, and you get a shorter but thicker muscle with the same overall weight.

For comparison, the standard engine in the Cessna produces a peak of 160 horsepower and weighs 117 kg. Thus, for a competitive ornithopter, ignoring all the problems associated with controlling flapping wings, we want somewhere around 130x the power output of an animal of similar size, and muscles that are about 9x stronger, weight for weight. You probably also want similar increases in bone strength.

Incidentally, if you assume the existence of such modified muscles, and put them on a large flying creature such as a dragon, you wind up with rather scarily fast creatures. If you take a human arm and double all of its dimensions, it winds up being 4x stronger (ST 20) and 8x more massive, it reaches the same total velocity on a swing and requires 2x as long, and thus has 4x the average power output. If we then multiply muscle strength by 9, the result is 36x stronger (ST 60), it reaches 3x the velocity on a swing and thus takes 2/3 as long as a human would, and produces 108x the power. In practice a flier would tend to work on minimizing its landing gear weight, and thus the front legs might be weaker than that, but also lighter, so it would remain extremely fast.

I have to know - how do you figure this?

I feel vastly uneducated.

M.