eathers to make it light!

With accurate data for making calculations, and a system of balance
effective in winds as well as in calms, we were now in a position, we
thought, to build a successful power-flyer. The first designs provided
for a total weight of 600 lbs., including the operator and an eight
horse-power motor. But, upon completion, the motor gave more power than
had been estimated, and this allowed 150 lbs. to be added for
strengthening the wings and other parts.

Our tables made the designing of the wings an easy matter, and as
screw-propellers are simply wings traveling in a spiral course, we
anticipated no trouble from this source. We had thought of getting the
theory of the screw-propeller from the marine engineers, and then, by
applying our tables of air-pressures to their formulas, of designing
air-propellers suitable for our purpose. But so far as we could learn,
the marine engineers possessed only empirical formulas, and the exact
action of the screw-propeller, after a century of use, was still very
obscure. As we were not in a position to undertake a long series of
practical experiments to discover a propeller suitable for our machine,
it seemed necessary to obtain such a thorough understanding of the
theory of its reactions as would enable us to design them from
calculations alone. What at first seemed a problem became more complex
the longer we studied it. With the machine moving forward, the air
flying backward, the propellers turning sidewise, and nothing standing
still, it seemed impossible to find a starting-point from which to trace
the various simultaneous reactions. Contemplation of it was confusing.
After long arguments we often found ourselves in the ludicrous position
of each having been converted to the other's side, with no more
agreement than when the discussion began.

[Illustration]

It was not till several months had passed, and every phase of the
problem had been thrashed over and over, that the various reactions
began to untangle themselves. When once a clear understanding had been
obtained there was no difficulty in designing suitable propellers, with
proper diameter, pitch, and area of blade, to meet the requirements of
the flyer. High efficiency in a screw-propeller is not dependent upon
any particular or peculiar shape; and there is no such thing as a "best"
screw. A propeller giving a high dynamic efficiency when used upon one
machine may be almost worthless when used upo