r hour resistance is by no means a negligible factor, so that
tangential is evidently a force of considerable value. In a higher wind,
which sustained the machine at an angle of 10 degrees, the pull on the
scales was 18 lbs. With the pressure normal to the chord the drift
proper would have been (.17 x 98) / .98 = 17 lbs., so that, although the
higher wind velocity must have caused an increase in the head
resistance, the tangential force still came within one pound of
overcoming it. After our return from Kitty Hawk we began a series of
experiments to accurately determine the amount and direction of the
pressure produced on curved surfaces when acted upon by winds at the
various angles from zero to 90 degrees. These experiments are not yet
concluded, but in general they support Lilienthal in the claim that the
curves give pressures more favorable in amount and direction than
planes; but we find marked differences in the exact values, especially
at angles below 10 degrees. We were unable to obtain direct measurements
of the horizontal pressures of the machine with the operator on board,
but by comparing the distance traveled in gliding with the vertical
fall, it was easily calculated that at a speed of 24 miles per hour the
total horizontal resistance of our machine when bearing the operator,
amounted to 40 lbs., which is equivalent to about 2-1/3 horse-power. It
must not be supposed, however, that a motor developing this power would
be sufficient to drive a man-bearing machine. The extra weight of the
motor would require either a larger machine, higher speed, or a greater
angle of incidence in order to support it, and therefore more power. It
is probable, however, that an engine of six horse-power, weighing 100
lbs., would answer the purpose. Such an engine is entirely practicable.
Indeed, working motors of one-half this weight per horse-power (9 lbs.
per horse-power) have been constructed by several different builders.
Increasing the speed of our machine from 24 to 33 miles per hour
reduced the total horizontal pressure from 40 to about 35 lbs. This was
quite an advantage in gliding, as it made it possible to sail about 15
per cent. further with a given drop. However, it would be of little or
no advantage in reducing the size of the motor in a power-driven
machine, because the lessened thrust would be counterbalanced by the
increased speed per minute. Some years ago Professor Langley called
attention to the great economy o