incidence of three degrees equals .545 of the pressure at 90 degrees is
too large, being nearly 50 per cent. greater than very recent
experiments of our own with a special pressure testing machine indicate;
(4) that the superposition of the surfaces somewhat reduced the lift per
square foot, as compared with a single surface of equal area.

[Illustration]

In gliding experiments, however, the amount of lift is of less relative
importance than the ratio of lift to drift, as this alone decides the
angle of gliding descent. In a plane the pressure is always
perpendicular to the surface, and the ratio of lift to drift is
therefore the same as that of the cosine to the sine of the angle of
incidence. But in curved surfaces a very remarkable situation is found.
The pressure, instead of being uniformly normal to the chord of the arc,
is usually inclined considerably in front of the perpendicular. The
result is that the lift is greater and the drift less than if the
pressure were normal. While our measurements differ considerably from
those of Lilienthal, Lilienthal was the first to discover this
exceedingly important fact, which is fully set forth in his book, "Bird
Flight the Basis of the Flying Art," but owing to some errors in the
methods he used in making measurements, question was raised by other
investigators not only as to the accuracy of his figures, but even as to
the existence of any tangential force at all. Our experiments confirm
the existence of this force. At Kitty Hawk we spent much time in
measuring the horizontal pressure on our unloaded machine at various
angles of incidence. We found that at 13 degrees the horizontal pressure
was about 23 lbs. This included not only the drift proper, or horizontal
component of the pressure on the side of the surface, but also the head
resistance of the framing as well. The weight of the machine at the time
of this test was about 108 lbs. Now, if the pressure had been normal to
the chord of the surface, the drift proper would have been to the lift
(108 lbs.) as the sine of 13 degrees is to the cosine of 13 degrees, or
(.22 x 108) / .97 = 24+ lbs.; but this slightly exceeds the total pull
of 23 lbs. on our scales. Therefore, it is evident that the average
pressure on the surface, instead of being normal to the chord, was so
far inclined toward the front that all the head resistance of framing
and wires used in the construction was more than overcome. In a wind of
14 miles pe