272 
ZAHM. 
The effect of friction is very manifest. Owing to it, the 
power reaches a minimum at about forty miles an hour. The 
mileage cost attains its least value at about fifty miles an 
hour and at an angle of less than three degrees. This latter 
relation is more clearly shown in figure 9, where the soaring 
angle and resistance are coordinated. The drift curve is 
nearly a straight line for the small range of angles plotted, 
but later turns rapidly upward, becoming infinity and verti¬ 
cal at an angle of ninety degrees. The friction curve begins 
at infinity, falls rapidly, and becomes zero at a soaring angle 
Fig. 9. —Soaring Angle and Computed Resistances for a Foot Square Plane 
Weighing One Pound. 
of ninety degrees. The total resistance is asymptotic to the 
others, and has its minimum at about two and a half de¬ 
grees. This angle and the corresponding speed are, there¬ 
fore, the most economical for a thin foot-square soaring plane 
weighing one pound. 
It will be observed in the last column that the maximum 
weight carried per tow-line horse-power is scarcely ninety 
pounds. This is very small, but may be increased in several 
ways: by lightening the load and letting the plane soar at 
a lower speed; by arching the surface like a vulture’s wing; 
by changing the foot-square plane to a rectangle and towing 
