Fig. 16.—Relative path of air particles past an airplane.
This diagram illustrates the general downward trend of the stratum of air met by the wings.
Lift.—Assuming that we have all the forward motion needed, let us now investigate the lift that results. Experimenters such as the Wrights and others have found out how to get this lift most conveniently. Lift depends upon the four following factors:
1. Area.
2. Density of air.
3. Angle of incidence.
4. Speed of motion.
1. Relation of Area of Wings to Support.—Consider a small wing; suppose it to be held by hand outside a train window in a given attitude, its area being 1 sq. ft. It tends to lift a certain amount, say 5 lb. Now increase its size to 2 sq. ft. and it will lift with 10-lb. force, tending to get away from your grasp. Rule: When only the area of a wing is changed, its lift varies with the area. If, as above mentioned, you can get 5 lb. of lift from each square foot of wing surface, you can by the same sign get 10-lb. of lift from 2 sq. ft. And if you have 500 sq. ft. of surface you can get 2500 lb. of lift.
Fig. 17.—Diagram showing that in fast airplanes wings are small; in slow airplanes wings are large.
(Above) Small wings; speed 115 mi. per hr.; for fighting. One seat.
(Below) Large wings; speed 80 mi. per hr.; for reconnaissance. Two seats.