Learning to fly in the U.S. Army

by E. N. Fales

Regarding area of wing surface, the pilot does not have to worry in a flight since he can do nothing to change it anyway. All he needs to know is that in different airplanes small wing area accompanies high speed and small weight-carrying capacity, as in the case of the Fokker and Sopwith speed scouts (see Fig. [17]). Conversely, large wing areas are used for heavy load carrying and slow speed (see Fig. [18]). Speed and weight-carrying capacity thus appear to be antagonistic and can not both be attained with efficiency, but only at the expense of enormous power. The incompatibility between high speed and weight carrying keeps the designer busy in efforts toward a reconciliation.

Fig. 18.—Diagram showing use of large wings for heavy airplanes, and small wings for light airplanes.

2. Density.—The second factor affecting the lift is the character of the air itself. I refer to the density of the air. The heavier each particle of air becomes, the more reaction it can furnish to the wing that drives it downward; so on days when the barometer is high the wing will lift more than on other days. Now the air is heaviest, or most dense, right near the ground; because in supporting the 50 miles or so of air above it, it becomes compressed and has more weight per cubic foot. Therefore, the wing gets more lift at a low altitude than at a high. Some airplanes will fly when low down but won’t fly at all high up. In Mexico, for instance, when the punitive expedition started out they were already at an altitude of several thousand feet above sea level. The airplanes had been built for use at places like New York and England, close to sea level, and when our army officers tried to fly with them in Mexico, they would not fly properly, and the factory had to redesign them.

Regarding density, the pilot should know that for a low density he should theoretically get a high speed. As density decreases, high up in the air, the speed tends to increase, and moreover he gets more speed for the same amount of gasoline. Unfortunately, at an altitude the motor power falls off, so that nowadays the speed is not faster high up than low down; but when the motor builders succeed in designing their motors to give the same horsepower at 20,000 ft. as they do on the ground, airplanes will be able to reach terrific speed by doing their work above the clouds.

It is found desirable to give large wings to airplanes which are going to fly at high altitudes, so as to offset the lack of density by an increase in area, thus leaving the angle range—that is, the speed range—as large as possible. The army airplanes in Mexico mentioned above were simply given a new set of larger wings to offset the lower air density in Mexico, and thereafter flew better.

3. Angle of Incidence.—The angle of incidence is defined as the angle between the wing-chord and the line of flight. The line of flight is the direction of motion of the airplane, and is distinct from the axis of the airplane which corresponds with the line of flight only for a single angle of incidence. If the line of flight is horizontal, the airplane may be flying tail-high, tail-level, or tail-low; that is, its axis may have varying positions for a given line of flight. This is true, if the line of flight is inclined, as in climbing. It is a mistake to confuse the line of flight with the axis of the machine.

The angle of incidence of the wings of the U. S. training machine may have any value from 15° down. When the angle is smaller the lift of the wings is smaller. Consider the model wing held out of a train window; if its front edge is tilted up to an angle of 15° with the line of motion it will lift say 1 lb.; if reduced to a 10° angle, it will lift less, say ⅔ lb. A model of the training-machine wing could be tilted down to an angle several degrees less than zero before its lift disappeared, because it is a curved, not a flat wing; this angle would be the “neutral-lift” angle; notice then that O° is not a neutral-lift angle, and therefore may be used in flight.

If the model wing were tilted up to an angle greater than 15°, the lift would not increase any more, but would be found to decrease. For this wing, 15° is called the critical, or “Stalling” angle, beyond which it is unwise to go.

4. Velocity.—If the model wing which is imagined to be held out of the car window, is held now in a fixed position at a given angle of incidence, any change of the train’s speed will result in a change of lift; should the speed rise from 30 miles per hour to double this value, the lift would increase enormously, fourfold in fact.