Learning to fly in the U.S. Army

by E. N. Fales

Cause of Resistance.—Wing resistance, which is affected, as mentioned previously, by the wing curvature, can not be decreased unless new and improved sorts of wings are invented. As to deadhead resistance, it may be decreased in future by methods of construction which eliminate unessential parts. In a high-speed airplane in this country an attempt was made to eliminate the wires altogether and most of the struts (because the wiring is one of the largest single items of deadhead resistance); so far the attempt has failed for structural reasons. In the monoplane type of airplane of course the struts are eliminated, which is an advantage from the standpoint of resistance.

Fig. 25.—Diagram illustrating advantage of stream-line shape.

Note large eddy disturbance and vacuum behind round shape, causing high resistance.

As long as struts, wires, etc., are used at all, the minimum resistance can be secured by giving them a proper “stream-line” shape. The stream-line shape is one in which the thickest part is in front and tapers off to a point in the rear, like a fish. If, for instance, we take round rods instead of the struts of the training machine above mentioned and having the same thickness, the resistance might be 80 lb. instead of 20 lb.; and if we take a rod whose shape is elliptical with its axes in a ratio of 1 to 5 the resistance might be 40 lb. instead of 20 lb.; and if we took the stream-line struts out of the training machine and put them back sharp edge foremost, the resistance would be increased. The advantage of the stream-line shape is that it provides smooth lines of flow for the air which has been thrust aside at the front to flow back again without eddies to the rear. This is not possible in the case of the round strut, behind which will be found a whirl of eddies resulting in a vacuum that tends to suck it backward. By fastening a stream-line tail behind the round rod the eddies are greatly reduced, as is the vacuum. The wires of the airplane are subject to the same law and if the training machine above mentioned had stream-line wires instead of round wires we might expect them to have less than 70 lb. resistance. The fuselage should always be given as nearly a stream-line shape as the presence of the motor and tanks will permit; and it must all be inclosed smoothly in “doped” fabric in order that the air-flow phenomena may operate. As for the wheels, they must of necessity be round, but by enclosing them with fabric the air flow past them is more easy and the resistance may be halved.

Total Resistance.—The necessity has been explained of discriminating between wing and deadhead resistance; if we are talking about wings we may ignore everything except the wing resistance (commonly called “wing drift”), but if we are talking about the whole airplane, we then must refer to the total resistance, which includes all the others and is overcome by the propeller thrust. “Skin-friction” resistance has not been mentioned nor need it be more than to say that any surface moving through air attributes part of its resistance to the actual friction of the air against it, and therefore should be as smooth as possible.

Motor Power Required for Flying.—The reason resistance interests us is that motor power is required to propel the airplane against it; more and more power as the resistance and speed increase. Obviously, the power required is least when the resistance is small, i.e., when the speed is intermediate between minimum and maximum. It takes more power to fly at minimum speed than at this intermediate speed. Of course it also takes more power to fly at maximum speed, where again the resistance is high.

Maximum Speed.—Ordinarily, for moderate speeds, airplanes have a margin of power at which the throttle need not be opened wide; should speed be increased the resistance and horsepower required will increase steadily until the throttle is wide open and motor “full out;” this establishes the maximum speed of an airplane; there is no margin of power, no climb is possible. The only way to increase speed is to use the force of gravity in addition to the motor force. It may be interesting to know what is the maximum possible speed in the case of a vertical dive with the motor shut off; it will be about double the maximum horizontal speed as may be readily seen from the fact that the thrust in the direction of motion is now no longer horizontal and equal to the resistance but is vertical and equal to the weight of the machine; that is, the thrust may be increased fivefold, and the speed resulting will be increased correspondingly, if the motor be running in such a vertical dive the velocity may be slightly increased though at this speed of motion the propeller would not have much efficiency.

There is danger in such high speeds; the stresses in the machine are increased several times merely by the increased resistance, and if the angle of incidence should be suddenly brought up to a large value at this high speed the stress would again be increased so that the total stress increase theoretically might be as high as fourteen times the normal value, thus exceeding the factor of safety. It is for such reasons that the maximum strength is desirable in airplanes; holes must not be carelessly drilled in the beams but should be located if anywhere midway between the top and bottom edges, where the stress will be least; initial stresses, due to tightness of the wires, should not be too great.

Climbing Ability.—Climbing ability refers to the number of feet of rise per minute or per 10 min. In order to climb, extra horsepower is required beyond that necessary for more horizontal flight. The machine can, for instance, fly at 56 miles per hour at which speed it requires 43 hp. If now the throttle is opened up so as to increase the horsepower by 22, making a total of 65 hp., the machine will climb at the rate of 380 ft. per minute, maintaining approximately the same flight speed. If instead of 65 hp., it were 54 hp. the speed of climb would be about one-half of the 380, or 190 ft. per minute; the flight speed again remaining approximately as before; that is, any margin of horsepower beyond the particular value of horsepower required may be used for climbing without material change of the flight speed. It is necessary here to state that lift does not increase during climb; and while for the instant that a climb commences there may be, due to acceleration, more lift on the wings than balances the weight, this does not remain true after a steady rate of climb is reached. To illustrate, in a wagon drawn uphill by horses the wheels which support the wagon do not exert any more support than on the level, and the entire force to make the wagon ascend is supplied through extra hard pulling by the horses. Thus in a climbing airplane the propeller furnishes all the climbing force and lift is no greater than in horizontal flight. In fact, the actual lift force may be even less, as the weight of the airplane is partly supported by the propeller thrust which is now inclined upward slightly.