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History of Aviation - Chapter 3.1

PRINCIPLES OF FLIGHT

Support of an Airplane by Its Wings.-An airplane is supported just as definitely as though on top of a post, and by the same law, namely reaction. If you try to sweep the air downward with a wing held at a slight angle, the air just before it consents to be pushed downward, delivers a momentary reaction which is upward. If you have a bag of air in your hand it exerts no push upward of course; but the minute you give it a quick push downward it resists, due to its inertia, thus delivering an upward "reaction" against your hand.

Whenever you move anything, it reacts an amount just equal to the force that is moving it; if you move a bullet out of a gun, just before starting the bullet reacts and you have "kick." If you should shoot a thousand guns downward, the reaction would be considerable, and for the instant might be sufficient to support heavy weight.

The airplane is a device for pushing downward millions of little, bullets, made out of air and exceedingly small and light. The wing of an airplane sweeps through these bullets, or molecules, of air like a horizontal plow, wedges the particles downward in vast numbers and in a continual stream, making up in amount what is lacking weight; so that as long as the airplane rushes along there are many thousands of cubic feet of air force down beneath its wings, delivering up a reaction that results in complete support for the machine This reaction is just as definite and secure as thou the machine were supported from the ground o wheels, but it disappears entirely when the airplane is at rest. Part of the whir of a training machine as it glides back to earth is made by the air driven, downward from the wings; the same phenomenon may be noticed when a bat flies close to your ears at night, and if you were a few feet below the airplane as it flew, you would feel the rush of air driven downward from its wings (see Fig. 16).

The net result of all the reactive pushes from this air is lift. It may amount to several pounds for every square foot of the wing surface. This is all that need be said about why the air supports an airplane; all you have to remember is that as long as you have the forward sweeping movement, you will have the lift.

The forward movement is absolutely essential, however, and to maintain it requires a lot of horsepower and gasoline. For it is by means of the engine and propeller that this forward movement is maintained. The engine is a device for creating forward movement-the propeller drives the machine ahead in exactly the same way as is the case in a torpedo, or steamboat.

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.
2. Regarding area of wing surface, the pilot does not have to worry in a flight since lie 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 weight4 carrying capacity thus appear to be antagonistic~ and can not both be attained with efficiency, bu4~ only at the expense of enormous power. The incompatibility between high speed and weight carrying keeps the designer busy in efforts toward ~ reconciliation.

3. 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.
4. 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 be gets more speed for the same amount of gasoline. Unfortunately, at an altitude the motor power falls oft', 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 defined as the angle between the wing-chord and the line of flight. The line of flight is the direction 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 1ine 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 degrees 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 degrees with the line of motion it will lift say 1 lb.; if reduced to a 10 degree angle, it will lift less, say 2/3 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 00 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 15degrees, the lift would not increase any more, but would be found to decrease. For this wing, 15degrees 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.

Lift varies as the square of the speed. Thus any increase or decrease of speed results in a great in-crease or decrease of lift. Interdependence of Angle of Incidence and Velocity.-The four factors above mentioned all contribute to the lift; if in an airplane wing each _ factor be given a definite value, the resulting lift is determined according to the formula:

L = KrAV2
where L is lift.
K is a coefficient referring to the angle.
A is the area.
V is the velocity.
r is the density.

Two only of these quantities change materially in flight, the angle and the velocity; the lift itself remains substantially the same under most normal circumstances. The angle always changes simultaneously with the velocity, increasing when the velocity decreases. Thus the drop of lift due to velocity decrease is balanced by gain of lift due angle increase, and the lift remains unchanged w -speed changes.

Speed change then requires that the pilot alter the angle of incidence simultaneously with the. throttle; so there are two things to do, unlike the case of the automobile where only the throttle is altered.

Minimum Speed.-When, in slowing up an airplane, the angle of incidence reaches the 15degree limit, no further decrease of speed is allowable; therefore, the critical angle determines the minimum limit of speed. If for any reason the machine exceeds the 15degree limit, it must speed up to gain support; that is, the pilot has to increase angle and speed simultaneously instead of oppositely.

Efficiency of Airplane Wings.-I said at the beginning of this chapter that the airplane was a device for pushing down an enormous quantity of air. A certain amount of force has to be furnished in order to keep the airplane moving, and this force is furnished by the engine and propeller. The propeller by giving a certain amount of push in a horizontal direction to the airplane wing enables this wing to extract from the air ten or twenty times this amount of push in a vertical direction; that is, the airplane wing will give you 10 lb. or more of lifting in exchange for 1 lb. of push.

The propeller push is necessary to overcome the drift or resistance of the wings to forward motion. It appears then that the airplane wing as it moves through the air has two forces on it, one acting straight up and called "lift," the other acting straight back and called "drift" (see Fig. 19). The lift is several times greater than the drift, and the situation is quite analogous to that of a kite, which rises upward in the air due to its lift but at the same time drifts backward with the wind due to its drift. In the case of the kite the string takes up an angle which just balances the joint effect of the lift and drift.

The efficiency of an airplane wing is indicated the ratio of lift to drift, and for a given lift, efficiency is best, therefore, for small drift. If lift is 1900 lb. and the wing drift 190 lb.,

Lift or weight 1900
Wing efficiency = wing drift = 190 = 10

Factors Determining Best Efficiency.-It without saying that an airplane wing should the best efficiency it can, and there are several w of doing this.

The first relates to the question of angle of incidence; we have already discussed the effect of angle on lift, but when we come to discuss its effect on efficiency we find that there is only one angle at which we can get the best efficiency. This is a -small angle, about 30 to 6~; at this angle the lift is nowhere near as much as it would be at 100 or 150, but the drift is so small compared to the lift that it is found desirable in airplanes to employ these small angles for normal flight. As the angle increases -above this value of maximum efficiency, the efficiency drops off, and when you get up to the stalling angle, the efficiency becomes very low indeed (see Fig. 20).

The second way to get good efficiency is to choose -the shape of the wings properly. For instance, early experimenters tried to get results with flat wings, and failed completely, for the flat wing proved to be very inefficient. When it was observed that birds had curved wings, this principle was applied to early experiments and then for the time man was able to obtain support in a 11 * machine. The fundamental principle of efficiency in wings is that they must be curved, or cambered, -it is sometimes called. This is because as the wind rushes onward it wants to sweep the air downward smoothly and without shock, as can be done only when the wing is curved (see air flow, Fig. 21).

The question of wing curvature is exceedingly important then; we find that the curvature of its upper surface is particularly so. We notice that airplane wings all have a certain thickness in order to enclose the spars and ribs; it is not necessarily a disadvantage for them to be thick, due to the fact that the upper curve of the wing does most of the lifting anyway, and the lower side is relatively unimportant. You can make the lower surface almost flat, without much hurting the effect of the wing, so long as the upper surface remains properly curved. However, the upper surface must be accurately shaped, and is so important that in some machines we find cloth is not relied on to maintain this delicate shape, but thin wood veneer is used (I refer to the front upper part of the wing). In general, then, wings are thick toward the front and taper down to a thin trailing edge.

You may wonder how it was found that the upper surface of the wing was the most important; and I will say that this was one of the interesting discoveries of the early history of aerodynamics. People at first thought that a wing sweeping through the air derived its support entirely from the air which struck the bottom of the wing, and they assumed that if the bottom of the wing were properly shaped, the top did not matter; that is, all the pressure in the air was delivered up against the bottom' surface. But a French experimenter conceived the idea of inserting little pressure gauges in various points around the wing. He found, it true, that there was considerable pressure exert in the air against the bottom of the wing; but found a more surprising fact when he measured the condition above the wing. When he applied h~ gage to the upper surface of the wing, it read backward, that is, showed a vacuum, and a very pronounced one. He found that there was a vacuum sucking the top part of the wing upward twice ~ hard as the pressure underneath was pushing, s that two-thirds of the total lift on this wing was due to vacuum above it (see Fig. 22).

Aspect Ratio.-The third factor in wing efficiency has to do with the plan shape. It was early found that square wings were not much good, and that if you made them wide in span like those of a bird, the efficiency was best (see Fig. 23). Aspect ratio is the term which gives the relation of the span to the fore and aft dimension of the wing, and this relation is usually equal to six or so. The reason why large aspect ratios are advantageous is as follows:

The tips of all wings are inefficient, because they allow the air to slip sideways around the ends, and there is all the trouble of disturbing this air without extracting any considerable lift from it. In a wide- span wing these inefficient wing tips are only a small percentage of the total area, but in a small-span wing they may be an important consideration (see Fig. 24).

Wing Arrangements.-All the foregoing remarks in this chapter have applied only to a single wing. They apply in general to double or triple wings (biplanes and triplanes), but the matter of arranging multiple wings affects the efficiency.

The monoplane with its single layer of wings is the most efficient type of flying machine. We find if we arrange wings into the biplane shape that the presence of the upper wing interferes with the vacuum formed above the lower wing, and the efficiency decreases (see Fig. 22). The same is true of the triple and the quadruplane arrangement. I all we wanted in airplanes was efficiency, we would use mono lanes, but the biplane is pretty popular now in spite of its low efficiency; this is because it can be much more strongly trussed than the mono-plane, and also because of the fact that sufficient area may b secured with less span of wings.

It may b said that the low efficiency of the bi.-. plane can be somewhat relieved by spacing the upper and lower wings at a considerable distance apart; ~ but if they are spaced at a distance much greater ~ than the chord, it requires extra long struts and wires, and the resistance and weight of these will offset the advantage of wider spacing; so that practically biplane-wing efficiency may be taken as 85 per cent. of monoplane efficiency.

It remains to mention the tandem arrangement, used in all airplanes, where the tail is a tandem surface in conjunction with the wings. A surface located in the position of an airplane tail is at a disadvantage and shows low efficiency for flight purposes. This is because the main wings deflect the air downward and when the tail comes along it meets air which has a more or less downward trend, instead of encountering fresh, undisturbed air (see Fig. 16).

Resistance of an Airplane to Motion.-Earlier in this chapter the support of an airplane was explained