Aeroplane Construction and Operation

by John B. Rathbun

Principles of the Aeroplane. In its elementary principles, the aeroplane can be compared with the kite, as both are supported by the impact of a horizontal stream of air. In Diagram 1, the kite surface is indicated by X-X with the relative air stream W-W-W-W moving from left to the right as indicated by the arrow heads. On striking the surface, the air stream is deflected vertically, and in a downward direction, as shown by the streams lines R-R-R-R. The reaction of the air deflection produces the lift shown vertically and upwards by the arrow L. The kite surface is held against the impact of the air stream by the string S so that there is relative motion between the air and the kite, and so that the surface will not be carried along with the air current toward the right. If the kite were allowed to drift with the wind there could be no relative motion between the surface and the air stream, hence the kite would fall as soon as it attained the velocity of the wind. The horizontal force exerted by the wind tending to carry the kite toward the right is indicated by the arrow D and is known as the "drag" or "drift" force. There are thus three forces, the lift (L), the drag (D), and the resultant of the two forces indicated by the string (S). The forces of lift and drag are nearly at right angles to one another. The kite tail T is simply a stabilizing device whose purpose is to maintain a constant angle between the surface and the wind and it performs an almost negligible amount of lift.

A few more words in regard to the "relative velocity" between the surface and wind. In the figure, the kite is assumed as being stationary, while the wind moves from left to right. With a thirty mile per hour wind, the relative air velocity in regard to the surface would be 30 M. P. H. If the air particles are now considered stationary, and if the kite is towed toward the left (opposite to figure) at 30 miles per hour, the relative velocity between the surface and air would still be 30 M. P. H. In other words, the kite may be stationary, or may move in regard to the earth, but its lift is unaffected as long as the relative motion between the surface and air remains constant. The motion between an aeroplane and the earth depends upon the difference of the aeroplane and wind velocities. For example, a aeroplane with a relative speed of 60 miles per hour, flying against a headwind of 30 miles per hour, moves 60-30 = 30 miles per hour in regard to the earth. The same aeroplane flying with the above wind would have a velocity of 60+30=90 miles per hour past a fixed point on the earth's surface, yet in both cases, the relative velocity of the surface in regard to the air would be the same.

Fig. 2 is a diagram of an aeroplane that shows the connection between the kite and aeroplane principles. In this figure, the wing surface of the aeroplane, X-X corresponds to the kite surface X-X. The relative air W-WW-W striking the wing from the left is deflected down along the arrows R-R-R-R and results in an equivalent lift force L, and a drag force D as in the case of the kite. The resultant force required to maintain the relative velocity between the air and wings is indicated by D¹, opposite and equal to the drag force D. The resultant required for overcoming the drag is provided by the screw propeller P instead of the string S shown in Fig. 1. The propeller thrust (D¹) is parallel to the air stream instead of being inclined as in the case of the string, but the total effect is the same since both are "Resultants of the lift and drag." To sustain the aeroplane, the lift (L) must be equal and opposite to the weight shown by M. The fact that M and L are opposite and equal makes it only necessary for the propeller to overcome the horizontal drag, and hence the thrust can be made parallel to the air flow—or nearly so. The aeroplane is provided with a small tail surface (T) that corresponds to the kite tail (T). It maintains the lifting surfaces X-X at a given angle with the air stream. The tail may, or may not aid in supporting the machine, but in modern machines it is common to employ a tail surface that is non-lifting under ordinary conditions of normal flight. The body (B) contains the pilot, motive power, fuel, and such useful load as it may be necessary to carry.

Fig. 3. Caudron Monoplane. Side Elevation.

Fig. 3 shows a Caudron monoplane in side elevation. This view illustrates the application of the principles shown by Fig. 2, except for the vertical rudder at the rear. The latter is used for steering in a horizontal direction. Fig. 4 shows the construction even more clearly since it is a perspective view. The machine is a Morane "Parasol" monoplane with the wing placed over the body. This location of the main lifting surface is for the purpose of improving the view of the pilot and in no way affects the principles just described. The wires shown above the wing are bracing stays. The tail is hinged near the rear so that the angle of the rear portion can be changed (Elevator flaps), and permits the angle of the main wings to be altered in regard to the air stream, thus causing the machine to ascend or descend. The tail also damps out oscillations or vibrations due to irregularities in the air current. The wheels and running gear (Chassis) allow the machine to be run over the ground until the relative air speed is sufficient to support the machine in flight.

Fig. 4. Morane Umbrella Type Monoplane. Courtesy of "Flight."

Multiplanes. In order to support a heavy load, and at the same time have a small compact machine, it is necessary to have more than one "layer" of wing surface. It is evident that the wing length or "span" can be made much less than that of the monoplane surface shown, if the total area could be divided into two or more parts. A machine having its main lifting surface divided into two or more separate sections is known as a "multiplane," this term becoming "Biplane," "Triplane," or "Quadraplane," depending on whether there are two, three or four independent lifting surfaces. There is almost a limitless variety of arrangements possible, but the most common arrangement by far is that of the biplane, in which there are two superposed surfaces as shown by Fig. 5. In this type, the two lifting surfaces are placed over one another with a considerable "gap" or space between. The body is placed between the wings and the tail surfaces and chassis remain the same as in the monoplane. This is known as a "Tractor" biplane since the propeller is in front and pulls the machine along while Fig.6 shows a "Pusher" type biplane in which the propeller is mounted behind the wings and therefore pushes the machine.