Aeroplane Construction and Operation

by John B. Rathbun

The wings must be separated by a sufficient distance so that the rear set will not be greatly influenced by the downward trend of the air caused by the leading wings. As the rear surfaces always work on disturbed air they should be changed in angle, increased in area, or be equipped with a different wing curvature if they are to carry an equal proportion of the load. Usually, however, the areas of the front and rear wings are equal, and the difference in lift is made by changes in the wing form or angle at which they are set. In some cases the wings are approximately the same, the difference in lift being compensated for by moving the load further forward, thus throwing more of the weight on the front wings.

Fig. 8. Langley’s "Aerodrome," An Early Type of Tandem Monoplane.

The Aeroplane in Flight. Up to the present we have only considered horizontal flight at a continuous speed. In actual flight the altitude is frequently varied and the speed is changed to meet different conditions. Again, the load is not an absolutely constant quantity owing to variations in the weight of passengers, and variations in the weight of fuel, the weight of the latter diminishing directly with the length of the time of flight. To meet these variations, the lift of the wings must be altered to suit the loading and speed—generally by altering the angle of the wings made with the line of flight.

Fig.9 shows an aeroplane in horizontal flight and lightly loaded, the machine traveling along the horizontal flight path F-F. With the light load, the angle made by the wings with the flight path is shown by (i), the tail and body remaining horizontal, or parallel to the flight path. With an increased load it is necessary to increase the angle of the wings with the flight line, since within certain limits the lift increases with an increase in the angle of incidence (i). Fig. 10 shows the adjustment for a heavier load (W₂), the angle of incidence being increased to (i'), and the body is turned down through a corresponding angle. The increased angle is obtained by turning the elevator flaps (T) up, thus causing a downward force (t) on the tail. The force (t) acts through the body as a lever arm, and turns the machine into its new position. It will be noted that when the angle of incidence is great that the rear of the body drags down and causes a heavy resistance. This position of a dragging tail is known to the French as flying "Cabré." With high angles cabré flight is dangerous, for should the propeller thrust cease for an instant the machine would be likely to "tail dive" before the pilot could regain control. This sort of flight is also wasteful of power. Cabré flight is unnecessary in a variable incidence machine, the wing being turned to the required angle independently of the body, so that the body follows the flight line in a horizontal position, no matter what the angle of incidence may be. In this type of machine the wings are pivoted to the body, and are operated by some form of manual control.

Figs. 9-10-11-12. Showing the Use of Elevators in Changing Angle of Incidence. - Machine Shown in Four Principal Attitudes of Flight. As the Body and Wings Are in a Single Unit, the Body Must Be Turned for Each Different Wing Angle.

In Fig. 11, the large angle (i’) is still maintained, but the load is reduced to the value given in Fig. 9. With an equal load, an increased angle of incidence causes the machine to climb, as along the new flight line f-f. With the load (W) equal to that in Fig. 9, the angle of incidence will still be (i) but this will be along a new flight line if the large angle (i’) is maintained with the horizontal as shown by Fig. 11. With the wings making an angle of (i') with the horizontal, and angle of incidence (i) with the flight line, it is evident from Fig. 11 that the new flight line f-f must make an angle (c) with the original horizontal flight line F-F. This shows how an increased angle with a constant load causes climbing, providing, of course, that the speed and power are maintained. With a given wing and load there is a definite angle of incidence if the speed is kept constant. Should a load be dropped, such as a bomb, with the wing angle kept constant, the new path of travel will be changed from F-F to f-f.

Fig. 12 shows the condition when the rear end of the body is elevated by depressing the elevator flap T. This occasions an upward tail force that turns the wings down through the total angle (i'). With the former loading and speed, the angle of incidence is still (i) degrees with the new flight path f-f, the new flight path being at an angle (c) with the horizontal F-F. The body is turned through angle (i'), but the angle (i) with the flight path f-f is still constant with equal loads and speeds.

To cause an aeroplane to climb, or to carry a heavier load, the elevator "flap" is pointed up. To descend, or care for a lighter load, the elevator is turned down. In normal horizontal flight the machine should be balanced so that the tail is horizontal and thus creates no drag. When the elevator must be used to keep the tail up in horizontal flight, the machine is said to be "tail heavy."