Aeroplane Construction and Operation

by John B. Rathbun

The dihedral would be very effective in still air, but in turbulent air, and with the body swinging back and forth, the dihedral would act in the nature of a circular guiding path, and thus tend to allow the swinging to persist or increase rather than to damp it down, as would be the case with level straight wings. Again, with the wing bent up at a considerable angle, a side gust as at (S) would tend to throw the machine still further over, and thus increase rather than diminish the difficulty. In practical machines, the dihedral is usually made very small (d = 176 degrees), the angle of each wing with the horizontal being about 2 degrees, or even less. I think the advantage of such a small angle is rather more imaginary than actual, and at present the greater number of war machines have no dihedral at all. In the older monoplanes the angle was very pronounced.

Fig. 15 shows the dihedral applied both to the upper wing (U) and the lower wing (L), the usual method of applying dihedral to large biplanes. Fig. 16 shows the method of applying the dihedral to small, fast machines, such as speed scouts, the dihedral in this case being used only on the lower wing. The dihedral on the bottom wing is usually for the purpose of clearing the wing tips when turning on the ground rather than for stability. A lower wing with a dihedral is less likely to strike the ground or to become fouled when it encounters a side gust in landing or "getting off." The use of straight upper wings makes the construction much simpler, especially on the small machines where it is possible to make the wing in one continuous length.

Ailerons and Wing Warping. Since the dihedral is not effective in producing lateral stability, some other method must be used that is powerful enough to overcome both the upsetting movements and the lateral oscillations caused by the pendulum effect of a low center of gravity. In the ordinary type of aeroplane this righting effect is performed by movable surfaces that increase the lift on the lower wing tip, and decrease the lift on the high side. In Some cases the lateral control surfaces are separate from the wing proper (Ailerons), and in some the tip of the wing is twisted or "warped" so as to produce the same effect. These control surfaces may be operated manually by the pilot or by some type of mechanism, such as the gyroscope, although the former is the method most used. The lateral control, or side to side balancing of an aeroplane, can be compared to the side to side balancing of a bicycle in which the unbalance is continually, being corrected by the movement of the handle bars.

Fig. 17 shows the control surfaces or "Ailerons" (A-A’), mounted near the tips, and at the rear edge of the wing W. As shown, they are cut into and hinged to the main wings so that they are free to move up and down through a total angle of about 60 degrees. In a biplane they may be fitted to the upper wing alone or to both top and bottom wings, according to conditions. For simplicity we will consider only the monoplane in the present instance.

In Fig. 18, a front view of the monoplane, the machine is shown "heeled over" so that the wing tip (w) is low. To correct this displacement, the aileron (A) on the low side, is pulled down and the aileron (A') on the opposite end is pulled up. This, of course, increases the lift on the low end (w) and decreases the lift on the high side (w'). The righting forces exerted are shown by L-L’. The increased angles made by A-A’ with the wind stream affects the forces acting on the wings, although in opposite directions, causing a left hand rotation of the whole machine. In Fig. 19, conditions are normal with the machine on an even keel and with both ailerons brought back to a point where they are level with the surface of the wing, or in "neutral." Fig. 20 shows the machine canted in the opposite direction with (w') low and (w) high. This is corrected by bringing down aileron (A') and raising (A), the forces L-L' showing the rotation direction. By alternately raising and lowering the ailerons we can correspondingly raise or lower the wing tips. It should be noted here that in some machines the ailerons are only single acting, that is, the aileron on the low side can be pulled down to increase the lift, but the opposite aileron remains in the plane of the wings, and does not tend to "push down" the high side. Since all of the aileron resistance in a horizontal direction is now confined to the low side, it turns the machine from its path, the high wing swinging about the lower tip with the latter as a pivot. In the double acting control as shown in Figs. 17 to 23, the resistance is nearly equal at both tips and hence there is no tendency to disturb the flight direction. With single acting ailerons, the directional disturbance is corrected with the rudder so that when the aileron is pulled down it is necessary to set the rudder to oppose the turn. On early Wright machines the rudder and lateral controls were interconnected so that the rudder automatically responded to the action of the ailerons.

Fig. 21 is a detailed front elevation of the machine and shows the control wheel (C) and cable connections between the wheel and the ailerons A-A’. When the wheel C is turned in the direction of the arrow K, the aileron A' is pulled down by the flexible cable (i), and a corresponding amount of cable (h) is paid off the wheel to the rising aileron A. Aileron A is pulled up by the connecting cable (e) which is attached to A' at one end and to A at the other. Pulleys (f) and (g) guide the interconnection cable. On turning the wheel in the opposite direction, aileron A is pulled down and A’ is elevated. In flight, especially in rough weather, there is almost continuous operation of the control wheel. Figs. 22 and 23 are sections taken through the wing W and the ailerons, showing the method of hinging and travel. Fig. 22 shows the aileron depressed for raising the wing in the direction of L, while Fig. 23 shows the aileron lifted to lower the wing. In normal flight, with the machine level, the aileron forms a part of the wing outline (in neutral position).

In the original Wright aeroplane, and in the majority of monoplanes, no ailerons are used, the rear of main wing tip being bent down bodily to increase the lift. This is known as "wing warping," and is practically a single acting process since the depressing force on the high tip is seldom as effective as the lift on the low. Warping is not generally used on modern biplanes since it is impossible to maintain a strong rigid structure with flexible tips. The control warping and twisting of such wings soon loosens them up and destroys what remaining strength they may have had.

Figs. 17-23. Showing Use of Ailerons in Maintaining Lateral Balance.

Banking and Turning. In making a sharp turn the outer wing tip must be elevated to prevent slipping sidewise through the effects of the centrifugal force (side slip). This is known as "banking." The faster and sharper the turn, the steeper must be the "bank," or the angle of the wings, until in some cases of "stunt" flying the wings stand almost straight up and down. Should the bank be too steep there will be an equal tendency to slip down, and inwardly, since the end resistance against side slip is very slight. Some aeroplanes assume the correct angle of bank automatically without attention from the pilot since the extra lift due to the rapid motion of the outer tip causes it to rise. On other machines the natural banking effort of the machine is not sufficient, and this must be increased by pulling down the aileron on the outer wing tip. Machines that have a tendency to "over-bank" must have the ailerons applied in the reverse direction so as to depress the outer tip. In cases of under, or overbanking machines it formerly required experience and judgment on the part of the pilot to obtain the correct banking angle. There are now "banking indicators" on the market that show whether the machine is correctly banked or is side-slipping.