Aeroplane Construction and Operation

by John B. Rathbun

With a single motor, the torque tends to turn the aeroplane in a direction opposite to the rotation of the propeller. Lateral stability is thus interfered with when the motor is cut off or reduced in speed. With right-hand propeller rotation, for example, the machine will be turned toward the left, forcing the left tip down. To maintain a horizontal attitude, the left aileron must be held down by an amount just sufficient to overcome the torque. In some machines one wing tip is given a permanent increase in incidence so that the down seeking tip is given permanent additional lift.

Lateral Stability. When an aeroplane is turned sharply in a horizontal plane, or "Yaws," the outer and faster moving wing tip receives the greater lift, and a lateral rolling moment is produced about the fore and aft axis. In the opposite condition, a lateral rolling moment tends to yaw or to throw the aeroplane off a straight course. Below a certain critical speed, the lateral or rolling oscillations increase in amplitude, with a strong tendency to side slip, skid or spiral. The tail fin or rudder retards the tail velocity in a side slip, and thus turns the slipping or skidding machine into a vertical spiral or spinning nose dive. This spin increases the angle of bank and hence the side slip. This in turn increases the turning or yawing velocity, and the spiral starts. This tendency toward a spiral dive can be corrected by a vertical fin placed forward, and above the center of gravity, or by raising the wing tips. An upper fin of this type will give a force that tends to break up the bank when side slip starts and thus will prevent spinning.

Sperry Gyroscopic Control System for Automatic Stability. The Gyroscopic Control at the Left Controls the Movements of the Electric Servo-Motor at the Extreme Right. This Motor Operates the Control Surfaces Through the Pulley Shown. A Small Electric Generator Between the Servo-Motor and Gyroscope Provides the Current and Is Driven by a Small Wind Propeller.

At normal speeds the rolling is damped down by the wing surfaces, and can be further controlled by the application of the ailerons. At the lower critical speed when the machine is stalled, one wing tip has no more lift than the other, and hence the damping effect of the wings and the action of the ailerons becomes negligible.

Dutch Roll. In "Dutch Roll," the rolling is accompanied by an alternate yawing from right to left. This is aggravated by a fin placed high above the C. G., and hence corrections for spiral dive conflict with corrections for Dutch roll. The rolling is accompanied by some side slip, and the motion is stable providing that there is sufficient fin in the rear and not an excessive amount above the C. G.

Degree of Stability. Excessive stability is dangerous unless the control surfaces are powerful enough to overcome the stable tendency. Since a stable machine always seeks to face the relative wind, it becomes difficult to handle in gusty weather, as it is continually changing its course to meet periodic disturbances. This is aggravated by a high degree of static stability, and may be positively dangerous when landing in windy weather.

Control Surfaces. A non-lifting tail must give no lift when at a zero angle of incidence. It must be symmetrical in section so that equal values of lift are given by equal positive and negative angles of incidence. Square edged, flat surfaces are not desirable because of their great resistance. A double cambered surface is suitable for such controls as the stabilizer, elevator and rudder. It has a low resistance, permits of strong internal spars, and is symmetrical about the line of the chord. Some tails are provided with a cambered top and a flat bottom surface so that the down wash of the wings is neutralized. Under ordinary conditions this would be an unsymmetrical lifting surface, but when properly adapted to the wings the lifting effect is completely neutralized by the down wash.

The curvature of the section should be such that the movement of the center of pressure is as small as possible. With a small movement of the center of pressure, the surface can be accurately balanced and hinged on the center of pressure line. It is desirable to have the maximum thickness of section at, or near to the C. P., so that a deep spar can be used for the support of the hinge system. Usually the movement of the control surfaces is limited to an angle of 30 degrees on either side of the center line, as the lift of all surfaces start to decrease after this point is reached. The surface movement should be limited by the maximum lift angle of the section in any case, since an accident will be bound to occur if they are allowed movement beyond the angle of maximum lift.

In locating the control surfaces, careful attention should be paid to the surrounding air conditions so that they will not be unduly affected by the wash-down of the wings or body. The effectiveness of the tail surfaces is very much reduced by bringing them close to the wings, and the lift is always reduced by the wash of a covered fuselage.