Aeroplane Construction and Operation

by John B. Rathbun

A Spanish Aeroplane Using a Peculiar Form of Upper Fin. These Fins Also Perform the Duty of Vertical Rudders as Well as Acting as Stabilizers.

The longitudinal stability decreases with a decrease in the speed, the fore and aft vibrations becoming more rapid due to the decreased effect of the tail surfaces, and to the reduction of wing lift. Instability at low speeds is common to all aeroplanes, whether inherently stable or not, and at a certain critical speed the machine becomes absolutely unstable in a dynamic sense. If a machine is to be stable at low speeds, it must not fly at too great an angle of incidence at these speeds, and it should have a very large tail surface acting at a considerable distance from the wings. Hunsaker states that the lowest speed should not require more than 80 per cent of the total lift possible.

Inertia or Flywheel Effect. The principal weights should be concentrated as nearly as possible at the center of gravity. Weights placed at extreme outer positions, as at the wing tips, or far ahead of the wings, tend to maintain oscillations by virtue of their flywheel effect. The measure of this inertia or flywheelage is known as the "Moment of Inertia" and is the sum of the products of all the masses by the squares of their distances from the center of gravity. A great amount of inertia must be met by a large damping surface or control area if the vibrations are to be damped out in a given time. In twin-motored aeroplanes the motors should be kept as close to the body as the propellers will permit.

Wind Gusts and Speed. A machine flying at high speed is less affected by wind gusts or variations in density than a slow machine, since the disturbing currents are a smaller percentage of the total speed. In addition, a high speed results in smaller stresses due to the gusts.

Gyroscopic Instability. The motor gyroscopic forces do not affect the stability of a machine to any great extent, and in twin motored aeroplanes the gyroscopic action of the propellers is almost entirely neutralized. At one time the gyroscopic torque was blamed for every form of instability, but on investigation it was found that the practical effect was negligible.

Instability Due Power Plant. The power plant affects stability in a number of ways. The thrust of the propeller may cause a fore and aft moment if the center line of thrust does not pass through the center of resistance. This causes the machine to be held head up, or head down, according to whether the line of thrust is below or above the C. G. If the propeller thrust tends to hold the head up in normal flight, the machine will tend to dive, and assume its normal gliding velocity with the power off, hence this is a condition of stability. With the effect of the thrust neutral, or with the thrust passing through the center of resistance, the machine will not tend to maintain the speed, and hence it is likely to stall unless immediately corrected by the pilot. With the line of thrust above the C. G., the stall effect is still further increased since with this arrangement there is a very decided tendency for the machine to nose up and increase the angle of incidence when the power is cut off.

Steel Elevator and Rudder Construction Used on a European Machine. The Elevators Also Act as Stabilizers, the Entire Surface Turning About the Tube Spar.

The slip stream of the propeller has a very decided effect on the tail surfaces, these being much more effective when the propeller slip stream passes over them. With lifting tails, or tails that normally carry a part of the load, the stoppage of the slip stream decreases the lift of the tail and consequently tends to stall the machine. Non-lifting tails should be arranged so that the slip stream strikes down on the upper surface. This tends to force the tail down, and the head up in normal flight, and when the power ceases the tail will be relieved and there will be an automatic tendency toward diving and increase in speed. On a twin aeroplane, a similar effect is obtained by making the upper tips of both propellers turn inwardly. The air is thus thrown down on the tail.