Aeroplane Construction and Operation

by John B. Rathbun

In landing in small fields it is necessary to bring the machine to rest as soon as possible, and this stopping distance depends to a great extent upon the attitude of the machine when it first touches the ground. If it is landed so that the chassis wheels and tail skid strike the ground simultaneously, the incidence is so great that the wings act as air brakes. On landing, the angle in any case should be quickly increased past the angle of maximum lift. The lift is much reduced and the drag is increased by quickly pulling the control toward the aviator. This also reduces the tendency toward nosing over.

A normal landing in a large field can be affected by first starting down at the normal gliding angle, and when from twenty to thirty feet above the ground the elevator control is pulled back so that the machine will describe a curve tangent to the ground. In student's practice the curve should not be exactly tangent to the ground, but tangent to a level two or three feet above the ground. The machine is now losing speed, and to prevent settling the elevator should be pulled back a trifle. The speed continues to decrease until it settles down through the small remaining distance with the elevator full back. The points of support should strike simultaneously. It is difficult for the beginner to make this sort of a landing, as there always seems to be an uncontrollable desire to jam the machine down on the ground. If a puff of wind happens to strike the machine when a few feet off, the student becomes rattled by the suddenly increased elevation and jams her down doubly hard.

Wind Flying. The nature of wind at low altitudes is determined to a great extent by the contour of the ground. Eddies are caused by trees, embankments, fences, small hills, etc., which tend to disturb the equilibrium or change the course of the aeroplane. As the altitude increases, the effects of these obstructions are less pronounced, until at from 2000 to 3000 feet the effect is practically negligible. Winds that may be "Bumpy" near the ground are fairly regular when 3000 feet is attained. At the higher altitudes the velocity increases, and if the machine is flying against the wind the progress will naturally be much slower at the higher altitudes. When starting in a strong wind it is advisable to attain an altitude of at least 300 to 400 feet before turning. Turning in with the wind carries the possibility of a drop or stall.

A short gust striking the machine, head on, tends to retard the velocity in regard to the earth, but in reality increases the relative air speed and thus causes the machine to climb momentarily. A prolonged head gust may produce a stall unless corrected by the elevator or met with by reserve power. A rear gust reduces the relative wind velocity and tends to make the machine stall, although there are a few cases where the gust velocity has been great enough to cause a precipitate drop. The higher the speed, the less the danger from rear gusts.

The gusts are much more pronounced with low winds, say winds of about 5 to 15 miles per hour, and hence it is usually more tricky to fly in a wind of this velocity than with a higher wind. It is not the speed of the wind so much as it is its variation from the average velocity. One should start to work on a "bump" at the moment it first starts to appear.

When flying with the wind, the total speed in regard to the earth is the sum of the wind speed and the aeroplane speed. When flying against it is the difference between the aeroplane and air speeds. Thus, if the air speed of the aeroplane is 60 miles per hour, the speed in regard to the ground will be 75 miles per hour with a following wind of 15 miles per hour, and 45 miles per hour when flying against a 15-mile wind. The speed when flying across the wind would be represented by the diagonal of a parallelogram, one side of which represents the aeroplane speed, and the other side the wind speed. The angle of the diagonal is the angle at which the machine must be pointed. When viewed from the ground, an aeroplane in a cross wind appears to fly sideways.

Turning. After the beginner is able to maintain longitudinal and lateral balance on straight away flights, he next attempts turns. At first, the turns must be of great radius. As the radius is gradually shortened, the effects of centrifugal force become greater, increasing the tendency toward skidding or outward side slip. To prevent skidding, the outer wing tip must be raised so that the lift will oppose the centrifugal force. The shorter the turn, and the faster it is made, the greater will be the banking angle. Should the bank be too steep, the gravitational force will pull the machine down, and inwardly in a direction parallel to the wings. This is known as an "Inner side slip." The banking may be performed by the natural banking tendency of the aeroplane or may be assisted by depressing the aileron on the outer wing tip. Unless the speed is well up to normal, the machine will be likely to stall and drop on a turn, as the head resistance is much greater under these conditions. For safety one should take a short downward glide before starting the turn, so that the speed will surely be sufficient to carry it around the turn. A turn should never be attempted when climbing unless one has a great reserve power. The combined effects of the turning resistance, and absorption of energy due to the climb, will be almost certain to stall the machine. There are banking indicators on the market which will prove of great service. These operate on the pendulum principle and indicate graphically whether the aeroplane is being held at the correct angle of bank.

Proper Flight Speed. An aeroplane should always be provided with an air speed meter, giving the speed of the machine in relation to the air. When flying with the wind the pilot is likely to be confused by the tremendous ground speed at which his machine is flying. While the machine may be moved at a fast clip in regard to the earth, it may be really near the stalling speed. This error is particularly dangerous when one turns in with the direction of the wind, after flying against it for some time. The sudden increase in the earth speed, when fully in with the wind, always creates a sudden desire to throttle down at the very time when the relative air speed has already been greatly reduced by the turn. Stalling due to this cause has resulted in many accidents, and the beginner should always attain an altitude of a least 500 feet before he tries turning in with a strong wind. An accurate speed indicator eliminates this danger to a great extent, but it should be proved that the instrument itself is accurate before too much reliance is placed on it.

Before the advent of the indicator, pilots were compelled to estimate the speed by the sense of feel, some depending upon the feel of the wind pressure on their faces, and others by the relative resistance offered to the movement of the control surfaces. The sense of "Air feel" developed by the late Lincoln Beachey was marvelous, for without instruments he would repeatedly climb nearly into a stall when only 50 feet from the ground, and then recover with his chassis nearly dragging in the weeds.

Gliding (Fr. Vol Plan). "Gliding" is a descent along an inclined path without power, and is possible with any aeroplane. By suitably inclining the wings with the horizontal, gravity is made to produce a forward propelling component that moves the machine forward at the expense of a loss in altitude. The angle of the gliding path made with the horizontal is known as the "Gliding angle," and indicates the efficiency of the aeroplane, for with machines having very low head resistance the angle is very "flat," and more nearly approaches the horizontal. The best or flattest gliding angle is an inherent feature of the aeroplane design, and this cannot be exceeded by any effort on the part of the pilot. It is generally expressed in terms of the ratio of the descent to the forward distance traveled, thus a gliding angle of 10 means that the aeroplane travels 10 feet horizontally for every foot of descent. Any angle steeper than the flattest angle can be produced by pushing forward on the elevator controls, thus depressing the elevator tips.