Such a drift indicator is, of course, useful only when the ground is visible. The pilot knowing the angle between the airplane axis and the line of motion and therefore knowing the deviation between the supposed course and the actual course is able to make corrections and steer the machine in its proper direction. This may be done by altering the “lubber-line” or his compass just enough to offset the side drift of the machine; after which the desired course may be followed by simply keeping to the proper compass bearing. An instrument has been devised wherein the rotation of the drift-indicator telescope simultaneously alters the lubber-line zero. The operator then has merely to take an occasional observation of the apparent drift line of the ground, which observation automatically shifts the lubber-line and navigation proceeds as if there were no side wind blowing whatever. Knowing the angle between the direction of movement and the airplane axis, the pilot may then compute the speed of motion in a manner analogous to the graphical method previously mentioned; or he can make use of a chart for the determination of this speed.
Navigation over Water.—In flying over water the presence of waves is a valuable guide to the aviator, for he knows that these waves extend in a direction normal to the wind. Moreover, he knows that the velocity of the waves bears some relation to the velocity of the wind. In order to estimate the velocity of the waves it is only necessary to know their wave length, that is, the distance between two consecutive wave crests. The rule is that for a wave length of 10 ft. the velocity is 10 miles per hour, and will vary as the square root of this wave length; that is, if the wave length is half, the velocity will be 10 divided by the square root of 2, or 7.1 miles per hour.
CHAPTER VI
THE RIGGING OF AIRPLANES
Object.—The object of this chapter is to teach the elementary principles of correct rigging. It is not expected that the student will become an expert mechanic, but with this treatment as a basis and through practice he will be able to judge whether or not a machine is correctly and safely rigged. In other words, he will not have to depend on someone else’s judgment as to whether panels, wires, controls, struts, etc., of a machine are in good order, but he will be able to observe understandingly that they are. If the engine goes wrong he can land, if the rigging goes wrong he is in great difficulty. Moreover, if the rigging is wrong, speed is lessened and the stability is uncertain.
The first thing to be learned in rigging is a knowledge of the peculiar terms which have come into use in aeronautics defining different parts of the machines. Our present list of terms is derived, partly from French, partly from English, and partly from American terms. Thus different names may refer to the same part.
NOMENCLATURE
1. Tractor.—An airplane that is pulled through the air by a propeller situated in front of the machine, is called a tractor.
2. Pusher.—If the propeller is back of the main lifting planes the machine is called a pusher.
3. Fuselage or Body.—The main body of the airplane in which the pilot sits and to which the landing gear, motor, controls, and sustaining surfaces are fixed. A small body, especially in pusher types of machines, is called a Nacelle.
4. Cockpit.—The openings and space in the fuselage where pilot or observer sits.