A French aerocycle, Fig. 38, a comparatively inexpensive machine, is also useful for practice in balancing and in short, low flights. The French apparatus in question may accordingly be considered an advance, not only over the English machine, even of the type shown in Fig. 39, which has a double control, and is especially designed for the teaching of beginners, but very much over the practice of attempting to actually fly for the first time in a strange machine, as it provides the necessary practice in the handling of the motor and the lateral steering. The machine can make high speed over the ground, but is perfectly safe for the beginner, as it is incapable of rising. Having gone through the stages represented by either of these contrivances, the best course for the learner to follow is to try gliding, taking short glides to attain the ability to quickly meet varying conditions of the atmosphere.
The fact that these glides are of extremely short duration at first need not be discouraging when it is recalled that, after several years of work, the Wright Brothers considered that great progress had been made when, in 1902, they were able to make glides of 26 seconds. During six days of the practice season of that year, they made 375 gliding flights of various distances, most of them comparatively short, but each one of value in familiarizing the glider with the conditions to be met. It is not material whether gliding or manipulation of the control levers is taken up first, as both should be mastered as far as possible before attempting to fly a regular machine.
Use of the Elevating Plane. So many things are necessary to the control of an aeroplane that thinking becomes entirely too slow a process—the aviator must be endowed with something approaching the instinct of the bird; he must be so familiar with his machine and its peculiarities that a large part of the work of controlling it is the result of subconscious movement. The control levers of many machines are so arranged that this subconscious movement on the part of the aviator directly operates the balancing mechanism. There is no time to think. When a machine rises from the ground, facing the wind as it should, its path of flight should be a gradual upward inclination, this being something difficult to accomplish at first, owing to the sensitiveness of the elevating rudder, the tendency almost invariably being to give the latter too great an angle of incidence. At this stage, the maximum velocity of flight has not yet been attained and care must be taken to keep the angle of ascent small. Otherwise, the power of the engine, which may not have reached its maximum, would not be sufficient to cause the machine to ascend an inclined path at the starting speed. If the speed of flight be reduced by the increased resistance at this point, the whole machine will slide back in the air, and if a sudden gust of wind happens to coincide with the attempt to rise at too great an angle, there is danger of it being blown over backward.
Where the machine is just leaving the ground and the elevator has been set at an excessive angle, the rear end of the skids or the tail may slap the ground hard and break off, or they will impose so much resistance upon its movement by scraping over the turf that the machine can not attain its soaring speed. It must be borne in mind, of course, that remarks such as the present can be only of the most general nature, every type of machine having its own peculiarities—in some instances, the extreme opposite of those characterizing similar machines. For example, in the Voisin 1910 type, the very large and powerful light tail tends to lift before the main planes, and if this be not counteracted, the whole machine may turn up on its end. In order to offset this tendency, the elevator must be raised so as to keep sufficient pressure beneath it; the moment of this pressure about the center of gravity must be at least equal to the pressure under the tail planes about the center of gravity of the machine, or the tail will rise unduly in the air. At least that is the theory of it—naturally, only practice with that particular machine would suffice to enable an aviator to familiarize himself with that particular peculiarity. Again, some machines are "tail heavy." But there is great difficulty in even approximating the degree of relative motion, for which reason it has been suggested, under "Accidents and Their Lessons," that a gradometer, or small spirit level, in plain sight of the aviator, should form part of the equipment of every machine. The Wrights long ago adopted the expedient of attaching a strip of ribbon to the elevator to provide an indication of motion relative to the wind.
Aeroplane in Flight. The sensation of motion after the machine leaves the ground is almost imperceptible, and it is likewise extremely difficult to tell at just what moment the aeroplane ceases running on the solid ground and takes to the air. There is a feeling of exhilaration but very little of motion. Whereas 40 miles an hour over the ground, particularly in an automobile, brings with it a lively appreciation of the speed of travel, the same speed in an aeroplane is a very gentle motion when high above the ground. If there be no objects close at hand, with which to compare the speed, the sense of motion is almost entirely lost.
Center of Gravity. The static balance of a machine should be carefully tried before commencing to fly, and particularly that of a biplane of the Wright type, in which the aviator sits behind the engine. When provision is made for carrying a passenger, his seat is placed in the center line of the machine, so that his presence or absence does not materially affect the question of lateral balance. As men are not all of the same weight, in cases in which the aviator only partly balances the engine about the center line, his weight being insufficient for the purpose, extra weights should be placed on the wing tip at the lightest end until the true balance is secured, otherwise a permanent warping, or gauchissement as the French term it, is required at this side in order to keep the machine on an even keel. In other words, the machine will carry what sailors term a port helm where the left side of the machine is lighter than the right, and vice versa, and it will be necessary to keep the rudder over to that side slightly during the entire flight to counteract this tendency.
In aeroplanes fitted with tails, the center of gravity is usually in the vicinity of the trailing edge of the main planes and, of course, should be on the center line of the machine. The center of gravity of the aviator on a monoplane should approximately coincide with that of the machine. If this be not the case, the stabilizers or the elevator must be permanently set to produce longitudinal balance. Much downward set, or the increase of the angle of incidence of the tail, will create undue resistance to flight and should be avoided when possible by bringing the weight farther forward. The center of pressure should coincide with the center of gravity, and balance will result.
Before even ground work is attempted, the position of the center of gravity should be determined in the manner shown in Fig. 40, the approximate location for four types of machines being shown. At what point the machine must be suspended, so that it can tip only frontward and backward and be evenly balanced, is a question that must be answered in order to ascertain the probability of the machine's pitching forward whenever mud, grass, or rough ground is encountered in alighting. If the center of gravity should lie in front of the axles of the ground wheels in a machine of the Farman type, trouble is sure to follow. Always consider the relation of the center of gravity to the wheels, in order that you may gain some idea of the distribution of the weight on the running gear when the machine is tipped forward 10 degrees. If the wheels are not forward far enough there will be trouble in running on the ground. The elevators must correct whatever variance there may be from the correct center of gravity and position of the wheels, and the manipulation of the elevators for that purpose requires skill. If the tail be very heavy, the elevator may not be able to counteract that defect.
Fig. 40. Method of Determining Center of Gravity of Different Types of Machines