FINE POINTS OF CONSTRUCTION
A GREAT many experiments have been made to find whether the flat or curved wings give the best support, and how sharply the curve should be drawn. The wings of birds are curved slightly upward, and in the end, after all the experiments, it has been found that this curve is just the right one. All forms of aëroplanes will fly more swiftly and steadily if the planes be slightly bowed or flexed. After you have built your aëroplane with flat wings it will repay you to replace them with flexed planes, and you will find that the experience in building models will make this construction very simple.
The lighter and more flexible materials, such as bamboo or cane, are best for the curved planes. After you have decided upon the dimensions of the wings cut the pieces for the ends slightly longer than the width of your planes. These pieces may then be bent by steaming them over a kettle of boiling water and bending to the desired curve. When dry they will hold their shape remarkably well. Another plan is to use a flexible strip and pull the ends together by a strong thread or wire until the wood is bowed to just the right curve. A corset steel or whalebone may readily be curved in the same way. It is a common mistake to curve the plane too sharply, when the resistance offered to the air will be greater than that with the flat plane.
A plane two or three feet in width cannot be held in shape merely by curving the end pieces. A series of ribs must be added at equal distances, each having, of course, exactly the same upward curve. The ribs may be fastened to the sides of the planes with small brads or simply with glue or wire. The covering should then be drawn down. A very smooth covering may be made of rice-paper. Cut the sheets the proper size and lay them for a few minutes between moistened cloths. Now stretch the paper carefully over the frame and glue in position. When dry the paper will contract and leave a smooth, taut surface like the head of a drum.
Much depends upon the curve of the plane. A wing whose curve is not a perfect arc of a circle, but which is rounded just back of the front edge and flattened at the rear, will be found to offer the least resistance to the air. The best plan is to study the curves in the aëroplanes or models and imitate them. Different models require different planes. It is a problem which each young aëronaut must work out for himself.
A Model Aëroplane Built from the Drawing (Plate C, Chap. IV).
The question of rudders or guiding planes is very important. It is too much to expect of even the best model that it will fly in an unswerving line. Any simple vertical plane which may be turned from side to side and held in position will act as a rudder. There is great difference of opinion as to the proper size and position of these guiding surfaces. It is argued by some aviators that the rudder should be placed above the plane, where the air is undisturbed, while others believe that the partial vacuum created above the wings in flight makes the propeller ineffective. Still others argue that a rudder placed back of the planes affords a leverage, and is therefore more effective. Try a rudder in each position. It is impossible to lay down a law for all models.
The larger models should be equipped with twin propellers. In building these the greatest care should be taken to have them exactly the same size, weight, and pitch. Twin propellers should, as a rule, be placed at the front of the machine, that is, they should pull and not push the planes. If by any accident the motor of one should fail, the second propeller will continue to keep the aëroplane afloat and break its fall on descending. With the propellers at the stern of the little airship, the failure of one would cause the plane to pitch downward, and the remaining propeller would drive it down to possible disaster.
In winding up the two motors, care should be taken to give both the same number of turns. The aëroplane may be launched by holding a propeller in either hand and releasing simultaneously. The double motor insures a steadier as well as a longer flight. Always turn the propellers in opposite directions. In flying they must spin around either toward each other or away from each other. If they turn the same way they will give the model a torque which no rudder could possibly overcome.
The efficiency of your motor depends more upon its length than its diameter. In constructing the motor-base, especially for the larger models, arrange to have the strands of rubber bands extend the entire length of your aëroplane, and if necessary, project well forward of the front plane. Such a motor in unwinding will exert a more sustained force. The shorter strands of greater diameter will unwind much more quickly and give very short flights.
With a little experience you will soon learn to gauge your motor to the needs of your air-ship. It is, of course, absolutely necessary that the force exerted by the motor should be sufficient to keep your aëroplane in rapid motion, but it is easy to make it too powerful. If it were possible to attach a “governor” to your motor, this would not matter so much. But since this is practically out of the question, the motor itself must be very nicely proportioned to the demand made upon it. You will soon be able to judge between the steady whir of a good motor, and the buzz of a propeller which races. There is a distinct note for each.
The motor is, at present, the great problem of the model aëroplane. The rubber bands are, at best, only a make-shift. It is practically out of the question to get a flight of more than fifteen seconds in this way, so that the distance is limited to a little more than two hundred feet. It is doubtless only a question of time before a much more efficient form of motor will be invented. Very probably, some amateur aviator will be the first to apply a new means of propulsion, which would be an important achievement indeed.
The simplest form of motor after the rubber bands would seem to be some form of metal spring which could be wound up. Long before the days of automobiles, as we now know them, wagons were built with motors of springs, and some surprising runs were obtained. The spring lends itself to many forms of construction, and is not expensive. It will be necessary to control its action in some way, however, to prevent it from racing and running down in almost no time, like the too heavy rubber motors. It might be found interesting to experiment with the spring to be found in the ordinary roller-shade. The weight of these springs is not too great to be carried by a good aëroplane model, which, of course, is a great factor in their favor.
Detail of Rudder and Propeller of Model Built from Drawing (Plate C).
A number of experiments have been made in France to equip aëroplane models with compressed-air motors. The compressed air is carried in a hollow tube in much the same position as the rubber bands. Many believe that the motor problem, for the toy aëroplane will be solved in this way. A number of interesting models have also been equipped with clock-work motors. A small movement, such as may be borrowed from some mechanical toys, will run for a minute or more. What glorious flights would be possible if our models could be kept aloft—say five times as long as at present. When you feel that you thoroughly understand your model, borrow the clock work from some old toy and make the experiment. It is possible to buy motors for model aëroplanes. The smallest of these develops one half horsepower, weighs seven pounds and will run for fifteen minutes.
The best covering for the wings still remains largely an open question. Although your model will make successful flights with almost any kind of covering, you will find that its stability will be increased and the flight lengthened by a little attention to this detail. According to the Wright Brothers, the most successful covering is the one which offers the greatest resistance to the air. The pressure of the air upward under the planes tends to force its way through the meshes of even the finest cloth. The addition of a coat of varnish will prevent this leakage. A light parchment will also be found effective. It will be well to experiment with a variety of coverings.
A very light, serviceable frame may be made for your motor-base by using hollow shafts or sticks. Procure a very thin, light wood, such as is used for veneering, and after cutting it carefully into strips, glue them together to form a hollow shaft about an inch square. Although the shell may be only one sixteenth of an inch thick, the frame will be found strong enough for all practical purposes. A hollow frame of this kind will save several ounces of weight.
The builder of aëroplane models will find a good friend in aluminium. It is strong enough for all purposes of the model air-ship and, even when used freely, adds almost nothing to the weight. The metal costs ninety cents a pound, but it is so light that, at this rate, it will be found a very cheap material. Comparatively thick pieces may be used for braces or for angles, thus making the frame absolutely rigid, while adding but a fraction of an ounce to the weight. The metal, being comparatively soft, is easily worked, and simple castings may be made at little expense.
Many builders of aëroplanes waste time and ingenuity quite unnecessarily in constructing sets of wheels for carrying their models. The time would be better employed in looking to your planes. The amount of friction saved by attaching wheels, even good ones, to your model, is after all very trifling. Should the wheels jam or stick, which is likely to be the case with such small models, they are worse than skids, and besides, add appreciably to the weight. A light skid is better than a clumsy wheel. If your model fails to rise from the ground, the fault is not at all likely to be in the skids, but in the thrust or lifting-surface.
An excellent plan for guiding the flights is to add square frames of soft lead wire to the front or cutting-edge of your front planes. Bend a piece of wire to form three sides of a square, each two or three inches long, and fasten the loose ends to the plane. By bending these up or down, the center of gravity may be altered at a touch. If your model goes askew, you may bend one of these up and the other down, until you get the desired balance.
In actual practice, the soaring- or floating-planes seem to add greater stability to the model and effect to a marked degree the length of the flight. It is difficult to tell exactly why. The planes in passing may create an eddy in the air, a following wave, as it were, which tends to retard the flight, while the floating-plane smoothes this out. In any event, here is an experiment well worth trying.