Under perfect condition, in absolutely undisturbed air, an aëroplane may be made to come down so lightly that no bones, even the smallest, will be broken. A gust of wind, however, may ruin all your calculations and bring the aëroplane down with a dislocating shock. The skids must be designed to meet extreme conditions, the worst that can possibly befall. It has been pointed out that these skids or supports should be high enough to give the propeller clearance so that the propeller blades will not touch the ground. By using a light flexible cane for the purpose, and bending them under, a spring may be formed which will take up the shock of a violent landing. Some builders go further and rig up the skids with braces of rubber bands to increase this cushion effect. A variety of constructions are shown in the photographs of the various models. Your skids should enable your model to withstand any ordinary shock of landing, without breakage of any kind.
The life of your motor can be greatly increased by careful handling. The rubber strands are likely to be worn away against the hooks at either end. The wire used for the hooks should be as heavy as possible to keep it from cutting through. Be careful that the wire which comes in contact with the rubber is perfectly smooth and flawless. A little roughness or a spur on the wire will soon cut through the rubber. It is a good plan to slip a piece of rubber tubing tightly over the hook and loop the rubber bands of your motor over this cushion.
The first break in the rubber bands is likely to come near the center of the strand. A number of loose ends appear. The broken ends should be knotted neatly and the loose ends cut away. If the strands come in contact with any part of the motor base, a breaking will quickly follow, and your strands soon become covered with a fringe of loose ends. Be careful to tie up all loose ends and trim them away, since the ends in twisting serve to break other strands. Although the finer strands of rubber give the greater thrust, do not buy them too small, since they are easily broken.
PLATE XI.
A Well Thought Out Monoplane.
The length of your motor base beyond the front plane should be carefully calculated. It is very easy, of course, to run your shaft too far forward. The center of gravity is easily shifted in this way, and your model soon becomes unmanageable. An aëroplane with this fault will not rise, but merely pitches forward under the thrusts of the motor. It is almost useless to attempt to balance this by weighting the machine. The front plane should be placed further forward, and if the lifting surface does not seem sufficient, cut away the front of your motor base, once for all. A too short motor base, on the other hand, will cause your model to shoot upward at a sharp angle, and waste much valuable propelling power before it rights itself and takes a regular horizontal flight.
In the model aëroplane there is only one point where friction affects the flight, namely, along the propeller shaft. One can hardly be too careful in the construction of the axle. The thrust of the rubber at best, is limited, and this power must be exerted without loss of any kind. A faulty propeller shaft will use up a surprising amount of energy. Your rubber motor should unwind to within one or two turns.
Bear in mind that one of four things is likely to be responsible for your trouble. The planes may not be properly placed on the frame, they may not be properly flexed, they are not set at the proper angle of elevation, or your motor is at fault. Watch these points, and you will soon have your machine under perfect control. In the extremely complicated models it is often difficult to locate the fault. Build your model so that these parts may be adjusted in a moment without taking apart. After you have built an aëroplane model, even a very simple one, the pictures of other aëroplanes will have a new meaning for you. Every new model you see will give you some new idea. A number of the most successful aëroplane models in the country are shown in the accompanying photographs. Study these carefully, and you will learn more from them of practical aëroplane construction than from any amount of reading.
PART II