The Boys' Book of Model Aeroplanes / How to Build and Fly Them: With the Story of the Evolution of the Flying Machine

by Francis A. Collins

FAULTS AND HOW TO MEND THEM

YOUR model, perhaps a beautiful one, finished in every part, may twist and tip about as soon as it is launched and quickly dart to the ground. The fault is likely to be in the propeller, being too large for the size and weight of the machine. This may be remedied by adding a weight to the front of the machine, by wiring on a nut or piece of metal. Should this fail to steady the aëroplane, the propeller must be cut down.

When your propeller is too small the machine will not rise from the ground, or, if launched in the air, will quickly flutter to earth. If the model on leaving your hand, with the propeller in full motion, fails to keep its position from the very start, the blade should be made larger. There is no use in wasting time and patience over the machine as it is.

Many a beginner, with mistaken zeal, constructs a too powerful motor. The power in this case turns the propeller too swiftly for it to grasp the air. It merely bores a hole in the air and exerts little propelling force. An ordinary motor when wound up one hundred and fifty turns should take about ten seconds, perhaps a trifle longer, to unwind. It is a good plan to time it before chancing a flight.

Bad bracing is another frequent source of trouble. The planes should be absolutely rigid. Test your model by winding up your motor and letting it run down while keeping the aëroplane suspended, by holding it loosely in one hand. If the motor racks the machine, that is, if the little ship is all a-flutter and the planes tremble visibly, the entire frame needs tuning up. It is impossible for an aëroplane to hold its course if the planes are in the least wabbly. The braces should be taut. A loose string or wire incidentally offers as much resistance to the air as a wooden post.

The flight of your model aëroplane should be horizontal, with little or no wave-motion. Your craft at first may rise to a considerable height, say fifteen or twenty feet, then plunge downward, right itself, and again ascend, and repeat this rather violent wave-motion until it strikes the ground. To overcome this, look carefully to the angle or lift of your front plane or planes and to the weighting.

The explanation is very simple. As the aëroplane soars upward, the air is compressed beneath the planes, and this continues until the surface balances, tilts forward, and the downward flight commences. Your planes should be so inclined that the center of air-pressure comes about one third of the distance back from the front edge. The center of gravity of each plane, however, should come slightly in front of the center of pressure. After all, the best plan is to proceed by the rule of thumb, and tilt your planes little by little, and add or lessen the weight in one place or another, until the flight is horizontal and stable.

If your aëroplane does not rise from the ground, but merely slides along, the trouble is likely to be in your lifting plane. Tilt it a trifle and try again. The simplest way to do this is to make the front skids higher than those at the back. If the front skids are too high, the plane will shoot up in the air and come down within a few feet.

The most carefully constructed model is likely to go awry in the early flights. The propeller seems to exert a twist or torque, as it is called, which sends it to the right or left, or up or down, even in a perfectly undisturbed atmosphere. It is assumed that your model is symmetrical. An aëroplane not properly balanced, which is larger on one side than the other, or in which the motor is not exactly centered, cannot, of course, be expected to fly straight. However, to be on the safe side, go all over the machine again. Measure its planes to see that the propeller is in the center. Hold it up in front of you right abeam, and test with your eye if the parts be properly balanced.

If it still flies badly askew, flex the planes by bending the ends up or down very slightly by tightening or loosening the wire braces running to the corners. At the same time add a little weight to counteract the tipping tendency. A nut or key may be wired on the edge which persists in turning up. It may require much more weight than you imagine. The difference should begin to show at once. Even after a model appears to work fairly well as a glider, the addition of the motor may so change the center of gravity that it will “cut up” dreadfully.