The first requirement of a model aeroplane is that it shall fly. The first essential for a machine to fly well is that it must be simple. Simplicity usually insures success and is synonymous with efficiency. A complicated scale model having as its prototype one of the most successful man-carrying machines usually will not fly. If it does fly, it does not do so well. Miniature steam engines, motors, etc., can be constructed to exact scale and will justify their existence by actually working and performing duty, but in most cases a model aeroplane made to scale will not fly well until it begins to approach full size.
The next indispensable feature might be called lightness, but at the same time it must be borne in mind that strength is also "second to none" and it would be fatal to sacrifice the one for the other. The hard knocks and battering which a model usually receives at the hands of a novice will soon wreck any flimsy construction.
To design model aeroplanes will at first seem like "robbing Peter to pay Paul," that is, no one part can be developed to an extreme without seriously affecting the efficiency of the other parts. The successful machine is a sort of "happy medium" arrived at solely through experiment. A thorough understanding, however, of the part played by each individual member of a model and its characteristics will make it possible to avoid much unnecessary work in that connection. It is therefore well to carefully read the following chapters before commencing to carry out any original ideas or to make any radical departure from the designs offered in this book.
From these statements it must not be inferred that the successful model aeroplane builder is necessarily an individual possessed of consummate skill in the handling of tools or a person of unusual judgment. A few simple tools and trifling mechanical ability will enable any one to build the simple little machines herein described. The greatest asset required in the work is patience, patience spelled with a capital "P." Not only patience in building the machines, but patience in adjusting them and patience in flying them. Making haste with a model aeroplane is poor policy. It never pays to use slipshod methods. Take the time to make sure every part is the best that you can make it. Care with the little details will insure success.
Model aeroplanes are exasperating to the extreme. A new model will swerve to the right and left or dive with unerring precision to the ground or nearest object. They seem to defy all attempts to make them behave and in the first few flights usually perform a "new one" every time. This is the point where success comes to the model aeroplanist who possesses patience and perseverance. One must learn to adjust and fly a model aeroplane by practice just as he must also learn to swim or ride a bicycle by repeated trials. A little persuasion will soon make a model soar in a surprising manner.
The question of resistance is the first consideration of the model aeroplane designer. An aeroplane should pass through the air in such a manner that it leaves that medium in as motionless a state as possible. All motion of the surrounding air represents so much power wasted. It is obvious that a boat with a square prow will offer more resistance than a ship having a sharp bow. The latter causes considerably less disturbance of the fluid in which it moves than the former.
FIG. 5. The disturbance created in the air by a square object. The arrow points in the direction of motion. The space in the rear of the object is the scene of violent eddy.
The resistance of an aeroplane is made up of:
- Aerodynamic resistance.
- Head resistance.
- Surface resistance.