§ 2. It should (theoretically) be launched into the air with a velocity equal to that with which it flies. If it launch with a velocity in excess of that it becomes at once unstable and has to "settle down" before assuming its normal line of flight. If the velocity be insufficient, it may be unable to "pick up" its requisite velocity in time to prevent its falling to the ground. Models with wooden aerofoils and a high aspect ratio designed for swift flying, such as the well-known Clarke flyers, require to be practically "hurled" into the air.
Other fabric-covered models capable of sustentation at a velocity of 8 to 10 miles an hour, may just be "released."
§ 3. Light "featherweight" models designed for long flights when travelling with the wind should be launched with it. They will not advance into it—if there be anything of a breeze—but, if well designed, just "hover," finally sinking to earth on an even keel. Many ingenious pieces of apparatus have been designed to mechanically launch the model into the air. Fig. 50 is an illustration of a very simple but effective one.
§ 4. For large size power-driven models, unless provided with a chassis and wheels to enable them to run along and rise from the ground under their own power, the launching is a problem of considerable difficulty.
§ 5. In the case of rubber-driven models desired to run along and rise from the ground under their own power, this rising must be accomplished quickly and in a short space. A model requiring a 50 ft. run is useless, as the motor would be practically run out by that time. Ten or twelve feet is the limit; now, in order to rise quickly the machine must be light and carry considerable surface, or, in other words, its velocity of sustentation must be a low one.
Fig. 50.—Mr. Poynter's Launching Apparatus.
[Reproduced by permission from the "Model Engineer."]
§ 6. It will not do to tip up the elevator to a large angle to make it rise quickly, because when once off the ground the angle of the elevator is wrong for actual flight and the model will probably turn a somersault and land on its back. I have often seen this happen. If the elevator be set at an increased angle to get it to rise quickly, then what is required is a little mechanical device which sets the elevator at its proper flight angle when it leaves the ground. Such a device does not present any great mechanical difficulties; and I leave it to the mechanical ingenuity of my readers to devise a simple little device which shall maintain the elevator at a comparatively large angle while the model is on the ground, but allowing of this angle being reduced when free flight is commenced.
§ 7. The propeller most suitable to "get the machine off the ground" is one giving considerable statical thrust. A small propeller of fine pitch quickly starts a machine, but is not, of course, so efficient when the model is in actual flight. A rubber motor is not at all well adapted for the purpose just discussed.
§ 8. Professor Kress uses a polished plank (down which the models slip on cane skids) to launch his models.