In order to get the best results, the propeller and the motor must be suited to each other. Some motors which “race” with a propeller which is slightly too small, work admirably with one a little heavier, or with a longer diameter. The question as to whether one propeller, or two, is the better practice, has not been decided. The majority of aeroplanes have but one. The Wright and the Cody machines have two. The certainty of serious consequences to a machine having two, should one of them be disabled, or even broken so as to reduce the area, seems to favor the use of but one. Chapter XI. MODEL FLYING MACHINES. Awakened popular interest--The workshop’s share--Needed devices--Super-sensitive inventions--Unsolved problems--Tools and materials--A model biplane--The propeller--The body--The steering plane--The main planes--Assembling the parts--The motive power--Flying the model--A monoplane model--Carving a propeller--Many ideas illustrated--Clubs and competitions--Some remarkable records. It is related of Benjamin Franklin that when he went out with his famous kite with the wire string, trying to collect electricity from the thundercloud, he took a boy along to forestall the ridicule that he knew would be meted out to him if he openly flew the kite himself. Other scientific experimenters, notably those working upon the problem of human flight in our own time, have encountered a similar condition of the public mind, and have chosen to conduct their trials in secret rather than to contend with the derision, criticism, and loss of reputation which a sceptical world would have been quick to heap upon them. But such a complete revolution of thought has been experienced in these latter days that groups of notable scientific men gravely flying kites, or experimenting with carefully made models of flying machines, arouse only the deepest interest, and their smallest discoveries are eagerly seized upon by the daily press as news of the first importance. So much remains to be learned in the field of aeronautics that no builder and flyer of the little model aeroplanes can fail to gain valuable information, if that is his intention. On the other hand, if it be the sport of racing these model aeroplanes which appeals to him, the instruction given in the pages following will be equally useful.