FLYING MODEL AEROPLANES.--Model aeroplanes contain their own power and propellers which, while they may run for a few seconds only, serve the purpose of indicating how the propeller will act, and in what respect the sustaining surfaces are efficient and properly arranged. It is not our purpose to give a treatise on this subject but to confine this chapter to an exposition of a few of the gliders and model forms which are found to be most efficient for experimental work. AN EFFICIENT GLIDER.--Probably the simplest and most efficient glider, and one which can be made in a few moments, is to make a copy of the deltoid kite, previously referred to. This is merely a triangularly-shaped piece of paper, or stiff cardboard A, Fig. 84, creased in the middle, along the dotted line B, the side wings C, C, being bent up so as to form, what are called diedral angles. This may be shot through the air by a flick of the finger, with the pointed end foremost, when used as a glider. _Fig. 85. Deltoid Glider._ THE DELTOID FORMATION.--This same form may be advantageously used as a model aeroplane, but in that case the broad end should be foremost. _Fig. 86. The Deltoid Racer._ Fig. 86 shows the deltoid glider, or aeroplane, with three cross braces, A, B, C, in the two forward braces of which are journaled the propeller shaft D, so that the propeller E is at the broad end of the glider. A short stem F through the rear brace C, provided with a crank, has its inner end connected with the rear end of the shaft D by a rubber band G, by which the propeller is driven. A tail may be attached to the rear end, or at the apex of the planes, so it can be set for the purpose of directing the angle of flight, but it will be found that this form has remarkable stability in flight, and will move forwardly in a straight line, always making a graceful downward movement when the power is exhausted. It seems to be a form which has equal stabilizing powers whether at slow or at high speeds, thus differing essentially from many forms which require a certain speed in order to get the best results. RACING MODELS.--Here and in England many racing models have been made, generally of the A-shaped type, which will be explained hereinafter. Such models are also strong, and able to withstand the torsional strain required by the rubber which is used for exerting the power. It is unfortunate that there is not some type of cheap motor which is light, and adapted to run for several minutes, which would be of great value in work of this kind, but in the absence of such mechanism rubber bands are found to be most serviceable, giving better results than springs or bows, since the latter are both too heavy to be available, in proportion to the amount of power developed. Unlike the large aeroplanes, the supporting surfaces, in the models, are at the rear end of the frames, the pointed ends being in front. _Fig. 87. A-Shaped Racing Glider._