A propeller of very high pitch was employed, but with the same results. Evidently a spring of this nature is not practical. Experiments have been made in Germany, however, with coil springs better suited for model aeroplanes, with satisfactory results. Since trolley cars have been driven for considerable distances by energy wound up in coil springs, the model aeroplane should be able to take advantage of this elasticity. Doubtless another year will find model aeroplanes flying by energy stored in this way. The Petrol motors are now offered for sale in sizes suitable for model aeroplanes by several firms. A motor developing one-half of one horsepower weighs but little. The principal difficulty of these little power plants is that they set up a more or less violent vibration which racks the entire machine and renders a stable flight difficult. To set up one of these engines and adjust it to so delicate a task requires the highest engineering skill. Unless one has had such experience it will be well to avoid such experiments for the present at least. As a rule the engines are used to equip scale models which are not intended to fly. The beautiful model illustrated on the cover of this volume built by Mr. Karl H. De Leon is equipped with a rotary Knome engine especially imported. CHAPTER XII LESSONS OF THE MAN-CARRYING AEROPLANES THE builder of model aeroplanes will do well to keep in touch with the development of the passenger-carrying air craft. The development of both types of machines will always be more or less parallel. It is especially important that you watch the design and position of wings, and the modeling of propellers, since here the problem confronting both classes of builders is practically the same. The use of ailerons and vertical surfaces and all improvement in steering the large machines again may have a direct application on model aeroplane buildings. The photographs of the new models of the large machines published in the newspapers will often give valuable hints, while the technical aeronautical papers will, of course, give more detailed information. In a few years, at the present rate of model aeroplane development, the designers and pilots of large machines may be looking to the new designs of model aeroplanes with much the same interest. There is at present but one passenger-carrying monoplane, the Valkyrie, flown "tail first." It is built in two sizes, the smaller, fitted with high-powered engines, being intended for racing. The general lines of the Valkyrie resemble those of many of the model aeroplanes, but there are some features which deserve careful study. The main plane of the Valkyrie is set at a pronounced elevation, and has a high aspect ratio.