Oneof the latest developments in the field of model flying is the McMahon compressed air driven monoplane. This model was built to be used as either a tractor or pusher, but in view of its ability to balance more easily as a pusher most of the experiments have been carried out on this machine as a pusher. The machine in itself is simple and inexpensive to construct, the chief portion of the expense being involved in the making of the engine. By using the machine as a pusher a great deal of protection is afforded both the propeller and engine, and this protection helps to avoid damaging the propeller or engine, which would mean an additional expenditure for repairs, thus minimizing the cost of flying the model.
The frame has been made to accommodate both the tank and engine, and this is done by using two 30″ strips of spruce, each ¹⁄₄″ wide by ³⁄₈″ deep, laid side by side, a distance of three inches apart, up to within 10″ of the front, as shown in the accompanyingphotograph. No braces are used on the frame, as the tank, when securely fastened between the frame, acts in that capacity.
The wings are made in two sections, each section measuring 24″ in span by 8″ in chord, consisting of two main spars, ³⁄₁₆″ in diameter, one for the entering edge and one for the trailing edge. To these edges, at a distance of three inches apart, are attached bamboo ribs, 18 in all, each measuring 8″ in length by ¹⁄₈″ in width by ¹⁄₁₆″ thick. The wings are round at the tips, and have a camber of approximately one-half inch, but they are not set at an angle of incidence. Light China silk is used for covering and after being glued over the top of the wing frame is given two coats of dope to shrink and fill the pores of the fabric. A good“dope” for the purpose can be made from celluloid dissolved in banana oil. The wing sections are attached to the frame and braced by light wire. The forward wing or “elevator” is made in the same manner as the main wing, but should measure only 18″ × 3″. Instead of being made in two sections as the main wing, the forward wing is made in one piece.
The chassis is made by forming two V struts from strong steel wire sufficiently large enough so that when they are attached to the frame of the model the forward part will be 9″ above the ground. One V strut is securely fastened to either side of the frame, at a distance of 8″ from the front. A 7″ axle is fastened to the ends of these struts. On the axle are mounted two light wheels, each about 2″ in diameter. The chassis is braced by light piano wire.
The rear skid is made in the same manner as the forward skid, only that the ends of the struts are brought together and a wheel 1 inch in diameter is mounted at the bottom ends by means of a short axle. The struts are not more than 7¹⁄₂″ long, thus allowing a slight angle to the machine when it is resting upon the ground.