A SIMPLE COMPRESSED AIR ENGINE
During the past few years model flyers in America have shown a tendency toward the adoption of compressed air engines for use in connection with model aëroplanes. Hitherto, England has been the home of the compressed air engine, where a great deal of experimenting has been carried on, to a considerable degree of success. Flights of over 40 seconds have been made with models in which compressed air power plants were used. But, however, the desire on the part of a large majority of model flyers in America to build scientific models, that is, models more closely resembling large machines, has made it necessary to find a more suitable means of propulsion; rubber strands being unsatisfactory for such purposes. Many different types of compressed air engines have made their appearance during the past few years, among which the two cylinder opposed type is very favorably looked upon, because it is perhaps one of the easiest to construct.
To make a simple two cylinder opposed compressed air power plant, as illustrated in [Figure 1 of diagram 16], it is not necessary that the builder be in possession of a machine shop. A file, drill, small gas blow torch and a small vise comprise the principal tools for the making of the engine.
The first things needed in the making of this engine are cylinders. For the making of the cylinders two fishing rod ferrules, known as female ferrules, are required. And for the heads of the cylinders, two male ferrules are required. Such ferrules can be secured at most any sporting goods store. The female ferrules should be filed down to a length of 2″, cut down on one side a distance of ³⁄₄ of the diameter, then cut in from the end as shown in [Figure 7]. When this has been done the two male ferrules should be cut off a distance of ¹⁄₈″ from the top as shown in [Figure 7-a], to serve as heads for the cylinders.
Diagram 16
A hole ¹⁄₈″ in diameter should be drilled in the center of each head so as to enable the connecting of the intake pipes. By the use of soft wire solder the heads should be soldered into the ends of the cylinders as shown in [Figure 1-d].
The pistons should now be made; for this purpose two additional male ferrules are required. These should be made to operate freely within the cylinders by twisting them in a rag which has been saturated with oil and upon which has been shaken fine powdered emery. When they have been made to operate freely they should be cut down one-half inch from the closed end as shown in [Figure 5-a]. For the connecting rods, 2 pieces of brass tubing, each ¹⁄₈″ in diameter by 1¹⁄₄″ long, are required, and, as illustrated in [Figure 6], should be flattened out at either end and through each end a hole ³⁄₃₂″ in diameter should be drilled. For the connecting of the piston rods to the pistons, studs are required, and these should be cut from a piece of brass rod ¹⁄₄″ in diameter by ¹⁄₂″ in length. As two studs are necessary, one for each piston, this piece should be cut in half, after which each piece should be filed in at one end deep enough to receive the end of the connecting rod. Before soldering the studs to the heads of the pistons, however, the connecting rods should be joined to the studs by the use of a steel pin which is passed through the stud and connecting rod, after which the ends of the pin are flattened, to keep it in position as shown in [Figure 5-a].
For the outside valve mechanism and also to serve in the capacity as a bearing for the crankshaft, a piece of brass tubing ¹⁄₄″ in diameter by 1¹⁄₂″ long is required. Into this should be drilled three holes, each ¹⁄₈″ in diameter, and each ¹⁄₂″ apart as shown in [Figure 4]. Next, for the valve shaft and also propeller accommodation, secure a piece of ³⁄₁₆″ drill rod 2″ long. On the left hand side of the valve shaft, as shown in [Figure 3], a cut ¹⁄₃₂″ deep by ¹⁄₂″ in length is made 1″ from the end. Another cut of the same dimensions is made on the right side only; this cut is made at a distance of ³⁄₈″ from the stud end.
As shown in [Figure 1-f], the crank throw consists of a flat piece of steel, ³⁄₃₂″ thick, ³⁄₈″ in length by ¹⁄₄″ in width. At each end of the crank throw a hole ³⁄₁₆″ in diameter should be drilled, the holes to be one-half inch apart. Into one hole a piece of steel drill rod ³⁄₃₂″ in diameter by ¹⁄₄″ long is soldered, to which the connecting rods are mounted, as shown in [Figure 1-f]. Into the other hole the stud end of the crank throw is soldered.
Schober pusher type compressed air driven monoplane
Schober compressed air driven biplane
Before making the tank it is most desirable to assemble the parts of the engine, and this may be done by first fitting the pistons into the cylinders as shown in [Figure 1-b], after which the cylinders should be lapped one over the other and soldered as shown in [Figure 1-a]. When this has been done a hole one-fourth of an inch in diameter should be drilled half way between the ends of the cylinders, and into this hole should be soldered one end of the valve casing shown in [Figure 4]. For the inlet pipes as shown in [Figure 1-c] secure two pieces of ¹⁄₈″ brass tubing and after heating until soft, bend both to a shape similar to that shown in [Figure 1-c]. When this has been done solder one end to the end of the cylinder and the other in the second hole of the valve shaft casing. The valve shaft should now be inserted in the valve shaft casing and the connecting rods sprung onto the crank throw as shown in [Figure 1-d]. To loosen up the parts of the engine which have just been assembled it should be filled with oil and by tightly holding the crankshaft in the jaws of a drill the engine can be worked for a few minutes.
The tank is made from a sheet of brass or copper foil 15″ long by ¹⁄₁₀₀₀″ thick. This is made in the form of a cylinder, the edges of which are soldered together as shown in [Figure 2]. Sometimes this seam is riveted every one-half inch to increase its strength, but in most cases solder is all that is required to hold the edges together. For the caps, or ends, the tops of two small oil cans are used, each can measuring 2¹⁄₂″ in diameter. To complete the caps two discs of metal should be soldered over the ends of the cans where formerly the spouts were inserted, the bottoms of the cans having been removed. The bottom edges of the cans should be soldered to the ends of the tank as shown in [Figure 2]. Into one end of the completed tank a hole large enough to receive an ordinary bicycle air valve should be drilled. [Figure 2]. Another hole is drilled into the other end of the tank, into which is soldered a small gas cock to act as a valve. [Figure 2]. This should be filed down where necessary, to eliminate unnecessary weight. To connect the tank with the engine, a piece of ¹⁄₈″ brass tubing 3″ long is required, the ends of which are soldered into the holes in the valve shaft casing nearest the cylinders, as shown in [Figure 1-ee]. As shown in [Figure 1-ee], a hole ¹⁄₈″ in diameter is drilled in one side of this piece, but not through, in the end nearest the tank. Another piece of brass tubing ¹⁄₈″ in diameter is required to connect the tank with the engine, one end of which is soldered to the cock in the tank, the other in the hole in the pipe which leads from the engine to the tank, illustrated in [Figure 1-ee], thus completing the engine.
In conclusion it is suggested that the builder exercise careful judgment in both the making and assembling of the different parts of the engine in order to avoid unnecessary trouble and secure satisfactory results. After having constructed an engine as has just been described, the constructor may find it to his desire to construct a different type of engine for experimental purposes. The constructor therefore may find the descriptions of satisfactory compressed air engines in the following paragraphs of suggestive value.