Signs are not wanting that compressed air as a motive power for model aeroplanes will become equally as popular as the twisted skein of rubber, which has practically held the field since it was introduced about the year 1870 by Alphonse Penaud.

One of the chief disadvantages of the rubber motor is that experiments of a full-size scale nature cannot be undertaken, owing to the length of frame required in order that the necessary power and duration of run may be obtained, and also owing to the disposition of weight, and consequently of the centre of gravity, not being tantamount to that obtaining in full-size practice. With a compressed-air plant these disadvantages are eliminated, since the weight can be kept well forward, thus making possible the designing of a model which represents in essential proportions a full-size machine.

Particulars are here given of a highly successful plant, for which the machine described in the next chapter was especially designed. Several of the illustrations in this present chapter are exaggerated to render the construction clear, and it is thought that the details given will be found comprehensive.

Fig. 136 gives a plan view of the engine, which is rotary with, of course, a stationary crank-shaft. The five cylinders are soft-soldered to what may be termed the crank-case, which consists of two circular brass discs of the gauge indicated. In order that the cylinders may be accurately located round the plates, a wooden jig should be made with slots to receive the cylinders, and a recess to take the plate. Five lightening holes are drilled in the two plates as shown. The front plate, that is, the one carrying the propeller bolt, is, however, left off until the pistons, crank-shaft, and sleeve are assembled.

Fig. 137 gives a side elevation of the engine, with the crank-shaft and sleeve shown in section. It will be seen that the sleeve butts to a shoulder, a slight undercut being given to the shaft when turning this portion to ensure a good joint. From this figure the inlet and exhaust principle will be manifest. It will be noticed that as each inlet pipe coincides with the right angular inlet in the shaft, so does it receive a charge of compressed air. The pressure on the piston revolves the engine, thus shutting off inlet to that particular cylinder and bringing the next cylinder in line with the inlet. As soon as the first cylinder nears the bottom of its stroke it begins to exhaust through the diametrically opposed exhaust port. Needless to say, the crank-shaft and sleeve must be turned a good running fit, otherwise there will be considerable waste of power. The best method to employ is to turn the shaft a push fit within the sleeve, and then to grind it in with rottenstone. When soldering the inlet pipes into the sleeve, care must be taken to ensure that they do not become “choked” with solder. The sleeve should afterwards be reamed out to remove all superfluous solder. When soldering the sleeve into the back plate care must also be exercised to ensure that it is truly at right angles to the plate.

It must be clearly understood that the engine revolves with the sleeve as a bearing. The five holes which are drilled round the sleeve to receive the inlet pipes must be equidistant, so that the periods of inlet are synchronous.

Fig. 138 gives an enlarged view of the crank-shaft and sleeve, and is self-explanatory. Observe that the exhaust port is larger in diameter than the inlet.

Details of the pistons are shown by Fig. 139. The connecting-rods are soldered to tubular distance pieces, which rock on the ¹/₃₂-in. silver-steel gudgeon-pins, which pass through the pistons, being cut shorter than the outside diameter of the piston to avoid possible scoring of the bore of the cylinders. The gudgeon-pins are soldered into position, the superfluous solder being scraped from the piston walls. To ensure airtightness of the pistons and cylinders, cupped leather washers are fixed to the piston-heads by means of tin tongues soldered to them, and which are forced through the washer and bent over. The ordinary cycle-pump washer is admirably suited to the purpose, but the height of the washer when within the cylinder should not exceed ⅛ in.

Fig. 140 gives dimensions of the crank and throw, to exaggerated scale, to avoid crowding the details. The important point to bear in mind when beginning this portion of the construction is to obtain the correct stroke, since the cylinders are designed to take a stroke of ½ in. only. See also that the crank-pin revolves truly, that is, at 180° to the shaft.

Fig. 141 is a longitudinal section of the cylinder. As there shown, the cylinder-head is “let in” the head and soldered there. The inlet pipes should be packed with resin prior to bending, this being afterwards melted out. The connecting-rods are shown by Fig. 142, the important dimension, obviously, being the centre distance of the holes for the crank-pin and gudgeon-pin respectively. These are of No. 20 b.w.g. brass. The tin clips used to secure the cupped leather washers to the piston head (four of which are used for each piston, so that twenty in all will be required) are shown by Fig. 143. They are of No. 30 s.w.g., and are bent along the dotted line to a right angle, the ⅛-in. portion being the end to be soldered to the piston.

The compressed-air container shown by Fig. 144 is made from copper foil of the thickness shown. This is folded round a wooden former of circular cross-section, and tied tightly in place while the lapped joint is being soldered. The two faces of the joint that are in contact should first be tinned, using Fluxite or resin as a flux; spirits of salt should on no account be used, as this has a deleterious effect on metal of so fine a gauge; and a mediumly heated iron should be used to solder the joint.

Wind the body with the No. 35 s.w.g. piano wire, soldering each spiral at each revolution so that it maintains its correct pitch. Now attach one of the half-balls (which for preference should be provided with a stepped flange as shown) while the body is still on the wooden former, first tinning the two surfaces in contact, and then “running” the solder round with a mediumly heated soldering bit, and so sealing the joint. Prior to attaching the second half-ball to the other end, a tension wire must be attached to the flange, either of the valve or the tap (according to which half-ball was attached first), by soldering. This is then passed through the body of the container (the wooden former, of course, now having been removed), and threaded through a hole drilled in the half-ball at a convenient point near the centre. Tension is now applied to the wire and the second half-ball eased into position, and while still pulling on the wire it is soldered into the hole through which it passes, afterwards being cut off sufficiently long to form a coil on the end.

It will, of course, be clear that the valve (of the Lucas type) and tap are soldered to the half-ball before the latter are affixed to the container body.

The container should be inflated and immersed in paraffin to test for leakages, and when these are stopped up the container and engine may be connected by a short length of tubing. The engine is then ready for running. Thin machine oil should be used for lubricating purposes, and where necessary the connecting-rods must be staggered for clearance.

In conclusion, it should be pointed out that the plant should not weigh more than 10 oz. complete, and is capable of flying a machine weighing 2 lb., provided that it is efficiently constructed. The container should be inflated to a pressure of not less than 100 lb. Fig. 145 shows a similar compressed-air model aeroplane engine complete.

The accompanying photographic reproduction shows a model compressed-air plant for model aeroplanes which is similar in general design to the one illustrated. The difference is that inlet takes place through hollow connecting-rods, which are ball-ended and fit into ball seatings. The cylinders oscillate, and the connecting-rods, being rigidly attached to the pistons, by their angularity during revolution form the inlet and exhaust mechanism. The propeller is geared up in the ratio 2: 1.

The design is analogous to a very early French engine much in evidence in the early days in model experiments.

It is thought that the photograph will give the reader an idea of the general arrangement of the plant previously described.

Driving Small Biplane.—Fig. 145 is a front elevation, with the front plate removed, of a 3-cylinder engine on similar lines, which would be sufficiently powerful for models up to 12 oz. weight. As will be obvious, the three cylinders are fixed (by solder) to two circular discs of No. 20 gauge brass forming the crank chamber. The cylinders and various component parts should be assembled before the front plate is fixed. A length of brass tube is soldered into the back plate, and equidistant round its periphery three ⅛-in. holes must be drilled to receive feed pipes which pass to the cylinder heads. A piece of brass rod to form the crank-shaft must be turned to make a good running fit within the tube and inlet, and exhaust holes drilled as indicated by the dotted lines. The pistons should be made an easy fit. Pieces of by-pass tubing are soldered into the small ends of the connecting-rods. Through these tubes pass the gudgeon-pins which are anchored to the piston walls. The position of the connecting-rods in relation to the piston is thus maintained. The container (into which air is compressed with a foot pump to from 100 lb. to 120 lb. per square inch) is constructed from copper foil of three-thousandths (·003) of an inch thickness, and is of the same dimensions as the five-cylinder one.