[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.
Fig. 145A.—Compressed-air Plant for Model Aeroplane
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.