Fig. 145.—Three-cylinder Engine
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.
CHAPTER XIV
Biplane Driven by
Compressed-air Engine
The model aeroplane illustrated by [Fig. 148] has been designed to suit the compressed-air plant fully illustrated and described in the preceding chapter. It is from the results obtained from the testing of the plant that the dimensions of a suitable model for it are determined; and while the design may suit the majority of the plants constructed from the illustrations shown in [pp. 95 to 101], it is chiefly given to show the correct method of designing a “power-driven” machine, since the power unit (unlike the elastic motor) cannot be varied, and recourse to some established line of reasoning becomes essential.
The first thing to do, then, once the plant has been “tuned up,” is to ascertain the thrust obtainable from it. This is found by suspending the plant by the valve on a balance, with a container fully inflated, the weight registered being carefully noted. The container pressure should now be released, and the weight registered when the motor is running observed. By subtracting the former from the latter the thrust is obtained.
Thus, assuming the plant, at rest, to weigh 8 oz., and when running 12 oz., it is clear that the thrust is equal to 4 oz. Now, it is necessary to know the average thrust developed, since, as hitherto explained, the thrust is not constant, but gradually diminishes as the density of the air in the container approaches normal atmospheric conditions; that is, 14·6 lb. per square inch (known as an atmosphere). It is possible to obtain some very interesting data by plotting a graph of the thrust given off at various moments from the release of the pressure in the container. Meanwhile it can be taken as a good rule that the thrust registered after one-third of the effective run of the motor represents approximately the average thrust; and the figure given above (4 oz.) will serve for the purpose of illustration.
It is next necessary to know the weight of the model it will lift. It is well established that a plant will fly a machine weighing from four to six times the weight of the thrust it develops, although, of course, much depends on the efficiency of the model; the greater the complexity of frame members the lower the lift drag ratio, and consequently the lower the ratio between the thrust and the weight of the model. Compromising, and taking 5: 1 as the ratio, 20 oz. is obtained as the total weight of the plant and model.