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Aviation History
1914
1914 - 1202.PDF
JycHf] DECEMBER II, 1914, <^S^£7 «*F by V. E. Edited Models Driven by Compressed Air. (Continuedfrom page 1184). As well as the various experiments already detailed, some were also made with respect to leakage. Even with 120 lbs. pressure this was found to be very slight, occuring mainly at the tap, which (as already stated) was of the ordinary cone shape pattern, and turned quite freely. In any tap suitable provision should always Fig. 1.- FIGI -Three-cylinder compressed air motor, are hollow. The pistons be made for properly tightening up the same, as in small power plants (unless a properly fitted needle valve tap be used) this is the most common source of leakage. How the Three-Cylinder Motor Works. The motor consists of three single acting cylinders, arranged symmetrically round a circle, i.e., the axis of each cylinder making an angle of 120° with the other two. There are no separate valve chests, slide, rotary or otherwise,^?- each cylinder is made to act as the slide valve to the cylinder next following it in order of rotation. Thus if the motor be supposed rotating anti-clockwise as shown in the sketch, the pipes or tubes, P, P', P2 are all connected with the main air supply, and are always full of com pressed air whilst the motor is working. The tubes Q, Q1, Qs, which connect the forward part of each cylinder with the rear or closed portion of the next succeeding cylinder, act both as supply and exhaust pipes, supply by means of the grooves or slots or some such arrangement, X, X1 and X2 in the three pistons, O, O1, O2. In the sketch the piston O1 is being driven forward ; it is approaching the end of its stroke ; O has already reached it. As soon as O1 has advanced a little further, O recedes and shuts ofi supply, but O' now begins to supply O2, and that travels forwards. O1 after a brief stationary period begins to move back. The cylinder A has ex hausted through Q. When the pistons travel towards the back or the cylinders it is evident they must not uncover any of the tubes, P, P1, P2—i.e., they must be sufficiently long, and the tubes, P and Q', &c, must be nearly enough together for this not to happen ; nor must the piston travel forward too far, so as to allow air from P, P1 and P2 to get into the closed end of the cylinders. The sketch is merely intended to explain the principle, and must not be taken as a scale drawing. S is the axis of rotation, and S T the throw in this case ; S T is about /8 of an inch, so the travel of the pistons backwards and forwards is § of an inch. To work efficiently like any other type of engine, it is evident the little motor must be well made, cpecially the fitting of piston and cylinder. The reason why the motor must not be worked at a lower external temperature than about 550, is that ice particles may form on the inner walls of the cylinder, and, in consequence, have an injurious effect on the fit of piston and cylinder. Paraffin should be used as a lubricant. Obviously this type of motor can be built in a very light form ; it is also symmetrical, and lends itself for attachment to the cylindrical reservoir; it is also powerful, more powerful than an JOHNSON. M.A. ordinary double-acting slide valve motor of same sized cylinder, &c, because in this case we have three impulses per revolu tion instead of two, but it uses more steam or compressed air as the cate may be, and supposing we made our double-acting slide valve motor of slightly larger bore, &c, so as to make it the equal of the three-cylinder one, this could, I think, undoubtedly be made the lighter of the two. The three-cylinder one is, however, (•elf-starting, which is an advantage. Model aeroplanes have now been made to fly successfully with many types of motors, and could no doubt be made to do so with practically any type of engine, if built light enough and at the same time of sufficient strength and accuracy to avoid loss by leakage. We should like to hear from various readers of any types of engines with which they have obtained successful results. The Question of Heating the Air. This question was already referred to in last week's issue. For laboratory experiments any efficient type of heating can be employed. The engine or motor should be mounted at a little distance from the motor with a coil of copper (for a commencement) tubing interposed between connecting reservoir and motor. The writer intends if possible to carry out some experiments of this character during the next fortnight. Inject into the reservoir say 50 or 100 pump strokes, time the run of the motor, &c, with the coil unheated. Next pump the same quantity of air (using a pressure gatoge if possible) into the reservoir and heat the air—if you possess the means to measure the temperature so much the better j and again, note the length of run, &c. When, however, it comes to the question of the model in free flight, there only appears to me to be one satisfactory method of heating and that is by means of a small blow-lamp : not altogether owing to the higher temperature which such a.flame gives, but also on account of the great difficulty of keeping any other flame alight under the circumstances, but we shall be glad to receive any sug gestions from our readers on this score. With regard to the question of keeping the flame alight, all boxed-in arrangement must be avoided as much as possible, both on account of Fig. 2.—Powerful blow-lamp and reservoir by Mr. H. H, Groves. extra weight and air resistance which they give rise to. The use of blow-lamp possibly entails the employment of steel instead of copper tubing and a brazed or at any rate silver soldered motor ; on the other hand, the quick expansion of the highly compressed air entails a heavy fall of temperature, and the resultant temperature might not be too high for a soft soldered engine, provided only a small blow-lamp was used. There is no doubt, however, which is best. Blow-lamps can be made very light. The one shown in Fig. 2 was made for the writer quite a good time ago by Mr. H. H. Groves. Its weight (complete but empty) is 4 ozs. only. Now nothing like such a large lamp is needed for a small plant; one weighing not more than 1J ozs. would be ample to try experiments with, carrying, say, | oz. of benzoline ; moreover, a. commencement might be made with a much simpler type. (To be continued.) 1202
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