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Aviation History
1914
1914 - 1220.PDF
I/H! DECEMBER 18, 1914. Edited by V. E. Models Driven by Compressed Air, (Continued from page 1202). The Container or Reservoir. THE writer has just completed the construction of a small con tainer, cylindrical with hemispherical ends, of the following dimen sions and weight:—Length (cylindrical) 26 ins., diameter 1 J^th of an inch, total weight 3 ozs. 4 grms. This weight includes an ordinary bicycle valve, a fitting for the use of a pressure gauge, and the tap for the engine. It ie constructed of steel, steel piano wire bound, with brass hemispherical ends. I was strictly limited as to the size of the container by the piece of steel in my possession, which was given to me by Mr. Colver when I went to Sheffield some two years ago. The narrow piece of steel band had to be first cut into two equal pieces and then soldered, in order to make a band of sufficient width to form a container of even this small diameter The thick ness of the steel is about TfjV<fth of an inch, and its weight is about equal to that of thin brass foil of the same size. I cannot at present say what pressure this container (which is soft soldered) can be safely charged up to ; so far it has only been tested with a gauge up to no lbs. per sq. in., above which pressure " exertion " becomes necessary—but I should have no fear of charging it up to 150 lbs. One advantage in the use of such thin, light material is that if it does burst there is no " material " of consequence to fly about and hurt you. Before describing how the the container was constructed, let us consider for a moment: The Question of Diameter. The following formulae will be found extremely useful in all calculations relating to cylindrical containers with hemispherical ends:— Volume of cylinder = height where h — and r = area of base = h height of cylinder, radius of circular base. 22 ,, — 7~, 7 Volume of sphere 4 22 3 7 where r is the radius of the sphere. Lateral surface of a cylinder (excluding the end?) = height x 22 circumference of base = 2 . r k. 7 Surface of a sphere = 4 . 2Z r'2. 7 If we double the length of our cylinder, we obviously double the volume, but we also double the weight; if, instead of doubling the length, we double the diameter, we double the material, but we have four times the volume, since the areas of the circular bases are to one another as the squares of their diameters, i.e., if we take two cylinders, both 3 ft. long, one I'5 in. in diam. and the 01 her 3 ins., then the latter has only twice the weight, but four times the volume, of the former. This shows us how important it is to let the diameter be a* large as possible. Softe early French models on the market were fitted with two cylinders of equal size, placed one above the other. This principle was clearly a wrong one, because the weight was doubled as well as the volume. One cylinder of a diameter to give double the volume of the original cylinder should have been used. If d be diam. of the original cylinder, and D of one we desire to have twice the volume, then we have T>—d «/2 = rfx 1-42 approx. It will thus he seen that the container described in our isme ot December 4th, has about three times the capacity of the writer's. I do not know the weight of the larger container, or the motor, described in that issue, but it has been on the market for some time, and undoub'edly some readers of FLIGHT possess this plant, and could, perhaps, supply the information, together with any results obtained or experiments that they have made. The Construction of a Compressed Air Container. The following method was employed by the writer in the con struction of the 3 oz. 4 gramme reservoir, and since it answered well in this case, and is one which can be made use of in other instances, we give fairly full details:— After the steel band was cut and soldered together again as already described, a piece of wood some 27 ins. long was turned up in a lathe until its diameter was such that, when the steel band was wrapped round the wood, its edges overlapped to the extent of about TVh of an inch. The wood was smoothed down and rubbed with blacklead and the steel oiled (on the inner side), it was then JOHNSON, M.A, wrapped round the cylinder of wood, tied at intervals and then soldered carefully all down the join. It was then slipped off the wood just to examine it ; it had been fitted rather too tightly and only came off with difficulty. A fine cut was taken off the wood before replacing the steel cylinder on it. The cylinder was then wrapped with No. 26 steel piano wire from end to end, the distance between the coils being about ^th of an inch; each coil was soldered to the cylinder twice every time, in the same place as* where the steel was already soldered together. About a foot of wire was left over at each end. The two ends were hemispheres of brass, taken from two 2\d. light brass knobs, one was soldered to the steel cylinder whilst it was still on the wood, which was then withdrawn, and the other half knob then soldered on; the wire ends were then wrapped round a ridge in the hemi spheres and soldered, and the ends passed round and fastened to the bicycle valve and tap, which had previously been soldered to the top or summit of the dome-shaped knobs before fixing these latter to the c) linder. Only that portion of the bicycle valve which was absolutely necessary was used, the rest being dispensed with or sawn off with a hack saw. The tap was a light ordinary one, and was, as a matter of fact, part of the whistle of an old small toy locomotive. Testing for Air-tightness. Having completed the construction of the container, it was necessary to look for leakages. A V-shaped metal trough was next constructed, in which the reservoir, whilst still connected with a bicycle foot pump (previously tested for leakages and remedied), could be placed. About a pint of paraffin was poured into the trough ; water was not used on account of its rusting properties. Several leaks were found and remedied, until finally there only remained a very slight leakage at the tap, which was not cone- shaped and could not be properly tightened up. The particular thinness of steel with which the above was constructed cannot be obtained commercially, the small coil from which the little piece given to the writer was cut was valued at some ,£15 odd, but copper or brass foil can easily be obtained, and although these (if of equal weight) would not have more than half the strength, if that, of steel, still if well wrapped with steel wire, I see no reason why such cannot be safely used up to a pressure of no or 120 or even 150 lbs. The hemispherical ends are of great strength, in order to prevent the ends blowing out entire, and to further strengthen the container, a piece of steel piano wire of about 20 gauge should pass through the cylinder from end to end, out at or near the dome summits at both ends, and be securely fastened either round or to suitable fittings, such as the valve and tap. It is extraordinary how rigid one of these fine tubes are when completed, and there is no trouble on that score at all. The above container is being fitted with planes, chassis, engine, &c., and the model (which comes out in about the nature of a 9-oz. weight only) is nearly completed. Unless anything unforeseen occurs it will be illustrated and described in the next issue. ( 7o be continued.) A Single-screw Self-rising Tractor. By JAMES C. BALDEN (Scottish Aero Soc. Model Aero Club). Weight. Main plane, 25 ins. by 5$ ins. (gap in centre of 1^ in.) J oz. Tail plane, 10 ins. by 4 ins. ... ... ... f ,, Rudder plane, 5^ ins. by 3 ins. ; body, 33 ins. by $ in. by \ in. ; chassis, 19 gauge tempered steel wire, J-in. round cane, and \ in. by £-in. flat cane ; wheels, aluminium, if in., distance apart 8J ins. 2§ ozs. Propeller, 10ins. by \\ in., Murray, Sons and Co. f oz. Pitch, \2\ ins., approx. Rubber, 6 strands, TVin. strip \ ,, 19«»UGt UMPEBEB STU1WIS* FINE 5HA5S TUBINC INTO WHICH TH* W1NCS FIT H>U*XEI> HEW ^S^ 44 ozs. ELASTIO) 3cfc Method of attaching wings (on left) In Mr. Balden's single screw tractor, and on right elevation showing how angle of incidence is obtained. 1220
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