FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1914
1914 - 0583.PDF
MAY 29, 1914. \fBn Edited by V. E. Some Experiments with Steam Plants for Models. THE writer has recently been making some experiments with aerial propellers driven by steam in which the boiler (of the ordinary type) is fitted with a steam gauge in order to acquire some really reliable knowledge of the steam or air pressure necessary to drive a propeller at some given speed. The boiler used was of copper, brazed, vertical with one central tube ; some 3 to 4 ozs. of water were used. The boiler was heated by a gas ring containing 16 holes, fed by a half-inch gas pipe, connected in turn with a larger one direct off the main. The engine used was a double acting single cylinder one of the toy locomotive type, fitted with a slide valve of the cylinder type. Its weight, 2J ozs. The propeller used was a 12 in. one, central type. In one case in a J-min. run, in which the pressure fell from 60 to 40 lbs., the mean being 50 lbs., the number of revolutions of the propeller was at the rate of 1,200 per min. In another test in which the mean steam pressure was 52 lbs., and the length of run the same, the r.p.m. were 1,360. In another trial in which the initial pressure was 100 lbs., falling to 50 lbs. at a mean pressure of 70 lbs., the r.p.m. were also 1,360. At first sight this seems curious, but can be accounted for, I think, on the score of leakage on the part of the engine, which was not, of course, of the really high-pressure type. In another case in which a pressure of approximately 60 lbs. was maintained throughout the run, the r.p.m. were about 1,560. The speed indicator used, so far as its moving parts were concerned, was of almost no weight or friction, and could not have appreciably delayed the number of revolutions per minute. The thrust of the propeller at the above speeds has not yet been taken, but this will be done later on. The employment of a pressure gauge is most interesting and instructive, and we hope others will carry out similar experiments with the same. The greatest difficulty was experienced in keeping up the pressure. The cubical contents of the boiler was 6 ozs. of water. The gas stove gave a run of 3 minutes and a half at a mean pressure of 40 to 45 lbs., using 3 ozs. of water. The r.p.m. of the propeller in such a case was between 1,200 and 1,300. The little engine worked well and showed no signs of giving out even at 100 lbs. pressure. In some earlier experiments an engine 075 of an ounce in weight, single acting oscillating, was used, and without difficulty drove the same propeller up to 1,000 r.p.m. The steam pressure was not taken. The heating agent was a vapour lamp, Mr. C. L. Pratt's Olympia steam-driven model. methylated spirits being the fuel. This engine was soft soldered, and although I have not as yet taken the other to pieces it is, I presume, the same. Some Deductions from the Above. Let us assume for a moment that a centrale propeller 12 ins. in diameter at 1,360 r.p.m. gives a thrust of 2 ozs. [perhaps some reader who knows from experiment ust what it is will kindly send JOHNSON, M.A. it along]. This is sufficient to fly an 8-oz. model. Now, taking a model of the worst type of flying-stick. Let us put the plant at 4J ozs. and the fuel at I i ozs. This leaves 2 ozs. for the rest ; not much certainly, but still possible. Now for the power plant. The engine presents no difficulty, and can be put down at 4 oz. The lamp ditto at I oz. We have then 3 ozs. left for the boiler, and in this we appear to have but little choice other than a very small pressure-fed one of the flash boiler type and one of a very light tubular type. In the latter case it is extremely difficult to keep down the weight. The tube must be seamless and brazed. The water fuel cariied would only be 1 oz. and the heating fuel J oz. Length of flight 30 to 40 sees. I am perfectly prepared for this idea to be ridiculed, but as a matter of fact I have progressed much nearer its solution than in the experiments related above. The ordinary type of flash boiler and lamp is constructed on altogetheriwrong principles so far as obtaining anyihing like maxi mum uniform heating effect. The manner in which the coils are arranged greatly interfere and shield one another, with the result that whilst parts of the tubing are at red or almost white heat, parts are comparatively speaking cool. Heating by means of flame is one which is extremely wasteful of energy. Some remarkable results have been obtained during recent years with what is known as surface combustion or heating, in which the burning gases are caught in suitable substances and rendered white hot (like an incandescent mantle), which latter in turn heat the desired body— round which they are packed—in a far more economical method than when bare flames are employed, in which enormous losses are entailed by radiation only. Some Further Experiments. Some further experiments were carried out on the losses entailed by friction in the case of very small steam engines. These were found in several cases to amount to more than fifty per cent., which I think goes far to explain how it is that good results (at high speeds) can be obtained from engines whose pistons rattle in their cylinders. Of course I do not maintain that such do not waste steam. But which is the worst evil—loss of efficiency due to wasted steam, or to friction owing to tight packings, &c. ? So far as my experiments go (always, of course, in the case of small motors), the former is the lesser evil. In highspeed engines the actual rush or momentum of the steam or gas is all-important. Its mass may be very small, but it moves at a high velocity. A really loose-fitting piston for a low-pressure engine would of course be useless. I have referred to the difficulty of keeping up a high steam pressure, and the manner in which, in boilers of small capacity, the pressure falls ; a 20 lb. drop in 10 sees, was quite common. This shows (in the case of compressed air motors) how rapidly the pressure must fall, for in this case Mr. D. A. Pavely's Olympia steam-driven model. nothing is being done, as in the case of the water boiler, to keep it up. Olympia Steam-driven Models. Quite a number of steam-driven model aeroplanes and plants were exhibited at Olympia, although these were unfortunately unable to give a good account of themselves at the official tests, such, that is, as were submitted ; we take their advent at Olympia as an extremely healthy sign. It clearly shows that there are a number 583
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
