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Aviation History
1914
1914 - 1184.PDF
Generally speaking, the same number of pump strokes gave exactly the same number of lbs. pressure, thereby showing there was practically no pump leakage. Now in the above table 50 pump strokes give a pressure of 50 lbs. per sq. in., but with 50 pump strokes the model was capable of free night, therefore a pressure of 50 lbs. per sq. in. is sufficient to maintain an efficient model and power plant in free flight. This conclusion is, I think, a most important one, and, so far as I know, it is the first time that it has been demonstrated experi mentally. I might perhaps add, although it should scarcely be necessary, that all the expeliments were made with the greatest care an3 can be thoroughly relied on; moreover, the conclusions have been carefully kept on the right side. Conversely to the above conclusion, since the above model was capable of flying with a 50 lb. pressure, whilst carrying a load per sq. ft. of 7 "5 oz., it clearly demonstrates that the model is in every respect an efficient one, i.e., it has an efficient general design, efficient planes (although we have heard much criticism in the pastas to why in the Bragg-Smith model the trailing edge is practically as thick as the from), efficient air resistance, efficient propeller, and last, but not least, an efficient engine—efficient, that is, for the purpose for which it is intended. You can drive the engine and propeller round by blowing in the con tainer, it runs so easily, but loss of efficiency owing to wasted air (at high pressure) is more than compensated for by consequent less of friction, which, as I have stated before, in small models can easily amount to more than 50 per cent. The above tests were not carried out to higher pressures because we all know very high pressures will fly any type of model. My desire was to find out what lower pressures, if efficiently applied, are capable of. In conjunction with the above tests, and in their way as a verifi cation of them, the following experiments on the static thrust given by the propeller at certain pressures were made. The results given are the mean of a number of experiments. Friction was eliminated by placing the machine on a slight incline, until it did just not run down. The pull was taken not by means of a spring balance, which is liable to error in some cases, but by means of a fine thread running over a freely moving pulley and a vertical lifted weight. Practically speaking, the fame pressures invariably gave the same results, the differences being quite minute and due mainly in all probability to the wheels not being perfectly round and the surface not perfectly level, the latter being a thinnish sheet of (tinned) iron. When 50 strokes of the pump (i.e., a pressure of 50 lbs. per sq. in.) was put into the container, the propeller gave a static thiust of 8 ozs.—a thrust, be it noted, well sufficient to maintain an efficient machine of 1 lb. 13 ozs. (the complete weight of the model) inflight. In another experiment, when the pressure was 60 lbs., the propeller thrust was over 10 ozs., this particular weight being raised freely. At no lbs. pressure, i.e., 130 pump strokes, the thrust was over I lb., a i-lb. weight being raised easily and maintained raised for some time. In these and other experiments of a similar nature, the weight was well lifted several inches or more; no undecisive result was taken. It is worth noting that whereas a 60 lbs. pressure produces a 10 oz. thrust, a pressure of no lbs. only produces one of 16 ozs. In other words, high pressures are not so efficient with this type of motor. From the foregoing we see that an initial pressure of 75 to 80 lbs. would be ample even for an r.o.g. model, especially if means could be devised to maintain that pressure, or a pressure not falling below that of 60 lb. per sq. in. This can be done if the compressed air be heated in its passage from the container to the engine. At the present moment it is also superfluous to speak of the deadly efficiency of the torpedo. Now, the torpedo is driven by compressed air, and some of the best modern torpedoes are said to be capable of no less a speed than 43 knots, and have a range of over 1,000 yds. The Whitehead torpedo carries two propellers running in opposite directions in close proximity on concentric shafts ; that is to say, one shaft is tubular and the other (which passes through it) is solid. One of the latest improvements has been the heating of the com pressed air with which they are driven, thereby increasing both their speed and range. This can at once be seen from the following con siderations. Let us suppose that we are using air contained in steel cylinders capable of withstanding a pressure of 300 to 600 lbs. per sq. in., i.e., 20 to 40 atmospheres. This is quite ordinary, for steel cylinders can be obtained con taining air compressed to the enormous pressure of 120 atmospheres —practically liquid air—and even so high a pressure as 250 atmo spheres has been used. The most economical way of using highly compressed air is to admit it direct from the reservoir to the motor cylinders ; but this means an enormous range in the initial working pressure, entailing not-to-be.thought-of compound enginei, variable expansion gear, &c. We have therefore to relinquish the DECEMBER 4, 1914. advantages of our high initial pressure and pass the air through some form of reducing valve, whereby an approximately constant pressure of, say, some 150 lbs. per sq. in. is maintained. By a variation in the ratio of expansion the air can be worked down to considerably lower than this if desired. Now the initial loss entailed by the use of a reducing valve may be counteracted in a great measure by heating the air before using it in the motor cylinders. By heating it only to a tem perature of 3200 F., the volume of the air is increased by one half and the consumption is reduced in the same proportion, i.e., the air would last twice as long and the range of the torpedo would be doubled. Therefore what has already been done in the case of the torpedo now requires to be done in the case of model aeroplanes driven by the same power. Leaving the practical consideration of this question for a moment, let us return to the flying tests. The day was unfavourable, a heavy atmosphere and a now and then puffy wind, condition of ground very wet, soft, and uneven grass, never theless when the model had been correctly adjusted (after several very vigorous rams at mother earth which in no way damaged the machine) it never failed to rise off the ground when travelling with the wind, and several quite good and steady flights were made. With the no lbs. pressure the model is capable of rising off decent ground and making a good half minute's flight, and with the con tainer more pumped up, i.e., up to 120 to 130 lbs., a duration of three-quarters of a minute can be obtained. The instructions with the container are that it is not to be charged beyond 150 lbs., which should give about a minute's flight ; up to now it has not, however, been tested up to this. The minimum temperature in which it should be flown is 55°, another reason for healing the air. The writer is at present constructing a similarly designed plant out of some very light scrap material he has by him, and whatever the rt suits full particulars will be given later. In our next issue a full explanation with sketches will be given of how the very ingenious and simple little motor works, together with the commencement of a series of articles on how to construct this type of plant, in which the heating question will be fully dealt with. (To be continued. The Paddington Club and Research Work. We have just received the following very interesting and en couraging communication from Mr. W. E. Evans (Hon. Sec, Paddington and District Aero Club), to which we will refer again in our next issue :— " Several members of our club are about to take up research work in connection with rubber-driven models, and I should be glad if you could suggest what lines might be followed with advantage. One subject which is sure to be taken up is that of propellers, testing by means of duration the relative efficiencies of large diameter and small pitch as against small diameter and large pitch; also whether wide or narrow blades are better in a model designed for slow speed. Another point requiring investigation is the ratio of camber to the chord of a main plane ; also the best angle of incidence for main plane, with corresponding angle for elevator or tail. " Should the weight 01 rubber be governed by the total weight of the model ? Should the propeller disc area have any specific relation to the weight of the model? Given an equal rotational speed, does the thrust vary as the disc area ? If any of these problems have been settled many aeromodellists would greatly appreciate the information concerning the results arrived at. If a discussion could be opened in the columns of the model section of FLIGHT, perhaps some of these questions could be elucidated by the leading aeromodellists. " Four of our members are constructing compressed air models, but at present these will not be far enough advanced for purposes of research work. WTe want reliable models for this purpose, and only those rubber-driven which are capable of doing, say, 60 sees, duration repeatedly will be employed for this work. It seems practically certain that rubber-driven models will be discarded by the best model flyers next season, which will give a much- needed prestige to model aeronautics in the future. " I also seek your advice concerning a wind tunnel which I pro pose to construct. I can obtain the necessary power to drive a 2 ft. 6 in. or 3 ft. propeller to create the necessary current of air. How can I best measure the air speed in the tunnel ? Would an anemometer be indispensable for this, or are there any other means by which it could be measured or calculated ? Could you give me any information as to how a balancing arm could be constructed, and what would be the simplest method of measuring, say, the lift and drag of an aerofoil ? " Any information or advice concerning the above points would be greatly esteemed by the writer." 84
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