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Aviation History
1909
1909 - 0422.PDF
JULY 17, 1909. MODEL PROPELLERS- RESULTS OF A "FLIGHT" CHALLENGE. THOSE who have followed our correspondence columns will have observed the challenge issued by Mr. Cochrane a few weeks ago and will also have noticed its acceptance by Mr. E. M. Wildey. Those who are interested in the question of design for small model propellers will doubtless desire to know what subsequently transpired between these two gentlemen, as also what manner of The Cochrane propeller for models consists of two flexibleblades made of corrugated aluminium riveted to a strip of the same metal. The material of the blades is lapped roundthe driving arm in order to give a thickened entering edge. The corrugations slant diagonally from the extremitiestowards the centre. .- --.•.. propellers they were that led to so great a paternal confidence in them. These things are easily reported upon by us inasmuch as our services were enlisted to act in the capacity of judge when the trial of the two types of propeller were carried through in our presence one day last week. Incidentally there are certain points in con- nection with the whole question of small propeller design which may to advantage be mentioned at the same time. To recall the terms of Mr. Cochrane's challenge, he claimed to be able to obtain more thrust from a Cochrane propeller of a certain weight than could be obtained by any other make of propeller of equal or less weight when fitted to his g-h.p., 200 volt, electric motor. A rival propeller had, in fact, to weigh less than 5^ ozs. without its boss, or less than 6 ozs. with its boss, and had then to show a greater thrust when driven by Mr. Cochrane's motor if it were to win his proffered five- pound note. It is true that originally he spoke of " best thrust for a given weight and horse-power," and it is also a fact that subsequently the motor in question was said by him to take 1 ampere when running at 970 revs, per minute on a 200 volt circuit; but nevertheless no actual stipulation was made as to brake horse-power that was to be absorbed nor as to speed of revolution, and consequently Mr. Cochrane's challenge meant little or nothing on the score of actual efficiency, or even on the score of weight in relationship to capacity or mechanical strength. This point was brought forward by us and was admitted both by Mr, Cochrane and by Mr. Wildey prior to any tests being carried out, but at our suggestion both an Elliot speed recorder and an ammeter were requisitioned in order that some definite figures of a useful character might be obtained while the various propellers were on trial. At the same early stage in the proceedings, too, Mr. Wildey disclaimed any hope of obtaining good results with his propellers at the relatively low speeds at which Mr. Cochrane's motor would run, since the very nature of the Wildey propellers precluded the possibility of heavy thrusts until very high speeds indeed had been attained. As regards this type of test, in which a propeller is simply used as a fan not as a propeller on a machine that is being propelled through the air) it must be borne in mind that no really reliable figures can, even under the very best of circumstances, be obtained that are applicable to aeroplane practice or even for the design of helicopteres. The propellers were merely spun round by the motor, the latter being mounted in one pan of a pair of scales, while the downward thrust of each pro- peller was measured in turn by ascertaining the increased weight needed in the other pan to restore equilibrium to the scales. How little bearing this kind of test of very small propellers—particularly those having a varying pitch like the Cochrane and the Wildey—can have upon any type of flyer may be judged from the very excellent article on " Propeller Mathematics For Novices " which we published a couple of weeks ago. Actual Figures Obtained. For what they are worth, however, we give herewith the data obtained from the Cochrane and Wildey "fans " on the occasion when Mr. Cochrane retained the five- pound note he had voluntarily hasarded. If the figures do nothing else, they will bring to the notice of some experimenters the fact that the current consumed by an ordinary direct-current shunt-wound electric motor increases considerably at lower speeds of revolution and that therefore the energy expended (i.e., the horse-power developed) is—unlike a petrol or steam engine—greater at lower speeds than at high speeds. We also place on record details of the two types of propeller to which these tests refer, the accompanying illustrations, taken in The "Wildey propeller for models is made up entirely ofthin sheet steel, two strips of which overlap at the boss. The extremities, which are enlarged to form the blades, arecurled round nearly 180 degrees. When in action, the air pressure on these extremities tends to flatten them out sothat they automatically adjust the pitch. The propeller is intended for use at very high speeds. conjunction with their respective inscriptions, enabling a thoroughly lucid conception to be obtained of each. -_-.-. Propeller. Cochrane. C. 2. Wildey. W. 2. W. 3. Total weight Si Weight without boss 5§ Overall diameter . Exerted thrust Current absorbed . Equivalent e.h.p. . Thrust per e.h.p. . Speed of motor . 22J . 68 • 1-85 •496 • 137 . 1040 2i 1* 144 2*>i •80 •215 i34 1640 544 14* 16 •88 •235 68 1600 *i12! 12 •62 •166 72 1720 6f ozs. 5* >. 15! ins. 20 ozs. 1-12 amp. •30 67 ozs. I4ior.p.mIn the table of results the propeller to which the original challenge referred is that indicated by the word " Cochrane," while, C. 2 relates to another of the same make which was tested for the sake of obtaining comperative data and because its overall dimensions were identical with one of the three produced by Mr. Wildey. That indicated by the word " Wildey " was the 424
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