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Aviation History
1909
1909 - 0348.PDF
JUNE 12, 1509. NOTES ON PROPELLERS. APROPOS of the subject of aerial propellers, on which we published an editorial article in connection with the Paris Salon in our issue of January 9th, Mr. Gerald Stoney, M.Inst.C.E , has been good enough to send us the follow- ing notes relating to propellers in general. Mr. Stoney, who is associated with the work of the Hon. C. A. Parsons, has had unique opportunities for studying the action of high-spted turbine-driven screws, not merely when they run in water, but also when they are used in air for colliery ventilation purposes. The following notes affect, it will be noticed, two or three of the leading queries which are most commonly propounded on this subject at the present day. One is, as to whether it is desirable to in any way shroud the blades, by putting a ring round them, in order to keep the air from flying off under centrifugal force; another is, as to whether any advantage might be expected to accrue from guiding the air to and from the propeller through a fixed casing ; and the third is, as to whether wide blades are more advantageous than narrow ones. Shrouding ]NTo Use. Mr. Stoney writes as follows: — "In marine propellers it has been found that no sort of shrouding or ring round the blade tips, or the use of a coned propeller-chamber, or other guides either before or after the propeller, are much use, and what is universally used at sea is a simple propeller revolving as freely as possible in the water. The hull of the ship being so shaped as to allow it to be fed abundantly with water. " In marine work the pitch co-efficient, that is, the ratio of pitch of propeller to diameter, is generally about 1-3 in slow-speed propellers, decreasing in high-speed pro- pellers to about 0*9. It is also found that the slip should be about 10 to 20 per cent, for the most economical results. " Two-bladed propellers have also in some cases proved more economical than ones with three or four blades, but are much more liable to cause vibration; so that three-bladed are almost universally used at sea, although in some cases where there is trouble with vibration good results have been obtained by substituting four-bladed for three-bladed. Economy in Blades. " As far as economy is concerned, however, two-bladed are probably the best. In slow-speed propellers, where cavitation does not come in, narrow blades are generally used; but in high-speed propellers, where cavitation is liable to take place, the width of blade has to be increased, until in some cases the projected area of the blades is equal to as much as o-55 or o"6 of the nisc area. In the case, however, of aeroplane screws, it is quite evident that, as there is no such thing as cavita- tion in the air, narrow blades are the best; and it seems further, from experience at sea, that, unltss the blades are very narrow indeed, the thrust depends largely on the disc area, and not on the width of the blades, so that narrow blades again should be most economical. "The case of propeller fans such as are used in colleries driven by steam turbines, and fans used for other purpose0, is quite different from that of a propeller of a ship or aeroplane revolving freely in the air. There it is required to draw from a shaft of a colliery or some other definite place, and therefore casing leading up to the propeller is necessary. Similarly suitably shaped casing to absorb the velocity of the air and turn it into pressure is beneficial on the outlet side of the propeller, but, as mentioned before, all such arrange- ments are useless when working in a free atmosphere or fluid." CLOTHS AND FABRICS FOR FLYERS. AMONG the firms who are making a speciality of fabrics and cloths for use on flying machines, is included Messrs. Hutchinson, who have earned a high reputation for their pneumatic and other tyres. At present they are supplying four grades of double texture and five grades of single texture materials, and the fact that they have been adopted by the French Government testifies to the high quality of the fabrics. Egyptian cotton of the finest grade is used throughout. The finest specimen of the whole range is that which consists of an outer diagonal layer of cotton cloth, dyed yellow with chromate of lead to protect the rubber proofing from the sun's rays and a layer of rubber doubled with a straight layer of cloth, at an angle of 45 degrees, the latter also have a layer of rubber proofing. This fabric is made 42 inches wide and weighs 9§ ozs. per square yard. The other double texture cloths are somewhat similar, but some are without the inner layer of rubber and some have the threads of the warp of both layers of cotton running parallel to each oiher. The lightest weighs 9 ozs. per square yard. The single texture fabrics range from 2§ ozs. to 5§ ozs. per square yard in weight, some of them only having rubber on one side while others have it on both. When these cloths were tested by the French Government small pieces 5 cm. wide and 20 cm. long were taken and the breaking strain was 840 kilogs. per metre both in the warp and weft. With regard to impermeability, specimens were taken off which allowed for a circular piece of 25 mm. in diameter. This, after being rubbed in the hands, was placed in the frame of a hydrogen gas cylinder and submitted to a pressure of 30 mm. of water for 24 hour?, and the test stipulated that the loss should not exceed ic degrees, each degree repre- senting a loss of 1 cu. dm. per square metre. In the first trial the loss was only 3 degrees, in the second 5 degrees, in the third 4 degrees and m the fourth 3 degrees; in all cases being considerably less than the limit imposed. Messr. Hutchinson have made a special study of the materials for these purposes, and can supply the above mentioned var eties from r;tock, but are prepared to make material suitable for any requirement. BACK NUMBERS OF "FLIGHT." THE publishers have pleasure in announcing that they have secured a few of the back issues of FLIGHT, anrf any of our new readers who may wish to complete their sets may obtain the first twenty-three numbers for 3s. ad. post free, from the Publishers, 44, St. Martin's Lane, W.C. 35O
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