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Aviation History
1921
1921 - 0423.PDF
JUNE 23, 1921 SUPER-SPEED PROPELLERS* Air Screws with Blade Tip Velocity Above that of Sound AIR propellers are rarely driven with a tip speed even as high as 950 ft. per second, and there has been a view current among aeronautical engineers that a critical point exists at or near the velocity of sound, say, 1,100 ft. per second, which would act as a physical limi: and beyond which there would occur something like what is known with marine propellers as cavitation. Apparen'ly the only published attempts to explore the region of higher speeds are those which appeared in a paper issued March, 1919, by the British Ad- visory Committee for Aeronauiics, in which a tip speed of 1,180 ft. per second was reached with a two-blade nine-foot propeller, when the observaiion was made that " as the tip speed approaches the velocity of sound the usual air flow entirely breaks down, the slip s'ream rapidly diminishes, through that velocity. This result was obtained up to avelocity of nearly 1,600 ft. per second, beyond which he did not conHnue his researches for the time being. Mr. Reedthen had his models and tes ing apparatus reproduced on a working scale capable of absorbing 100 h.p., at the fac'.oryof the Curass Airplane and Motor Corporation, at Garden City, L. I , and Mr. Gilmore, Chief Engineer, placed the testsin charge of the Director of Research. Mr. Arthur L. Thurston, under Mr. Reed's direction. The Curiiss engineers found Mr. Reed's laboratory resultsfully verified, and the accompanying plot and photographs illustrate the test made April 6, 1921, with the 4-ft. propellerseen in the cut and which reached a tip speed of 1,508 ft. per second with a thrust of 186 lbs. This is 36 per cent. g S 0 H H H E 17-in. and 22-in. thin-bladed metal propellers, used in the early experi- ments of 1915 and 1916. H H 0 H IS H E 13 H 13 H and ultimately vanishes when the velocity of sound is reached. Air then appears to be sucked in on both sides of the disc and exhausted at or close behind the periphery ; presumably the thrust of the screw under such conditions becomes exceedingly small and may possibly vanish." Mr. S. Albert Reed, Ph.D., a retired engineer, while engaged in 1915 in private research work on acoustic pitch of high frequency, employed an apparatus which had a shaft rotating at over 600 revolutions per second, carrying a hub with radiating arms of thin metal with sharp edges. He observed Test at Curtlss plant, April 6, 1921, of a 4-ft. propeller, which reached a tip speed of 1,508 ft./sec, with a thrust of 186 lbs. that centrifugal force supplied the necessary rigidity to the arms. It then occurred to him that the field of high speeds for air propellers might be explored by a similar me hod, and he proceeded to conduct, systematically, a long series of experiments with propellers 20 ins. in length. He used a 10 h.p. electric motor of 1,150 r.p.m., geared up 12J to 1 to the propeller shaft, which therefore rotated at 235 r.p.s. The apparatus was carefully designed to permit accurate measurements of thrust, torque and speed, and he ascer- tained that the supposed physical limit at the velocity of sound does not exist, but that in fact the ratio of thrust to tip speed undergoes no appreciable variation in passing * Scientific American. beyond the velocity of sound, and is something between thespeed of a revolver bullet and that of a rifle bullet. The formula for centrifugal force, if applied, will show thatwith a propeller of 2 ft. radius and a peripheral speed of 1,000 ft. per second, 1 oz. will give a radial tension of about1,000 lbs., and at 1,500 ft. per second, about 1 ton. It is evident that a virtual or kinetic rigidity is thus supplied, whichmay sufficiently replace the structural rigidity of the usual air propeller and make possible a blade sufficiently thin andsharp to function properly at these enormous speeds. At the ordinary speed of about 600 ft. per second, the usualrather bulky and blunt propellers sufficiently meet the con- ditions ; but apparently they are not adapted to the super-speeds, which may be placed as beginning at about 850 ft. per second. In order to ascertain the efficiency of this type of propeller LB S Z K • TIP SPEED 1 - IN FE \ :T PER SECONC ' ; i 1 ' t ] Graph showing results of test of Reed's thin-bladed high-speed propeller, in which there was a steady increase of thrust as the blade-tip speed rose from 900 to over 1,500 ft./sec. for actual flying the experiment illustrated in the cuts was repeated under approximately wind-tunnel conditions, the wind being supplied by an aeroplane anchored close in front of the Reed propeller and delivering its slip stream axially down the slip stream of the Reed propellers, the wind velocity being carefully plotted with a Pitot tube, at about 42 m.p.h. The efficiency factors so determined were found to be high, and a 9-foot propeller of the same design is now nearly ready for testing without gear on an aeroplane, av<i both on the ground and flying. Mr. Reed has prepared a paper for one of the scientific societies, giving the details of his researches quite fully, and it will be published in the near future ; but there appears 423 -<«*•;:
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