02 



NATURE 



[July 13, 191 1 



the air velocity at which the results were obtained are 

 - null. 



The question is one which is at present mainly of theo- 

 il interest, and the importance of which lies in the 

 light it may throw on the comparison of water and air 

 mces. Lord Rayleigh in a second note, also printed 

 in the technical report, has discussed the mailer further, 

 and has pointed out some difficulties in reconciling the 

 general formula with certain conclusions from experiment. 

 Some evidence is furnished by results recently obtained by 

 Dr. Stanton in experiments on the flow of air in pipes 

 and by the study, by visual methods, of the flow past 

 obstacles in a water channel, but the matter demands 

 further investigation before a final conclusion can be 

 ai 1 ived at. 



Friction uj Mr in Pipes. — Among the reports included in 

 the technical report is a preliminary communication by Dr. 

 Stanton of some results obtained for air friction by means 

 of experiments on the flow in pipes, in which the effect of 

 cnanges in the dimensions and roughness of the pipes is 

 discussed. Some of the pipes tested were artificially 

 roughened by cutting right- and left-handed screws along 

 the inner surface of the pipes of pitch and depth propor- 

 tional to the diameters. It is interesting to note that the 

 dimensional relation for these artificially roughened pipes 

 is precisely similar to that found by Messrs. Bairstow and 

 Booth in their examination of the experiments on the 

 normal resistance of flat plates of different sizes, referred 

 to above. 



Whirling Table and Propeller Tests. — A description of 

 the whirling table and of the design of the dynamometer 

 was given in the report of the committee for the year 

 1909-10. A number of tests on propellers of different types 

 have been carried out with the apparatus there described, 

 and particulars of some of these tests are given below. 

 Recently, with a view to obtain increased propeller speeds, 

 up to 3500 revolutions per minute, and a greater range and 

 sensitiveness in the measurements, a motor of greater horse- 

 power has been provided to drive the propeller, and a new 

 dynamometer has been designed and constructed. A brake 

 has also been added, since at high propeller speeds the 

 propeller alone in some cases drives the whirling arm faster 

 than is desired. 



With the view of reaching as high an accuracy as 

 possible in the future tests, especially at the higher speeds 

 of translation, a careful study has been made of the motion 

 set up in the air of the whirling table shed by the rota- 

 tion of the whirling arm. As a result of the experi- 

 ments, it was found that when the end of the arm was 

 travelling at a speed of 35 miles per hour the mean velocity 

 of the air in the shed at the boundary of the circle described 

 by the arm was about 2 miles per hour, while the velocity 

 of the air into which the arm was entering was 16 miles 

 per hour. The air velocity was also found to be approxi- 

 mately proportional to the arm speed. The second figure 

 gives the air swirl correction to the arm speed at 35 miles 

 per hour required for the purpose of the propeller tests. 

 In all future tests a direct determination of the swirl 

 velocity will be made and the necessary correction applied. 



Effect of Blade Area on Propeller Efficiency. — At the re- 

 quest of Captain Sueter, a series of tests was made to 

 determine the effect on propeller efficiency of varving the 

 width of blade. The tests were made on model propellers 

 designed and supplied by Messrs. Vickers, Ltd., whose 

 representative visited the National Physical Laboratory for 

 a few days in order to take part in the work. Messrs. 

 Vickers wen also -,„„! enough to furnish the results of 

 tests made at Barrow on a full-sized propeller, in order 

 that these might be compared with the results of the small 

 model experiments mad* at the laboratory. 



For comparison with the full scale results, a tesf was 

 first made on the corresponding model propeller at a speed 

 of translation having a ratio to the test speed of the full- 

 sizr.l propeller equal to the ratio of the square roots of 

 their linear dimensions. It was found that, for the same 

 slip, the thrust and efficiency given by the model experi- 

 mi tits diffi red only by a small amount from the values they 

 should d duced by calculation from the full scale 



tests. The experiment is important from the point of view 

 of the prediction of full scale results from small model 

 tests, but the work so far done is not sufficient to jtisiifv 



no. 2176, vol. 8;] 



any general conclusion as to the validity of the " model " 

 law, which proved in this instance to be correct. It is 

 hoped that further comparisons may be carried out shortly. 



The further tests on the series of models were made at 

 the speed of translation suggested by this preliminary work, 

 and by reducing the width of blade from that used in the 

 above experiment an increased efficiency was obtained. It 

 was found that the maximum efficiency was reached at a 

 disc area ratio of approximately 0-19. 



Other Propeller Tests. — A series of tests has been made 

 for the superintendent of the Army Aircraft Factory en 

 some Ratmanoff propellers, to the design of M. Drzewiecki, 

 who also paid a visit to the laboratory. These tests are 

 not yet entirely completed, the intention being to carry 

 them up to speeds of the propeller tip in the model equal 

 to those occurring in practice with the full-sized propeller. 

 For this work the new apparatus recently installed is 

 required. 



The particulars given in the detailed account of the pro- 

 peller tests of the work so far done will, however, be found 

 ol interest. As is well known, the aim in the design of 

 this propeller is that each element of the blade should 

 strike the air at the same angle of maximum efficiency, 

 the propeller being run at a definite ratio of translational 

 speed to speed of rotation. The maximum efficiency 

 reached with any of the propellers tested, at the propeller 

 speeds attainable at the time when the tests were made, 

 was 67 per cent., at a speed of translation of about 30 miles 

 per hour. 



In addition to the above, other experimental tests have 

 been made, and some propellers have been tested for private 

 firms or individuals. 



Balloon and Aeroplane Fabrics. — A considerable amount 

 of work has now been carried out at the National Physical 

 Laboratory in connection with the testing of fabrics. The 

 materials tested have included rubbered fabrics by various 

 makers, oilskin, varnished silk, and other fabrics with 

 special proofing, goldbeaters' skin, &c. 



Strength Tests. — The apparatus employed for tensile tests 

 has been supplemented by a testing machine by Messrs. 

 Avery. This has been modified to enable wide variations 

 in the rate of loading to be obtained. The dimensions of 

 the test specimen now regularly employed are 20 cm. 

 between the jaws of the testing machine by 5 cm. wide, 

 and the usual rate of loading is such as to fracture the 

 specimen in not less than two minutes. 



Experiments to determine the effect of varving the rate 

 of loading have been made, and it was found, for a par- 

 ticular fabric, that the ultimate strength found by rapid 

 loading was about 14 per cent, higher than that found by 

 slow loading. The rate indicated above as that now 

 employed is practically equivalent to a dead-slow rate. 



The existence of the speed effect just mentioned sug- 

 gested the probability of a fatigue effect, and this question 

 was also investigated. Some difficulty was experienced in 

 devising a satisfactory method of test, as owing to the 

 large and unavoidable variations between one sample and 

 another, the usual methods of making fatigue tests are not 

 applicable. With the method finally employed it was found 

 that the strength of the particular fabric tested to with- 

 stand repeated applications of stress was about 1 1 per 

 cent, lower than the strength taken on a single specimen 

 loaded to rupture. 



Bursting Tests. — Difficulties were originally found in 

 making bursting tests owing to the fact that most of the 

 earlier cylinders tested broke at the join. Finally, a 

 cylinder of diagonally doubled material was obtained which 

 did not burst at the join, and which broke at very high 

 stresses. Damage done in this test led to the redesigning 

 of the apparatus, and in the new apparatus arrangements 

 have been made to enable the cylinder to be subjected to 

 longitudinal tension in addition to internal pressure. 



An account is given in the technical report of an interest- 

 ing series of tests carried out with this apparatus. In 

 these tests the ratio of the circumferential to the longitudinal 

 stress varied from 2 : 1, corresponding to pure bursting 

 test, to 0:1. corresponding to a pure tension. The tests 

 appeared to indicate that the strength in warp or weft is 

 approximately independent of stress applied in the direc- 

 tion at right angles. The behaviour of fabrics under 

 various ratios of compound stress is being further examined 



