3IO 



NA TURE 



[MaV 12, 1910 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for opinions 

 expressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the ivriters of, rejected 

 manuscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous communications.] 



The Stability and Efficiency of Kites. 



In Nature of March 17 Mr. Dines suggests that the 

 instability of kites may be due to changes in the relation 

 of the weight to the velocity of the wind, or to deforma- 

 tion of the kite by excessive pressure, or that there is a 

 critical velocity at which the forms of the stream-lines 

 become so altered that instability results. 



My own experience leads to the opinion that deforma- 

 tion b}' a strong wind is practically the only important 

 cause of instability. At the Blue Hill Observatory, where 

 the conditions for experimenting are unusually severe, the 

 Clayton modification of the Hargrave kite is the only one 

 that can be employed. In this form the longitudinal sticks 

 are continuous from one cell to the other, and the lateral 

 sticks form the front and rear edges of the cells which 

 prevent the fluttering of the cloth unavoidable in kites 

 the rigidity of which depends upon the tension of the 

 covering. The Clayton-Hargrave kite is rigid, but there 

 is no strain or tension anywhere except when flying, and 

 it has proved itself to be sufficiently stable. Relief from 

 sudden and excessive strain is necessary, and to secure 

 this the bridle is made elastic, so that in strong winds 

 the angle of inclination becomes smaller. Thus equipped, 

 the kite is not uniformly efficient in strong winds, for, as 

 the angle of inclination becomes smaller, the pressure of 

 the wind upon the edges of the cells becomes relatively 

 greater, and the altitude is reduced. A normal altitude 

 of 55° to 60° may be lowered to 40° by an increase of 

 wind of 15 to 25 metres per second. If the front cell of 

 the kite is equipped with rigid curved lifting surfaces the 

 efficiency is greatly increased, the mean altitude exceeding 

 60°, and the loss due to increase of wind is unimportant 

 in velocities up to 25 metres per second. 



Some of the lightest of these kites have flown in the 

 strongest winds encountered while experimenting, the 

 velocity in some instances having exceeded 30 metres per 

 second. An interesting example of this kind occurred on 

 April 14 during the international ascension. Two kites, 

 weighing 600 and 850 grams per square metre, and having 

 lifting surfaces of 11 and 7 square metres, respectively, 

 were employed to lift the line. The outer section of the 

 line was 1500 metres of wire having a tensile strength of 

 140 kilograms, and the next was 2500 metres long, having 

 a tensile strength of 180 kilograms. The large light kite 

 was placed at the outer end of the line, and the other at 

 the junction of the two sections. At a height of 2000 

 metres, with 3500 metres of line out, two gusts of wind 

 resembling thunder-squalls were encountered, the mean 

 velocity for twenty minutes exceeding 30 metres per second, 

 and the maximum reaching 33. The strain on the line 

 at the ground did not exceed 90 kilograms, and, allowing 

 for the weight of the line, probably did not exceed no at 

 the second kite. The pul' of the larger kite in a lo-metre 

 wind is usually about 45 kilograms, and that of the 

 smaller about 35, and, allowing for the pressure of the 

 wind on the line, t'lis, apparently, was not greatly ex- 

 ceeded. The large kite will fly in a wind of 5 metres per 

 second, and was perfectly steady in a velocity of 33 metres 

 per second. The pressures corresponding to these velocities 

 are, respectively, 2 and 80 kilograms per square metre of 

 surface exposed normally ; hence it seems improbable that 

 a well-made kite could become unstable through dispro- 

 portionate weight or some unusual property of a high 

 wind. It should be said that the velocities given are 

 " true " velocities, and not to be compared with those 

 from the large Robinson anemometers, in which the factor 

 3 is employed. The maximum velocity referred to, ex- 

 pressed in English units, becomes 74 miles per hour " true " 

 velocity, 90 when reduced to the U.S. Weather Bureau 

 standard, or about 100 miles per hour when reduced to 



NO. 2 115, VOL. 83] 



tlie same scale as the Kew pattern when the factor 3 is 

 employed. 



In 1900, while comparing different wires for use as 

 kite-lines, I found that, theoretically, the lyger wires were 

 the more efficient, although slightly weaker, weight fo" 

 weight, than the smaller. The reason for this is that the 

 pressure of the wind is more effective upon the small wins 

 than on the large. A No. 10 wire weighing 2-16 kilograiii- 

 per 1000 metres usually breaks at 85 kilograms. lis 

 diameter is o-bi mm., and the surface presented to the 

 wind is i square metre for each 1650 metres of length. 

 If we wish to double the strength of our line we employ 

 a wire 093 mm. in diameter, weighing 5 kilograms per 

 1000 metres of length. The cross-section, however, has. 

 increased only one-half, the surface presented to the wind 

 being i square metre for each iioo metres of length. 



An opportunity to secure experimental data did not 

 present itself until January, 1908. Since then, in conduct- 

 ing the monthly kite ascensions at Blue Hill, I have 

 employed small kites flown with small wires, and large 

 kites flown with large wires, to determine the relative 

 efficiency of the two systems. The results show very con- 

 clusively that the system of large kites and large wires is 

 the more efficient, not only for the sizes experimented with, 

 but very probably for much larger sizes. The lifting 

 surface of the kites employed has varied from 3 to 13 

 square metres, and the line has been made up of pieces, 

 of wire varying from No. 10, of 85 kilograms, to No. 21, of 

 235 kilograms, tensile strength. 



The opinion, held by many, that large kites are inferior 

 to small kites in meteorological work is not sustained by 

 these experiments. The Clayton-Hargrave kite when built 

 with three sections can be made stronger for the same 

 weight than when made with two or four sections. The 

 increase of weight as the size increases is unimportant in 

 meteorological experiments, for kites with lifting surfaces 

 exceeding 15 square metres need not weigh more thai> 

 650 grams per square metre. The ability of these larger 

 kites to withstand high winds apparently is greater than 

 that of small kites, for the large and heavy sticks necessary 

 in the framework, like the large wires, present relativcl>- 

 a smaller cross-section to the wind for a proportionate 

 weight and strength. 



Increased stability may be secured by placing two 

 diverging vertical planes in the rear cell of a kite. If 

 these planes are adjustable, the kite may be caused to fly 

 on either side of the mean direction of the wind, or any 

 errors of flight may be corrected. 



The entire question of stability appears to be one of 

 eliminating unequal strains and unnecessary resistances. 



F. P. Fergusson. 



Hyde Park, Mass., U.S.A., April 20. 



I AM much interested in Mr. Fergusson's letter, and his 

 long experience with kites, about double my own, makes 

 me very diffident about expressing an opinion contrary to 

 his. 



Doubtless deformation is a very fruitful source of in- 

 stability, but after carrying out some thousand kite ascents 

 from a steamer and on land, I, am of opinion that it is 

 not the only cause. However, my position is that we do 

 not know with certainty the cause of instability, and it is 

 very desirable at the present time that we should know 

 beyond dispute. 



I agree with Mr. Fergusson as to the advantages of 

 large kites ; they are more stable than small ones, and, as 

 he has shown, since the wind resistance on the wire is 

 the one serious obstacle to reaching great heights, it is 

 obviousl}- desirable to make that resistance small in pro- 

 portion to the other forces. But there are practical objec- 

 tions. Large kites and thick wire require a stronger and 

 more expensive outfit, and more assistance at starting 

 and landing ; also, should an accident occur, the risk of 

 its being serious is far greater. 



I do not agree with Mr. Fergusson that the Clayton- 

 Hargrave kite is the only one that can be used when the 

 wind is strong. The conditions in England in the winter 

 are probably more severe than at Blue Hill, and have been 

 particularly severe during the last winter. Nevertheless, 

 the strength of the wind has on no occasion prevented our 



