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Aviation History
1914
1914 - 0223.PDF
FEBRUARY 28, 1914. This effect is doubtless small when the angle of attack of the wing is small, but is probably of great value at high speeds and may perhaps largely account for the flying powers of many insects. It is, however, the first point which I wish to call more special attention to, viz.:— The mean pressure on a surface due to a stream whose relative velocity varies periodically is higher than that due to a steady stream whose velocity is equal to the mean velocity of the unsteady one. A complete and formal proof of this statement is not possible, but if it be assumed that during a periodic change of velocity the conditions at any instant are the same as if the velocity at that I/OCHTj 01 02 OS Oi 05 06 07 06 O-fl 1-0 instant were steady, then the mere fact that the square root of the mean of a number of squares exceeds the mean of the roots of those squares shows that the effect of variation in velocity is to increase the effective value of the mean velocity, the pressure at any instant being proportionate to the square of the velocity at that instant. To illustrate this point with a simple numerical example, let it be supposed at a series of moments the velocity, in any convenient units, is 4, 5, 6, 7, 5, 3. Then the mean velocity is the sum of these (30) divided ty 6=5. Since, however, pressures (subject to the above mentioned assumption) vary as the square of the velocity at any instant, the mean p?-essure will be proportionate to the mean square = (16 + 25 + 36 + 49 + 25 + 9)/6 = 26-66, and the equiva lent velocity, i.e., that which, if steady, would give the same pressure, is sq/rt of 2666 = 5-16. In this case the equivalent velocity is 3-33 per cent, higher than the mean velocity, and the pressure is 6 66 per cent, higher than that of a steady wind of the same mean velocity. This use of the " Root-mean-square" is similar to that which occurs in alternating current calculations, but it must be remembered that in the latter there is, as it were, nothing but gustiness, so that the formulas in common use will not apply to this case. It may be shown when the gustiness is " harmonic " in its varia tions that, subject to the above mentioned assumption, the mean pressure varies as the sura of the square of the mean velocity and half the square of the maximum excess or deficiency of a gust from that velocity, so that the equivalent steady velocity is V = JW? + £A V2 ® ® The Britannia Airship Fund. AT a meeting of the Britannia Airship Fund Committee, Capt. Hawtrey Cox, who presided, said that a donation of ^750 had been promised by the Marconi Wireless Telegraph Co., making a total amount of ^1,900 received or promised in addition to the dona tion of Mr. Ralli announced at the inaugural meeting. The " Montgoifier " at Maubeuge. THE new French military dirigible of the Clement-Bayard type, the " Montgoifier," on the 20th inst., went from Lamotte Breuil to her new station at Maubeuge. There were eight persons en board, and the journey was made in an hour and three-quarters, the average speed working out to 75 kiloms. an hour. The " Adjudant Vincenot" at Lamotte Breuil. BY way of returning the call made at Issy by the new Clement- Bayard airship " Montgoifier," the "Adjudant Vincenot," which, it will be recalled, is of similar design, on the 17th. paid a visit to the Clement-Bayard works at Lamotte Breuil, carrying eleven officers on board during the voyage to and from Issy. Aerial Manoeuvres at Cologne. DURING last week some aerial manoeuvres were carried out at Cologne, and a typical day's work was that on the 19th inst., when the Zeppelin "Z 2" went to Dusseldorf, Duisburg and Essen, and back to Cologne, while Lieut. Appell made a 4-hr. flight to Marburg- sur-Lahn, and Lieuts. Foly and Hantelmann, each with passengers, 3-hr. trips to Frankfort. where V, = mean wind velocity, and AV = maximum excess or deficiency of gust above or below mean wind velocity, if we write AV = k. V0> then V = V„ */(I + Jk") (k = ratio of excess or deficiency of gust above or below mean velocity to that mean velocity). The following table and the three diagrams will perhaps be useful to illustrate the ideas involved :— Coefficient of gustiness ("k "). O'O O" I 0-2 0-3 0-4 o-5 06 0-7 o-8 0-9 1 0 Ratio of mean pressure to pressure of steady wind of same mean velocity. I 'O 1 '005 i"oa 1-045 1-08 via 118 1-245 132 1-405 ir5 Ratio of equivalent mean velocity to actual mean velocity. 1 0 1-0024 1-0099 I"0222 1-039 I-058 (See i-086 Fig. 1).. 1 • 116 I-149 1-185 1*224 Students of hydrodynamics will, of course, notice a certain weak ness in the fundamental assumption. It has been assumed that in an unsteady stream at any instant the conditions are the same as in a steady stream in which the relative velocity of the wind surface to the general body of the stream is the same as that which occurs in the unsteady one at that instant. I believe the analysis of the stream-lines in this unsteady stream transcends the present powers of mathematicians, so that investigation along that line would seem to neither confirm nor deny the result given. Analogy suggests, however, that the reactions cannot well be less than the values suggested, and that almost certainly when the accelerations have been taken into account the reactions will be appreciably more. This, then, leads us back to the question of the acceleration of the virtual mass. In my little book on the " Force of the Wind" (p. 24) I briefly referred to this problem, but did not clearly dis tinguish between the velocity effect and the acceleration. During the increase of velocity there is an excess of pressure equal to the product of the virtual mass and the acceleration, and again during the decrease of the velocity there is a decrease of pressure. The laws of harmonic motion show that the acceleration varies harmonically if the velocity does so and also varies as the product of the frequency and the amplitude. If the variation of velocity is harmonic (as in the case of a beating wing) then all that requires to be known is the mass of air affected. Doubtless experiment would provide some information as to this, and I suggest that laboratories - possessing wind tunnels would do well to experiment with a pulsating stream. The discharge from a reciprocating air-pump might serve the purpose. The results would probably be applicable to structural design as well as to aeronautics. ® ® Oversea Voyage by the "Sachsen." THE Zeppelin airship " Sachsen," which is at the disposal temporarily of the German Naval authorities, made a trip from Hamburg to Heligoland and back in the small hours of Tuesday morning. She was seen passing Cuxhaven at 3.30 a.m., and an hour later exchanged flashlight signals with the airship station on the island. She arrived back at Hamburg at 8.30 a.m. « ® ® 6JrJ The Slack Fund. THE following further contributions have been received:— Messrs. A. M. Forman, £$ $s. ; S. Pickles, £2 2s. ; Glasgow I.C.S. Staff per W. Keer, £1 10s. ; M. Desoutter, C. J. Sabiston, J. W. Dunne, £l Is. each ; Victor Lapeyre, F. Bjorklund, £l each ; E. E. Clarke, A. D. Frampton, R. W. Fryer, F. W. Moore, R. D. Scoular, 10s. 6d. each; R. P. Baker, W. W. Odell,'E. B. Seymour Norton, Anonymous, H. S. Brough, Mrs. Parke, J. S. Batty, II. F. Jackson, 5.?. each ; Miss K. Henn, 4s. ; Messrs. D. Y. Ferguson, A. Bourne, J. T. Holding, zs. 6d. each ; Anonymous, is. To Help the Benevolent Fund. A PRACTICAL move to assist the Benevolent Fund for aviators which has been started by the Royal Aero Club has been made by Mr. B. C. Hucks, who has decided to charge a fee of 6d. for each autograph which he gives, the money to go to the fund. Mr. J. C. Savage, manager for Mr. Hucks, estimates that during the last three years Mr. Hucks has signed his autograph over 9,000 times. 223
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