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Aviation History
1914
1914 - 0768.PDF
THE FLYING MACHINE AN EN By FREDERICK WILLIAM (Concluded The work that has been done on the Continent on the subject of stability does not in sum amount to much, moreover it frequently appears to suggest complete ignorance of what has been done in this country ; in this particular matter it would seem that the Continent has become insular and our island cosmopolitan. For example, we find the work of Georges de Bathezat, " Etude de la Stabiliti de P Aeroplane" (Dunod, Paris, 1911), described by Mr. Painleve as " the first to give an exact and complete discussion of the stability of the aeroplane," and when we examine the work so described we find the subject not more than half dealt with, and at that in so ineffective a manner that scarcely one of the conclu sions can be regarded seriously. R. Knoller (" Ober Langstabilitdt der Drachenflugzeuge," 1911) and Reiszner (" Einige Bemer- kungen zur Seitenstabililat der Drachenflieger," 1912), though interesting, do not materially advance the subject. R. Soreau deals with the subject of longitudinal stability under two distinct headings, equilibrium, and stability; so far as the former is concerned his conclusions as formulated will be found in their entirety published by me in 1897, with the rationale clearly set forth. (Patent Specifi cation 3608, 1897, or compare also Aerial Flight, vol. ii, p. 353-) Soreau, however, scarcely carries the matter as far as in my previous publication. Incidently Soreau gives two propositions, i.e., minimum tractive force, and minimum horse-power, which, except for differences of notation, appear to be identical with two propositions previously given by me in Aerodynamics, 1907, § 164. When we come to the question of stability it will suffice to state here that his conclusions on the subject of longitudinal stability are gravely at fault; briefly, Soreau states that the moment of inertia must not be too small for fear of oscillations becoming to rapid, whereas the only oscillation of importance—my " phugoid oscilla tion "—is virtually independent for its period on the value of the moment of inertia. On the questions of lateral stability and direc tional stability, Soreau's views (as pointed out by Bryan) are entirely at fault; the whole question of asymmetric or rotative stability is lost sight of, and the fact that in directional stability the centre of gravity cannot be treated as a static pivot (compare Aerial Flight, vol. ii, §§ 95 to 100; also Bryan, Stability in Aviation, chap, vii) is ignored. The work of Captain G. A. Crocco (" Sulla stabilita laterali degli aeroplani" also "Perfezionamenti nella stabilita longitudinale degli aeroplani," "Rendiconti delle Esperienze e degli Studi eseguiti nella Stabilimento di Costruzioni Aeronautiche del Gcnio Anno II") is of interest. In the main he follows established mathematical lines of treatment ; I have made no attempt to follow his work in detail. FROM GINEERING STANDPOINT. ppu»*jt. sumrajcg ^ LANCHESTER, M.Inst.C.E. from page 739.) In general the work has, in the past, been treated on too closely mathematical lines to be of immediate service to the engineer ; in many cases the writers have clearly suffered from their want of appreciation of the real conditions. It is my deliberate opinion that there is very little room for useful work to-day on the subject of stability unless it be rigidly and directly supported by experimental work, and from our standpoint as engineers I think we may in the future confidently look to the exellent work being accomplished at the National Physical Laboratory, and at the Royal Aircraft Factory, to keep us in touch with that which is essential in this important branch of the subject. APPENDIX I. The subject of skin friction where air is concerned has been one of considerable controversy. The quantities to be measured are so small and the apparatus employed until recent years has been so insensitive that until the work of Zahm in 1904 very little was known on the subject. Langley in his Experiments in Aerodynamics, 1891, asserted skin friction to be a negligible factor in its relation to flight. Dines about the same date expressed the same view ; in my Aerial Flight, vol. i, which appeared in 1907 (not at that time aware of the work that had been done by Zahm), I published some determinations of skin friction and attacked Langley's views, pointing out that skin friction is one of the controlling factors in the economics of flight. I also introduced the practice of expressing skin friction as a coefficient representing the resistance of a thin lamina in tangential motion in terms of its resistance at 90 degrees ; the coefficient so expressed is the double surface coefficient, and in my work is represented by the symbol £. In the greater part of my experimental work planes or laminae of mica were employed of but a few square inches area ; the largest area used by him in any of his determinations was approximately | square foot. Now it is well established that the coefficient of skin friction in a plane of small area is sensibly greater than in one of large area, consequently my values were on the whole considerably higher than those of experimenters working to a larger scale ; however, the following passage may be cited as the summary of experiments made with planes of about J square foot area and of smooth surface : " It is therefore to be concluded that for a well- varnished surface or for polished metal, under the conditions of experiment, the effective value of £ is approximately OTJ09 with a probable error of less than 10 per cent., plus or minus." (Aerial Flight, vol. i, p. 389.) According to the best estimate that can be made to-day the actual value of the double-surface coefficient under the conditions of the experiment in question should be 00081, showing an error of precisely the 10 per cent. which I allowed myself. .SI«£i.*^±u_2.' a 0 0 u I OOT [ t- M "003 U. k u 002 0 0 v H- •*-*„ | * * * Mr ,-m 1 •~—_^1B_ *•» SKKa. ~££!2ii_«uSr *****<L« SUB FA >--5'J1«£E_ _£!£«£ wee f CE ACE Fig. 38a. Captain Crocco's conclusions on the whole appear to be far sounder than those of most Continental writers ; his work is evidently worth careful study. _ The above may be taken as a brief summary of the existing literature of the subject. Excellent abstracts of the work of the foreign authors cited will be found in the appendices to the various reports of the Advisory Committee; excepting in the case of the French writers, which have been consulted in the original, I have relied on the abstracts in question for the summary here given. 768 Fig. 38b. It has been frequently stated that my results were in entire dis agreement with those of Zahm ; sometimes those making this statement ignored my lower values and took his highest, which admittedly were too high ; in other cases they read his double- surtace coefficient as a single-surface coefficient, and so made his values twice as great as they really are. In a communication to the Advisory Committee (Memo. No. 15, June, 1909), I pointed out that my own results and those of Zahm for r. and the results obtained many years ago by W. Froude for
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