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194 AIRCRAFT ENGINEERIN G July, 1937 Th e Effect of Slipstream on Longitudinal Stability- Mor e Information on Gust Loads. calculation on slipstream effect. It was b y no tained in this paper would appear to answer to Longitudinal Stability and Slipstream means intended that direct use should be made a considerable degree your plea for more infor T o THE EDITOR of the equations. I said in my article that mation relative to gust loads. DEA R SIR,—I read with much interest Mr. there were so many factors not amenable to You will note that the paper contains little J . H. Crowe's article on Longitudinal Stability, theoretical treatment tha t we mus t rely entirely or no information on the correlation between which was published in the March issue of on wind tunnel research. This paragraph may the strength of the gusts and the weather or AIRCRAFT ENGINEERING. It forms a useful be unfortunately worded, bu t wha t 1 mean t to other pertinent conditions. This unfortunate summary of present-day knowledge on the convey was th e exceedingly complex nature of situation arises from the fact tha t the transport glider stability problem, but is rather mis the factors governing power on stability and pilots have not found it practicable to make th e leading in the section devoted to the effects of the calculation was in fact more in the nature necessary observations except in extreme cases, slipstream. of a demonstration of the inadequacy of any such as the line squall encountered by a DC-2 To particularize my criticisms—equation (52) simple theory. aeroplane of Eastern Airlines. We tried, early is correct as far as it goes, but begs the very A more recent report, N.A.C.A. 578, also in the programm e of acceleration measurements importan t question of the variation of the notes a tendency to power on instability and on transport aeroplanes, to persuade the pilots amoun t of tail surface affected by the slip the A.R.C. report for the yea r 1935-6 says that t o fill out, a t the end of each ilight, a tabular stream. The variation of the affected tail sur this deficiency in the stability of monoplanes form which would give the desired data on face—considered quantitatively with respect to appears to be mainly due to an interference weather, etc., bu t go t no results. The principal incidence—may increase or decrease according effect of the airscrew slipstream as it is most difficulty, aside from the labour and book to the vertical position of the tail with respect conspicuous with power on. keeping that would be involved, seems to have to the slipstream, owing to the change in As regards the variation of the amount of been that the pilots did not know where to vertical displacement of the slipstream with tail surface affected by the slipstream, this is, "dra w the line " between gusts worth noting respect to the tail due to downwash and as Mr. Stafford points out, a factor which should and those not worth noting. In this connec incidence changes. Mr. Crowe may have be taken into account. One would have tion, the chief pilot of one of the transport intended to cover this effect in his statement thought this, however, a subsidiary effect. On companies suggested recently the development tha t " the presence of interference factors makes a typical machine, assuming the thrust axis of an instrument for "drawin g the line," and the prediction .. . uncertain." It would have set along the wind direction for top speed the we are accordingly now constructing instru been thought, however, that this effect was change in downwash angle at a C of 1∙4 due ments for indicating to the pilot when " 2g" sufficiently important to have been given to slipstream is abou t 3 cleg., which means very and " 0 g " are exceeded in the positive and prominence in the discussion. approximately 9 in. on th e tail plane of a 7-ft. negative directions, respectively. The various Passing on t o equation (53), even if we neglect chord aeroplane. With a normal arrangement companies have promised their co-operation in the fact that s ma y be a function of C , it cannot of tail plane the affected portion should not making and recording observations correspond be denied that R the "sli p factor" is also a change excessively. ing to accelerations in excess of the set values, function of C , so tha t (53) is not the complete and since the programme of acceleration R can certainly be expressed as a function of differential coefficient of (52). Similarly, the measurements is t o be continued because of th e C and in dynamic stability calculations must continuing advent of larger and faster transport expression for obtained by differentiation always be taken into account, as is done by aeroplanes, we may expect, eventually, to S. B. Gates in R. & M. 1118, bu t in static of equation (51) is no t correct, as the variation obtain a certain amount of the desired data. stability estimates surely it is legitimate to of R with C is neglected. If the differentia take the partial differential of moment with Meanwhile, a t Langley Field, we ar e attempt tions are carried out completely, the final respect to incidence keeping other variables ing to obtain by somewhat more "scientific" such as speed constant. Perhaps it should expression for is much more complex methods a quantitative picture of the relation have been made clear that the calculations ships between gust intensities and gradients, than is shown by equation (54), and it is not referred only to static conditions. and the kind and severity of conditions giving correct t o draw the conclusion tha t the influence I have checked equation (54) and have rise to atmospheric turbulence. We lack the of slipstream on the tail surfaces is to give the obtained the same result as given in the article. proximity of mountains for this work, but on aeroplane greater stability, in fact in many In conclusion, I trust I may have cleared up an y the other hand we have a choice collection of cases the slipstream is decidedly de-stabilizing. misconceptions for which I was responsible as mild and strong wind gradients and thermal Even neglecting the variations R and s with regards the validity of these calculations. conditions (what with tropical hurricanes, C , I find that equation (54) should read :— Wind tunnel results are, of course, indispensable. thunderstorms, and line squalls) and we should, Yours faithfully, therefore, get a t least a few answers. Yours very truly, 274, Monument Road, J. H. CROWE. I point out these features, not in any carping RICHAR D V. RHODE, spirit, but to save others from being misled in Edgbaston, attemptin g to make direct use of the equations Aeronautical Engineer. Birmingham, 16. under review. Undoubtedly, the question of Langley Memorial Aeronautical Laboratory, June 10, 1937. slipstream effects on tail characteristics should Langley Field, Hampton, Va. be dealt with by wind tunnel research. Data on Gusts May 25, 1937. Yours faithfully, To THE EDITOR " Pax," R. S. STAFFORD, A.F.R.Ae.S. DEA R SIR,—In reference to your editorial in 7, Leighton Avenue, [We understand that accelerometers to gather the May, 1937, issue of AIRCRAFT ENGINEERING, Pinner, Middx. data on the volocity af gusts are being carried on I a m takin g the liberty of sending you herewith May 31, 1937. the Europe to East Indies lines and also on two copies of a paper published in the Journal Mr. Crowe's Reply certain internal lines in India.—EDITOR], of the Society of Automotive Engineers for March, T o THE EDITOR 1937, describing the work of the N.A.C.A. on DEA R SIR,—Mr. Stafford has probably mis acceleration measurements in flight during * "Gust Loads on Airplanes," by Richard V. Rhode. S.A.E. understood my intention in giving that short transport operations.* The information con Journal. Vol. 40, No. 3, March, 1937, pp. 81-88—EDITOR. R.Ae.S . OFFICERS FOR 1937-1938 The Council of the Royal Aeronautical Mr. A. H . Hall, C.B., C.B.E., M.Inst.C.E., Vice-Presidents : Society have elected the following for the year M.I.Mech.E., F.R.Ae.S. October, 1937—September, 1938:— Mr. F . Handley Page, C.B.E., F.R.Ae.S. President of the Society: Mr. A. H. R. Fedden, M.B.E., M.I.A.E., Mr. D. R. Pye, C.B., M.A., F.R.S., Mr. H. E . Wimperis, C.B., C.B.E., M.A., M.I.Mech.E., F.R.Ae.S. M.S.A.E., F.R.Ae.S. M.I.E.E., F.R.Ae.S.
Aircraft Engineering and Aerospace Technology – Emerald Publishing
Published: Jul 1, 1937
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