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Aviation History
1930
UNTITLED0 - 0091.PDF
FLIGHT, JANUARY 3, 1930 CORRESPONDENCE [The Editor does not hold himself responsible for opinions expressed by correspondents. The names and addresses of the writers, not necessarily for publication, must in all cases accompany letters intended for insertion in these columns.] INTERNATIONAL COMPARISONS [2255] In his letter No. 2244, of December 20, Mr. C. C. Walker has taken up the cudgels in defence of the British aircraft manufacturer. Unfortunately, the method he adopts acts like a two- edged sword, and shows some perfectly worthy British aircraft as not being eligible for a Certificate of Airworthiness. The graph given in the letter purports to indicate whether or not an aircraft will satisfy certain take-off and climb conditions. The variables are the wing and power loading ; for the purpose of generalising, airscrew efficiency, aspect ratio and parasite drag coefficient are presumably taken as constants for constant values of wing and power loading. The stalling speed is one very important factor in both the run to take-off and the rate of climb. Mr. Walker has assumed this to be only a wing-loading function. The curve he gives, therefore, is only valid for one particular value of maximum lift coefficient. A change of maximum lift coefficient from 0'73 to 0-52 involves an increase of 18 per cent, in the stalling speed, for the same wing loading. Thus the optimum climbing speed a 0 / 30 25 15 o CLJQVETO CL INI B" MB3 10 20 13Mlf75 FT 30 4 STALLING \ \ CUDVE "A TO CLEAR 68 54C YDS FROM \ C 50 6O SPEED M P \ 70 H F RE \ T ST \ 8O 90 for the high lift wing will be about the stalling speed for the low lift wing. The climb of the high-lift wing (other things equal) will therefore be greater than that for the low lift wing, but Mr. Walker's graph does not take this into consideration. From the cases I have investigated, it appears that the curves refer to machines having a maximum lift coefficient of 0-52 (approximately R.A.F. 15). I have replotted the curve against stalling speed in m.p.h. instead of wing loading basis on this assumption. This should give more satisfactory comparisons than the earlier graph. Below a stalling speed of 65 m.p.h. curve B is the criterion of performance, and not curve A. At the present moment stalling speeds much over 60 m.p.h. do not seem to be popular, so can be ruled out of the ques- tion, and curve B only need occupy our attention. A simple formula I use to find the approximate maximum power loading to satisfy the climb requirements is W ^ 2220 r • P "** 80 -j- V, W = All up weight, lbs. P = Normal b.h.p. Vs = Stalling speed, m.p.h. Latest News of Sir Alan Cobham SIR ALAN COBHAM, who is on a flight to the Cape, is now(January 1) at Ndola (Northern Rhodesia). According to reports received in Bulawayo, he is unable to take off owing This seems to be similar to curve B. As pointed out by Mr. Walker, there will be many excep- tions to curve B, although the variation should not be more than ± 5 per cent, for machines, which will just satisfy requirements (on a stalling speed basis). This will probably modify Mr. Walker's remarks about the foreign aircraft " P," which, if of Continental design, is probably a high-lift monoplane. W. R. ANDREWS. Technical Department, A. V. Roe and Co., Ltd. December 23, 1929. ACCELERATION J2256] Originally Mr. Russell said that an acceleration produces a force, now he says that the lift produces the acceleration O-f the aeroplane ; surely both statements cannot be true. May I suggest that the sound way of looking at it is this, that it is the force being transmitted to the various masses via the structure that produces the stresses in the structure ; just as it seems unsound to say that the accelera- tion of a train causes loads in all the couplings—when the couplings are transmitting the engine pull. Now about St. Paul's : acceleration is rate of change of velocity, so if a body has no velocity and continues to have no velocity it can have no acceleration. Certainly there are two forces acting on St. Paul's, but what the force of gravity produces is the reaction, not an acceleration. Likewise there were two forces acting on Mr. Russell's lift—but what I queried was his statement that it had two accelerations. The pince-nez, too, produced two equal and opposite forces in a horizontal plane and therefore no velocity and no accele- ration ; for there can be velocity without acceleration but no acceleration without velocity. No matter how many forces act on a body it can only move along one path and have one acceleration. It could only have two accelerations if it could travel along two paths simultaneously, which is absurd—though I admit that a lift that could be at different floors simultaneously might be useful. However, this is a question of physics, not meta- physics, and I cannot credit that an engine can have two simultaneous and separate existences, paths, and rates of change of velocity. Mr. Russell says finally that the " nett acceleration " is the difference between the load factor and gravity ; this is so in horizontal flight, but the statement is untrue all the way round a loop ; and in defining load factor he evidently refers to his (imaginary) gross acceleration. Surely it would be much simpler to define load factor as applied load divided by machine weigh!. W. E. GRAY. Edgware, Middlesex. December-23, 1929. AIR SURVEY [2257] There is an error in the article appearing in the issue of FLIGHT dated December 27, under the heading of " A New Air Survey Contract in Northern Rhodesia." This occurs on page 1344 when referring to the survey of the Irrawaddy Delta. It is stated that Major Cochrane Patrick joined the undersigned and carried out the flying and technical supervision of the survey. While it is a fact that Major Cochrane Patrick in this instance carried out the whole of the survey flying very efficiently, he had nothing whatever to do with the technical supervision, and 1 think it will be only fair to those who were responsible for the technical supervision, that the error should be corrected. AIR SURVEY CO., LTD., R. C. KEMP, Managing Director. London, S.W.I. December 30, 1929. <3> <S> to the condition of the ground which has been rendered swampy by several days' rain. Another Soviet Air Line A REGULAR air service between Moscow, Baku, and Teheran was opened on January 1. 91
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