FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1917
1917 - 0111.PDF
FEBRUARY I, 1917. ANSWERS CORRESPONDENTS [As a number of letters reach us signed with initials only some of which do not give a complete address, we would point out that such communications cannot be dealt with in 0«r columns. Full name and address, which will not be pub- lished, must always be given.—ED.] R. R. (London). In applying the results of tests on model wing sections to the wings of a full-sized machine, account must be taken of the fact established by experimental research that, whereas the lift coefficient is scarcely affected by changes in size and speed, the same does not apply to the drift coefficient, or, as it is now frequently called, drag coefficient, the word " drift " already having another meaning in connection with the flight of an aeroplane. Experiments at the N.P.L. have shown that the drag or resistance of a wing does not increase with Vs but at a slightly less rapid rate. The drag coefficient is not, therefore, a constant, but is subject to variation with I to IS to s V &e I N •S I-O IS 2-0 2S 3-0Los. L.V. speed. This, of course, affects the lift/drag ratio, which v ill be slightly higher in a full-size machine than that indicated by tests on models. In other words, the full-sized wing will be rather more efficient than the model. In order to deter- mine how lift/drag changes with size and speed (L.V.), experiments were carried out at the NP.L on a model of the R.A.F. 6 section, and the results of these experiments are shown in the accompanying graph, the application of which you say you do not quite follow. In this figure the ordinates are the lift/drift ratios for the R.A.F. 6 section, plotted against log L.V. as abscissa. The reason for choosing log L.V. instead of L.V. itself is purely one of convenience in plotting. L = length of chord in feet, and V «= velocity in feet per second. From the graph it will be seen that at the angle corresponding to maximum L./D., the value of L./D. is increased from about 8 for log L.V. = -5 to about 18 for values of log L.V. greater than 1 • 6. Now, in practice, experiments are rarely made on models so small and at velocities so low as to give a value of • 5 to log L.V. For instance*, a very usual chord length for models tested in wind tunnels is 3 ins., or 0-25 ft., and veloci- ties of 30 feet/sec, are quite common. Taking these figures for a basis, it will be seen that L.V. =0-25 x 30 = 7-5, and therefore log L.V. = log 7-5 = o-88. Looking at the graph, it will be seen that the difference in the value of L./D. at the angle of maximum L./D., 40, between log L.D. = o-#8 and a full-size machine of 5 ft. chord flying at 100 ft./sec.—corre- sponding to log L.D. = 2-69—is from 15 to 18, or an increase in L./D. value of 20 per cent. For other angles the difference is somewhat smaller. In other words, the lift/drag ratio of the full-size wing at the angle of maximum L./D., may be expected to be some 20 per cent, higher than that indicated by the model tests. These experiments, however, cover too narrow a range to make them directly applicable to all sections, since it does not necessarily follow that variations in L./D. ratio with L.D. is the same for all sections. They serve, however, to give an approximate estimate of how much greater the efficiency of the full-size wing may reasonably be expected to be than that found for the model. B. G. F. (Royal Fusiliers). An explanation of the problems that are troubling you would be too lengthy for our correspondence column, but you will find them dealt with in the following books • " Aero- plane Design," by F. S. Barnwell, and " The Design of Aero- planes," by A. W. Judge. Both are obtainable from " FLIGHT " offices. The prices are 2s. lod. and 9s. 6d. respec tively, post free. C. C. (Widnes). You are quite right in assuming that the phenomena known as ',' air pockets " are not " holes in the air " or spaces of rarified air, but are rather to be considered as currents and gusts of various direction. For instance, one may speak of head gusts, rear gusts,-up currents, down currents and rotary gusts. The head gust virtually increases momen- tarily the air speed of a machine, and therefore results in an upward swoop. The up current tends to lift the machine bodily, while, of course, a down current causes it to drop. A rear gust also tends to cause the machine to drop, since it lias the effect of decreasing, the relative speed of the machine through the surrounding air. Little is at present known re- garding rotary gusts, but it is conceivable that a machine entering such a whirl would be affected both by the whirl and by the linear velocity generated by it, and therefore experience a greater disturbance than if it had met a pure head-on gust. F. P. W. (Coalaston). (1) The speed required to lift a machine with a loading of 11 lbs. per square foot depends on the section employed. (2) The thrust required in relation to the weight depends on the " fineness " of the machine. If the gliding angle is 1 in 8 at a certain speed, then the thrust required at tha't speed is one-eighth of the total weight. (3) The size of spars for a loading of if lbs. per square foot and span and chord of 25 ft. and 3 ft. respectively should be : Depth, as great as wing section will allow; width, about 1 ins., spindled out to an I section between points where struts are attached to spars. Exact dimensions cannot be given without further particulars of the design. F. B. (South Baling). The outside diameter of the 100 h.p. Gnome single-valve engine is about 38 ins. For methods of mounting rotary engines, see our " Constructional Details " series published in " FLIGHT " during 1916: E. W. P. (Openshaw). You should apply to the Admiralty for the necessary form, and, having filled it up, send it to the Director of Air Services, Admiralty, S.W. G.W. Q. (Purley). We presume you mean qualified for the Royal Aero Club certificate, as it is not issued to anyone under the age of eighteen. It is hardly likely that any school would take an instructor so young. J. W. (Shipley).You must first obtain your C.O.'s permission, and, having got that, you should be able to get into the cadet school. J. J. W. (Brighouse). IUnder the circumstances, you appear to be doing more useful work where you are. If you are passed for generalservice, you would probably not be able tp join the K.1 .('. W. A. V. (Southport). There appears to be no reason why you should not be able to join the R.F.C. Cadet Corps. If passed for general service, you would not be able to enlist in the R.F.C. If you are given a commission, you would be taught at a Government school. C. T. (Hollinwood). The fact that at present you have " had no experience with aeroplanes " need not prevent you applying for admis- sion to the R.F.C. Cadet Corps if you are physically fit. You can obtain full particulars from Adastral House, F..C. Ill
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
