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Aviation History
1922
1922 - 0577.PDF
Those wishing to get in touch with others interested in matters relating to gliding and the construction of gliders are invited to write to the Editor of FLIGHT, who will be pleased to publish such communications on this page, in order to bring together those who would like to co-operate, either in forming gliding dubs or in private collaboration. SEVERAL more entries have now been received for the Daily Mail gliding competition at Itford Hill, Sussex, and a considerable number of machines, although not definitely entered at the moment of writing, will undoubtedly be entered before the closing date, Saturday, October 7. Com mander Perrin, who has just returned from France, learned that at least four French gliders will be entered. One of these is the Farman flown by Bossoutrot in the French competition at Combegrasse, and another is the Dewoitine monoplane on which Barbot (whom the Daily Mail will •insist on calling Barpot) flew for 20J minutes. This machine is a cantilever monoplane with thick wing section, and is stated to be very scientifically designed. At present it is tnot known what the other two machines will be. * * * MR. H. E. WAITE, of Morecambe, Lancashire, has entered a monoplane with some form of flapping wings, having a span of 54 ft. Mr. Waite is the constructor of the machine, and will pilot it himself in the competition. * * * MR. C. FROBISHER, of Sheffield, has entered a monoplane of 30 ft. span, the wings of which, it is stated, will be worked by pedals. This machine has been built by the Sheffield School of Aeronautics and Engineering. The weight is given AIR-SAILING as about 100 lbs. The machine will be piloted by Mr. Frobisher. * * • A BIPLANE of 26 ft. span, and weighing approximately no lbs., has been entered by Mr. A. P. Maxfield, of Birming ham, who will pilot the machine himself. * * * IT seems that flapping wing machines will be much in evidence at the competition. Another ornithopter mono plane, whose wings will be operated by foot-power, has been entered by Mr. H. S. Dixon, of Ealing Common. The machine is stated to weigh but 50 lbs., although the span is 30 ft. We hope Mr. Dixon will win in the foot-power versus foot-pounds match. * * * MIJNHEER FOKKER has intimated his intention of entering his two biplane gliders for the competition. It was on one of these, it will be remembered, that Fokker made a flight of 13 minutes' duration (hors de concours) in the Rhon. The machines are characterised by great simplicity, and it is stated that they took only 10 days to build. * * * THE de Havilland glider, a full description of which appears in this issue, has not yet been entered, but it is practically certain to be among the starters on October 16. In addition to the entries mentioned above, there is reason to believe that at least another half-dozen will be entered by Saturday, so that, given reasonably favourable weather, the competition should provide a great deal of sport, even if no sensational flights should be made. WHICH IS THE "BEST" WING SECTION FOR A GLIDER? Some Fundamental Considerations in Choice of an Aerofoil FROM the number of enquiries which we have received, it appears that among those who are interested in gliders and gliding there are many who find considerable difficulty in visualising what, exactly, are the characteristics one must aim at to get the best results. In taking all factors into consideration one is, as a matter of fact, faced with almost as many conflicting problems as in the design of power-driven machines, but by leaving out some of the factors it is, at any rate, possible to fix one's ideas and to sort out some of the more important features which must be incorporated in a design if good results are to be obtained. From enquiries received it would appear that there are two pitfalls into which the unwary are apt to fall. One is to assume that if a certain wing section has a very high value of the L/D ratio it must necessarily make a good glider, because the gliding angle is very small. The other view is that one must have a high-lift wing so as to get a iow gliding speed. The actual facts are, of course, that what one should aim at is to get a wing section which will combine both at the same angle of incidence. This should be clear if one remembers that a high L/D, or small gliding angle, may give a very flat glide, but that if the lift coefficient corresponding to this gliding angle is small the gliding speed will be high, and consequently the rate of descent may also be high. Now in order to make use of winds of relatively small velocity and of small upward trend, the rate of descent should be as small as possible, and this is attained by making the gliding angle and the gliding speed small, at the same angle of inci dence. Therefore, the wing section which combines the highest L/D with the highest lift coefficient will give the lowest rate of descent. It may not be without interest to examine, briefly, the considerations which lead to this conclusion, and to establish the very simple ratios which give the best results. In doing this we shall attempt to be as clear as possible without going into " mathematics " other than the most simple. The only thing which we take for granted is a slight knowledge of the fundamental principles of trigonometry and algebra. The gliding angle € is, of course, given by the well-known trigonometric ratio tan e = —, where D is the drag and L the lift of the wing. Expressed in terms of the drag and lift coefficients, ko and hi. respectively, the gliding angle is tan e =-^r • The rate of descent, which we will call Vf i.e., vertical velocity), is given by Vv — V sin e, where V is the air speed of the machine. For small angles, such as would come within the range of gliding angles of a glider, the sine and the tangent of the angle are sufficiently nearly equal to allow of substituting tan e for sin t, and we can, therefore, write Vo = V tan e instead of writing V« — V sin e. In order to find the gliding speed V we make use of the well- known fundamental formula W = ftLp A V2, where W is the weight of the machine in lbs., ki, is the lift coefficient of the wing section used (in " absolute " units) p is the density of the air, A is the wing area in square feet and V the velocity, either in m.p.h. or in ft./sec. When V is in m.p.h. the value of p in the equation is o-oo5i. When V is in ft./sec. the, value of p is 0-00237. Now it will be seen that this fundamental formula can be written V -J, W •, from which V can be found V fa. X p X A' at any loading and lift coefficient. We have already estab lished the formula V» = V tan e, and have seen that tan « == —. If we substitute these two values we get w V >t X p X A X ?2., or V„ fa -</; \v tin p X A fa3/* The quantity under the square-root sign is the wing loading. Consequently the rate of descent is smallest (for that wing fa8 loading) when the ratio — is a maximum. This ratio can, ku of course, be written as U) X fa, or as (L/D)2 x hi.. In order, therefore, to determine which wing section gives the lowest rate of descent for a given wing loading we must find the one which gives the highest value of the ratio (L/D)1 X fa. An examination of the characteristics of a very great number of gliders reveals the fact that the great majority have a wing loading of approximately 2 lbs./sq. ft. If, therefore, this figure is assumed as a fair average, we can compile tables of gliding angles, gliding speeds, rates of descent, etc., for a number of different wing sections. This wing loading has been assumed in the following tables, in which the first column gives the angle of incidence; the second gives the L/D ratio at that angle of incidence, and the third the corresponding lift coefficient in " absolute " units. In the fourth column are tabulated the squares of the L/D ratios, and in the fifth the square of the L/D ratio multiplied by the lift coefficient. The sixth column contains the velocity corresponding to the different lift coefficients (obtained from the equation v-y; w -). Finally, ^ fa. X p X A' the last column contains the value of the rate of descent 577
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