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Aviation History
1927
1927 - 0456.PDF
44 TOFLIGHT JUNE 23, 1927 THE AIRCRAFT ENGINEER efficiency of about 70 per cent., which would give a thrust urn v, , at 110 m.p.h. of X 110 - = 239 lb. For the total loaded weight assumed, this would represent a value of L/D of approximately 6 • 7, which is rather high in view of the light wing loading necessitated by the low minimum speed of 35 m.p.h. In fact, a rough estimate of the wing drag of a good modern aerofoil section, of the area necessary, indicates that the thrust horse-power required to overcome wing drag only, at the top speed.Jwould be in the neighbourhood of 40 h.p., or, again, assuming 70 per cent, propeller efficiency, approxi- mately 57 b.h.p. This would leave but 43 b.h.p. for over- coming the drag of fuselage, undercarriage, tail, and wing bracing. In other words, at the angle corresponding to 110 m.p.h., it will be no easy matter to attain an L'D of 6- 7 or so. The majority of '" normal " aeroplanes with the figures for which the writer is acquainted appear to have an L/D, at a corresponding attitude, of somewhere in the neighbourhood of 4. And that figure refers to machines much more heavily loaded than will be those entered for the Guggenheim competition. The problem comes to this, then, that what is wanted is not so much a machine having a high value of maximum L D as one having a high L'T) at small angles of incidence. In this connection it is of interest to examine the very efficient design of the Martin " PM3," published in THE AIRCRAFT ENGINEER of April 28, 1927. This machine, it will be recollected had a very high maximum L'D (about 18 full scale, 17 model), but as has been shown, this in itself avails us little. The polar curves of the Martin P.M.3, obtained at the Gdttingen laboratory, are very nearly vertical over a con- siderable range of lift coefficients at the lower end of the scale. In other words, the drag remains reasonably constant over a fair range at the high-speed end, while the maximum L/D occurs at a fairly low lift coefficient. Of course, due to the fact that the speeds corresponding to these low lift coefficients are fairly high, the power required, as distinct from the drag, goes up fairly rapidly, but on examination the curves seem promising. To be on the safe side, and in view of the fact that two engines have to be used in the Martin P.M.3, one should probably allow slightly more for total loaded weight than was done in the case of the 100 h.p. machine previously examined. In order to get at anyrate a rough idea of how such a machine might be expected to work out, let us assume that the total loaded weight of a machine of the Martin P.M.3 type is 1,800 lb. The maximum lift coefficient, according to the Gdttingen curves, is about 100. In British " absolute" units, this corresponds to a lift coefficient of 0-5. and as the minimum speed is to be not more than 35 m.p.h., the wing loading becomes 3 12 lb./sq. ft. This is, unfortunately, very light, but in spite of any increase in kL max. that may be ex- pected owing to scale effect, it will probably not be safe to assume a higher loading. Accepting this figure, therefore, a total wing area of 577 sq. ft. is obtained. The original Martin P.M.3 model as tested as Gottingen had an aspect ratio of 6. With our wing area this would correspond to a span of approximately 59 ft. and a chord of 9 ft. 10 in. By no means a small machine. With a wing loading of 3-12 lb./sq. ft., the lift coefficient corresponding to 110 m.p.h. will be 0 -Ooin British " absolute " units. From the Gottingen curves on p. 30 of THE AIRCRAFT ENGINEER of April 28, 1927, it is found that at a Ca of 10, the C(j of the Martin P.M.3 model is approximately 2-2, or 0-011 " absolute." In other words, the model L/D is 4 • 545. This is a good deal less than was found to be required. For our 1,800 lb. machine, the drag would be 396 lb. at 110 m.p.h., corresponding to a horse-power required of 116-2 h.p. Again, assuming a propeller efficiency of 70 per cent., the actual power required would be 166 b.h.p. Thug even the efficient Martin monoplane would not, appar- ently, cover the required speed range with an engine of 100 h.p., or rather with two engines of 50 h.p. each. If, how- ever, it be possible to build the machine for the weight assumed, 1,800 lb., when fitted with two engines such as the " Cirrus Mark II," the top speed seems to be just attainable. Assuming 170 h.p. maximum for the two engines, and the same consumption as before, fuel and oil for three hours would weigh 306 lb. which with weight of pilot and observer, would make a total of 626 lb., leaving 224 lb. to be carried a;', ballast. This ballast would require 22-4 cu. ft. of space, or 11 -2 cu. ft. in each fuselage, which would not be difficult to arrange for. The doubtful point then narrows down to the weight figure of 1,800 lb. Any serious increase on that would reduce the margin of possibility of attaining 110 m.p.h. How a machine like the Martin P.M.3 would fare in the steep glide, manoeuvrability and controllability tests the writer does not profess to know. Presumably by fitting lateral controls of the Handley Page slot-aileron type, the lateral control might be made very powerful for stalled glides, but of that probably Mr. Handley Page could tell us something in a forthcoming issue of THE AIRCRAFT ENGINEER. A type of machine which appears to offer distinct possibili- ties, for which, in fact, the Guggenheim competition seems to have been specially designed, is the Hill " Pterodactyl" tailless monoplane. In his paper read before the Royal Aeronautical Society, and published in the Society's Journal of September, 1926, Capt. Hill gave some curves and tables relating to the characteristics of his machine. There appears to be a slight discrepancy between the two. Thus on the curve, Fig. 5. on p. 526 of the Journal, the maximum L/D is shown to be just under 11. In the table on p. 540 a maximum L/D of 14-21 is given. According to the curve, the maximum L/D occurs at a lift coefficient of 0'4, whereas according to the table it corresponds to a kL of 0-259. Possibly the explanation is that the curve refers to the complete model, while the table represents the figures for wing only. If that is the case, the L/D of the model at the lift coefficient corresponding to 110 m.p.h. and kL max. of 0-5 is only about 2-5 to 3, which is a good deal lower than was found to be required. It should be remembered, however, that the original model had a modified airscrew 4 wing section, with consider- able " wash-out " of the wing tips. Presumably, by using a more modern wing section with small travel of the c.p., and by making use of every other refinement which modern knowledge may suggest, Capt. Hill could improve considerably on the efficiency of his first machine. If, for instance, he could bring the full-size efficiency generally up to that shown in the table, and by designing specially for high LI) at top speed, it would seem at least not without the bounds of possibility to produce a machine of 100 to 150 h.p. or so that would cover the required speed range. The tailless machine, with its small nacelle, appears to offer excellent opportunities for reducing body drag. The absence of marked stalling point, the use of the wing tip controllers, the wing rudders used as air brakes for varying the gliding angle, and so forth, are all features which tally exactly with what those responsible for drawing up the regulations for the Guggenheim competition have obviously been aiming at. How the machine would recover from ''normal" and "abnormal" attitudes one does not know, but all the controls appear to be so ample that presumably the desired stability could be attained readily enough. The writer commenced these notes by expressing dis- agreement with the view that the " Autogyro " seemed the only solution. He will conclude by expressing the opinion that the tailless machine appears to offer the most promising line of attack on the pro blem. It is to be expected that a great many aircraft designers will not share this view, but if they will come forward and show how a better and more promising design can be produced, their arguments will doubtless .)o followed with interest by readers of THE AIRCRAFT ENGINEER (The Editor would point out that " Marco Polo,' who desires to remain anonymous for reasons best known «•> himself, has no connection with, nor interest, other than a. scientific, in the "Pterodactyl." We know him to be perfectly unbiased, and that in writing these notes he hasi bej- ~ actuated solely by the desire to examine how the probtet^ of fulfilling the Guggenheim conditions may possibly be nu.- We shall be pleased to have the views of other aircr^ designers on this subject, and shall be glad to publish these i- the next issue of THE AIRCRAFT ENGINEER.) A\Ad
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