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Aviation History
1937
1937 - 0085.PDF
JANUARY 14, 1937. FLIGHT. 33 Well, there isn't one. Just painstaking attention to detail. 1 contend that all the engines I have mentioned have a much harder time than any aero unit, whether engaged in long distance races or club flying, but should be relieved and happy it anyone could put forward a logical argument to show that 1 ani wrong in putting these frequent failures down to inefficient servicing. Wishing Flight every success in 1937- STEVE HILL. London, S.E.13 [Our correspondent overlooks the fact that his motor cycle ni"ines are not called upon to run at very nearly their full output for perhaps twelve hours non-stop, as is the engine of a long-distance record-breaking aeroplane. And is he justified in his accusations of unreliability? We cannot recall more than half a dozen aircraft engine failures in major record attempts and races during 1936—whereas engine failures invari ably account for a large number of retirements from such motor cycle events as the T.T. Races.—ED.] TAPERED WINGS. IN my letter to Flight, published in the issue of December 17, I asked Mr. Stanley Evans for rational reasons which made him such a convinced champion of the elliptical plan form. Mr. Evans' reply published in Flight, issue of December 31, leaves that question unanswered. Alone, the sarcasm in which he indulges would not have drawn a farther reply from me; but I consider it necessary to correct certain statements and obvious misleading distortions of the meaning of my own words. (1) My letter was directed not against his suggestion of a re search programme dealing with elliptical wings. I referred very clearly only against his previous statement that trape zoidal plan forms—whose advocate Mr. Evans calls me, much against mv own wish—were " every bit as bad as those rect angular plan forms of the past." (2) His remarks in regard to the eflect of the slope of the lift curve on Glauert's calculation of the induced drag of various plan forms are beside the point. As the slope enters the calculations in form of a constant, the theory is not prin- cipallv affected—as far as the relative effect of various plan forms is concerned—whether the theoretical value for —3 a a is introduced or a slightly smaller value. Recent pressure distribution tests in the N.A.C.A. full- scale wind tunnel on a tapered wing fitted with partial flaps have very satisfactorily confirmed span loading distributions calculated on the basis of the Lotz method which is, after all, but a mathematical development of *he original Glauert method. (Journal of the Aeronautical- Sci&ucet, -Volume 3, 'Number 5, page 161.) (3) Gottingen wind tunnel experiments on various plan forms were not quoted for the sake of the absolute values obtained but for the relative difference observed with various plan forms. What Mr. Stanley Evans calls a " vivisection " of ray re marks is a clever distortion of the obvious meaning of my words, namely : that these tests showed no practical difference between the profile drag of elliptical and trapezoidal wings. The relatively low absolute value of the profile drag coefficients resulting from the Gottingen tests must be attributed to the turbulence in this canal. The words, " careful analysis " mean that the coefficients of induced drag were not simplv expressed by —'-r as' in the original Gottingen publication, but were corrected for the respective plan forms. If Mr Evans thinks fit to ridicule my reference to experi ments done by such painstaking and exact research workers as the Gottingen group simply because the Reynolds number was rather low compared with present-day standards and be cause they date back some twelve years, I can only remind him that although bigger and better wind tunnels have been built since then in U.S.A. and elsewhere, certain important problems connected with profile drag have not yet been satisfac torily solved in the large tunnels, and no responsible designer ar aerodynamic engineer would consider the available evidence on profile drag derived from experiments in large wind tunnels *s a really accurate and quantitatively trustworthy basis to predict full scale performance. The results of full scale tests ^eceutly carried out on the influence of thickness on drag have own a remarkable discrepancy from prediction based on ( Thnd exPeriments at h'gh Reynolds number, to tV IV fact t'lat designers in other countries have taken trie elliptical plan form is no proof in itself. Nothing is so infectious in aeroplane design as a fad started by somebody. (Vide the large fillet.) As far as one of the aircraft quoted by Mr. Stanley Evans is concerned, I have definite information from an authoritative source that she is as unpleasant when stalled as any other aircraft fitted with ordinary tapered wings. (5) As to Mr. Evans' hope that " the ramparts of Crickle- wood " might one day be bewitched by the magic circle of his " fellow-sorcerers," I should like to say that we prefer to remain on firm ground, but we shall always be amused to watch a good trick. G. V. LACHMANN. Cricklewood. PUTTING A FINGER ON—OR IN—AIRSCREW NOISE TNDICATOR " has raised an interesting question with J- reference to airscrew noise on pusher machines. I notice- he attributes the noise to the prop cutting the vacuum behind the trailing edge, which undoubtedly does make a certain amount, but I believe by far the greater part is due to the prop passing close to the engine mounting. I know that, whichever it is, it isn't a matter of life and death, but I thought it might be of interest to a few designers to pass on a simple test I tried out myself a short time ago. I partly own a Drone and was rather tired of the only thing which spoilt an otherwise particularly interesting pas time, so I first of all fixed up an extra large exhaust silencer —which didn't have the slightest effect. When it came to the propeller I couldn't make up my mind at what point it created the beat, for, although it seemed obviously the wing, there were several reasons why it might not be so. To begin with, the prop didn't beat "fiat against it," but started at one end and sliced across it—also, if it made a noise coming down on to it. it would also make one on the underside coming up, and, as the wing is well below the boss, the sound wouldn't be regular. Then, again, if it was the prop tip passing the cut out section of the wing it would again form an irregular note. Judging by the note it appeared to be about equal to a four- cylinder engine running at the same speed, or, in other words, the equal of each blade passing one point per revolution. On account of this uncertainty, the best way seemed to be to remove the prop, get.it running in free air, and then search for the noise with a piece of wood. So off it came and was rigged up in a workshop. The results weren't particularly startling, but 1 was surprised to find that objects brought near the trailing-edge side created practically no noise, but on the leading-edge side made fifty times as much—also, you could bring objects close up to the tip without noise, which ruled out the closeness of fuselage to prop tip as a possible source. This, of course, means that the front of a prop throws out a very strong bow wave, the eflect of which, from the point of view of noise, is out of all proportion to the back—an object held roughly a foot away in front is about equal to 3in. at the back. • - -: • ; . Excessive noise was made only when the wood was held roughly on the radius line—to bring it within an inch or so created a' deafening din quite equal to any open-exhaust car engine, but when held in the Drone wing position, about half wav between boss and tip, there was nothing to grumble about at all. So I took it back and remounted it on the machine —hwking for a single object, near the tip and running along the radius. Well, you can guess—the only one I could find was the engine mounting, coming up from the fuselage and passing about.iiin. away. If only designers would keep close objects off the radius line I don't see why pushers should be any noisier than tractors—they are certainly much more pleasant to use, for if you can't get a good outlook you might as well travel under ground or underwater for all the fun there is in flying. With apologies to "Indicator" if I'm wrong—I may very easily be, because flying isn't my beef and pudding. Huddersfield. P. PRIEST. New Coventry-Victor Engine AN improved aero engine is shortly to appear from the tin entry-Victor works. On similar lines to the firm's flat-twin and four-cvlinder models, this redesigned engine will have four cylinders with increased bore and side valves. Dual ignition is to be used, and the aim is to produce a unit giving about 45 h.p. without resorting to a high power-weight ratio. Thus it is hoped to make the price about £60 and secure a degree of freedom from maintenance comparable with that of a normal light car engine. An official 100 hour type test is to be carried out during the next week or two.
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