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Aviation History
1920
1920 - 1281.PDF
DECEMBER 16, 1920 NOTB.—All communications should be addressed to the Model Editor. A stamp should be enclosed for a postal reply Wing Tips—Why they are Swept Forward A READER has written pointing out that in most of the designs given in FLIGHT the wings have longer trailing than leading edges, and he enquires the reason. The accompany- ing sketch renders the matter clear ; with a rectilinear plane the air, indicated in the sketches by arrows, has a tendency li 111 t 1, I 1 TIL* \ iTiiTt ^ v end /oss \ a/rf/ow no end /oes 7u\\ to leak endwise from the plane. By shortening the leading edge, however, the airflow, whilst maintaining its original line of motion, is prevented from leaking over in the manner shown in the lower sketch. This end loss is known technically as " end loss." Winding Propellers IT is unwise to wind new rubber to its full elastic limit. Modellers have frequently written to me complaining of the fact that "the rubber I bought from So-and-So's broke the first time I used it." Investigation has found that the reader has been using well-worn rubber that he knew would stand about 1,000 turns, and upon replacing it with the new has given it 1,000 turns straight away. No rubber can be expected to stand this ; the machine should have been' given several flights with only 300 turns, several more at 400, and so on up to the limit. With twin-screw machines do not place all the turns on one screw at once; place 100 on one, then 100 on the other, and so on alternately up to the limit. Dihedral Angles and Warping A READER complains that he cannot get his model to fly satisfactorily owing to the wing bowing upwards in mid-air. This is chiefly caused by using wood of too small a cross section in relation to the span. It will have been noticed that on nearly all of my models top diagonal bracing only is given ; this is to prevent the dihedral from flattening out, the wing itself being made sufficiently rigid to remain true when acted upon by air-pressure. A wise disposition of the ribs will often eliminate this flexing difficulty ; securing the fabric neatly to each rib acts as a brace, and also prevents flexing or warping. Wings tapering towards the wing-tip also have a less tendency to distort when in motion. I have found results so impaired when a multiplicity of bracing is employed that I consider it advisable to place a little extra weight in the wing construction and do away altogether with wing bracing other than maintaining or keep threads. When a wing is found to warp it is necessary to first find the correct position of the main plane, and then brace it top and bottom. For planes more than 30 ins. span and under 3 ft. 6 ins. (assuming a constant aspect ratio of 6 to 1), the cross section should be -fc-rn. X -sVin. The spans should be tapered off cantilever fashion toward the tips. The dihedral should not be more than T'sth of the span, that is to say, the versed sine of the curve should not be more than 1 in. for every 15 ins. span. Compressed Air Motors MR. H. BAILEY, of Leicester, writes as follows :— " After a great deal of experimental work with compressed air models, I was much impressed with the exhausting pump- ing up of the container of models needing high pressures necessary to obtain free flight. So much so, I determined if possible to design a model requiring a minimum amount of energy in pumping up, combined with a maximum result in performance. The completed model under actual flying tests has turned out highly satisfactory. I have briefly outlined particulars of same below :— " Particulars.—Tractor mono. O.A. length 36 ins. ; span, 40 ins. ; air screw, 14 ins. ; dia., 20J pitch ; air container, 18 ins. x 2 ins. (3J ounces) ; engine, 3 cylinder stationary (| in. bore x i in. stroke), ij ozs. (less tape) ; total weight of model 8| ounces (the container was buit up of the lightest brass foil obtainable (purchased locally) 003., " The foot pump used with model under test was 1^ in. bore X 10 ins. stroke, with 20 pump strokes in the container. The model rose off short grass, this showing the minimum number of pump strokes to obtain flight off ground. " On 60 pump strokes the model rapidly climbed off ground to a height of 30 to 40 ft., making a stable and powerful flight of 25 seconds. Such results, I think, are worthy of mention. Has such a model as herein described been built and tested, the results publicly published ? [Yes, many times ; 71 seconds is the record.—ED.] " Personally, I consider the 25 seconds duration obtained could be improved upon, providing a reducing valve was used to throttle down the engine during the initial high pressure in the container. Unfortunately, I have been unable to find information relating to same to enable me to carry my tests further. " I should be more than pleased to receive any suggestions as to the most useful form of reducing valve which could be used with the above plant." Replies to Correspondents A. M. (Leicester).-—I append herewith a drawing of a suitable valve for your steam plant. The angles of cut-cff and inlet are most important. CLOSES OFIii' VIEW " PORT FACE PORTS IN OF A. M. C. (Blandford).—I duly dispatched the drawing. Please write if in doubt or difficulty.
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