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Aviation History
1910
1910 - 0019.PDF
I got an 8-in. Cochrane on it it flew 80 yards first attempt. I got several fine flights with it since. I use a geared elastic motor of my own make. Perhaps it might be of service to some other FLIGHT readers. Birkenhead. G. R. CAMPBELL. [267] I am building a small model biplane but can get no information as to how much twist an elastic motor will stand. I intend to have 32 strands of fg in. rubber 3 ft. in length. Can you or any of the readers of your interesting paper inform me how many turns such a skein ought to give ? Woolwich. AERO.SPORT. [We do not happen to have experimented on the twist capacity of rubber strands, but possibly some of our readers may be able to supply an estimate of the motor in question.—ED.] MODEL PROPELLERS. [268] I should be greatly obliged for information as to method employed for ascertaining thrust of small screws (12 in.) so as to take into account all conditions tending to produce thrust. A calculation based on the velocity of the blast seems to ignore possible suction at the front surface of convex blades. Is there any means within the reach of an ordinary amateur by which thrust can be directly measured at the spindle without introducing friction ? Also, what is a good way of finding the power used in driving the screw ? Although destitute of mechanical and theoretical knowledge, I have been keenly interested by the articles and corre spondence in FLIGHT, and should be grateful for information which would enable me to experiment. N. Finchley. F. C. HARROP. [The useful thrust developed by a propeller is wholly represented by the force required to produce the blast which it creates. Air from a state of rest is accelerated to a state of motion and a force proportionate to the acceleration and the mass is required to create the change. That force has, by Newton's law, an equal and opposite reaction which in this case is known as the thrust of the propeller and as the force that drives the machine. Losses occur all along the transmission system which prevent the whole of the energy available on the crank-shaft of the engine being converted into thrust; among these losses is that due to the resistance of forcing the blades of the propeller edge on through the air. Our correspondent asks, how can this loss be ascertained ? Possibly some of our readers may have devised some rough-and- ready method which gives a fair indication of its practical value, and if so we hope they will communicate it to us. A really accurate investigation of this point in connection with full-size propellers is one that has not yet been carried out; but we believe engineers are at work upon it, and we have little doubt that their results will be satisfactory. They should go far towards an improvement in pro peller efficiency, for they should provide data of a fundamental and reliable character, capable of being used as a ruling factor in design. To the question of measuring thrust the above remarks also apply, and we would again draw attention to the importance of avoiding confusion between a propeller and a fan, the former being designed to create a thrust while moving and the latter to create a thrust while stationary.—ED.] . [269) Could you inform me through the medium of your paper on the following:—What is the best form for propeller blades of model aeroplanes? One in which the blade slopes away from 90° at the axis to 280 at tips, or (a) 280 uniform over whole blade, or (iii) any other form ? If you could recommend a book that would answer the above and similar questions, it would probably save space in your columns. Thanking you in anticipation, Shettleston. PROPELLER BLADES. [The constant pitch screw is more correct than the constant angle screw, but it is common practice in full-sized propellers to change the pitch at some point along the blade according tv the designer's ideas on the subject. There is no book with which we are acquainted that would give just 'he sort of information our correspondent appears to require.—ED.] AN EARLY MODEL. [270] Ten or fifteen years ago I made a model to my own designs, which I called a brovoplane, of which I have pleasure in enclosing photographs. My only difficulty was to get a light engine, and I am now preparing to build a machine on the same lines measuring 46 ft. long by 40 ft. span. The principle combines the dirigible and the aeroplane, gas bags, having a lifting power of about 150 lbs., being provided ; the body and the wings are to be of aluminium. Norton-on-Tees. G. SMITH. BRASS V. ALUMINIUM. [271] You and yorr readers may be interested to hear that in building models a much more satisfactory material than aluminium is small brass tube. It may seem strange to say that this saves weight, but I find that building two models of the same size and strength that made of brass tube is considerably lighter. This is due to, first, the greater resisting strength of brass itself, and secondly, to the greater facility of making firm joints, both of which enable one to use tube of not more than half the diameter. For my model, which is now nearly completed, and of which I will send you full particulars in a week or so, I am using brass tube of -$f in. diameter, making the joints with " Tinol," combined flux and solder in paste form. The length of body is about 28 in., and I am certain that it would be impossible to build a model of this size and strength with any lighter material of the same diameter. Ludgate Hill, E.C. F. ALLEGDER ROBERTS. FLYERS ANALYSED. [272] I worked out a table some time ago similar to one pub lished by Mr. Evans, only I based the calculations on the official table you gave in FLIGHT of the aeroplanes at Rheims, and it differs somewhat from Mr. Evans'. I wonder which is right ? I enclose list of machines I worked out. One of your correspondents wants details of Farman's machine. I measured the camber at Blackpool, and made it about 3£ ins. Since then I have received a cutting from the Royal Engineers' lonrnal, which gives the following information :—"The ribs are o[ poplar, weigh about l| lbs. each, and are spaced 18 ins. apart ; the canvas on the ends and partitions is laced to the framework with whipcord and stiffened with slats, similar to those used in yacht sails. The supporting surfaces are formed to a curve fore and aft, similar to that of a bird's wing, the camber being about 4 ins., at 18 ins. from the front edge tor the length of 7 ft., and diminishing from this point to nil at back and front. In the Goupy type the ribs of main planes are fixed at an angle to the uprights, so that when the machine is horizontal the planes are set at a lifting angle of about 1 in 10." (This means 5 per cent.) From suitable photographs, I worked out, by getting the angle oi machine, that the Voisin was set at an angle of 12 per cent, when the machine was running along the ground. The Antoinette wings at the tips are apparently at an angle (dihedral) of 4 per cent., i.e., if you drew a straight line from centre of machine to tip of wing, the latter would be raised 4 per cent. 15
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