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Aviation History
1917
1917 - 0293.PDF
MARCH 29, 1917. GHT AMSWERS CORRESPONDENTS J. R. B. (New Southgate). Apparently you are labouring under some misapprehensionas regards the use of the formula P = Cpw2. In this formula P = the pressure in whatever units of mass, length and timeare employed. In this country it is usual to employ lbs. and square feet. Hence P = lift in lbs. per square foot =Cpv2. C is the " absolute " lift coefficient of the particular wing section employed ; p is the density of the air. Whenft. per sec. is used for the velocity the value of p is 0-00238 ; when m.p.h. is used for velocity p = 0-0051 ; v is the velocityin one or the other of these units. In the answer to a corre- spondent, a cutting of which you enclosed in your letter,C, or the " absolute " lift coefficient, is the quantity sought, P, p aad v being given. In our article on " Units Employedin Recording the Results of Wind Tunnel Experiments," P, the lift in lbs. per sq. foot, is the quantity sought, C, p and vbeing given. Therefore in the first case the formula is p written C = -. and in the second case it is written P x v1,P = Cpv". In the answer to a correspondent the loading— or, in other words, P—was assumed to be known to be 4 lbs.per sq. ft. and v to be 60 m.p.h., and it was desired to find the " absolute " lift coefficient, C. This was done by snb-P v stituting in the formula C= 2 thus :— O-22.s X602 0-0051 X3,600 Therefore the " absolute " lift coefficient of that particular wing section was o-22. How you arrive at 6 as the result we are at a loss to understand. In order to make this example analogous to that shown in the article on " Units," we will take it that C and v, or in other words the " absolute " lift coefficient and the speed, are known, and that it is desired to find P, which is the lift in lbs. per sq. ft., or, putting it another way, the loading in lbs. per sq. ft., as the lift and loading must be equal if the machine is to follow a horizontal flight path. We therefore have C=o*22; p = 0-0051, and v = 60 m.p.h., and can write P = 0-22 x 0-0051 x 602 = 4. P, or the lift per sq. ft., is therefore 4 lbs. We fail to see any traces of the inconsistency you refer to. T. S. K. I. (Bristol). Dope and fabric for model aeroplanes may be obtained from Messrs. A. E. Jones, Ltd., 97, New Oxford Street, to whom we should recommend you to write for prices, &c. For an explanation of the term " lift coefficient " we would refer you to an article published in the January 25th issue of " FLIGHT," 1016, entitled " Units Employed in Recording the Results of Wind Tunnel Experiments." W. D. K. (Sanderstead). The instruments on an aeroplane usually include : Alti- meter, for indicating the height ; clock ; compass ; revolu- tion indicator showing the speed of the engine ; speedometer, indicating the air speed of the aeroplane ; petrol and oil gauges, indicating the amount of fuel in the tanks. On military machines other instruments, such as wireless sets, may be fitted, but regarding these no information can be published at present. Aero (Kingston). The name "Parasol" was, we believe, first given to a Morane-Saulnier monoplane in which the wings were mounted above the pilot's head, hence the name. No simple formula exists by means of which the stability of an aeroplane can be calculated. The subject is a very complex one, and cannot be dealt with in this column. We should recommend you the book " Aeroplane Design," by F. S. Barnwell, which can be obtained from the offices of " FLIGHT." The price is 2s. iod., post free. H. G. (Langley). Generally speaking, the centre of thrust, the centre of resistance, the centre of gravity, and the centre of lift should all coincide. It will frequently be found, however, that it is not possible to get the line of resistance to coincide with the line of thrust. In that case, if the thrust is below the resistance the centre of lift should be such a distance to the rear of the centre of gravity that the thrust-resistance couple is equal to the lift-weight couple. Both these couples should be kept as small as possible. The chief objection, from a practical point of view, to having the engine in front and the screw at the rear, is that this arrangement entails a long shaft, which, if the power to be transmitted is of any great magnitude, means adding very considerably to the weight. L. J. P. (R.N.A.S.). Up to the present nothing definite has been agreed upon as regards what constitutes a " clockwise " and " anti- clockwise " engine. It is general practice, however, to so, term an engine according to its direction of rotation as seen from the pilot's seat in a tractor. Thus, for instance, a Gnome would be termed a " clockwise " engine, since it rotates in the same direction as the hands of a clock. If the same engine is taken out of a tractor and put in a pusher, it would still be a clockwise engine, as the direction of rotation of its cylinders in regard to the crank shaft would remain the same as before. The Renault, acoording to above definition, is a clockwise engine, but as its propeller is mounted on the cam shaft and therefore revolves in the opposite direction to the engine, the screw fitted to a Renault in a tractor is left-hand or anti-clockwise. On the other hand, if the Renault is placed in a pusher it still remains a clockwise engine but the screw will, in that case, be right-hand or clockwise. The best known example of single spar wing con- struction is the pre-war type Breguet tractor biplane. This machine had a single steel tube spar, and the ribs were a loose fit on the spar, to which they were secured by steel springs so that they were, to a certain extent, free to alter their incidence according to the pressure of air on them. Unfor- tunately these springs used to give trouble and weaken after being in use for a certain period, with the result that after a steep dive the springs were no longer able to sustain the load, and the machines got a reputation for uncontrollable nose dives. This arrangement also had the disadvantage that, owing to the travel of the centre of pressure, the spars were subject to very great twisting stresses. Of machines with a variable incidence, that is to say machines in which the incidence could be altered during flight—which was not the case with the Breguet—the French Paul Schmitt biplane is probably the most successful, and established a number of records before the war. This machine, which was fully described in " FLIGHT " of October 30th, 1914, had a very large balanced elevator, but no fixed tail plane. G. M. R. (Leeds). So far as we are aware there is no publication, weekly or otherwise, which deals with the making of propellers. The theoretical side of the question has been fully dealt with in a number of publications, but the practical side tan, we think, only be learned in a propeller factory. G. W. (Burgess Hill). Write to Major Mitchell, R.F.C., The Polytechnic, Regent Street, W. K. S. D. (R.N.R.). Apply to your Commanding Officer for permission, and then send in your application to the Director of Air Services, Admiralty, London, S.W. J. B. (Cheltenham). There appears to be no reason why you should Hot apply for a commission in the R.F.C. when you are 18. J. C. (Dover). Your age would almost certainly bar you from obtaining a commission in either service. If accepted you would be taught at the Government's expense. S. P. (Edinburgh). We understand that the pay is the same as that of a private. L. E. G. (Enfield). If you are passed for General Service you would not be accepted for the R.F.C. as an -unskilled mechanic. H. G. (Plumstead). Your training would probably help you in an application for the R.N.A.S. 293
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