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Aviation History
1915
1915 - 0861.PDF
mmm NOVEMBER 5, 1915. The most important defect of the majority of modem aircraft, when regarded as fighting machines, is the restriction of forward gun fire imposed by the presence of the tractor screw. For the effective use of a gun nothing has yet been built to equal the " pusher," but this type itself tends to be deficient in speed. It is unreasonable to expect one type of aeroplane to fulfil all the requirements of war. Reconnaissance is ihe primary function of military aeronautics, but such aircraft need at lean the protection of a fast scout, while their complete freedom of the air can only be established by an absolutely superior real fighting fleet. These three classes are fundamental: there remains to be fulfilled uich special purposes as bomb-dropping and other incidental duties that may be dictated by the changing phases of a campaign. Both as an offensive and defensive quality, speed is probably the most important factor in aeroplane design, and it is when considering the conditions that regulate the speed of aircraft that we enter the realm of theory by one of its most interesting doors. Aeroplane Resistance. Air resistance. Motion being the consequence of force overcoming resistance, and speed being merely the measure of its rate, the basis of this discussion must necessarily be founded on the principles underlying resistance to motion in the air. And it may be said, without much deviation from the truth, that the investigation of these laws has for several years constituted the chief occupation of scientific aeronatical research. The absolute and primary importance of such information is obvious. Knowing the resistance to lie overcome, the engine power required to attain a given speed resolves itself into a problem of arithmetical simplicity. Without this knowledge, the potential capabilities of an aeroplane remain unknown : its design is sheer guess work or an expression of purely per.-onal opinion. Already, the resislances of a great number of forms have been tested experimentally, and the ablest mathematical minds have studied the theoretical side of the problem in all its bearings. In England the modern starting point in a long series of carefully conducted researches was the investigation of the resistance of small square plates in a uniform current of air* followed by similar experi ments on large boards exposed to the wind. It is self-evident that a flat plate forms the simplest model with which to commence an experimental campaign into the resistances opposed by solid forms to the flow of a current of air. Equally is it apparent that a very thin flat plate, hrst placed facing the stream and then placed edge-on to the stream, should afford comparisons of two extreme conditions of fundamental interest and importance. In the former case, the plate offers what is apparently the maximum possible direct obstruction. In the latter case, the resistance presumably attains its least possible value. Moreover, the mind readily imagines a marked distinction between the natures of these two resistances. In the case of the plate facing the wind, there is the effect of a barrier, or dam, causing a definite interruption of the air flow, whereas in the case of the plate edge-on to the line of motion, the fluid is parted as by a knife, and the resistance is caused by the friction of the air molecules rubbing along the surface. If, in the first of the al>ove described experiments, the effect of the obstruction presented by a flat plate of area A were to bring to * See Stanton's " Resistance of Plane Surfaces in a Uniform Current of Air," and " Experiments on Wind Pressure," published in 1907 and 1908 by the National Physical Laboratory, Bushey House, Teddington, England. ® ® LIGHTER She was a Good Sport. THE lady was taking a ride in one of the open-front machines, where there is only a stick as foot-rest. Her skirts had been tied round her limbs before starting, but were working loose, and the air was fillirg the kinks. The ambidexterous pilot undertook to control the machine with one hand and call her attention to what was happening with the other. He first pointed to the inflated garments, then described a parabolic line with the one free hand. She shook her head; and he thought her face was somewhat flushed, and he realised that there wete possibilities of his being misunderstood. Then he looked for a suitable landing-place and cut off the motor. " It is ballooning," he remarked, as they glided down, alluding to the inflating garmenls. " Would make good parachuting," she acknowledged, quickly. "Good sport;" he exclaimed; and, turning on the motor, he straightened up and made for a pink cloud that was lazily floating in the azure expanse. A woman who could contemplate the possible use of her skirts as parachutes in case of a fall deserved an extra long ride ; and he gave it to her.—Aerial Age. rest the air of density r flowing at a velocity r, then the resistance of the plate would be expressed approximately by the equation Force -Constant * density K Area x Velocity" /' = C x p x A x »•* and the value of the constant would t>e unity. Actually, the constant C is in no case unity. Dr. Stanton, of the National Physical Laboratory, established some years ago the following experimental values for square plates, and they arc still the standard coefficients: Small plates. 0=0507. I-arge plates. C—0"6a. In a more convenient form, the wind pressure on a large flat surface standing normal to the stream may be expressed : /(lbs./sq. ft.)~r» 306 where J "is the air speed in miles per hour. From the variation in the above coefficients with the linear dimen sion of the plate, Lord Rayleigh pointed oult that the " v!" law of resistance, hitherto assumed true, could not be strictly accurate ; in short, that if, as had been proved, there was a variation of F with / (/ being a linear dimension of the plate), then there must also be a variation of /'with 0 and with v.t Very great importance attaches to this result,§ because exigencies of time, space and economy make it necessary to use models in experimental research. It is highly important to lie able accu rately to adjust the scale effect when applying the results of such- research to full-sized machines. For rough approximations, how ever, the tr law is commonly regarded as sufficiently correct to cover moderate speed ranges, || Turning to the edge-on position of the thin flat plate, the p* law is found definitely not to apply, nor is the resistance even directly proportional to the area exposed to contact with the stream. To Dr. Zahm the credit is due for experimentally establishing the law of frictional resislance in the air."' His fundamental equation is as follows :" Force • Constant x Area":l i Velocity''"" When A is the total resistance of both surfaces of a plate measuring ./ square feet in single surface area ; the value ot /• croooooSa. There is thus no constant relationship between the face pressute and surface friction of a plate, but it may help to lix ideas' to calculate a particular case, e.g., for a single face area of 1,170 square feet, moving normally at a uniform speed of 80 111.ph., the ratio of single-face pressure to double-suiface friction is 30010 1. Such a vast difference very forcibly diaws attention to possible economies in the disposition c-f surfaces that have to l>c driven through the air. It is apparent that the case of the flat plate facing the wind must be avoided at all costs, and the conditions represented by the plate edge-on approximated t<> as nearly as possible. (To be continued.) t See "Technical Report of the Advisory Committee for Aeronautics," Vol. 1909-10, [*age 38. ! The symbol v denotes what lui^ been termed the kinematic viscosity of the fluid. It might better have been expressed as the Specific Viscosity, for it is viscosity divided by density. Its dimensions are/." / 1. S Commonly referred to as the law of dynamical similarity. ll For charts illustrating the change in the coefficient with change of velocity over a wide speed range, see the second " Wilbur Wright Memorial Lecture " by Dr. K. T. Glasebrook, C.B., F.R.S., published in the Aeronautical Journal, Vol. XVIII, p. 376. In the Technical Report, Vol. 1911-11, p. 74, it is pointed out that " no scriuus correction is necessary to the lift values obtained, but that from 15 per cent, to 20 per ceot. should be added to the maximum lift drift ratio recorded to make them applicable to full-scale areofoils." 1 For Dr. Zahm's original paper, see " Atmospheric Friction on Even Surfaces," published by the l'hilo»<i'hical Society of Washington. • This is the form in which the formula is given in the " 'technical Report of the Advisory Committee for Aeronautics." See Vol. 1911-12, pags* 33, 3t. ® ® THAN AIR. FINANCIERS should feel at home in the Air Service, where a go:>d balance is secured by careful banking. t t I THE COOK : " Sir ! Sir! There's a Zcp'lin outside, and if you don't come wi' the keys of the cellar, we'll all be in— in— Heaven in a couple o' minutes !" THE DEAN I " God forbid !" J ! J "'WHEN LONDON SLEEPS' (Last Nights).'" Nonsense! We have a lot more faith in our Anti-Aircraft Corps than that. — l.cndon Ofitnion. ! ! t Those Hen Monoplanes Again. JAGWEI.L; "What makes that hen in your back yard cackle so loud ?" WIOWAV : " Oh, they've just laid a corner-stone across the street, and she's trying to make the neighbour* think she did it."
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