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Aviation History
1910
1910 - 0119.PDF
FEBRUARY 12, ioid. [351] Mr. F. C. Harrop (329) and Mr. G. H. Challenger (328) may better solve the problems of the screw propeller by considering the actions from a " velocity " point of view instead of a " difference of pressure " point. The pressure which propels the screw forward exists only on the surface of the propeller-blades, and is equal and oppositely opposed between the blades and the air in contact at any moment with their aft surface. The propeller is working in a fluid which presses in every direction with a pressure of about 15 lbs. per sq. in., and air moves with high velocity under small difference of pressure, so that no great, or even very small pressure difference can exist between two points in the air only a foot or less apart. It is a velocity problem. The air enters the screw at a velocity, S, equal to that of the machine it is driving. The blades then accelerate it to a higher velocity, V, while it is in contact with them ; and this acceleration is the cause and measure of the thrust of the screw. Glasgow. RANKIN KENNEDY. [352] In your lucid and informing article on Aerial Propellers, which is just what many readers must have long desired, I note this remark (p. 36, under head, " Pitch of a Propeller "):— "The angle of the blade is neither constant from root to tip, nor constant from leading edge to trailing edge. A constant blade angle from root to tip would result in a variable pitch, and a constant blade angle from leading edge to trailing edge would produce a flat blade, which, from the analogy of the aeroplane, might well be expected to be less efficient than one suitably cambered." In the recently published " Des Helices Aeriennes," by S. Drzewiecki, I find a passage (pp. 26-27) which appears adverse to this form of screw :— " In our opinion, it is an error to give to the blades of aerial screws the slight concavity which is usually given to sustainers, in order to increase their carrying power. For sustainers there is certainly a gain in increasing by all means possible the lift per unit area, at the risk even of increasing also the drift, for every diminution of the supporting surface signifies a proportionate diminution of the weight that it has to carry; and, moreover, the solidity of the structure cannot but be improved ; the only compensation necessary is to increase a little the power of the motor. But it is by no means the same for screws ; their business is to utilise for propulsion the power of the motor to the best possible advantage, a reduction in the weight of the serew has only an insignificant influence on the total weight, while on the other hand every increase of resistance to rotation entails, as we have seen, a very notable diminution m the efficiency of the propeller. The defenders of the practice of curved blades often invoke, in favour of their thesis, the argument that the filaments of air meet without shock the surface of a concave blade tangentially to the entering edge, and that these filaments are deflected progressively till they emerge along the trailing edge, after having, by their reaction on the blades, produced the maximum of thrust, after the manner of that which is produced in turbines with curved blades. In our opinion, the two phenomena are not comparable, because in turbines it is only a single bunch of isolated filaments which strikes the curved blade, and it is, in fact, deflected entire ; whilst the blade of the screw encounters parallel filaments of air all along its depth, and if those which enter tangentially to the angle of entry are ' progressively deflected by the concave surface of the blade, as in a turbine, it is by no means the same with the other filaments which strike the surface of the blade all across its breadth, and especially towards the rear part, where meeting the increasing and excessive angles of incidence at the point of maximum pitch, this diminishing considerably the effectiveness of the propeller. This is why, until the contrary is proved, we think it preferable to give to the blades of propellers helicoidal-plane surfaces, and not concave " I enclose original, as a means of checking my inexpert translation. Maxim, in his " Artificial and Natural Flight," quotes his experi ments as leading to a similar conclusion, he finally giving his screws only sufficient camber to counteract deformation by air pressure. , Your article was of necessity limited in space, but this question of flat or curved blades is so fundamental that I am sure an elucida tion and final pronouncement would be welcomed by many readers. On a later page of " Des Helices Aeriennes " the author observes that bench tests of screws, without travel through the air, are only applicable to the supporting screws of helicopters, and applied to propellers are absolutely fallacious, the indications obtained having nothing in common with those arrived at under proper conditions. He recommends the placing of the testing apparatus in a tunnel of moving air. Apologising for the length of this. F. C. HARROP. [Our correspondent raises an extremely interesting point on the subject of propeller design—viz., whether it is worth while cam- I/QGHT] bering the faces of the blades on the ground that inasmuch as a propeller-blade is an aeroplane it should be designed on similar lines. Our correspondent quotes an article of our own in support of cambering, and also brings forward a quotation from Drzewiecki's " Des Helices Aeriennes," in which the author argues that flat surfaces are preferable to cambered surfaces for propellers, although he admits that the contrary is the case for aeroplanes. While the point at issue remains a question of theory, our opinion differs from that of M. Drzewiecki, although we would add, as he does, the proviso "until the contrary is proved." M. Drzewiecki's argument as quoted above by our correspondent is, we think, hardly logical. He speaks of the deflection of the filaments of air as if one and all gained free access to the face of the propeller blade; in other words, he assumes as his hypothesis the Newtonian medium for air, which is known to be inexact, and was so admitted by Newton himself. Athough it is convenient to give to the mind imaginary pictures of intangible objects, it is very important not to use them too far as a basis for practical deductions. The Newtonian theory, for instance, is a most useful hypothesis, primarily as a means of proving itself inexact. Air is not an inviscid fluid as the Newtonian medium supposes, consequently, when the "filament" of air is deflected by some solid object like an aeroplane or a propeller blade, which we contend are fundamentally the same in principle, other filaments in the vicinity are deformed by the first. Thus, after the tur bulence immediately following the setting in motion of some aerial system has subsided, the air forms itself into streams of considerable depth and extent, the streams being composed of an infinite number of filaments. Only one stratum can slide along in actual contact with the face of the propeller blade or aeroplane as the case may be, the other strata being deflected on top of the first, and conforming to its path in every way. The depth of the strata thus influenced depends on various factors, prin cipally the compressibility of the air, and its effective value is at present an unknown quantity, upon which information is much wanted. It determines the gap between superimposed planes in biplanes and triplanes, and it is a fundamental factor in any formula that may be devised for the calculation of lifts and pressures from the velocity of air in motion. The above is, it is true, mainly a theoretical point of view, but it is a very reasonable hypothesis as a little consideration will show. No matter in what light the actual constitution of the air may be regarded, it is perfectly evident that its mass cannot be ignored. If, therefore, a "lump " of air is assumed to have struck a propeller- blade, its subsequent movements must be satisfactorily accounted for in order to deduce a reasonable theory on the dynamics of its action. To argue,' as it seems to us is done by M. Drzewiecki, that every lump of air in a stream actually hits the face of the propeller is to suppose that each lump vanishes at the point of contact, as if it were actually absorbed within the material of the propeller itself, or had passed through the blade to the other side. If, as is more reasonable, it is supposed that such particles of air as happen to actually strike the face of the propeller-blade slide along it until they fall over the other edge, it will be seen that the surface of the blade is continuously covered by an air film, comprising particles that are undergoing gradual deflection. While those particles are there, the face of the propeller itself cannot be ns
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