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Aviation History
1910
1910 - 0447.PDF
JUNE II, 1910. IfiiQWT] HOW AN AEROPLANE LIFTS. IN our first article entitled " How Men Fly," which appeared in our issue of February 6th last year, we drew attention to the leading phenomena associated with the •flight of an aeroplane, and just as that article may be taken as an account of what happens, so may this be •considered as a continuation embodying an explanation -of why it happens. The theory associated with the lifting power of an •aeroplane is one which puzzles a good many people who have not hitherto had occasion to study the laws of fluid dynamics, and although the subject is one which has been •dealt with in FLIGHT before, we purpose going into the 'matter again from a different point of view at the request of a correspondent whose letter appears in this week's issue. Mr. Hickman asks how it is that the deck of an aero plane travelling very nearly edge on to the air is subjected to a pressure reaction much about the same as he calcu lates it would sustain if set vertically across a wind of the same speed. Simple and straightforward as Mr. Hick man's question has the appearance of being, it neverthe- f- NE\NTON'6 FIRST " Rest" Fi<3 1 LAW- •NEWTOriS SECOND L*W" ftcceleratior\ / <*- Force F fig 2 iess covers a very wide field, and makes it necessary to •evolve a satisfactory answer step by step. The starting point in all dynamical problems is Newton's laws of motion, and for the benefit of those who have forgotten their physics it can hardly be out of place to recall the wording of these three simple rules, which are of absolutely universal application throughout the whole sphere of physical existence :— Newton's Laws of Motion. Law 1. (Law of Inertia). Every body continues in a state of •rest, or of uniform motion in a straight line, except in so far as it may be compelled to change that state by external force acting on it <Fig. 1). Law 2. (Law of Force and Motion). Rate of change of momentum is proportional to the force which causes it, and takes place in the direction of the force (Fig. 2). Law 3. (Law of Reaction). To every action there is an equal and opposite reaction (Fig. 3) : or when two bodies mutually act upon each other the momenta •developed at the same time are equal but opposite in direction. Reading through these simple statements, the natural mechanical mind grasps them at once as obvious, but that by no means implies that the mind is alive to the significance of their importance and use as laws. To say that such and such a thing is so in a case where it is otherwise apparent, is one thing, but to say that the same thing must be so in a case when appearances are to the contrary, is a very different matter altogether, and as the theories which enthusiastic inventors daily thrust before our notice only too often contain violations of Newton's laws, we cannot too strongly urge upon those who are making a study of the science of flight to thoroughly absorb their meaning to the last word. The Basic Reason. They constitute the sole and complete reason, so far as it is possible to give a reason for natural phenomena, as to why an aeroplane can exert a lift, and is therefore capable of flight. Newton's first law states that it requires a force to change a state of rest to a state of motion, or to change a state of motion in one direction to a state of motion in the other, or to change a state of motion to a state of rest. Now air having mass to the extent of y^th lb. per cubic foot (air weighs approxi mately 13 cubic ft. to the lb.) is a body within the meaning of Newton's law, and if we assume that the air is in a state of rest to begin with, then it will require a force to set that air in motion, and by Newton's third law this force will experience an equal and opposite reaction. It is this third law of Newton's which is perhaps the most elusive of all, because it is so thoroughly obvious in certain cases, and yet apt to be not apparent in others. No one, for instance, could imagine himself pushing against a wall (Fig. 4) without experiencing this equal and opposite reaction of which Newton speaks, but when it comes to dealing with in animate forms, and especi- f4EWTOrNS THIRD LAW NExNTON 6 THIRD LAW ."Resistance R= Force F.. ReevCrlC)Pv R. FOTCe F Fig 3 • ftg 4 ally when they are working in a somewhat abstract" body" like the air, there is, perhaps, not always quite the same readiness to appreciate the force and its opposite. The Wedge Action. An aeroplane—which for the moment we will assume consists of a simple flat plate—pushed horizontally through the air with its leading edge tilted slightly above the level of the rear edge so that the attitude of the plane is slightly inclined, must obviously either push the air along in front of it as the scavenger's squeegee sweeps the liquid mud from a dirty street, or ~nust press the air downwards beneath it like a wedge forr 3 its way through some lightly-resisting substance, fy' . matter of fact, it is the latter point of view which is correct, and the analogy of the wedge is very often a useful way of bring ing aeroplane problems down to a concrete form. Once having established that the air is set in motion at all—and that is a matter which is susceptible of very easy proof for it is only necessary to experiment with a (SE.WITONS FIR5T LAW force required to change direction of air- stream F«3 5 NEWTONS THIRD LAW Force • Reacttorv — Ltft Fi£6 piece of cardboard in order to make a draught in the re quired manner—it is at once necessary to apply Newton's first law, and to say that this change of state needed a force to bring about (Fig. 5), and further that this force by Newton's third law, experienced an equal and opposite 445 C 2
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