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Aviation History
1913
1913 - 0551.PDF
SCIENTIFIC INSTRUMENTS, THEIR DESIGN AND USE IN AERONAUTICS.* By HORACE DARWIN, M.A., F.R.S. BEING THE FIRST WILBUR WRIGHT MEMORIAL LECTURE. " THE chief cause of failure in operation is the ill determination ** and measurement of the forces and actions of bodies,"—Francis Baion. I have been asked to give the first Wilbur Wright Memorial Lecture. I feel this an honour and a responsibility, and I hope that what I shall say may be of some interest to the many able men now working at the science of aeronautics. No memorial lecture is required to make us all admire the •character of the man, his brilliant engineering work, the scientific method by which he obtained his results. Each step forward was secured by careful reasoning based on former trials; each step was tested separately ; all available data were used. An account of the method and results of his original experiments has not yet been published, and would be of extreme interest. May I express a hope, which I know you will share, that when the appropriate time •comes it may be published ? All Wilbur Wright's work was done in the closest co-operation with his brother Orville. We do not know how much each did, and we do not want to know j Orville probably says that Wilbur did most, it is equally probable that Wilbur would have said that Orville did most. We, at any rate, know that they together did a very great piece of work. Natural and Artificial Flight and Locomotion. The Wright brothers made careful observations of the flight of "birds, and found their observations valuable. It is interesting to consider the resemblance and differences of the manufactured aero plane and the living bird. The resemblance may be simply the result of copying the bird, or it may be that similar designs have been arrived at independently by birds and men. The wings of both are roughly the same shape : of wide span, and narrow in the direction in which the bird flies ; both have concave wings with thick leading edges. In many aeroplanes hollow spars are used like bones and like the quills of the feathers of birds. We copy plants also in this respect, for they too have learnt the economy of material in the use of hollow spars. The bodies of airships are similar in shape to the bodies of swimming animals, with the greatest width towards the head. The bodies of birds are of similar form. These resemblances are remarkable, but there are great differ ences. The Wright brothers found no biplane bird to copy, and did not flap their wings. No flying animal uses a continuously rotating propeller to drive him forward on soaring wings, and it is perhaps hardly too much to say that if birds only knew how, they would now copy the Wright brothers. Muscular action and the circulation of the blood, however, put supreme difficulties in the way of the development of the continuous rotation of a part of an animal. Cranks and connecting rods, as well as rotating valves to allow the circulation of the blood, would be required. No animal has succeeded in developing wheels instead of legs, although their development might have enabled him to run about with less consumption of fuel, anyhow in a country with good macadam •roads. There is a beetle who has made use of something in the nature of a wheel. He collects his food (manure) and carefully fashions it into a ball larger than himself, and then rolls it along to a convenient place, buries it, and lives on it at his leisure. This is not a wheel, but it has many of the advantages of a wheel, and we may consider this beetle as the pioneer transport engineer. The development of the power of flight in birds has been so slow that we cannot realise the time taken, or form the roughest estimate in years ; but the perfection of these adaptations and ihe beauty of their skill, strength and movement must strike anyone who has ever watched their flight. Some less advanced animals have only learnt to glide, and are now in the same stage of development as the Wrights were a few years ago. Perhaps these gliders developed more slowly or perhaps only began to learn the art many ages after birds had learnt to fly. A few plants also have developed wings to their seeds so that they can glide away to more suitable places for germination and growth. The evolution of those remarkable flying animals, the Wright Brothers, has been enormously more rapid because it has not depended on the method of trial and error, and because each trial does not correspond with the lifetime of an individual. But the •difference is even more far-reaching, since the material on which they worked was knowledge, or, in other words, the experience of man kind handed down from one generation to the next. And more * Read before the Aeronautical Society of Great Britain, May 21st, 1913. important still, their mental powers enabled them to test the accuracy of this knowledge and to increase its amount. It is interesting to MM that in the opinion of biologists, birds, bats, fishes and insects all learnt to fly independently j inheritance from a common stock played no part in their development. There are no large flying animals now and the great majority in exi are extremely small. These facts would lead us to expect diffi culty in making a very large aeroplane, and theoretical considerations confirm this view. The Ostrich, the largest existing bird, standing $ ft. high, has no power of flight; nor had the Moa of New Zealand (now ertinct) standing 13 ft. high. It is probable, however, that both had flying ancestors. The only vertebrate animals which developed flight during fossil times were Pterodactyls. The largest of these had a body a little larger than a swan and a span of wings of over 2a ft. This is less than the smallest aeroplane, but large compared to an Albatross, the span of whose wings is over 11 ft. - probably exceeding that of any other bird. The whole family of Pterodactyls have long ago become extinct. On the other hand there is a very small spicier who makes a kind of flying machine out of the simplest materials. In the autumn he desires to emigrate, and as he is very small a land journey would I it- slow and difficult. He selects a calm and sunny day, on which we should expect to find local upwards currents of air ; he climbs ID the tip of a blade of grass and spins a thread which is blown out by the wind, and at the right time he lets go and is carried away by the wind, he knows not where. This clearly could not be done by a large animal. But we had better not attempt to copy flying animals too closely. We shall learn nothing from the spider's success in aeronautical engineering. Would it not also !«• just as great a mistake to try and fly with flapping wings as it would be to propel a ship by a flapping tail like a fish, or to make a motor 'bus trot about the streets on four legs? I think it would, but my ancestor Erasmus Darwin thought differently. In 1781 he wrote :— Soon shall thy arm, UNCONQUER'D STEAM ! afar Drag the slow barge, or drive the rapid car ; Or, on wide-waving wings expanded bear The flying-chariot through the fields of air. A few years ago I did not believe that we should see the essential part of his forecast about flying fulfilled, and I may be just as wrong about the waving wings. The Design of Scientific Instruments. The subject of my lecture to-night is Scientific Instruments— their use in connection with flying—and some general considerations with regard to their design. The Wilbur Wright Memorial Lecture will be given annually, and I believe the most useful results will be obtained if the lecturer is allowed considerable latitude in the choice of a subject. All I ask is to be allowed to speak on a subject at which I have worked for many years. Instruments used in Aeroplanes. It is important to realise beforehand the difficulties of using instruments on aeroplanes during flight and the errors thai may be introduced in the readings. The aeroplane shakes, it does not remain level, and is subject to acceleration in all directions. The instrument should be so designed as not to be affected by any of these disturbances. A vertical acceleration has the same effect as a change in the amount of the downward pull due to gravity, the tilting of the aeroplane changes the direction oi the downward pull, with regard to the instrument. A lateral or longitudinal acceleration has the effect of both altering the direction and amount of gravity. But vibration is a greater difficulty still. The hand of an instrument may move so much and so rapidly that it is difficult to estimate the mean reading on the scale, and sometimes it is quite impossible to do so. And this may happen when the quantity, which is indicated by the position of the hand, only varies slowly and by small amounts, Consider a part of an instrument that can rotate about a vertical axis, and suppose that its centre of gravity is not on the axis, then a sudden lateral movement of the whole instrument will tend to rotate the part relatively to the instrument, the side-ways force will act at the axis, and the resistance of this force due to inertia will act at the centre of gravity of the part. The farther the centre of gravity is from the axis, the greater this tendency to rotate ; the tendency will also be greater tbe greater the mass of the moving part. To make this tendency small, the centre of gravity should 573
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