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Aviation History
1912
1912 - 0479.PDF
silenced, the only noise now heard being that of the tractor. It is amazing to see it banking and gliding with the inimitable G. de H. at the wheel. Its controls are so beautifully sensitive that it can taxi on terra firma as easily as a motor car. It would do your correspondent a world of good to visit Farnboro' in order to see and then believe. LE PETIT OlSEAU. Limiting Stresses in Wings. [*558] It is not certain from his letter, No. 1538, in your issue of the 4th inst., how far Mr. Flanders wishes to press the point brought forward by him at the Aeronautical Society discussion that the limit of stress which could be put upon an aeroplane wing at a given speed of flight is represented by the reaction which such wing would experience if driven normally through the air at the same speed. I am not aware how far the following considerations have been taken into account by the constructors of aeroplanes, but as they have not been mentioned in this discussion I set them out below. In the first place it must be remembered that the text book values for the resistance experienced by aerofoils moving relatively to the air deal only with uniform motion—variations in velocity being avoided as far as possible in the experiments on which these values are founded. In other words, the phenomena dealt with are relatively stable, readings only being taken when such is the case. But when we change the attitude, and consequently the angle of inclination of the wings of an aeroplane in free flight, we have to take account of a different type of phenomenon, including an accelerating rotating movement of the aerofoil. The effect of this additional factor on the resistance of the aerofoil has not yet been properly investigated. It is well known in hydraulics that if a body be plunged into a uniformly advancing fluid stream the reaction experienced by the body at the instant of immersion is greater than subsequently, when a stable stream-line motion round the obstacle has been established. In the case of air, it is, of course, known that when a screw-propeller is started up a greater reaction is experienced at first, although the rate of revolution is less than it will eventually be, than when the full speed of revolution has been attained lor some time. Lilienthal and others (Kouchino, &c.) have also made experiments which show very clearly that, as consideration would lead us to expect, a stream-line system following the lines of least resistance which will be eventually reached cannot be set up instantaneously, and that while it is being set up an enhanced reaction is experienced by the body causing the setting up of the system of flow. Very many important results follow from this fact, which is due to the same properties of the air which enable us to fly, but we are only concerned here with its bearing on the wing-stress question in aeroplanes. To obtain some valuable experimental data to apply it to this question we should have to drive a plane through the air at nil angle of inclination, and then, as quickly as possible, change the angle to 90°. We should not be able to turn the plane instantaneously, because in attempting to do so an acceleration factor would be introduced into the reaction opposing the turn, which would be enhanced sufficiently to slow down the rotary movement. However, we could, no doubt alter the angle of attack quickly enough to get an acceleration effect in the resistance experienced by the advancing plane. That is to say, the resistance at the moment when it reached 900 would be greater than after it had been at that angle for a short time, and correspondingly if the turn were arrested at smaller angles than 90", notwithstanding that the rate of advance were kept constant. The exact amount of added resistance which would accrue for a given speed and degree of turn we have not sufficient data to determine. Experiment is urgently needed on this point. It is obvious that if an aeroplane is flying with its wings at the ordinary small positive angle and if, in order to dive, this positive angle be suddenly turned to a negative one, the system of flow appropriate to the first angle must be suddenly wiped out and a new system, appropriate to the second angle, must be set up. The wings will therefore experience during the transition from one system to the other, not a reaction which can be ascertained by reference to an ordinary pressure curve, but an enhanced reaction which might be considerably in excess of that which they would receive at normal presentment in a uniform current of the same speed as their rate of advance. How much greater this enhanced reaction may be depends upon the quickness of change of attitude, and its degree. In ordinary flying in still air the change would not be unduly quick nor extended over a wide enough range to lead to undue stressing. We are not, however, considering ordinary flying, but rapid evolutions, and it is clear that during these, whether they be dives, or sudden turns to the right, left or upwards, we have to reckon with the acceleration factor in the aerodynamic reactions which are set up. Aerial disturbances which cause sudden changes in the attitude of the aerofoil relative to the wind must also introduce a similar acceleration effect. The type of force set up in the above circumstances is one which may have a very great intensity but will have always a very limited range through which it acts. In the extreme case, it is of the nature of a percussion. Consequently a rebound effect comes into play when such a force is applied to an aeroplane's structure. The latter " gives" to the first impact, it is to be hoped no further than is good for its elastic limit, and then we should in favourable circum stances have a repercussion of the wing and the air, which would give a series of shocks of diminishing intensity. It might occur, of course, that the series was of increasing intensity and then the aeroplane would be severely tested. Hydroplanes have, in some what analogous circumstances, been smashed as effectively as if they had struck a rock. There are two further considerations arising from what has been said above. Firstly, as the enhanced acceleration reaction mentioned is due to the fact that the steady stream lines have not yet been reached, the steam-line system which does actually exist at the instant of acceleration may give a differently directed resultant from the later ones and so lead to different location of the centre of pressure. This fact may lead to undue stressing of the wing. Secondly, the actual force set up by the rotation of the wing while its attitude is being changed may add a quota to the complex forces, to which it is subject at the moment of change, sufficient to lead to undue stressing. In conclusion, I think I ought to say that the acceleration problem which has been mentioned above is under the consideration of the Research Committee of the Aeronautical Society, by whom a report will shortly be issued. The Committee will be glad to hear from anyone who has new matter to contribute towards the elucidation of this problem. Tooting Graveney, S.W. BERTRAM C. COOPER. Planes, Ltd. [I559]. I notice in your article " From the British Flying Grounds" last week, a paragraph under the head,—" Freshfield, the Mersey Aeroplane Co.," which, though in the main thoroughly accurate would tend to convey the impression that the Mersey Aeroplane Co. are the aviation department of Planes, Ltd. On the contrary they are simply licensees and tenants of Planes, Ltd., and have purchased on certain conditions Planes, Ltd's. latest machine, which in large part was designed by Messrs. Fenwick and Swaby when in the employ of Planes, Ltd. as managers. PLANES, LTD. WM. P. THOMPSON, Chairman. 6, Lord Street, Liverpool. Model " Records." [1560] In reply to Weston Hurlin Co.'s letter (No. 1551) re the World's duration record, our Mr. Trykle will be pleased to meet Mr. Weston to contest for this honour. The contest to be flown under the K. and M.A.A. Rules, at any time and place suitable to Mr. Weston. If, however, we may be allowed to make a suggestion, we should suggest Whit-Monday, May 27th, as Mr. Trykle will be visiting London on that day in connection with the Birmingham Aero Club's visit to the Paddington and Districts Aero Club. Trusting this meets with Weston, Hurlin Co.'s approval, OVERTON, KING & Co. 2, Stanley Street, Coventry. Natural Stability. [1561] I have written to Messrs. Weston, Hurlin and Co. re their acceptance of my challenge in the following terms :— Eccles. WILLIAM BOOTH. (Enclosure). " I am delighted with your acceptance of my challenge and willingly agree with your stipulation re Kite and Model Aeroplane Association rules and observers. " Re conditions. I would suggest that the K. & M.A.A. be asked to draw up a series of tests for lateral stability. I am content to leave this matter in their hands, provided the tests they impose are sufficiently stringent. " A'e time and place of contest. I am willing to fly at any time convenient to you after Whitsuntide, and as you are good enough to leave it to me to arrange conditions, I suggest the Manchester Aero Model Club's ground as the place of meeting." World's Records and Natural Stability. [1562] "A Question of the World's Record" (1551 > does not appear to us to be a question of the world's record at all, but rather of the British official record, which is quite a different thing. The world's records for distance and duration are to the best of our 479
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