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Aviation History
1912
1912 - 0369.PDF
AEROPLANE WING STRESSES. WE publish another interesting letter from Mr. Mervyn O'Gorman this week, accompanied by portions of a graphic record showine the instantaneous changes of attitude on the part of an aeroplane in flight. It is a very instructive communication, which deserves to be closely studied by all, notwithstanding the fact that the writer himself tends to underestimate its value in his own letter. The whole subject of aeroplane stresses in flight, which has been put on the tapis by M. Bleriot's little disclosure, has, of course, yet to be thoroughly threshed out, but of the many aspects of the case one that particularly deserves study is the centrifugal component caused by swerving from the straight path. This also has been the subject of investigation by Mr. Mervyn O'Gorman, who has drawn our attention to the importance of remembering that the centrifugal stress does not alone result from following curved paths of flight in a vertical plane, being equally induced by turning to the right or left in a horizontal plane. It is convenient in the study of there forces to reckon them in terms of the weight of the machine, because by doing so we can see at a glance the amount by which the factor of safety is diminished under given circumstances. Calculations of this order have already been made by Mr. T. W. K. Clarke some of which were given in his letter on the Bleriot report, which we published recently. They have since been checked by Mr. Mervyn O'Gorman, who has also as we have just mentioned, added the further calculation of the stress due to centrifugal force caused by turning horizontally. Thus, for example, suppose the radius of the flight path were 75 feet and the speed of flight 40 miles an hour. On the vertical circle, the ratio of the centrifugal force to the weight of the machine is 1 '42 to 1, which is to say that if the flight path is downward, i.e., the machine is diving, then the top pressure is (he centrifugal force minus the weight, viz., 0'42 times the weight. When the centrifugal force exceeds the weight, as in this instance, top pressure of some sort must obviously be employed to keep the machine to its track and the top pressure in question is, of course, the air pressure on the upper sides of the wings. The corresponding case with a horizontal circle gives a ratio of lift (normal to planes) to weight of machine of 174 to I, which indicates that there is a 75 per cent, increase in the under pressures on the wings. At 80 miles an hour on a radius of 150 feet the under pressures are trebled for the horizontal circle while the top pressure on the wings would be nearly twice the weight of the machine in the event of a dive over such a curved path at this velocity. Or, when flattening out from a dive the increase in stress on the wings, from beneath, is itself more than twice the normal stress. As Mr. Mervyn O'Gorman points out, these facts scarcely concern the conditions of normal flight, but some of them may very well apply to the state of things after a dive, when aeroplanes are quite commonly flying at speeds very largely in excess of their horizontal flight speeds—all of which goes to show that it is the pilot who is the principal factor of safety in flying a high-speed aeroplane. A particularly interesting discussion on this subject of wing stresses, with particular reference to centrifugal action, took place ® ® Fast Huntingdon to Brooklands Trip. HAVING decided to transfer his headquarters from Huntingdon to Brooklands, for the summer, Mr. W. B. R. Moorhouse brought his 50-h.p. Gnome-Bleriot down by way of the air on Tuesday morning. He left Huntingdon at five minutes past seven, and after an hour's flving, passed over Hendon at a height of 7,000 feet. At Hampton Court he ran into a thick mist and had to come down in order to pick up landmarks, but arrived safely at Brooklands at 8.25, having covered the 80 miles in as many minutes. Aquaplane Tests at Barrow. SOME good tests were made with the Avro aquaplane at Barrow Docks on the 18th inst., Mr. Sippe, first made a circular flight at a height of 80 feet, and then Commander Schwann piloted the craft for a trial above the Cavendish Dock. The Sanders Machine at Gosforth. THE flying week which was to have been held at Gosforth last week was brought to a sudden conclusion by a mishap to Capt. Sanders and his machine on the 15th inst. He had got his biplane in flying trim, and rising to a height of 60 ft. flew outside the aerodrome in the direction of Killingworth, but was soon brought down by an air pocket. He restarted again, only to be brought •down once more by the same cause, and this time the machine informally on Monday afternoon at the Aeronautical Society, during the course of which Mr. L. Howard-Flanders drew attention to a ]>oint that had not previously been emphasised. A given wing section, flying at a given speed, |>ossesses a characteristic curve of loading denoting the intensity of pressure per unit of area for any given angle of incidence. Without going into the peculiarities of individual shape, we will assume that the maximum pressure is attained when the attitude is 900 to the direction of motion, and we will further assume that the flying attitude of wing gives rise to a pressure intensity of, say, one-fifth the value corresponding to 900. The point Mr. Flanders urged was this, that by no conceivable means would it be possible to stress such a wing lit the given spied by more than five times the usual stress. Any attempt to pursue a curvilinear path such as would give rise to a centrifugal component exceeding live times the weight of the machine would in fact be frustrated by the machine itself side-slipping through the air. In other words, the wings are incapable of creating, at that speed, a pressure exceeding the above- mentioned maximum, and are consequently inea|iablc ol fanning an abutment against which superior forces to the above could jiossifoly be generated. In order to apply this principle in practice to any given machine, it is necessary to know the characteristic of its wings, but even, as a mere generality, it is obvious that this is an impmtaut and in teresting way of regarding the problem. It suggests, for example, that a steady going machine, flying at low loading and small angle of incidence, may have a latitude for possible stress that might exceed any reasonable factor of safety, whereas a fast heavily loaded machine might be automatically safe from too much excess stress, because both loading and angle are already great. Of course, there is little doubt that the latitude for excess stress available in any machine is considerable, and it would seem probable that the centrifugal component is at times capable of bunging the whole of the range into play. Hut this fact is of small account, compared with the importance of having a limit at all. So soon as it becomes justifiable to accept a limit, so soon will it be reasonable to talk about factors and safety in the wing spars, and to try and gel their design down to something like a satisfactory basis. There is, of course, the speed factor to be taken into account as a disturbing influence that may upset all calculations, and although the question of head resistance may tend to put a limit on this also, nevertheless it is probably of such a high value as to be of no account. In fact it is the pilot who must for all time be relied u|»on not to exceed the maximum speed for which the machine is designed, and who must forego his trick dives and other fanciful evolutions of small purpose and much danger if the machine is to lie spared. The human element comes into all problems, but the engineer does not ask more than that the liability from this cause should be limited to a fairly well-defined field. It would be already something if the manufacturer could feel that no manruuvre carried out within the proper speed limits of the machine would lie capable of wrecking its wing spars in mid-air, and it is an interesting question in the light of the foregoing argument to discuss whether a theoretical guarantee of this kind is not now available from the causes above mentioned. ® ® running into a hedge, was considerably damaged. The pilot was thrown out and sustained a broken rib. Busson Flies from Pau to Paris in the Day. NOT to be outdone by his confreres Vedrines and Prevost, Busson on his Deperdussin monoplane, made a "plcndid flight from Pau to St. Cyr on the 18th inst., and thus placed himself first by a very big margin in the competition for the Coupe Pomeroy. He left Pau at twenty minutes to six in the morning and at 9.45 landed at Chauviniere, close by Poitiers. He reported that his journey had been easy as far as Libourne, but after that he had lieen much troubled by the rain, and the wings of his machine were soaked. After taking a rest he restarted at two minutes past eleven, and at ten minutes past twelve arrived at Tours. lie had had such a trying time that he decided not to continue his journey to Paris until the following morning, but during the afternoon the weather changed, and so at five minutes past five he set out in the direction of Chartres. All went well with him, and at ten minutes past seven he landed on the military grounds at Versailles. On the following day he flew over St. Cyr to Issy. Three French Balloons for G.-B. Race. THERE will be no necessity for eliminating trials to select the team to represent France in the Gordon-Bennett balloon race, as only three balloons have been entered, viz., those of MM. A. Leblanc, E. Dubonnet, and Willy Jourdain. 369 C 2
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