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Aviation History
1913
1913 - 0155.PDF
FEBRUARY 8, 1913. gsgcfif pieces seen by the Committee appeared to have come from the trailing edge of a wing, and others to be fragments of the strips used in fastening the fabric to the ribs. It was not clear where these were picked up. The strap had holes for nine wood screws to fix it to one of the lower cross members of the fuselage. Two brass screws only, at the two extremities of the strap, had been in use. 14. The steel strap was provided for the attachment of the lower ends of the two inner lift cables, one to each wing. These lower ends of the cables were fitted with quick-release devices. On examination of the cables after the accident it was found that the thin tubular ferrule, intended to secure the quick-release attachment, was missing from the cable to the right wing. It could not pass over either of the fastenings at the ends of the cable, and therefore could only have been lost by breakage. 15. The Committee have examined the quick-release device, and the ferrule of the quick release to the left wing cable, which was found on the cable. This ferrule appeared to have been made by flattening circular steel tubing to the shape required, and was cracked along almost its whole length. The representative of the British and Colonial Aeroplane Co. stated that oval tubing was ordinarily employed, but that the circular tubing had been used by their employees at Salisbury Plain when preparing the machine for the military trials, at a time when the oval tubing was not available. The iron strap above referred to was also refitted at the same time. 16. The ferrule was held in place by the pull on one side of the cable, and on the other side of the lever of the quick release. It was not otherwise secured, and when the tension in the cable was relaxed could be shaken from its position, allowing the quick release to open. 17. The Committee are of opinion it is probable that in the first instance the ferrule in the right wing cable broke. The tension of the left wing cable was then sufficient to pull out shortly afterwards, the two screws holding the steel strap in position. Blows from the steel strap, hanging from the left wing cable, may have damaged the trailing edge of the left wing before the strap fell oft ; and the loose right wing cable may also have caused damage to the fabric of the right wing. 18. The flyer probably switched off his engine on finding some thing was wrong and commenced to plane down. The increase to a steeper angle of descent may have been due to the injury done to the wings, or possibly to the flyer's desire to land within Port Meadow. The augmented velocity due to this steep descent may have occasioned the rupture of the already damaged fabric, and have caused it to pull away from the ribs to which it was fastened ; or, alternatively, may have led to the failure of the rear structure of the wing. It should be noted that in this machine the loading on the wings was high, about 9 lbs. per sq. ft., compared with the more normal monoplane loading of 6 to 7 lbs. per sq. ft., and between 4 and 5 lbs. per sq. ft. in the high-speed biplane BE 2. These figures refer to the average loading in steady flight. It should, of course, be borne in mind that the loading on parts of the wings will be greatly in excess of them, while the average loading will be increased by the effect of sudden gusts, warping, change of attitude, &c. 19. Experiments and calculations made at the National Physical Laboratory have led to the conclusion that it is improbable fabric of good quality would have ruptured, even on a machine oi such high normal loading and under the increased stress due to the greater velocity, unless already damaged from some other cause. The Committee regret that portions of the actual fabric from the wings of the machine destroyed were not preserved for examination, but fabric supplied by the Company and stated by them to be similar to that used on this machine has been tested and found satisfactory. 20. Calculations have also been made at the Laboratory to deter mine the distribution of stress in the main spars of the wing under the abnormal conditions which prevailed when the wing finally gave way. The stresses on the back spar under such conditions would have been unusually high, and the possibility that this spar may have failed before the fabric cannot be regarded as entirely negatived by the results of the investigations made. The actual factor of safety depends, of course, on the breaking stress of the material of the spar, and the Committee are of opinion that the similar machine now at Larkhill (Bristol Monoplane, No. 262) ought not to be used without further investigation. 21. Accident to Major Gerrard's Machine.—This machine was a Nieuport monoplane, fitted with a 50-h.p. Gnome engine. The accident in this instance resulted in no serious damage except to the engine. When at a height of about 1,000 ft. a loud noise of breakage was heard from the engine. The flyer at once switched off and planed down, first at a small gliding angle, which was increased of necessity later. In the opinion of the Committee he owed his life to his prompt action and sound judgment. On subsequent examination of the engine, one connecting-rod was found to be broken in two places ; the gudgeon-pin* and its hush were also fractured. Other damage was done to the engine, which is described in Capt. Paine's report. 22. In the course of their inquiry into this accident, the Committer learnt that several cases had occurred of fracture of both gudgeon and big-end pins in Gnome engines. A numlxM of broken big-end pins from different Gnome engines were sent to the National Physical Laboratory for examination, as well as the broken con necting-rod and other parts of the engine of Major Gerrard's machine. 23. Tests on the bent connecting-rod proved the material of which it was made to be quite satisfactory. Some reference to the design is made later. Examination of the broken big-end pins showed them to be made of fairly soft steel, case-hardened on the outside to the depth of about I mm. A groove for an oilway had been ground out on one side of the pin after hardening, extending into the soft material, and very appreciably reducing the section on that side of the pin. It was at once evident that repealing bending stresses would be very likely to cause fracture at the groove, where all tlu- fractured big-end pins submitted had broken. Tests showed that the case-hardening had weakened the resistance of the material to impact. Photomicrographic examination of one of the broken pins indicated, further, that the hardening had been carried out from too high a temperature, producing cracks in the material. 24. General Conclusion as to the Three Accidents Inves tigated.—In the course of their inquiry into these three accidents, and their inspection at Larkhill of the monoplanes in the possession of the Royal Flying Corps, the Committee have had under con sideration a numl>er of special points in which improvement appears to lie desirable, and of general questions requiring investi gation. These are dealt with below, and the Committee venture to hope that the suggestions they have to make may be of assistance towards diminishing the risk of flying, not the monoplane alone, but both classes of aeroplane. With regard to the three cases specially investigated, however, they are of opinion that it has been clearly demonstrated that these accidents were not primarily due to causes dependent on the fact that the machines were monoplanes. The investigation into these accidents has, however, raised a number of important questions of a more general character which may now be considered. 25. Engine Breakages.—Of the three accidents specially investigated, one was due to the fracture of a gudgeon-pin and a connecting-rod in the 50-h.p. Gnome engine. There is a possibility that one of the other accidents may have l>een primarily caused by fracture of some part in the 100-h.p. Gnome engine. A recent case of importance has also been brought to the notice of the Committee. A 70-h. p. Gnome engine was opened up after running 18 hours to remedy a slight defect to a valve. On examination two big-end pins were found to have fractured at the groove, and a crack had begun to develop in the head of the connecting-rod into which one of these pins fitted. 26. It has further been established that in a number of other cases fracture of gudgeon and big-end pins in Gnome engines has occurred. As has been already stated, some of these pins have been carefully examined and tested at the National Physical Laboratory. The reports made by the Laboratory show clearly that the big-end pin needs modification in design, while, in addition, the heat treatment in the pin submitted to photomicrographic examination was faulty. 27. The material of the connecting-rod tested was found to be satisfactory. In certain respects the design of the rod is capable of improvement. The evidence showed also that the arrangements for locking the valve into position in the piston were such that it was possible for a careless or inexperienced workman to put considerable torsion on the connecting-rod. This would give rise to abnormal stresses in the connecting-rod during the subsequent running of the engine, and it appears possible that the fracture of the rod was due to this. 28. The view taken by the representative of the Gnome Engine Co., who gave evidence before the Committee, was that the latter explanation was the more probable. After the interview with this representative a number of questions were submitted, in writing, to the Company. In reply to one of the questions relating to the accident to Maj. Gerrard's engine, the Company write :— " It is probable that the cause of the accident is the fracture of the gudgeon-pin at the piston end of the connecting rod." It also appears from the replies that the design of the big-end pin has been modified lately. 29. The Committee are of opinion that it is of great importance that the attention of th<; Company should be called to the two sources * In view of the similarity of the pins in the Gnome engine, it is desirable to explain precisely the terms used in this Report in referring to them. The term gudgeon-pin is used to denote the pin at the piston end of the connecting-rod ; the pin at the other end of the connecting-rod is termed the big-end pin. The latter has an oil groove in it ; the former has no groove. i-55
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