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Aviation History
1956
1956 - 1696.PDF
858 FLIGHT, 30 November 1956 AUSTRALIAN AERONAUTICAL DEVELOPMENT.. and civil aircraft operators. V-g recorders were installed on aircraft on allthe major air routes in Australia and also to Japan. These results showed that the severity of the loads depends critically on the routefollowed, and also on the season; they give little hope of establishing a single average gust spectrum which is applicable to all aircraft typestravelling on all routes. In 1951, R.A.E. counting accelerometers and fatigue meters became available and these have superseded the V-grecorders which provided data of a restricted character. We are now obtaining information on routes from Australia to Japan, America andSouth Africa, as well as on a number of internal routes. Australian airlines have co-operated enthusiastically, while D.C.A. has made theinstallation of fatigue meters mandatory in all new big transpon types. V-g recorders have also been installed in aircraft of the R.A.A.F. tomeasure manoeuvring loads. In training aircraft the somewhat unexpected result was found that there is a distinct possibility of fatigue, particularlyin aircraft practising dive bombing. The number of repetitions of load is not large, but the pull-out load during each practice run is extremelyhigh. Another interesting piece of work was done in collaboration with the Ministry of Supply, whereby V-g records were obtained from anR.A.A.F. unit on combat duty in Korea. The laboratory work has continued energetically since its first incep-tion and the fatigue strength of three different types of aeroplane wing has been investigated. The first was the Mosquito, which was of gluedwooden construction. These wings sustained many thousands of repeti- tions of 90 per cent of the failing load and then failed under approxi-mately the same static load as virgin specimens. The conclusion was that the glued wooden wings were not susceptible to fatigue failure. Thematter was not pursued very far because wooden aircraft are obsolete, but the most likely explanation is that glued construction does not inducestress raisers, whereas, of course, riveted metal members have hundreds. There followed a series of tests on 14 metal wings from Boomerangfighter aircraft, the wings being of the single-spar type. The wings were tested at a number of values of fluctuating load amplitude and an S-Ncurve established. In order to define the tail end of the curve, where small loads were repeated many millions of times, a rig to vibrate thewing at its resonant frequency was devised. When compared with available data for riveted joints at low alternating stresses a reasonableagreement was obtained but, at stresses approximating to 40 per cent of the ultimate, the wing life was only ^, of that of the riveted joints. In 1950, a test programme on Mustang wings was begun and is nowcomplete. This sets out to determine the endurance under cyclic loading over the whole relevant range of alternating and mean loads, andto see how rates of crack propagation and types of failure varied with the load range. By this time, the importance of "scatter" in fatigue testresults was realized so that it became necessary to test a number of wings at each value of alternating and mean load; in all, about 100 wings weretested. Repeated load testing on such a large scale has, we believe, not been done elsewhere. In parallel with work on complete wing structures, specimens ofmaterial and forms of construction have been tested in the various types of testing machines. In the 1940s it was found that tests on the samematerial in different laboratories yielded results which could not be correlated or reproduced because specific details of the manner of testingand methods of preparation were not given. It was clear that the endurance was very sensitive to the condition of the surface and of thesubsurface layer, and an investigation was undertaken in our laboratories to determine the effect of various surface finishes on the fatigue strengthof aluminium alloy (24 ST) material. Other important investigations were made of strength reduction factors resulting from notches, and ofthe effects of testing at various rates of stress reversal. At the same time, more fundamental studies were made of the mechanism of fatiguefailure and of the development of slip bands and other metallurgical features during fatigue stressing. The importance of this wo:k was tragically highlighted in January1952, when a Dove aircraft failed in flight in Western Australia with a total loss of life, and it was discovered that another Dove which hadflown a similar number of hours had major cracks in the same critical member. It is believed that aircraft in America have also failed infatigue, although not much publicity has been given to the accidents. The fact that the Dove accident occurred in Australia was due notto the severity of service loads, but to the very high utilization of aircraft in this country; the two Doves referred to had exceeded 9,000 hoursbefore similar aircraft elsewhere had reached 5.000 hours. It might not be too strong a statement to say that the world in general became rapidlyfatigue conscious after this event, although Australia had been preaching this particular gospel for some years. Since the tragic Dove crash the Department of Civil Aviation has made a continuous study of the fatigue characteristics of all Australiancivil aircraft used in the airlines, whether their origin be American or British. In every case where there is any reason to fear an early failure,a detailed calculation of the life has been made and safe lives laid down. Already seven separate aircraft types have been affected by modificationsor replacements of components as a result of these studies. The Department has used the basic methods of estimation and detailedresearch data obtained at A.R.L., and gust data obtained in the airlines. It has combined these with a number of detailed refinements developedby its own engineers to keep the results in close accord with the practical experience so dearly bought. By always correlating the latest methodsof estimation of life with the application of the same method to the case of the Dove, the Department has been able to avoid the wide fluctuationsof estimated safe life which have occurred elsewhere and a steady and safe middle course has been steered down the years. Any estimate of life of a structure subjected to irregular load sequences,as in atmospheric turbulence, requires either a test under the actual conditions or some theory for the summation of the fatigue damagedue to various parts of the load history. Commonly the cumulative damage rule due to Palmgren and Miner is adopted, although it isknown to be inaccurate, while with a completely irregular sequence the whole method of interpretation of the rule is ill-defined. Work toelucidate the effect of irregular loading is proceeding at A.R.L. in two parallel series of experiments. The first is on material specimens, wherethe irregular load sequence originates in the random noise from a ther- mionic valve; this is amplified and force applied to the specimen by aloud-speaker type of vibrator unit. The wave form of the oscillation applied to the specimen can be controlled to produce a load spectrumsimilar to that encountered in atmospheric turbulence. The rate of loading is high and specimens can be tested to millions of cycles ina few days. The second approach is by testing full-scale wings, using a randomloading device. Operating through the normal system of hydraulic jacks this device selects in a random sequence a series of eleven upwardand eleven downward loads. The numbers and magnitudes of the loads follow a similar pattern to a typical gust spectrum. Unfortunately, thistype of test is rather slow and it will be some time yet before sufficient results are available to substantiate the current cumulative damage ruleor to suggest a new one. It should be stated that this random load investigation is a good example of Commonwealth co-ordination inaeronautical research. The R.A.E. was also aware of the importance of establishing the effect of random loading and had given thought tothe possibility of undertaking some tests. A meeting of co-ordinators in the field of structures was held under the auspices of C.A.A.R.C.in Australia in March 1955, and it was there agreed that Australia, being further forward in the devising and engineering of the testfacilities, should undertake this aspect of the work, leaving the R.A.E. free to concentrate on other things. Conclusion.—Returning, in his summing-up, to the topic of civilaviation and equipment, Mr. Coombes said:— "Finally, I would like to leave a thought suggested by one of theaircraft operators. British commercial aircraft have only been moder- ately successful in capturing world markets, despite the brilliant successof the Viscount. The British designer is second to none and the industry has a long record of military successes. The weaknesses, and here Itread upon dangerous ground, are two: firstly, the long period from the first conception to the aircraft entering airline service; and, secondly,lack of appreciation of the needs of operators in other countries. "Aircraft often take seven or eight years to perfect; this was trueof the Viscount, whereas the Convair took only four years by intensifying the development stage. Australian airlines, by reason of their wonder-fully suitable conditions of operation, have higher utilizations of aircraft than any other country m the world. No firm's tests will ever disclosethe weaknesses which airline operation will find; sheer hours on one unit, whether aircraft or engine, are a searching test, total hours on anumber of units being much less significant. Consequently, the French, in order to shorten the development period of the Caravelle, have puta prototype into simulated passenger service with Air France, although actually the aircraft only carries freight. Could not British types, quiteearly in their development, be handed to an Australian operator to find the bugs? At the moment, new aircraft are not seen in Australia untilthey are regarded as perfect. "Australian operators would also be glad to help in the specificationand conception stages of projected designs. They have long experience of aircraft operation under highly competitive conditions and are familiarwith both British and American types and with the thinking of both countries. I believe that, if consulted, they would bring a new viewpointto bear, a viewpoint which might well ensure the acceptance of the type in a much wider market than the European." _.. Nov. Dec. FORTHCOMIr 30. R.Ae.S.: Graduates and Students Section: Annual Dance. ' I. British Interplanetary Society: "Use of Probe Rocked" byC. A. Cross. MA. Dec. 3-8. Association Pour (.'Encouragement a la Recherche.Aero- naut ique: Exhibition of Rockets, Guided Missiles, and their Equipment, Paris. * . Dec. 6. Institute of Refrigeration: "Cooling Problems in High Speed Military Aircraft," by Miss B. Shilling, O.B.E., M.Sc. . ,.-.•• Dec. 7. R.Ae.S.: Graduates and Students Section: Film 'Shz*. Dec. 7. Helicopter Association: "Autcmatic.Pilots fcr Helicopters": _•... .. Part 1, "Theoretical Considerations," by H. Collomosse, •...•"'i. B.Sc; Part II, "Flight Development," by M. C. Curties, B.Sc., A.F.R.Ae.S. Dec. 8. Society of Model Aeronautical Engineers: Annual Dinner and Dance. Dee. 11. R.Ae.S.: Section Lecture: "Materials for Aircraft Struc- tures Subjected to Kinetic Heating," by Prof. A. J. Murphy, A.F.R.Ae.S. Dec. 20. R.Ae.S.: All Day Section Lecture on Transonic Wind Tunnel Testing Techniques: Papers by H. F. Vessey, F.R.Ae.S., R. Hills, A.F.R.Ae.S., J. A. Kirk, A.F.R.Ae.S., F. O'Hara, and F. E. Roe, Grad. R.Ae.S. R.Ae.S. Branch Fixtures (to Dec. 12) Dec. 3, Derby, "Development and Production of High Temperature Alloys for Turbine Engines;" by H. E. Hignert; Glasgow, "Supersonic Aircraft and Missiles," by P. J. Duncton. Dec. 5, Bristol, "Prospects and Problems in British Aviation," by P. G. Masefield; Luion, "Trends in Aircraft Electrical Installations," by H. Zeffert; Chester, "Problems Associated with Non-metallic Materials in Aircraft," by N. J. L. Meg- son and E. W. Russell; Christchurch, "Transfer Machining,'' by A. J. Sephton. Dec. 10, Henlow, Student Members' Lectures. Dec. 12, Leicester, Annual General Meeting and Film Shew; Christchurch, "American Production Methods," by J. A. Mackenzie; Coventry, Lec- tures by Junior Members; Merthyr Tydfil, Film Show; Preston, "Novel Methods of Take-Off and Landing," by T. J. Cummings; Southampton, Fourth Mitchell Memorial Lecture, "Noise and Aircraft Structures," by Prof. E. J. Richards.
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