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Aviation History
1955
1955 - 1692.PDF
FLIGHT, 25 November 1955 811 DESIGN ASPECTS DISCUSSED —at R.Ae.S. All-day Meeting: Structural "Fail-safe" Philosophy Questioned THE lecture theatre of the Institute of Civil Engineers atStorey's Gate, Westminster, was uncomfortably full onThursday, November 17th, for the so-called "Section lecture and discussion on aircraft design philosophy" organized by the Royal Aeronautical Society. Although seldom lapsing philosophical, the meeting raised a good num- ber of design factors for public airing, and provoked illu- minating comments from members of the aircraft industry, research establishments and the Air Registration Board. The all-day deliberations were firmly guided by Sir Arnold Hall as chairman, who pointed out that the theme of the four main aspects to be discussed would be their bearing on aircraft safety. Flight Loads was the first subject, on which a paper was pre- sented by R. H. Sandifer, assistant chief designer (structures) of Handley Page, Ltd. After tracing the historical development of knowledge in this sphere, Mr. Sandifer based his description of current requirements on illustrations of the basic flight envelope and basic gust envelope, in which the variations of acceleration factor and gust load factor, respectively, were plotted against speed. He compared the requirements for civil and military aircraft in each case, and also in the case of asymmetric manoeuvres; and referred to "the debatable question of fitting g-restrictors." Con- trol-surface loads caused by autopiloted flight, and cabin pres- sure loads, were also described. Turning to the future effects of flight at higher speeds and accelerations, die lecturer said that the most important factor which would influence flight-load requirements would be the effect of kinetic heating. Temperature gradients would need to be restricted, and V-g recorders (if not g-restrictors) might be demanded. Novel aerodynamic loads might result from such developments as the jet flap while, on the structural side, aero- elastic requirements would need to be restated in view of new wing-forms. Accident investigations had shown that less than five per cent of accidents in the past had been caused by struc- tural weakness—this was a fairly good record but care would be needed in the future. In the discussion following Mr. Sandifer's paper the need for the severe requirements for negative-g manoeuvres in the case of civil aircraft, and for "fishtailing" manoeuvres, was questioned. Other points raised in the discussion included the problem of autopilot malfunctioning; the fact that the military aircraft nega- tive-g requirement was based on V-g records; and the question of whether margins used in the past to cover pilot error were now necessary. In reply to a questioner who opposed the increase in stick forces as a means of restricting the accelerations which could be applied by the pilot, the lecturer stated that weight could be saved, particularly on highly manoeuvrable aircraft, if g-restrictors were used. Safety Factors, by Mr. J. K. Williams (head of the structures section, A.R.B.), was the next paper to be presented. In it the lecturer submitted that the whole approach to the specification of safety factors had been based on experience, compromise and judgment. The statistical approach was of no use while design and operating techniques were changing as rapidly as at present. Safety in civil aircraft, he stated, depended on the properties of structures (materials, workmanship, stress analysis and test methods), intensity and frequency of loads (gusts, pilot-induced, cabin pressure and landing), and operational technique (control of speed and altitude). The trend in the fatigue-strength properties of aluminium alloys, he continued, had been a decrease which had accompanied the improved ultimate tensile strength which had occurred over the last 20 years. Strength tests to destruction were still neces- sary as a basis for certification. Dealing with design cases, the lecturer referred to the static case, in which the safety factor was expressed in terms of a load ratio (including an analysis of the 1.5 factor); and the fatigue case, in which the safety factor was expressed in terms of a life ratio. After describing past experience and suggesting some future problems, Mr. Williams referred to safety aspects of "fail-safe" structures. Opening the general discussion, Mr. Bulman (Short Bros, and Harland) enquired how the fail-safe theory was related to the use of integrally constructed wings. Mr. Chichester Miles (Hunting Percival Aircraft) submitted that, if the design philosophy of the 1.5 factor were correct, it should be applied to the stress which produced fatigue failure. Mr. Legg (Short Bros, and Harland) suggested that present stiffness requirements in relation to flutter and to aeroelasticity in general were out of date, and Mr. Raoul Hafner (Bristol Aeroplane Co.) put forward a plea for the abolition of safety factors altogether. These factors were factors of ignor- ance, Mr. Hafner added, and when failure occurred it was not because the safety factor was wrong but because the design assumptions were wrong: assumptions that were more conserva- tive should be made. Mr. Williams commented that any new design must be shown by test to be safe—either by means of a fatigue test or by fail-safe demonstration. Fatigue was the first subject considered at the afternoon session, and the main paper on this was presented by Mr. H. Giddings (assistant chief engineer, Bristol Aeroplane Co.). A paper by Lundberg in 1947, he said, had given a very accurate forecast of the position today. The Comet accident in 1954 had shown how slowly fatigue work on wings had been extended: the fact was that no component could be excluded in considering fatigue problems. The speaker dealt in turn with fatigue loading conditions (loads having differing frequencies of application), methods of obtaining fatigue loads (calculation, he said, could be only approximate) and of minimizing loads, and the empirical nature of the problem of fatigue strength. After referring to fatigue damage ratios for various loads and the cumulative-damage theory, and pointing out that the scatter of test results could be decreased by pre-loading and more com- plete testing, Mr. Giddings turned to the question of fail-safe design. The critical crack-lengths in cases of fatigue and static failures were compared, and the relationship between mean stress and critical crack-length was studied. In conclusion the lecturer put forward the following programme for guarding against fatigue :(1) analyse the operating conditions of all components; (2) study the S-n data; (3) simplify the design of the structure; (4) by careful design ensure that the rate of crack propagation is not catastrophic; (5) carry out fatigue tests; (6) confirm the results by means of flight tests; (7) use counting accelerometers in flight; and (8) ensure a high standard of maintenance and inspection. During the discussion on the subject of fatigue, Mr. Giddings emphasized that there was no real conflict between concepts of integral construction and fail-safe design. Concerning possible use of reinforced plastics in place of light alloys, the lecturer said that his company had made tests on this; in some of the tests on reinforced plastics specimens, however, "a number failed as we were putting them in the test machines." He thought that a fail-safe structure could be achieved without any significant weight-penalty. Turning to the more general aspects of design philosophy, one sneaker asked whether sufficient attention was being paid by designers to pilot fatigue: in many accident cases where "pilot error" had been blamed, he suggested that designers should take their share of responsibility. Another questioner wanted to know whether cheaper civil-aircraft operation would be achieved by reverting to the use of materials having a lower ultimate strength but better fatigue properties. The lecturer replied that fatigue- proof aircraft could be built without economic penalty and without going back to low-strength materials. Structural Strength Testing was the subject of a comprehen- sive review by Dr. P. B. Walker, head of the structures depart- ment, R.A.E., who dealt in turn with static, fatigue and thermo- structural testing. In an earlier paper published in 1949 the lecturer had concluded that major static strength tests were essential to safety. On the basis of ten years' testing experience, some 50 per cent of structures as originally designed had failed to reach the designed standard of strength—although most of these aircraft reached this standard after one or two modifications. This paper had suggested that some designers were deliberately taking the risks in design in order to save weight, secure in the know- ledge that ultimate safety was ensured by the final test; and this philosophy of taking risks in initial design was now widely accepted. The great majority of premature failures, it appeared, were caused either by structural instability or by stress concentrations. It might never be safe to rely entirely on theoretical calculations to reveal all local stress-concentrations or to indicate their magnitude. As fatigue tests were far more effective than static tests in detect- ing stress concentrations it might be possible to dispense with major static-testing for this purpose, although the likelihood of structural instability was not indicated by fatigue tests. Concerning the fatigue hazard, there were three main con- tributors; gusts, cabin pressurization and manoeuvres. The two problems facing the designer were to decide what tests were necessary and how the testing should be carried out. Tests of components, and major tests of complete aircraft or large units,
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