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Structural Fundamentals

Structural Fundamentals Aircraft Engineering THE MONTHLY ORGAN OF THE AERONAUTICAL ENGINEERING PROFESSION different approach is to use factors based on fatigue properties. Either a factor may be applied to the fatigue life as such, or a stress OO late for a report to be included in this issue the Royal Aero­ factor to the fatigue strength over the designed life. If such predic­ nautical Society held a Section Lecture, or rather group of tions can be made accurately enough this seems a sound approach. Tlectures, on Aircraft Design Philosophy. This was in fact However fatigue strength can only be determined by empirical primarily concerned with the fundamentals of structural design. A methods, and is subject to scatter and the influence of a number of paper by M R R. H. SANDIFER examined the ways in which loads are intangible variables depending on material, maintenance conditions applied to an aircraft. Safety factors were the subject of a paper by and practices, and corrosion effects. The sense of lack of complete M R J. K. WILLIAMS, and MR H. GIDDINGS and DR P. B. WALKER dealt respectively with Fatigue, and control over these variables has led Structural Strength Testing. to the concept of the fail-safe air­ craft: the provision of more than one From these papers, and from the discussion arising out of them, it stress path, and the choice of ma­ To meet the continued rise in costs, including the could be seen that there is still a good terials in which cracks propagate recent increase in postage rates, it has become slowly, so that they can be found deal of controversy about the funda­ necessary to make a change in the price of before catastrophic failure occurs. mental principles upon which design 'Aircraft Engineering'. With the January 1956 This invites the criticism that stand­ for strength should be based, even if issue the price per single copy will be 3s. post free. there is a fair degree of uniformity in ards of structural design may fall if The annual subscription will be 35s. post free. practice. It may be that some speak­ the aim is no longer to produce a The rate for students will remain unchanged at fail-proof structure. Here we are ers were at cross-purposes in that 21s. per annum post free. back at one of our fundamental some were outlining desirable aims factors of ignorance: with what for the future, and others concen­ trated on what was immediately degree of certainty can we assert possible. that a structure is fail-proof? A further effect of increased awareness of the problems of fatigue In fact there is in each of the four subjects covered by the lecturers was shown by the growing suspicion of high-strength aluminium a greater or lesser factor of ignorance. Much has been learnt in recent alloys on the grounds of notch sensitivity. There was a general years about probable gust loads on various routes by means of route feeling that if higher factors of safety have to be applied when surveys using counting accelerometers and other similar instruments, these alloys are used, or if design is based on fatigue properties but the loads applied by the pilot are not so easily assessed, particu­ rather than ultimate static strength, the extra cost of high-strength larly in military aircraft which are subject to violent manoeuvres in alloys may not be worth while. the course of their operational duties, and may often be flown by less experienced pilots than civil aircraft. The g-restrictor, or the incor­ In the final paper D R WALKER pointed out the benefits in terms poration of suitable feel in the controls, are solutions of this problem, of design efficiency of using static strength tests to check the stress but there are dangerous manoeuvres which can, as it were, evade the calculations. Such testing not only prevents weaknesses due to controlling element, particularly when high angular accelerations in design errors from going undiscovered, but also enables the de­ roll or yaw are present. In civil aircraft the auto-pilot is sometimes signer to take risks which would not otherwise be justified, and so suspected of failure resulting in excessive control forces being applied, produce a more efficient structure. DR WALKER also recommended but with the servo-mechanisms now available it is possible to incor­ fatigue testing of the complete airframe as a unit, such tests to be porate devices which will anticipate and counter such loads. However continued until complete failure, cracks and other minor failures there will continue to be differences between those who have faith in being repaired as they developed. 'black boxes' and those who have none. As always in such gatherings many more questions were asked For some time safety factors have been the object of criticism. than were answered, and it was clear that much work remains to Some people advocate higher factors, others that they should dis­ be done before an agreed and unified basis of design can be devel­ appear altogether and their place be taken by other considerations. oped. As always among designers there was considerable criticism At this discussion the suggestion was made that factors should be of materials, and some blame was put on pilots, but it is becoming recognized that there are limits to the pilot's capacity, and more increased for highly loaded parts and possibly reduced for less highly loaded ones. It is not clear what would be gained from this if the must be done to help him. parts are equally vital to the safety of the aircraft, and have metal The papers given are to be published in full in the Journal of the thicknesses properly proportioned to the loads involved. A totally Royal Aeronautical Society. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Aircraft Engineering and Aerospace Technology Emerald Publishing

Structural Fundamentals

Aircraft Engineering and Aerospace Technology , Volume 27 (12): 1 – Dec 1, 1955

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Publisher
Emerald Publishing
Copyright
Copyright © Emerald Group Publishing Limited
ISSN
0002-2667
DOI
10.1108/eb032631
Publisher site
See Article on Publisher Site

Abstract

Aircraft Engineering THE MONTHLY ORGAN OF THE AERONAUTICAL ENGINEERING PROFESSION different approach is to use factors based on fatigue properties. Either a factor may be applied to the fatigue life as such, or a stress OO late for a report to be included in this issue the Royal Aero­ factor to the fatigue strength over the designed life. If such predic­ nautical Society held a Section Lecture, or rather group of tions can be made accurately enough this seems a sound approach. Tlectures, on Aircraft Design Philosophy. This was in fact However fatigue strength can only be determined by empirical primarily concerned with the fundamentals of structural design. A methods, and is subject to scatter and the influence of a number of paper by M R R. H. SANDIFER examined the ways in which loads are intangible variables depending on material, maintenance conditions applied to an aircraft. Safety factors were the subject of a paper by and practices, and corrosion effects. The sense of lack of complete M R J. K. WILLIAMS, and MR H. GIDDINGS and DR P. B. WALKER dealt respectively with Fatigue, and control over these variables has led Structural Strength Testing. to the concept of the fail-safe air­ craft: the provision of more than one From these papers, and from the discussion arising out of them, it stress path, and the choice of ma­ To meet the continued rise in costs, including the could be seen that there is still a good terials in which cracks propagate recent increase in postage rates, it has become slowly, so that they can be found deal of controversy about the funda­ necessary to make a change in the price of before catastrophic failure occurs. mental principles upon which design 'Aircraft Engineering'. With the January 1956 This invites the criticism that stand­ for strength should be based, even if issue the price per single copy will be 3s. post free. there is a fair degree of uniformity in ards of structural design may fall if The annual subscription will be 35s. post free. practice. It may be that some speak­ the aim is no longer to produce a The rate for students will remain unchanged at fail-proof structure. Here we are ers were at cross-purposes in that 21s. per annum post free. back at one of our fundamental some were outlining desirable aims factors of ignorance: with what for the future, and others concen­ trated on what was immediately degree of certainty can we assert possible. that a structure is fail-proof? A further effect of increased awareness of the problems of fatigue In fact there is in each of the four subjects covered by the lecturers was shown by the growing suspicion of high-strength aluminium a greater or lesser factor of ignorance. Much has been learnt in recent alloys on the grounds of notch sensitivity. There was a general years about probable gust loads on various routes by means of route feeling that if higher factors of safety have to be applied when surveys using counting accelerometers and other similar instruments, these alloys are used, or if design is based on fatigue properties but the loads applied by the pilot are not so easily assessed, particu­ rather than ultimate static strength, the extra cost of high-strength larly in military aircraft which are subject to violent manoeuvres in alloys may not be worth while. the course of their operational duties, and may often be flown by less experienced pilots than civil aircraft. The g-restrictor, or the incor­ In the final paper D R WALKER pointed out the benefits in terms poration of suitable feel in the controls, are solutions of this problem, of design efficiency of using static strength tests to check the stress but there are dangerous manoeuvres which can, as it were, evade the calculations. Such testing not only prevents weaknesses due to controlling element, particularly when high angular accelerations in design errors from going undiscovered, but also enables the de­ roll or yaw are present. In civil aircraft the auto-pilot is sometimes signer to take risks which would not otherwise be justified, and so suspected of failure resulting in excessive control forces being applied, produce a more efficient structure. DR WALKER also recommended but with the servo-mechanisms now available it is possible to incor­ fatigue testing of the complete airframe as a unit, such tests to be porate devices which will anticipate and counter such loads. However continued until complete failure, cracks and other minor failures there will continue to be differences between those who have faith in being repaired as they developed. 'black boxes' and those who have none. As always in such gatherings many more questions were asked For some time safety factors have been the object of criticism. than were answered, and it was clear that much work remains to Some people advocate higher factors, others that they should dis­ be done before an agreed and unified basis of design can be devel­ appear altogether and their place be taken by other considerations. oped. As always among designers there was considerable criticism At this discussion the suggestion was made that factors should be of materials, and some blame was put on pilots, but it is becoming recognized that there are limits to the pilot's capacity, and more increased for highly loaded parts and possibly reduced for less highly loaded ones. It is not clear what would be gained from this if the must be done to help him. parts are equally vital to the safety of the aircraft, and have metal The papers given are to be published in full in the Journal of the thicknesses properly proportioned to the loads involved. A totally Royal Aeronautical Society.

Journal

Aircraft Engineering and Aerospace TechnologyEmerald Publishing

Published: Dec 1, 1955

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