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STRESSING FOR FAILURE

STRESSING FOR FAILURE September, 1942 AIRCRAFT ENGINEERIN G 243 PRIBRAM'S paper is in the same category and the point he raises is of almost as much importance to th e general structural engineer as it is to the aeroplane designer. MISS PETTIFOR'S paper, which has the weight of the Forest Products Research Laboratory behind it, deals with a material which has in aeroplanes now been largely superseded, though there are still, and probably always will be enough machines having structural members in wood for the point he raises to be of continued interest. Comparison between the results of various methods of testing materials has always been a matter of difficulty—we attempted to help by publishing a graph of comparative values in our Data Sheet No. 10 last month—and this article is of importance in giving an authoritative method of relating results. It is based on the results of an analysis of a large amount of data obtained at the Laboratory itself and in industrial establishments both here and in E are privileged to publish in this issue two papers of con­ the U.S.A. It is, we imagine, just what has been needed and should siderable importance to structural engineers. The one be most valuable. A point that occurs to us, tha t is not dealt with strikes at the very root of the stressing of the modern aero­ in the article, is how far th e information is application to the new plane and the underlying regulations ; while the other, though more plastic materials and, more particularly, to what are commonly limited in scope, is none the less of great value to all employing known as " improved woods". If, as we suppose, the relationships timber, as it calls attention to the relative merits of different laid down between the various methods of measuring shock resist­ methods that are in common use for testing the impact-resistance ance apply equally to these synthetic materials the paper will be of quality of this material. immediate, and growing, importance to aeronautical engineers. Presen t Methods Inadequate No one with full knowledge of th e subject can have a completely THE SOFT PEDAL NEEDED easy conscience about the present system of making calculations for a stressed-skin wing and matters have only gone as well as they have The interesting article by DR . POSTLETHWAITE, on enemy bomb because of the strength requirements allowing, except in a few ex­ sights, of which the first instalment appears this month, raises, in an ceptional circumstances, for an adequate margin of error. Up to aside as it were, a point with which we have had it in mind to deal the present, in fact, the whole subject has been largely built up on for some time. He implies, rather than directly voices, a mild pro­ an empirical basis with, in th e absence of adequate time for research, test against the practice of "ove r calling" (to adopt a card-playing no very definite scientific basis behind it. The result has been, in term) in publicity announcements of new devices. With gentle irony the main, a tendency to caution which has led to what will probably he writes of th e American sight "capable of dropping bombs into a ultimately prove to be unnecessary weight in some of th e supporting barrel from the substratosphere". Each one of us can at will pro­ structural members. Many minds in all countries are concentrating duce a number of similar instances of outrageously fantastic claims on this problem and order is gradually evolving out of the scientific put forward—usually, to be perfectly frank, from U.S. sources. chaos into which the rather unexpectedly rapid development of th e There is, for example, the case of the Fortress bomber which, in the stiff covering for the main structural components of the monoplane form in which it first appeared, was in fact a very mediocre military threw the whole subject of stressing. Close attention to points of machine and even in its latest development is markedly inferior to detail has had the danger of causing the wood to be obscured by the any of the three contemporary British four-engined types in per­ trees. formance, bomb-carrying capacity and other important respects. This needs saying, because a flood of publicity matter, giving the com­ A Fundamental Point pletely false impression that this is the outstanding bomber of the war, continues to be poured out through the medium of the daily MR . PRIBRAM, with a valuable flash of insight, now appears with press and, we regret to say,th e British Broadcasting Corporation. an axe with which he a t one blow clears away the undergrowth and This is only a single instance of a large number of similar cases in lays bare a root. His shrewd demand for a satisfactory definition of regard to American aeroplanes; many of which, from the quoted "failure" will, we suspect, force many an engineering heart to quail, sources of the stories, emanate from individuals of standing in th e but it looks as if the issue cannot be burked and a serious attempt U.S.A. who ought to know better. To those of us who are in a will have to be made to face up to it . The requirements laid down position to know the facts it is a constant source of irritation to be in A.P. 970 have well served their turn but events have overtaken repeatedly assailed by these yarns repeated by the ignorantly gullible them and are now leaving them behind; with resultant headaches as an example of how much better things are done on th e other side for the "stres s office". of the Atlantic. This type of publicity is having a most undesirable effect on relations between the two countries, as it is alienating responsible opinion a t a time when it is most important that the two A Refresher Course nations should work in the closest possible accord. The British The article is, of course, it is now easy to see, the logical outcome aircraft manufacturer, to give another instance, knows full well, and of the epoch-making paper by DR . PUGSLEY, "Structural Research is justified in his knowledge, that in the last three years he has pro­ in Aeronautics," which we had the honour of publishing in June gressed so far in the quantity-production of aeroplanes that it is 1939, and to which MR. PRIBRAM refers. In tha t paper the expres­ the American producer who can now learn from him rather than the sion "Aero-Elasticity" appeared, in a published contribution, we converse—and he is not impressed by continued asseverations to the believe for the first time ; though it has since passed into the lan­ contrary. We hope that the soft pedal will be applied across th e guage and appears as the heading to a section in the "British Atlantic and these unnecessary offorts to "sell" America to her Standard Glossary of Aeronautical Terms." It is not wholly un­ allies be discontinued. related to MR. PRIBRAM'S conundrum; for which, and other in­ trinsic reasons, we recommend a re-reading of it . Speaking for our­ selves, we can certainly say that we are glad to have been sent back to it and have little doubt that others, like ourselves, will benefit The fact that goods made of raw materials in short supply owing to war conditions are advertised in AIRCRAFT ENGINEERING should not be taken as an indication that they are by refreshing their memories on it. It is one of those rare contribu­ necessarily available for export. tions which takes the reader right back to fundamentals. MR. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Aircraft Engineering and Aerospace Technology Emerald Publishing

STRESSING FOR FAILURE

Aircraft Engineering and Aerospace Technology , Volume 14 (9): 1 – Sep 1, 1942

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Publisher
Emerald Publishing
Copyright
Copyright © Emerald Group Publishing Limited
ISSN
0002-2667
DOI
10.1108/eb030936
Publisher site
See Article on Publisher Site

Abstract

September, 1942 AIRCRAFT ENGINEERIN G 243 PRIBRAM'S paper is in the same category and the point he raises is of almost as much importance to th e general structural engineer as it is to the aeroplane designer. MISS PETTIFOR'S paper, which has the weight of the Forest Products Research Laboratory behind it, deals with a material which has in aeroplanes now been largely superseded, though there are still, and probably always will be enough machines having structural members in wood for the point he raises to be of continued interest. Comparison between the results of various methods of testing materials has always been a matter of difficulty—we attempted to help by publishing a graph of comparative values in our Data Sheet No. 10 last month—and this article is of importance in giving an authoritative method of relating results. It is based on the results of an analysis of a large amount of data obtained at the Laboratory itself and in industrial establishments both here and in E are privileged to publish in this issue two papers of con­ the U.S.A. It is, we imagine, just what has been needed and should siderable importance to structural engineers. The one be most valuable. A point that occurs to us, tha t is not dealt with strikes at the very root of the stressing of the modern aero­ in the article, is how far th e information is application to the new plane and the underlying regulations ; while the other, though more plastic materials and, more particularly, to what are commonly limited in scope, is none the less of great value to all employing known as " improved woods". If, as we suppose, the relationships timber, as it calls attention to the relative merits of different laid down between the various methods of measuring shock resist­ methods that are in common use for testing the impact-resistance ance apply equally to these synthetic materials the paper will be of quality of this material. immediate, and growing, importance to aeronautical engineers. Presen t Methods Inadequate No one with full knowledge of th e subject can have a completely THE SOFT PEDAL NEEDED easy conscience about the present system of making calculations for a stressed-skin wing and matters have only gone as well as they have The interesting article by DR . POSTLETHWAITE, on enemy bomb because of the strength requirements allowing, except in a few ex­ sights, of which the first instalment appears this month, raises, in an ceptional circumstances, for an adequate margin of error. Up to aside as it were, a point with which we have had it in mind to deal the present, in fact, the whole subject has been largely built up on for some time. He implies, rather than directly voices, a mild pro­ an empirical basis with, in th e absence of adequate time for research, test against the practice of "ove r calling" (to adopt a card-playing no very definite scientific basis behind it. The result has been, in term) in publicity announcements of new devices. With gentle irony the main, a tendency to caution which has led to what will probably he writes of th e American sight "capable of dropping bombs into a ultimately prove to be unnecessary weight in some of th e supporting barrel from the substratosphere". Each one of us can at will pro­ structural members. Many minds in all countries are concentrating duce a number of similar instances of outrageously fantastic claims on this problem and order is gradually evolving out of the scientific put forward—usually, to be perfectly frank, from U.S. sources. chaos into which the rather unexpectedly rapid development of th e There is, for example, the case of the Fortress bomber which, in the stiff covering for the main structural components of the monoplane form in which it first appeared, was in fact a very mediocre military threw the whole subject of stressing. Close attention to points of machine and even in its latest development is markedly inferior to detail has had the danger of causing the wood to be obscured by the any of the three contemporary British four-engined types in per­ trees. formance, bomb-carrying capacity and other important respects. This needs saying, because a flood of publicity matter, giving the com­ A Fundamental Point pletely false impression that this is the outstanding bomber of the war, continues to be poured out through the medium of the daily MR . PRIBRAM, with a valuable flash of insight, now appears with press and, we regret to say,th e British Broadcasting Corporation. an axe with which he a t one blow clears away the undergrowth and This is only a single instance of a large number of similar cases in lays bare a root. His shrewd demand for a satisfactory definition of regard to American aeroplanes; many of which, from the quoted "failure" will, we suspect, force many an engineering heart to quail, sources of the stories, emanate from individuals of standing in th e but it looks as if the issue cannot be burked and a serious attempt U.S.A. who ought to know better. To those of us who are in a will have to be made to face up to it . The requirements laid down position to know the facts it is a constant source of irritation to be in A.P. 970 have well served their turn but events have overtaken repeatedly assailed by these yarns repeated by the ignorantly gullible them and are now leaving them behind; with resultant headaches as an example of how much better things are done on th e other side for the "stres s office". of the Atlantic. This type of publicity is having a most undesirable effect on relations between the two countries, as it is alienating responsible opinion a t a time when it is most important that the two A Refresher Course nations should work in the closest possible accord. The British The article is, of course, it is now easy to see, the logical outcome aircraft manufacturer, to give another instance, knows full well, and of the epoch-making paper by DR . PUGSLEY, "Structural Research is justified in his knowledge, that in the last three years he has pro­ in Aeronautics," which we had the honour of publishing in June gressed so far in the quantity-production of aeroplanes that it is 1939, and to which MR. PRIBRAM refers. In tha t paper the expres­ the American producer who can now learn from him rather than the sion "Aero-Elasticity" appeared, in a published contribution, we converse—and he is not impressed by continued asseverations to the believe for the first time ; though it has since passed into the lan­ contrary. We hope that the soft pedal will be applied across th e guage and appears as the heading to a section in the "British Atlantic and these unnecessary offorts to "sell" America to her Standard Glossary of Aeronautical Terms." It is not wholly un­ allies be discontinued. related to MR. PRIBRAM'S conundrum; for which, and other in­ trinsic reasons, we recommend a re-reading of it . Speaking for our­ selves, we can certainly say that we are glad to have been sent back to it and have little doubt that others, like ourselves, will benefit The fact that goods made of raw materials in short supply owing to war conditions are advertised in AIRCRAFT ENGINEERING should not be taken as an indication that they are by refreshing their memories on it. It is one of those rare contribu­ necessarily available for export. tions which takes the reader right back to fundamentals. MR.

Journal

Aircraft Engineering and Aerospace TechnologyEmerald Publishing

Published: Sep 1, 1942

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