Letters to the Editor

Letters to the Editor In some ways this would be desirable, but in other respects it would seriously affect the work of the committee. For since most aeronautical research work is sponsored by Governments for military purposes, inevitably most of the litera­ Fatigue of Helicopters — Further Evidence on Steel Lugs ture will be in the same category. Nearly all the recommendations of the committee can only be Authority for Documentation put into effect either by the agreement of the Government officials concerned (e.g., standardi­ zation of document size, inclusion of catalogue cards in reports) or by a Government grant (e.g. very serious effects on life, by initiating fatigue To the Editor international abstracting journal, aeronautical failure. Dear Sir, dictionary). It is doubtful if a documentation In structures or mechanisms embodying pin- 'Engineers must bow to facts' wrote Mr J. S. committee of UNESCO would be able to ob­ loaded lugs, therefore, a fatigue test ending with­ Shapiro (AIRCRAFT ENGINEERING, Vol. XXIV, tain implementation of its proposals as quickly out failure after (say) ten million cycles cannot No. 286, p. 376, December 1952). as has been the case with AG ARD . The main point clear a lug for infinite life unless adequate anti- I had quoted in an article on 'Fatigue of against the committee coming within NATO fretting measures such as re-boring and bushing Helicopters' (AIRCRAFT ENGINEERING, Vol. XXLV, is its restriction to certain countries. But this is are taken as soon as signs of attrition appear. No. 278, April 1952) a fatigue failure of a steel more apparent than real, for disregarding the In these circumstances, helicopter designers lug (B.S.S. S.l 1) after about one million cycles at Communist bloc, the only other countries doing employing lug connexions have two courses open: a nominal stress of 7·85±2·37 tons/in.2 to any appreciable aeronautical research are Aus­ illustrate my point that, where attrition (some­ 1. Keeping to extremely low working stresses tralia, Sweden and Switzerland. It is very doubt­ times called 'fretting corrosion') can occur, the and ful if the output from these countries together normal procedure of fatigue strength estimation would be more than five per cent, of the whole, 2. Assigning a replacement or overhaul life for pure fatigue (i.e. by using stress concentration even though it is of high quality and value. and insistence on periodic inspection for data and modified Goodman diagram) breaks attribition and crack detection. The first meeting of the AG ARD Documenta­ down. tion Committee was held immediately following I give below the complete loading history of Mr Shapiro, in reply, pointed out that, in the the second annual conference of the Aslib Aero­ the above lug, in descending order of alternating case of a lug, one should apply the stress con­ nautical Group, and its agenda was compiled from stress (not chronologically): centration factor (or whatever he wishes to call matters which had been discussed by that confer­ Mea n Stress* Alternatin g Stress* Numbe r of Cycles it) to the mean stress as well as to the alternating ence. It is of interest to note that representatives (tons/in.1) (tons/in. 1) stress. He also, quite reasonably, remarked that 2·8 0 1·58 383,000 of Australia, Sweden and Switzerland were pre­ 1·52 1·035 54,000 the lug in question, having a s.c.f. of 4, was an sent at this conference, that they are members of 2·72 1·02 270,000 unfavourable design. Hence, he concluded that 2· 5 0· 8 to 0·9 47,000,000 the Aslib Aeronautical Group, and that they 2·48 0· 7 to 0·8 18,500,000 . the example given did not prove my case. keep us informed of their views on aeronautical 2· 7 0· 6 to 0·7 3,400,000 Recently, another steel lug has failed in a 2· 0 0· 5 to 0·6 3,700,000 documentation. 1·55 0· 4 to 0·5 5,200,000 fitting used on a SchEnck testing machine after a 1·3 0· 3 to 0·4 50,000,000 There has been a plethora of international total of 128 million cycles. This time, the theore­ conferences on scientific information, trying to Total cycles 128,507,000 tical stress concentration factor was only 2·5 obtain agreement on many different aspects of the and the maximum nominal tensile stress was * Nominal stresses on transverse section through centre of hole. work. Many recommendations have been made 2·8±1· 6 tons/in.2 for 380,000 cycles only. Most concerning, for instance, international abstract One cannot but approve Mr Shapiro's sensible of the loading was at much lower levels. The effort to rescue helicopter designers from the bulletins, but very few have been put into effect. failure started at a fretting mark inside the hole. Slough of Despond, but it would be doing a dis­ A possible reason for these failures is that those The material again was S.11, and the lug had a service to the industry to allow designers to be participating in the conference were drawn from steel bush with about 0.001 in. interference fit lulled into a false sense of security. too wide a field, and had not the intimate know­ on a hole of 2·8 in. diameter. ledge of each other's problems. In the Aslib Yours faithfully, The peak local stresses at the highest load level Aeronautical Group, we believe that it is more w . A. p. FISHER were only 2-5 x (2·8±1·6 ) tons/in.2, i.e. 7±4 important for us to work in line with aeronautical Royal Aircraft Establishment tons/in.2. This makes complete nonsense of the documentalists in Holland or America, than that Farnborough contention that the Goodman diagram has any we should conform to the practice of docu­ Hants significance for such a lug. mentalists in other fields of knowledge in this November 16, 1953 This example appears to indicate that as long country.The AGARD DocumentationCommittee as there is any attrition (and I have yet to see is just one way (though possibly the most im­ a pin-loaded lug where attrition does not occur portant) of putting this policy into effect. To the Editor eventually) there is no real endurance limit. Mr Yours faithfully, Shapiro wrote that perhaps the 'biggest question Dear Sir, CYRIL CLEVERDON, mark' in the fatigue of helicopters was whether In the editorial 'Deliberation on Documenta­ Hon. Secretary, pure fatigue is an adequate representation of tion', you question the advisability of the AG ARD Aslib Aeronautical Group service conditions. He mentioned, and dismissed Documentation Committee continuing to be part somewhat lightly, corrosion and wear as possible of a military set-up, and suggest that it might The College of Aeronautics Library aggravating factors. There can be no doubt what­ more appropriately come under the aegis of Cranfield, Bletchley, Bucks ever that fretting corrosion is liable to produce UNESCO. December 7, 1953 2 0 Aircraft Engineering http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Aircraft Engineering and Aerospace Technology Emerald Publishing

Letters to the Editor

Aircraft Engineering and Aerospace Technology, Volume 26 (1): 1 – Jan 1, 1954

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Publisher
Emerald Publishing
Copyright
Copyright © Emerald Group Publishing Limited
ISSN
0002-2667
DOI
10.1108/eb032380
Publisher site
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Abstract

In some ways this would be desirable, but in other respects it would seriously affect the work of the committee. For since most aeronautical research work is sponsored by Governments for military purposes, inevitably most of the litera­ Fatigue of Helicopters — Further Evidence on Steel Lugs ture will be in the same category. Nearly all the recommendations of the committee can only be Authority for Documentation put into effect either by the agreement of the Government officials concerned (e.g., standardi­ zation of document size, inclusion of catalogue cards in reports) or by a Government grant (e.g. very serious effects on life, by initiating fatigue To the Editor international abstracting journal, aeronautical failure. Dear Sir, dictionary). It is doubtful if a documentation In structures or mechanisms embodying pin- 'Engineers must bow to facts' wrote Mr J. S. committee of UNESCO would be able to ob­ loaded lugs, therefore, a fatigue test ending with­ Shapiro (AIRCRAFT ENGINEERING, Vol. XXIV, tain implementation of its proposals as quickly out failure after (say) ten million cycles cannot No. 286, p. 376, December 1952). as has been the case with AG ARD . The main point clear a lug for infinite life unless adequate anti- I had quoted in an article on 'Fatigue of against the committee coming within NATO fretting measures such as re-boring and bushing Helicopters' (AIRCRAFT ENGINEERING, Vol. XXLV, is its restriction to certain countries. But this is are taken as soon as signs of attrition appear. No. 278, April 1952) a fatigue failure of a steel more apparent than real, for disregarding the In these circumstances, helicopter designers lug (B.S.S. S.l 1) after about one million cycles at Communist bloc, the only other countries doing employing lug connexions have two courses open: a nominal stress of 7·85±2·37 tons/in.2 to any appreciable aeronautical research are Aus­ illustrate my point that, where attrition (some­ 1. Keeping to extremely low working stresses tralia, Sweden and Switzerland. It is very doubt­ times called 'fretting corrosion') can occur, the and ful if the output from these countries together normal procedure of fatigue strength estimation would be more than five per cent, of the whole, 2. Assigning a replacement or overhaul life for pure fatigue (i.e. by using stress concentration even though it is of high quality and value. and insistence on periodic inspection for data and modified Goodman diagram) breaks attribition and crack detection. The first meeting of the AG ARD Documenta­ down. tion Committee was held immediately following I give below the complete loading history of Mr Shapiro, in reply, pointed out that, in the the second annual conference of the Aslib Aero­ the above lug, in descending order of alternating case of a lug, one should apply the stress con­ nautical Group, and its agenda was compiled from stress (not chronologically): centration factor (or whatever he wishes to call matters which had been discussed by that confer­ Mea n Stress* Alternatin g Stress* Numbe r of Cycles it) to the mean stress as well as to the alternating ence. It is of interest to note that representatives (tons/in.1) (tons/in. 1) stress. He also, quite reasonably, remarked that 2·8 0 1·58 383,000 of Australia, Sweden and Switzerland were pre­ 1·52 1·035 54,000 the lug in question, having a s.c.f. of 4, was an sent at this conference, that they are members of 2·72 1·02 270,000 unfavourable design. Hence, he concluded that 2· 5 0· 8 to 0·9 47,000,000 the Aslib Aeronautical Group, and that they 2·48 0· 7 to 0·8 18,500,000 . the example given did not prove my case. keep us informed of their views on aeronautical 2· 7 0· 6 to 0·7 3,400,000 Recently, another steel lug has failed in a 2· 0 0· 5 to 0·6 3,700,000 documentation. 1·55 0· 4 to 0·5 5,200,000 fitting used on a SchEnck testing machine after a 1·3 0· 3 to 0·4 50,000,000 There has been a plethora of international total of 128 million cycles. This time, the theore­ conferences on scientific information, trying to Total cycles 128,507,000 tical stress concentration factor was only 2·5 obtain agreement on many different aspects of the and the maximum nominal tensile stress was * Nominal stresses on transverse section through centre of hole. work. Many recommendations have been made 2·8±1· 6 tons/in.2 for 380,000 cycles only. Most concerning, for instance, international abstract One cannot but approve Mr Shapiro's sensible of the loading was at much lower levels. The effort to rescue helicopter designers from the bulletins, but very few have been put into effect. failure started at a fretting mark inside the hole. Slough of Despond, but it would be doing a dis­ A possible reason for these failures is that those The material again was S.11, and the lug had a service to the industry to allow designers to be participating in the conference were drawn from steel bush with about 0.001 in. interference fit lulled into a false sense of security. too wide a field, and had not the intimate know­ on a hole of 2·8 in. diameter. ledge of each other's problems. In the Aslib Yours faithfully, The peak local stresses at the highest load level Aeronautical Group, we believe that it is more w . A. p. FISHER were only 2-5 x (2·8±1·6 ) tons/in.2, i.e. 7±4 important for us to work in line with aeronautical Royal Aircraft Establishment tons/in.2. This makes complete nonsense of the documentalists in Holland or America, than that Farnborough contention that the Goodman diagram has any we should conform to the practice of docu­ Hants significance for such a lug. mentalists in other fields of knowledge in this November 16, 1953 This example appears to indicate that as long country.The AGARD DocumentationCommittee as there is any attrition (and I have yet to see is just one way (though possibly the most im­ a pin-loaded lug where attrition does not occur portant) of putting this policy into effect. To the Editor eventually) there is no real endurance limit. Mr Yours faithfully, Shapiro wrote that perhaps the 'biggest question Dear Sir, CYRIL CLEVERDON, mark' in the fatigue of helicopters was whether In the editorial 'Deliberation on Documenta­ Hon. Secretary, pure fatigue is an adequate representation of tion', you question the advisability of the AG ARD Aslib Aeronautical Group service conditions. He mentioned, and dismissed Documentation Committee continuing to be part somewhat lightly, corrosion and wear as possible of a military set-up, and suggest that it might The College of Aeronautics Library aggravating factors. There can be no doubt what­ more appropriately come under the aegis of Cranfield, Bletchley, Bucks ever that fretting corrosion is liable to produce UNESCO. December 7, 1953 2 0 Aircraft Engineering

Journal

Aircraft Engineering and Aerospace TechnologyEmerald Publishing

Published: Jan 1, 1954

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