Edition 1 Edward Meysztowicz January 2015
These notes are provided solely for the purpose of developing and sharing knowledge about steel strip construction techniques. I would welcome any questions, constructive criticism, feedback, literature or technical information that can advance knowledge in this science and art. These notes will be updated continuously as more information becomes available. Where information changes significantly, a new edition will be published.
Making aircraft out of steel seems unusual. But from this material some of the fastest and most elegant British aircraft were made between 1925 and 1935. At the Hendon Air Pageants of the 30’s the Bristol Bulldogs of the RAF performed spectacular synchronised aerobatics, their ungeared Bristol Jupiter radial engines spinning their propellers until their tips disrupted the air to create a wonderful doppler drum rip. You would certainly know when a “Vic” of Bulldogs flew overhead with a guttural, synchronised snap and growl.
The Bristol Bulldog was a light, acrobatic aeroplane, its large flying surfaces complimented by a powerful engine. Its structure was entirely made of steel alloy. This was accomplished by using an ultra thin steel alloy strip formed into corrugated sections. The steel alloy was high strength, flexible and highly resistant to fatigue. The work done in developing this steel alloy in the 1920’s and 1930’s created a product that still exceeds the performance of nearly all commercially available steel alloy today. Understanding this material is the key to the restoration of these remarkable engineering structures.
It is not possible to directly relate modern aerospace materials to these historic structures. In the first instance the science of metallurgy was in an infant and exuberant development cycle, with new alloys streaming out from development laboratories. Original literature describes these materials in experimental rather than emphatically proven terms. Where they did find practical application, they were quickly submerged under the cloak of commercial or defence secrecy. There are many oblique references to alloy performance characteristics within the literature of the day, but nothing empirical that clearly explains which materials were the standard at the time. Clear information may be obtainable from original manufacturing drawings, but this material is preserved, at best, partially. It is necessary to forensically reconstruct whole engineering facts from a number of datasets, contributions being made from drawings, surviving remnants and commercial and engineering literature of the day. It is important to realize that contemporary engineers were also dealing with novel materials and new information. The development of aerospace was so dynamic in the 20th century, that its practitioners seemed to shed obsolete information as quickly as they grasped the new. Strip steel aircraft technology was obsolete by 1935, and though it persisted in application until 1945, designers and material scientists had their minds in supersonic jets by then. The best time to ask the engineer of the 1930’s about the materials of the time would be to pose the question in the 1960’s. Unfortunately the thirty year old engineer of 1930 would now be 115 years old in 2015, so that a confident view on the materials of strip steel aircraft production is effectively lost. Strip steel was not the only steel used in these aircraft. Tubes, forgings and machinings were all made of steel alloys. These elements deserve their own separate analysis. For the moment we will focus only on strip steel. This defining element of strip steel aircraft design is the art and science most profoundly lost.
“It is appreciated that what holds good today need not be the case in quite a short time. The industry is very young, and manufacturers of raw materials, sections and component parts alike have been handicapped in a multitude of ways which are peculiar to the industry. Standardization has been impossible in many ways, and it is only of very recent years that metal aircraft have been turned out in series. Experimental types have been the rule instead of the exception, with the result that each firm has endeavored to try all the available materials possible.”
Pg 357 Materials of Aircraft Construction, 2nd Edit 1934 FT Hill, Pitman
As new materials became proven and made their way to the workshop floor, it became necessary to categorize and standardize them, a never ending effort that continues to this day.
“The BSI ( British Standards Institute) assumes responsibility for specifying a material only when it is in thoroughly established use. Comparatively new or experimental materials, or those that are only likely to be required for RAF especial use are specified by the DTD” (Directorate of Technical Development of the Air Ministry)
Pg 33 Materials of Aircraft Construction, 2nd Edit 1934 FT Hill, Pitman
Navigating through steel selection for historical aircraft requires both a recognition of the rapidly evolving nature of the historical industry and the materials it used and discarded. Secondly an appreciation of an ongoing metals industry mania for schemes to systemize the classification of materials. Relating modern classification systems to historical classifications sometimes requires two or three somersaults through nomenclature while trying to hang on to what in chemical and heat treatment terms is the same thing. The metallurgist of today will not follow the conversation of the metallurgist of the 1930’s, unless they are additionally an historian. What is easily possible is to put a few generations of metallurgists in the same room and make a joke out of it :
Two 1930’s metallurgists walk into a bar. The 1930s British metallurgist asks for a whiskey in a DTD 166A cup. The barman says “I only have Staybrite” and the Briton says. “that will do”. The 1930s American metallurgist says “I’ll have what he’s having, only I want it in 18-8” The barman says I only have “Staybrite” and the American says “that will do”. They drink for a while. A 1940’s British metallurgist walks in and asks for the same in a S520 cup or a 302 S32. “I only have Staybrite”. A 1940’s American metallurgist joins them and asks for an SAE 5517A cup, if that’s not available then an AN-QQ-S-772a CompG cup or Type 302 cup if there is no other choice. “I only have Staybrite” They drink a while longer. Two 1960’s metallurgists walk in “Say, can we have a whiskey in an SAE 30302 or UNS S30200 cup?” The barman sighs, “I only have Staybrite.”
In this particular bar there are no Germans or Frenchmen to further complicate the conversation ! Evolving schemes for classifying metals reflect the dynamic nature of twentieth century metallurgy itself. Much of the progress of aeronautics was the based on the progress of metallurgy, in creating materials capable of containing more horsepower in engines for less weight and in structures more resistant to deformation for less weight. Where do you start to build an understanding of antique metallurgy? I have chosen to begin with a chronology of steel strip material cited in the literature of the day. The next step is to locate one typical example of aircraft using strip steel within this chronology and from this base then range over other aircraft designs. In this case the common example selected is the Hawker Demon, part of the Hawker Hart family of aircraft, the archetype of British strip steel construction.
This choice is based on the existence of original drawings and aircraft remnants from the Hawker Australian Demon variant available for study by the author. An additional path of enquiry flows from the restoration of the different Hawker (UK) Demon I variant G – BTVE in the United Kingdom. In the Hawker Demon, the wing spars, the principal strength members, are made from a high strength alloy steel strip. What was this alloy steel ?
Chronology of steel strip materials
For the Australian Hawker Demon precise evidence exists in one Hawker Aircraft Ltd original drawing specifying the use of BS S88c material in the strip steel spar section. This material selection is confirmed in correspondence from the ultimate holding company of Hawker Aircraft Ltd, British Aerospace in 1979. A divergent approach was in the selection of ‘stainless steel’ spars for restoration work undertaken for UK registered Hawker Demon I G-BTVE, citing the use of stainless steel spars in the maritime Hawker Osprey variant of the Hart family. These two datasets are placed within the chronology, but a further attempt is made to develop and understand the context of these choices by referring to other historical literature. The Hawker Hurricane of Battle of Britain fame developed from the Hawker Hart family of aircraft in the late 1930’s. The Hurricane incorporates significant aspects of the strip steel spar design developed for the Hart family. In UK CAA Airworthiness Approval Notes supporting certification of flying Hurricane restorations, references are made to the use of BS S88c spars for Hurricanes, the same material used in the earlier Australian Demon. So it appears that BS S88c was used across a span of aircraft types for an extended period. It is an important material to understand.
The UK CAA notes detail that replacement material used in restored Hurricane spar booms is BS S535, a current UK Aerospace Standard, as the original material BS S88c is no longer available. I do not have access yet to the engineering work that supports this material substitution. A further search of restoration literature shows that this problem was also addressed in 1980 in support of the restoration of static Australian Demon A1-8 by the RAAF Museum. A basic comparative engineering analysis of spar materials introduced the US specifications NE 8630, SAE 4130 and BS S517 as potential substitutions. As a static museum aircraft, Australian Demon A1-8, reflecting constraints of time and money, was ultimately outfitted with facsimile timber spars, so the material substitution question was never resolved. So in the beginning there are some clear facts to develop an analysis from :
BS S88c was used in Australian Hawker Demon and Hawker Hurricane wing spars.
The specification for BS S88c is obsolete. The material is a nickel-chrome alloy no longer commercially available in strip form, so a modern analogue must be found or the original composition remade.
A literature search provides examples of substitution materials from medium carbon steels BS S517, BS S535, SAE 4130, NE 8630 and stainless steel S21 301.
In the Subritsky collection in New Zealand is a complete Hawker Hind wing set. The Hawker Hind variant is very similar to the Australian Demon. It always surprises me how these remarkably strong, large, braced wing structures can be picked up by one person. They are feather light. It is my reflection that what was achieved 80 years ago in lightweight, high strength steel aircraft structures still represents the state of the art in steel structural work today. The mathematical analysis and practical experimentation that supported this construction technique was extensive.
In this respect the modern steel structural engineer dealing in steel building frames or bridges may not have ready experience to apply to deciphering the logic behind these historical designs. I certainly don’t, and I suspect the modern aeronautical engineer dealing in aluminium or composites or stressed skin structures needs to go back to a different school of knowledge to grapple with it too. I have learnt that a patient approach to trying to understand historical material choices eventually yields some hidden logic that is worth digging for. It is necessary to go back to the basic functionality and application of the original material to assist in understanding the selection of a contemporary aerospace material that equals or exceeds the performance of the original material, where this is no longer available. And so we go back in time to the 1930’s, to see what clues can be found in the literature of the day.
The following are the materials most generally used in this country for the purposes stated :
Spars- Steel – Strip- HighTensile Steel : DTD 54A, 99, 100, 137, 138
High Tensile Non Corrodible DTD 46A, 60A, 166
Pg 215 Handbook of Aeronautics, Vol 1 1934, Pitman
DTD specifications are difficult to find. A DTD specification is not solely a chemical composition, but relates to any aspect of a material, eg heat treatment, performance tests. I understand that these were classified documents used solely for military applications, with limited and controlled distribution. What I have found is a range of historical literature that refers to DTD standard chemical compositions, performance characteristics and applications.
Most importantly, British Standard S88c February 1936 states that it replaces DTD 54A, and it is consistent that Hawker Australian Demon spar material should be specified using the newly promulgated S88c specification in a drawing dated December 1936. This logic is supported by the existence of spar drawings for Hawker Nimrods license assembled in Denmark over 1934-5, in which the spar material is specified as DTD54A. The Hawker Nimrod was a single seat naval biplane contemporary with the Hawker Australian Demon. In order to further verify the important identification of DTD54a with BS S88c other contemporary literature is examined for supportive evidence.
Running with this evidentiary link to the materials previous identity as DTD 54A opens up a great body of historical literature citing the use of the material, which was not previously apparent following its reclassification as BS S88c. DTD 54A is a high tensile steel strip of 65 tons proof stress (896 Mpa) that seems to find apparent use in many aircraft of the period.
Typical metal spar sections (used by) Westland Aircraft (recognized by author as Wapiti spar) DTD54A,
Bristol Aeroplane Co (recognized by author as Bulldog spar) DTD 54A,
Messrs Boulton & Paul DTD 46A (NB corrosion resisting),
Armstrong Whitworth Aircraft DTD 54A
Pg 254 Handbook of Aeronautics, Vol 1 1934, Pitman
The Designers selection of materials
High tensile steel strip DTD 54 (ordinary), DTD 46 (non corrosive) Application is for highly stressed parts not liable to fatigue failure, spar sections, butt tubing, tension members.
Pg 352 Materials of Aircraft Construction, 2nd Edit 1934 FT Hill, Pitman
DTD 137 High tensile carbon steel sheet 0.1% proof stress 50 tons/sq in
DTD 138 High tensile carbon steel sheet 0.1% proof stress 65 tons/sq in
DTD 54A High tensile Nickel Chromium strip 0.1% proof stress 65 tons/sq in
DTD 60A Non Corrosive High tensile steel sheet 0.1% proof stress 40 tons/sq in
Pg 266 – 270Aeroplane Design 1938 EWC Wilkins, Griffin & Co
DTD 46 Nickel Chromium steel strip 65 ton proof stress, 12% chromium, 1 % nickel
Pg 346 Fig 68 Materials of Aircraft Construction, 2nd Edit 1934 FT Hill, Pitman
Non Corrosive steels. There are two distinct forms, the straight chrome alloy steel, discovered by Mr H Brearley and the proprietary brands known as Anka, Staybrite, which are austenitic chromium nickel alloys and are not true steels at all. The first class can be hardened and tempered, and contains 12 % chromium and 1% nickel with 0.15 -0.35% carbon. These steels are subject to electrolytic action when placed in contact with other metals…the second class…hardens up when cold worked, materials in this class are DTD 166.
Pg 355 Materials of Aircraft Construction, 2nd Edit 1934 FT Hill, Pitman
In supporting the radical departure from timber construction to strip steel construction after WW1, the British Air Ministry played the role of industry sponsor. The costs to industry of abandoning its existing timber manufacturing capacity and adopting novel steel based processes and techniques in the face of the post WW1 aircraft building recession needed the financial wherewithal of the government to overcome. It did this in two ways. The first radical step was to specify that only aircraft made from steel would be accepted for RAF use after 1927. In reviewing the historical literature, a second great act of state sponsorship stands out from 1928 as the spur for both the steel industry and aircraft constructors to realign around the new technology : the building of the Airships R100 and R101. These massive undertakings brought into eminence the steel mill JJ Habershons & Sons as manufacturers of steel strip and the aircraft manufacturer Boulton Paul as manufacturers of closed joint tubing formed from stainless steel strip. The fate of R101 was tragic, crashing on its maiden voyage, effectively bringing to a close the UKs experiments with airships. The legacy of this government stimulus was in the promotion of scale strip steel manufacturing capacity, research and development into new strip steel alloys and strip steel technology transfer to new aircraft designs. Accordingly it is worth investigating the materials used in the construction of these airships.
In the R101 a mixed construction has been adopted, in which the boom tubes of the longitudinal girders are worked out in steel….The corrosion resisting steel used for the longitudinal girders of the R101 has a tensile strength of about 140kg/mm2.
Pg 18 -19 The Present State of Airship Construction, July 1938, Hans Ebner NACA Technical Memorandum 872
A great deal of experimental and research work has been carried out in an endeavour to produce stainless steel strip for the various specifications, and besides supplying the material used in the construction of the R101, large quantities of stainless material of the austenitic type of the very highest quality are being produced to meet the various specifications calling for this class of material. The future of these stainless steels, particularly for seaplanes and aircraft built to withstand the humid atmospheres in some parts of the world, will be readily appreciated. The following are specifications fulfilled by the austenitic type of stainless steel : 42 H and 42S.
Pg 584 Air Annual of the British Empire 1930, editor CG Burge, Gale & Polden
Not official, but deeply illustrative, are the hand written notes of an RAF fitter in 1935. These workbooks were associated with the training of airframe and mechanical fitters for the RAF, and often contain beautifully hand drawn breakdowns of components and quite detailed listings of airframe materials to support field repair. This practical information is not usually found in the official Air Ministry publications for specific aircraft types, which up to the outbreak of WW2 could be purchased by any member of the public. It is logical to assume that it was necessary for trainee fitters, subject to military discipline, to be given access to proprietary and often confidential manufacturing and defence information. These workbooks can be occasionally found when attics are cleared out and the books find their way to auction. In comparing the content of a number of workbooks from three different trainees the author has found the same material information for specific aircraft types. Further, given the necessity to train fitters for a range of aircraft then in service use, detailed information for a wide range of aircraft are given. These workbooks form a remarkable and comprehensive material survey that is obtainable only in piecemeal from official print literature.
Bristol Bulldog Repair scheme
Fuselage, rear portion, longerons and struts – DTD 99 (55 ton NCS strip)
(author’s note – longerons and struts are steel strips roll formed into circular sections)
Spars DTD 54 A (NC strip)
Spar web DTD 100 (40 -50 ton NCS)
Ribs DTD 100
Hawker repair scheme
Main planes spars DTD 54A
Tail plane spars DTD 54A
Side plates DTD 166A
(Westland) Wapiti Repair scheme
Spars DTD 54A
Ribs DTD 100
(Armstrong Whitworth) Atlas repair scheme
Spars DTD 54A
Ribs DTD 100
Leading and trailing edge DTD 100
RAF Notebook 1935 SJ Hardy
From these Notes extracted from contemporary literature emerges a selection of materials for spars and wings used for many RAF strip steel aircraft in the period 1925 – 1935 :
Non Corrosion resisting steels