1925-35 British Aircraft Structural Steel S88 – Hart, Bulldog, Wapiti Steel Strip

Below is a work in progress which I would welcome “peer review” in respect of corrections, elaborations and illuminations. It is long.
The purpose of this material is to understand 1925 – 1935 British steel aircraft construction techniques which evolved between WW1 and carried through to the wing designs of Hurricanes.
The material is a draft document with indicative footnotes (represented by A) to more voluminous appendices.
It is in two parts. This first part develops through known data about Hawker biplane and other contemporary type spar construction. A subsequent part explores a steel type ubiquitous in the steel strip construction technique.

I would be happy to supply anybody who is interested a copy of the final document with properly formatted tables & appendices once it has been completed.
I would be even happier if anyone had any material that they could share to shine a light on steel strip construction, relevant DTD standards or the aircraft of the period.

BRITISH STEEL STRIP & SPAR CONSTRUCTION MATERIALS 1925 – 1935 Edward Meysztowicz

“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 endeavoured to try all the available materials possible.”
Pg 357 Materials of Aircraft Construction, 2nd Edit 1934 FT Hill, Pitman

“The BSI ( British Standards Institute) assume 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” (Department 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 spars requires 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 ideally required is to put a few generations of metallurgists in the same room and make a joke out of it :

Two metallurgists walk into a bar. The 1930s British metallurgist asks for a drink in a DTD 166A cup. The barman says “I only have Staybrite” and the Brit says. “that will do”. The 1940s 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 Yank says “that will do”. They drink for a while. The Brit asks for a refill in a S520 cup or a 302 S32. “I only have Staybrite”. The Yank asks for a refill in 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. “Say, have you got SAE 30302 or UNS S30200?” The barman sighs, “I only have Staybrite.”

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. In trying to understand historical material choices for Hawker biplane spars the most comprehensible way to start is with a chronology of steel strip material cited in the literature of the day.

Non Corrosion resisting – Heated & Tempered Corrosion resisting – Cold Worked
HIGH Carbon MEDIUM Carbon LOW carbon
1925 DTD 137 – 138 DTD 54A DTD 46A

1930 SAE 4130 DTD 166A

1935 S88C

1940

1945

1950

1955 S520
S517

1960

1965

1970

1975 S535

1980

1985

1990

1995

2000

2005

2010

2015

Most specifically accurate evidence exists in one Hawker Aircraft planA specifying the use of BS S88C material in a Hawker Australian Demon spar section. This material selection is confirmed in correspondence from the ultimate holding company of Hawker Aircraft, British Aerospace in 1979A. A divergent approach was in the selection of ‘stainless steel’ spars for restoration work undertaken for UK registered Hawker Demon I G-BTVEA, citing the use of stainless steel spars in the maritime Hawker Osprey variant. 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 purpose of the chronology is to assist in the selection of a contemporary material that equals or exceeds the performance of the original material, where this is no longer available. The specification for S88C is obsolete. The material is a Nickel-Chrome-Moly alloy not commercially available in strip form, so a modern analogue must be found or the original composition remade.

In the Subritsky collection in New Zealand is a complete Hawker Hind wing set. It always surprises me how these remarkably strong, large, braced 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 tensile 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 old 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.

Note 1
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 assume that these were classified documents used solely for military applications, with limited and controlled distribution. I have yet to locate an original DTD standard from the period. What I have found is a range of historical literature that refers to DTD standard chemical compositions, performance characteristics and applications. Specifically British Standard S88C February 1936A states that it replaces DTD 54A, and it is perhaps consistent that a Hawker Australian Demon spar material should be specified using the newly promulgated S88C specification in a drawing dated December 1936A. To date, I have not located any other manufacturer’s or Air Ministry plan or data that specifies DTD 54A steel for use in Hawker Hart family spars. In order to verify the single datasetA 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 the path of 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.

Note 2
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 Bolton & Paul DTD 46A (NB corrosion resisting),
Armstrong Whitworth Aircraft DTD 54A
Pg 254 Handbook of Aeronautics, Vol 1 1934, Pitman

Note 3
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

Note 4
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

Note 5
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

Note 6
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

Not official, but deeply illustrative, are the hand written notes of an RAF fitter in 1935

Note 7
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 aircraft in the period 1925 – 1935 :

Specification Description Performance Comment
DTD 46A 12% Chromium
1% Nickel HT strip 65 Ton proof stress
heat treatable Subject to electrolytic corrosion
DTD 54 A –
BS S88C Nickel Chromium Moly HT strip 65 Ton proof stress
heat treatable Most accepted for spars, regular use demands promulgation as British Standard
DTD 60A Non Corrosive steel sheet 40 Ton proof stress
Uncommon
DTD 99 Nickel Chromium strip 55 Ton proof stress
heat treatable Identical chemical composition to DTD 54A (author – perhaps different heat treatment)
Used for Bulldog fuselage
DTD 100 40 Ton proof stress
heat treatable Identical chemical composition to DTD 54A (author – perhaps different heat treatment)
Used for Bulldog spar web & ribs, Wapiti ribs, Atlas ribs
DTD 137 HT Carbon steel sheet 50 Ton proof stress
heat treatable Uncommon
Identical chemical composition to DTD 138 (author – perhaps different heat treatment)

DTD 138 HT Carbon steel sheet 65 Ton proof stress
heat treatable Uncommon
Identical chemical composition to DTD 137 (author – perhaps different heat treatment)

DTD 166A Austenitic Chromium Nickel Non Corrosive 50 Ton proof stress
Cold worked Used as fish/joining plates in Hawker biplane fuselages and wings

In interpreting these Notes and the tabular summary I make the following interpretations :

1. Corrosion resisting steels.

1.1
DTD 60A, 40 tons Proof stress is not strong enough for wing structural application. Apart from Note 1, I have never seen a specific manufacturer’s reference to its use. I would discount is use in spars.

1.2
DTD 46A is a heat treatable material. Was this material too aggressive as an anode in combination with carbon steel or aluminium ribs ? Note 6 refers to electrolytic corrosion issues. One of the advantages cited for the use of cold worked DTD 166 sheet is the simplicity of using a material without the need for heat treatment. DTD46A appears to be an original, early composition of stainless steel. Apart from Note 1, I have never seen a specific manufacturer’s reference to its use. I would discount is use in spars.

1.3
DTD 166A is cold worked sheet used extensively in Hawker Hart family biplanes as fish plates for fuselage joints and bracing wire wing fittings.

The great advantage of Staybrite FDP steels to specification DTD 166A (high tensile) and DTD 171A (low tensile) is that they can be welded without heat treatment and are not liable to intercrystalline corrosion or brittleness under marine conditions. Although it may be admitted that for certain fittings and fuselage construction, especially on land machines, metals which are liable to corrosion can be effectively employed, such reliance is now placed on these two specifications that land machines are using DTD 166A for stressed fittings, as it obviates any form of heat treatment during construction. The Air Ministry specification 2S.4 (5 % nickel sheet) is actually being replaced wholesale by DTD 166A at a great first cost for the material, but the important consideration that this material can be worked up into fittings and be put straight into use without any form of heat treatment, far outweighs the first cost; and the fact that the fittings are essentially corrosion resisting makes for long life and economy when reconditioning.
Pg 432 Air Annual of the British Empire, Firth Vickers Stainless Steels Limited

DTD 166A has 50 tons Proof stress but up to 70 tons Ultimate stress, implying that further, careful, cold working may improve it’s tensile strength. Contemporary advertising illustrates a Bristol spar made of DTD 166AA. Approval notes for stainless steel spars in Demon G-BTVE in the UKA cite the use of DTD54A in Hawker naval variants, which in my interpretation is an incorrect, non stainless, specification to reference. It is my interpretation that the Hawker Osprey was equipped with DTD 166A spars, but I have no supportive evidence for this apart from the context of references provided. The use of DTD 166A is cited for repair patches for damaged spar booms in AP1461 Vol III Instructions for Repair for Demon aeroplaneA. In this respect Hawker Aircraft were familiar with the material and it was deemed acceptable as a patch material for spars made of S88C.

Accepting that DTD 166A is a suitable material for spars I understand it was uncommon, being used only for a small number of aircraft, typically for Fleet Air Arm use where salt environments advanced corrosion. Contemporary literature cites a threefold cost increase for the use of stainless steel material, being the obvious reason it was not adopted for wider use.

Note 9
DTD 54 fp (proof stress) 65, cost (pence per lb) 12
DTD 166 fp (proof stress) 50, cost (pence per lb) 36
Pg 359 Materials of Aircraft Construction, 2nd Edit 1934 FT Hill, Pitman

Approval notes for stainless steel spars in Demon G-BTVE in the UKA cite the use of stainless steel type 301 S21 for spars. The same approval on page 2 cites S524 as the replacement for DTD 166A fishplates, although on pg 3 what I assume is a typo lists this as S54.
Correspondence from the ultimate holding company of Hawker Aircraft, British Aerospace lists S524 as the replacement for DTD 166A in 1979A. DTD 166A was a material in extensive use that was encapsulated into BS S520 in 1957 and later BS S524.

2. Non Corrosion resisting steels.

2.1
DTD 137 & DTD 138 are high carbon, heat treatable steel sheet products. Apart from Note 1, I have never seen a specific manufacturer’s reference to its use. I interpret these materials as being a lower cost option than alloy steels, but subject to reservations about corrosion in service use. A further comment relating to the steels of the period A refers to distortion caused by oil quenching of high carbon steels, whereas alloy steels were able to be air hardened, minimizing distortion, a vital consideration for thin walled spar sections.

Engineers hand written notes from 1980A examining replacement materials for S88C spars in RAAF Australian Demon static restoration A1-8 detail that DTD 138 was encapsulated in BS S517. DTD 137, DTD 138 and S517 have identical chemical compositions and a 65 ton Proof stress, so interpret this material as a basic, high carbon, high tensile steel sheet suitable for spars where corrosion is not an issue, such as a static restoration.

2.2
The Society Of British Aircraft Constructors Technical Specification TS96 Issue 3 A lists S517 as a withdrawn standard. It further lists S535 as a current standard, being Cr-Mo steel analgous to SAE 4130.

SAE 4130, developed in the 1930’s, is the ubiquitous steel used in US aircraft construction with ready supply in strip form to this day.

This is a chrome molybdenum steel which has been generally adopted in aircraft construction for practically all parts made of sheet and tubing. The general use of this steel is due to its excellent welding characteristics, its ease of forming, its response to heat treatment and its availability in all sizes of sheet and seamless drawn tubing. It is customary to specify this steel for all parts of an airplane fabricated from steel unless some special property possessed by one of the other steels is required.
Pg 42 Aircraft Materials and Processes, Tittetrton, Grumman Aircraft, Pitman 1937

This report includes the use of 4130/S535 based on anecdotal reports of its use in Hawker Hurricane restorations, which share the same steel spar wing design concept as the earlier Hawker Hart family biplanes. In this respect the material warrants further investigation, not the least because of its ready availability.

2.3
In summarizing the heyday period of British steel strip aircraft production it seems one chemical composition of alloy steel strip – DTD 54A/ S88C- subject to variations of heat treatment, (DTD 99, DTD 100) was the predominant and characteristic material of the period.

Standard 0.1% Proof Stress Carbon Silicon Manganese Sulphur Phosphorus Nickel Chromium Vanadium Molybdenum Tungsten
BS S88 1936 65 T sq in 0.25-0.35 max 0.3 max 0.7 max 0.05 max 0.05 3.0-5.0 0.5 – 1.5 max 0.25 max 0.5 1
DTD 54 H 1934 65 T sq in 0.25-0.35 max 0.3 0.45-0.7 max 0.05 max 0.05 3.0-5.0 0.5 – 1.5 0.65 max 0.5 1
DTD 99 1934 55 T sq in 0.25-0.35 max 0.3 0.45-0.7 max 0.05 max 0.05 3.0-5.0 0.5 – 1.5 0.65 max 0.5 1
DTD 100 1934 40 T sq in 0.25-0.35 max 0.3 0.45-0.7 max 0.05 max 0.05 3.0-5.0 0.5 – 1.5 0.65 max 0.5 1

The table above is constructed from data within BS S88C and Materials of Aircraft Construction, 2nd Edit 1934 FT Hill, Pitman. In respect of the Fitters Note 7, it appears that all steel strip construction on Hawker Hart family biplanes, Bristol Bulldog, Westland Wapiti and Armstrong Whitworth Atlas was made from the one steel type subject to different heat treatments. In this case I would make the observation that the S88 composition was the key material supporting 1925-1935 the British steel strip construction philosophy in the same way that 4130 supported the US welded tube construction design approach.

S88 steel.

Apart from chemical composition the BS S88 standard A comprises a series of mechanical tests, including bend tests. The addition of Vanadium, Molybdenum and Tungsten is at the option of the manufacturer, so it is allowed within the standard to omit these elements subject to the material passing mechanical tests. In its basic form the material is a High Nickel –Chromium-Manganese alloy steel, in the full specification a High Nickel –Chromium-Tungsten- Manganese – Molybdenum alloy steel.

Too be continued.
December 2013
Edward Meysztowicz
[email]meysto@email.com[/email]