Historic Aviation Metallurgy : British Standard S2 steel structural components.

AGS 935

Here is AGS 935 from the 1930’s. Specified in nickel chromium alloy. I wonder what it was fitted to?

Why-o-why…

Why is knowing what S2 was ‘understood to be back then’ important ?
This is a helix of thoughts growing from the observation of different aircraft designs, the materials of construction used in them, and how wings were engineered.
In simple terms an aircraft is a cross shape, north-south a fuselage, and east-west a wing.

The fuselage is generally one continuous member or structure, while the wing might be a continuous member or structure.
Often the wing is not a continuous structure, but is made of two members joining at the fuselage.

In observing the materials and arrangements of these designs, it is necessary to carry the forces affecting one wing, through the fuselage, to the other wing, for structural integrity.

Many British designs of the piston era had nickel chromium alloy spars, and used nickel chromium fuselage to spar fittings to join to the fuselage. Where the fuselage was a tube based design, they also used nickel chromium cross tubes or bracing wires across the fuselage to effectively tie the wings together as ‘one structure’ reacting to forces in a consistent way. Examples of these designs are Gloster Gladiator, Hurricane, Typhoon and Tiger Moth, using S2 bracing wires.

These metal materials evolved from a foundation of timber, and how it performed in service. Timber is a wonderful material of construction, but it tends to shrink or swell with moisture, and not last. So what is a metal that is ‘timber like’ in performance, but longer lasting and more predictable?

The attributes of nickel chromium alloy are high strength, ductility and elasticity. So a simple focus, today, on high strength, may not be enough to translate the designer’s original intent with material selection and performance. Nothing flexes so much as a wing. It must be strong, and, within limits, elastic.

Jes thinkin’.

Another notebook

Another fitters notebook from 1929-30, dealing with Siskins, Virginias and Wapitis, everybody’s favourites ! They offer a clue as to why the chemical composition of S2 was never given, in that in 1929 the spec could be met by nickel chromium alloy or vanadium alloy, ‘as used in some well know automobiles’. Now vanadium is one of those subjects that you don’t generally come up in conversation at a bus stop. Vanadium could be the name of a heavy metal rock band (but the specific gravity of vanadium is less than iron). If you did have a conversation, say on a first date, or in a jammed elevator, I would venture that a few scoops of vanadium were more expensive than nickel and chromium, and overtly vanadium alloys went the way of the dodo. But S2 lingered on, open ended, while everybody understood this to be NiCr.

There is a precedent for this in T5 tube, which was supposed to be 50T ultimate strength tube, but everybody understood this to mean 45T ult strength in the 1920’s and 30’s. Then T5-3% nickel was brought introduced in the early 30’s to mean 50T, until this was re defined as T50 tube.

For interest, located within the pages of the notebooks, was a pre-weekend leave bollocking on sloppy barracks cleanliness, which shows nothing much has changed in human affairs….

Another historical reference to S2 being ‘nickle’ chromium alloy, from a pre war RAAF fitters notebook, from ANAM Moorabbin Archives, dealing with Wapitis up to Hudsons, so 1928 – 1939 era.

Nicko, nice. A frosty Coopers Red for you !

Aero Engineering (Newnes) has a table from Hadfields Ltd. It is again not useful in that it places a hyphen under C, Si, Mn and Ni. It has S and P at .05. I am curious that Ex Brat quotes ‘Sulphur = not less than 0.05%, Phosphorus = not less than 0.05%’. Aero Eng notes that S and P above .06 causes problems. For S, the steel becomes brittle, for example. As discussed above, I expect that the hyphens just mean that it is not specifically controlled. I expect that the S & P values are actually maximums. I haven’t pasted a picture because for some reason I can’t upload at present.

Looking at the Vampire Mk.30 SRM (AAP.851), I see that there are potentially some S.2 parts: G00653 Eye Bolt is listed as S.11 or S.2 bar. No help of course for the XRF unless you can figure which option they actually used. Some De Havilland parts do have the material spec marked on them.

I suspect that unless you cut up the part you are testing, XRF would not tell you if a part had been case-hardened if you only tested the outside surface; this goes back to having the drawing which would specify if the part is carburised or nitrided, to know the truth about composition.

There are a number of factors behind the decision to select a forging over a simply machined part, or one assembled or welded. To consider the initial thought in design: a forged part may be the lightest solution or it might be the most compact solution where the design has to be compact – if a part has too much bulk then required clearance to adjacent parts might be sacrificed – think in particular of moving parts that require clearance to surrounding fixed structure. After that initial thought the next one is whether the manufacturer has the capability to make forgings or can get the forgings supplied. The manufacturer may chose to invest in what is for them a new process. Later on, a manufacture develops a standard practice amongst their models where a certain undercarriage part, say, becomes standard in general design – the process has been developed, and the investment in the forge and process development needs to be paid off. Each new aircraft model that comes along, the manufacture uses the IP they have developed, and uses the same basic idea and manufacturing process. This may hold until the part actually has additional requirements placed on it or a great new process comes along that has benefits worth investing in, or even that the manufacturer discovers that after the part has been in service 15 years that it actually has shortcomings.

In terms of modern replacements for forgings, there is a compromise that modern airliner manufacturers use. These are forged blocks. A basic forged block may be fully machined to make an elevator hinge fitting, say. The benefit is that a hinge fitting is basically carrying a pin joint with a spherical bearing, so that the load directions on the lug are limited and moments aren’t transferred (different though at the actuator). The benefit then in the forged block is good directional properties. Although machining does cut the aligned properties the overall process is cost effective.

Aircraft Construction Handbook by Thom Dickinson (1943) lists all sorts of US steel, brass, bronze & aluminium specs

BHP Hynicro

Just another reference, from an Australian context, the manufacture of Bristol Beauforts, showing S2 as BHP Nycromax, a nickel chromium alloy.
Out of interest, S11, normally used in crankshafts, is also a NiCr alloy, Hynicro.