Results of XRF analysis and historic literature on materials used in Hawker stainless fishplates for pin jointed construction of aircraft structures are posted for whom it may concern. Hawker pin joint construction was developed by Fred Sigrist in the mid 1920’s and found application from the prewar Hart biplanes to Hurricanes and ultimately Typhoon. The initial use of carbon steel fishplates was replaced by the use of Staybrite/18-8 stainless to specification DTD166 in the early 1930s. The advantage of DTD166, a work hardening, non weldable, austenitic stainless was in the ability to cut and form fishplates without the necessity for heat treatment to build strength, simplifying production and maintenance in the field.This inquiry developed from the analysis of Canadian CCF Hurricane remnants in comparison to UK Hurricane remnants to build an understanding of material substitution applied in Canadian production. In analysis by XRF it was expected to find evidence of US manufactured stainless steel, identifiable by chemical composition.
The surprising discovery in analysing these remnants was in linking Hawker fishplates to stainless exhaust system materials. Specifically, Hawker fishplates were predominately made from exhaust type materials, as a secondary application to the main demand in engine production. This is evidenced by the predominance of titanium in Hawker fishplates, an alloying element necessary for sound welding of this type of stainless. Hawker fishplates and Hawker pin joint construction did not require welding. The welding of Hawker fishplates would severely weaken pin jointed structures. So why would a weldable grade of material,using scarce and expensive alloying elements, be used in pin jointed construction? An answer can be understood in following the development of materials for exhausts, between the use of Inconel and Staybrite. The key influence in this story is the massive demand on materials caused by the 1930’s arms race and the scarcity of alloys such as nickel, particularly during wartime. These engine exhaust materials are explored in the following analysis :
The logic of using exhaust materials for Hawker fishplates develops from the simple arithmetic of demand. Every single aircraft engine, whether radial or inline, required exhaust manifolds and piping. Though thousands of Hurricanes were built, the demand for stainless sheet fishplates would never equal the demand for stainless sheet exhaust material for every single aircraft engine. It would make more sense for a mill to cast an ingot of material suitable for exhaust application, roll this into a sheet to send to exhaust manufacturers and divert a small portion sideways into Hawker fishplate construction. By the chemical composition of both UK and Canadian Hurricane remnants, this is what is most likely to have occurred. While suitable for making into exhausts, it is obviously necessary that it would also meet the specifications for material suitable for fishplates.
DTD166 (50 T ultimate strength) is the original sheet material specification for Hart biplane and Hurricane fishplates. This is identical in composition to DTD 171 (35 T ultimate strength). They are in fact the same material, but DTD 166 is harder and stronger. Both materials start out as the same cast billet run through a rolling mill to form a sheet coil. In simple explanation the coil is cut in half and the first half is sent out as DTD171, soft enough to be formed in a rubber press into half shells that can be welded together into exhaust stubs and collectors. The half that remains is passed through the rolling mill to progressively work harden, until it meets the DTD166 specification. In this pathway there is no disadvantage in using exhaust type material to make Hawker fishplates, except the expensive alloying element titanium is wasted in this application. But the costs of running small volume casts and sheet rolling runs of fishplate type material without the titanium are far greater through lost production efficiency.
By reference to FT Hill ‘Materials of Aircraft Construction’ 1934 there is no titanium in DTD166, but in the 1941 revision DTD166B, titanium is optionally allowed for. So I tender that DTD166 evolved into an exhaust type material, and carried the application of Hawker fishplates along with it. In every wartime squadron maintenance store was always a few sheets of DTD166B, good for repairing a Hurricane fishplate as much as a weld repair to the exhaust collector of a Centaurus or Merlin stub. In 1956 DTD166b was formalized into British Standard S520. In turn, this was superceded in 1969 by S524 (titanium stabilized) and S525 (niobium stabilized). These are still extant standards, so sourcing materials for the repair of a damaged Hawker fishplate is not problematical. As CCF Hurricane production demonstrated, the use of weldable US type exhaust materials for fishplates has ample precedent.
In the photo below, 23 Hawker Hurricane fishplate and stainless fittings are shown, numbered and tested by XRF. Each piece is identified by Hawker part number, eg A58556 and manufacturers inspection stamp, eg HA for UK Hawker Aircraft and CCF for Canadian Car and Foundry. Additional samples are marked by Hurricane assembly sub contractors Gloster and stamping works Pressed Steel Company. These are random samples sourced over the years from online auction and appeal to key members. To those who have responded, I am grateful.
[ATTACH=CONFIG]247605[/ATTACH]
By: StevSmar - 13th November 2016 at 03:20
Very interesting P&P, thanks for sharing the results of your investigations!
Sign In
August 18, 2016 at 2:24 pm