Results of XRF analysis and historic literature on materials used in Bristol Bulldog, Hawker Demon and Boulton Paul spars and structures are posted for whom it may concern. These remnants have been sourced over a number of years from the UK, Australia and New Zealand and I am grateful for the assistance of forum members in arranging access to some of these. The purpose of these records is to form an additional engineering dataset that can confirm and support selection of modern materials for the restoration of this neglected class of aeroplanes to flight.
In simple terms these structures are formed from corrodible nickel chromium alloy strip steel, SAE type 33xx, more appropriately typified by British Standard S88 (1936) 65T strip steel. This material was first introduced as DTD54a before being formalized as BS S88. The key attributes of this material are high strength and ductility, allowing them to be roll formed into elaborate corrugated profiles.
The school of design thought that utilised this material had its genesis in the shortage of long lengths of spruce for spars for large bomber aircraft such as the Handley Page O/400 and V/1500 in 1917. The British steel industry was impressed into the task of creating steel spars to meet this need and this work evolved in the 1920’s into the task of developing alternatives to the usual timber structures for service aircraft.
On a strength to weight ratio, and for simplicity of use, timber was an excellent material, but it was subject to distortion and rapid wear in service. In 1928 the Air Ministry decreed that henceforth all service aircraft would be made of metal and the all steel Siskin III, steel winged Westland Wapiti and airships R100 and R101 were the immediate expression of this direction. The Bristol Bulldog, introduced in 1930, represents the most refined use of strip steel structural design while the Hart family biplanes adopted strip steel spars that persisted through to the Hawker Hurricane as the last expression of the technique.
An evolution of strip steel structures were composite aluminium steel spars used in late 30’s designs such as the Fairey Battle and Bristol Blenheim. A brief resurgence in steel construction for aerospace was associated with the Blue Streak rocket program and Bristol T188 high speed research jet in the late 50’s and early 60’s.While the welded steel tube aerostructures pioneered by Anton Fokker persist today, less is known about riveted strip steel structures. It is surprising to understand that many of the strip steel sections, including the spars in the Bristol Bulldog relied on precise friction fitting and this proved to be a sound technique that was easy to repair.
The way to understand original strip steel structures is to understand that designers sought a wood like material that was simply longer lasting and more predictable in use. These aerostructures were braced, box like, beam designs used on rough grass strips and through the dust and deserts of Empire. They were truss structures, built like a girder bridge, clad in a thin superstructure of timber and doped linen. The concept of elasticity within the steel core was important in creating structures that could survive abuse.
As more powerful engines were developed and airspeed increased the attribute of rigidity in structures became more important. Fabric covering was replaced by monocoque shell structures and the more rigid cantilever wings of high speed monoplanes. The materials used for these structures were different. So applying the materials of today back to strip steel structures must be sensitive to an entirely different design philosophy. This is resolved in the first instance by the question of ductility. Very few readily available high strength strip steel alloys will allow themselves to be roll formed without fracturing. In resolving this manufacturing question the rest resolves, as high strength, ductile structures also exhibit elasticity in service.
These alloys of 80 years ago are surprisingly complex, using additions of titanium, vanadium and tungsten to develop steels of remarkable performance. In the UK, the base ore chosen for these alloys was pure Swedish material. A wry criticism by one German observer of the British aircraft industry in the mid 30’s was that the much vaunted national advantage of Sheffield steel was utterly vulnerable to the cutting off of Swedish ore supply. The alloys, under probing by XRF, universally show evidence of ‘over salting’, the addition of expensive alloying elements to maximums where a range is given. You can admit, from a distance, that strip steel structures were too relatively expensive to be sustainable. The exuberant use of materials was soon curtailed in WW2, where alloys were simplified out of necessity. I can say with confidence that the materials of the 1930s were superior to those used in the War and are only equalled today by aerospace alloys processed through vacuum remelting.
The general rule of thumb for strip steel aircraft spars was 1 lb of weight for 1 foot of spar, and under this constraint elaborate corrugated sections were developed that are only crudely equalled in modern aluminium extrusions using much more material. If modern titanium strip was applied to some of these old designs quite extraordinary beam structures would result. The end of strip steel structures came in 1935, with the advent of higher strength duralumin alloy and the consequent final supremacy of monocoque aluminium structures. So the technique is largely forgotten, but this does not mean it is too complex or impossible or even prohibitively expensive to enter into. It just takes a bit of patience.
In simple terms you cannot trick past the essential prerequisite of commissioning a coil of material similar to the original, if the original structures are to be attempted. This obviously holds back the restoration of these types of aircraft. Another pathway is through the use of work hardening stainless steel strip material, which is more readily available. But lower first cost is eventually subsumed in the greater eventual cost of re-engineering structures. So much can happen if a coil of the original material becomes available. The analysis of original remnants supports this approach, to further confirm material specifications in original plans and the soundness of original engineering computations. Most importantly it shows proofs of structural solutions with a demonstrated history of safe use.
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September 22, 2016 at 11:22 am