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Hawker Hind type wing spars

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  • powerandpassion
    Never Be Afraid to Ask
    • Jul 2012
    • 1152

    Hawker Hind type wing spars

    These were roll formed from hardened SAE 3315 type nickel-chromium steel sheet, called DTD54a up to 1935 then British Standard S88 steel after 1935. This material was later used to make the similar but larger Hawker Hurricane spars. This sheet material is not commercially available today, effectively falling out of use from 1945, though similar material in the form of bar or billet is still used today in high strength, cyclically loaded, high fatigue life parts such as airliner undercarriages. In order to form bar into sheet to allow roll formed spars to be made, enquiries into the nature of the historical material have been conducted over the last seven years, in order to build engineering data supporting the restoration of original wings utilising these types of spars.

    In general, early 1930s British aircraft steel was based on high quality, relatively pure Swedish iron ore. Very high quality alloys are a characteristic of 1930's aircraft structures and engines. From the mid 30's , the pressures of rearmament and ultimately war resulted in lower quality alloys. Since the 1960's, vacuum smelting has resulted in very pure, high quality aerospace metals. A modern, vacuum remelted alloy steel made to the identical chemical composition of a 1930's steel will generally exhibit higher mechanical qualities than the original steel on this basis.

    The final mechanical characteristics of a steel alloy depend on careful treatment in casting and forming into a finished product. Careless handling through excessive mechanical deformation while forming a bar into sheet, or careless heat annealing between stages of mechanical deformation will result in a poorer finished product. Conversely, careful handling may result in a substantially better product. The combination of vacuum melting and careful manipulation of a cast billet into a finished sheet steel should result in a product far superior to the 1930's original. This is important where the design life sought for a restored historic structure is greater than the original design life. The 1930's aircraft was meant for a service life of perhaps 5 years, while the modern restoration might seek to be functional for 5 decades.

    Understanding the original material provides a realistic basis for understanding the potential of a modern material. A conservative approach with restored structures is to limit performance factors, but if this is done without a true understanding of original performance factors, the object of representing a historic aircraft in its true context can be lost. In the case of Hind - Australian Demon variants of the Hart aircraft family, these were originally designed as dive bombers, carrying significant wing loadings in service. Eliminating munitions significantly reduces these wing loadings. In this case pulling out of a dive, consistent with the angles employed in dive bombing, is not over stressing the wing structure. Where a better quality, modern, vacuum smelted spar material is used, this represents a further factor of safety. This is not an argument for the careless flinging of historic structures through the sky, rather against the careless placement of limitations on their considered handling that have no basis in engineering fact.

    The most conservative approach in restoring a historic structure is to 'do as they did' in respect of materials, moderated by an understanding of 'why' they did it. Using original material compositions capitalizes on the embedded engineering intellectual property in the material. This IP was costly to create, requiring the combined capital of government research laboratories and private enterprise employing hundreds of thoughtful personnel skilled in the arts of calculus and materials testing to arrive at a conclusion. This conclusion was then risked against the lives of test pilots and ultimately thoroughly proven in tens of thousands of hours of service use. Service use created a constant feedback loop evidenced by the development of improved variants of the same product. The original materials and engineering intellectual property was proven as safe. To try and re-create this mass of engineering intellectual property today is beyond the budget of time and money of an individual restorer or restoration concern. If a substitute material is sought, the invariable conservative corollary is to limit performance factors. A counter argument is to present historic aircraft in their true performance context, where original, proven materials of construction are utilized in restorations, that additionally benefit from factors such as vacuum smelting.

    For the sake of clarity, the use 80 year old, corroded original material, preferencing originality over safety, does not mean 'doing as they did'. New materials are safe materials.

    In order to develop this idea, a sound understanding of the original materials provides a starting context. While in general 1930's steel was high quality, it could also be low quality. By testing enough samples of Hart to Hurricane type spar material from a number of countries and a number of aircraft types, a realistic basis can be developed to consider what modern materials, benefiting from many decades of metallurgical development, can offer in terms of additional conservative factors of safety. The sample of original Hawker Hind spar examined below, from an aircraft which enjoyed a long and demanding service career and did not structurally fail during this time, shows a surprisingly poor quality steel. The aircraft entered service in 1935, at a time when rearmament was beginning to place pressures on material procurement and it could be surmised that a poor quality scrap iron was used as a feedstock rather than pure Swedish ore. The telltales, indicated by red arrows, are 100 micron long manganese sulphide inclusions, from which stress fractures could propagate. Today, a 5 micron long inclusion of this nature would cause the steel to be rejected for defense or critical performance use, yet a 20 fold increased risk made it into use as wing spar material in 1935. A further factor indication excessive and careless overheating in the mill forming the material into strip steel are excessively large grains. This was a steel carelessly cast and carelessly milled, that was nevertheless used as wing spar material in an aircraft used as a dive bomber for over 10 years.

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