The Historic Aircraft Collection has a number of single and two-seater Hawker Biplanes under restoration. The restorations are being carried out by HAC's sister company, Retrotec Ltd. HAC completed and flew their first restoration, a Hawker Nimrod I, S1581, in 2000.

The Hawker series of biplane fighters and military two-seaters developed from the Sopwith aircraft of the First World War. The availability of the Rolls Royce V12 "F" engine - later to become the famous Kestrel series of engines, and the rapid development of metal airframe structures, resulted in the sleek and beautiful silver-painted Fury fighters and Hart two-seater light bomber and training aircraft in the late '20's. Of course, the Fury directly led to the famous Hurricane fighter, whilst the Hart developed into many sub-variants, ending in the Hind, still in use all over the world during the outbreak of the Second World War.

Aircraft from the First World War period were usually wooden framed, wire braced and covered in linen fabric. Aircraft produced in the 1930’s were of transitional construction. This next phase of development was substituting the wooden frame for one of tubular steel or as was to become more common, sections made from formed rolled steel strip. Steel in this case being of a high tensile nature and therefore very light and rigid structures could be made. The problem that restorers of aircraft of this period face, is that the steel corrodes and being so thin is almost always unusable a second time around. Fortunately the fitch plates at all the junctions were made of stainless steel and these mostly survive in good order.  

In the case of the Hawker biplanes, the tubular structure used squared ends so that a joint could be made with the stainless steel plates. It was a requirement from the Air Ministry at the time that these aircraft should be easily maintained in outlying empire countries, where welding facilities may not be available. The structure was therefore held together with close fitting ferrules held together between the plates with flared mild steel tubular rivets.  

Nowadays, tubular rivets are not available nor are the high tensile strip steel and tubes utilised, so this has resulted in a major development programme at Aero Vintage Ltd to re-manufacture the material and re-create machines to produce these sections exactly as they were by Hawkers. 

The Hawker biplane had a faceted wing spar made of a rolled steel strip, closed and riveted together with a high tensile steel web separating the top boom from the lower boom; this produced a very strong structure. The machine used for this process is called a roll-forming mill and at the time the spars were re-manufactured there was only one or two companies left in the UK that had a machine sufficiently large to do this work. Both companies have now closed down. The special steel used had to be made for us by a Swiss steel company who arranged a unique smelt for Aero Vintage. Having solved both these problems we were able to go ahead and undertake the extensive engineering work required to rebuild the aircraft. 

One special feature on the Hawker biplanes is the squaring of round tubes where they meet at the pitch place. This was done at the time by a tube-squaring machine and unfortunately when the project was initiated, none were known to exist. A chance discovery of a set of rolls in a South African scrap yard, combined with photographs of the original machine, enabled Aero Vintage to remanufacture the machine. Other machines have also been installed to remanufacture the special streamline tubing used on the wing struts, and to produce the special tubular rivets.

One major challenge with the Hawker biplanes is the Kestrel engine.  In essence, this is a V-12 water cooled and supercharged engine, similar in design and layout to the later Merlin engine but there are very few of these around today and certainly almost no spares or engineering drawings. We have had to manufacture all the spares needed, and where possible anticipate problems that may develop – such as the rocker arms, which wear very quickly with modern oils.  In this case we have developed and tested a curved carbide insert that is brazed to the arm, and this provides a much better bearing surface for the camshaft.  Talking of bearings, almost every bearing on the Kestrel is special and has to be remanufactured –a slow and expensive process.  Few of the very many special tools survive either and so we have had to manufacture them from scratch, using pictures from manuals as a guide. Far from ‘restoring’ a Kestrel, we really ‘remanufacture’ them, as all spares have to be made. With our Hawker biplane projects we set out the challenge of finding twice as many engines as airframes, so that we could cannibalise others for spares if needed.  That was the idea, but it became apparent that they all had the same faults (such as rusting oil traps in the crankshaft).  After the engine is completed, we run them on our own test stand for 5 hours under load, before they are fitted to the airframe and then they are run for a further 5 hours in the airframe to test for any installation faults. Regular oil samples are taken to test for any signs of early failures. 

The smallest items to find, and some of the hardest, are the instruments.  In essence, the panel is a mixture of 1st and 2nd World War instruments. Apart from these, there are then a few special instruments only fitted to aeroplanes of this era, and they are extremely hard to find. Over the years enough have been located and overhauled, but that proved a hard challenge.  The rarest instrument of all was the boost gauge fitted to the Fury; we have never seen an example and no museum seems to carry one either, but bizarrely, one was found at a military vehicle show, on a Polish stand in the jumble section!   How it found its way to Poland is beyond imagination, let alone that it survived all these years. The beautiful stainless steel control handle, made by Dunlops, one would think to be a hard item to find also, but fortunately, few seem to have been discarded and many found their way into our stores, often in amazing condition, probably adorning pride of place in many homes over the last 80 years.