WORKSHOP
In the UK warbird world, two things have happened in parallel: the renaissance of the Hawker Hurricane, and the boom in customer rides. It was inevitable that, someday, a project would bring the two together. At its Elmsett base in rural Suffolk, Hawker Restorations is doing just that
Unless you were attached to the US Army Air Forces’ 350th Fighter Group in Sardinia 75 years ago, trained or instructed on the type in the wartime Soviet Union, or likewise in post-war Iran, you almost certainly won’t have flown in the back of a two-seat Hurricane. From next year, you should have the chance to join that rather exclusive list. When Peter Teichman was selling his well-known ‘Hurribomber’, the Canadianbuilt MkXII he had completed in that configuration and operated from 2009-17, he found a buyer in Hawker Restorations, the firm that originally returned this machine to flight. Upon taking to the air again, it won’t look too different from the outside, but there will be one substantial change: the second cockpit and rear seat.
The project represents a longheld ambition on the part of Hawker Restorations. Ideas of building a two-seat Hurricane first entered the mind of the company’s then owner, Tony Ditheridge, many years ago. This magazine reported in August 2001 how he intended to complete Battle of Britain veteran MkI P3717 in such a fashion, its cockpit configured to resemble one of the 350th FG machines. In the event, plans changed, and P3717 was fittingly restored to authentic 1940 condition. The concept, though, was never forgotten. Much more recently, Hawker Restorations — now under the ownership of David Wenman — saw the opportunity to turn it into reality.
First came an extensive feasibility study, as little technical detail was available on the original two-seaters. According to Hawker Restorations director Andrew Wenman, “There were a few e-mails back and forth with the design organisation that we use, David Starkey and John Tempest at Acro Aeronautical, just discussing the plausibility of it: would it be possible? They said they thought it would be.
“When we decided we were going to go for it, the original plan was that we were going to restore a two-seater from the ground up. That would take somewhere in the region of three years to do. We were always going to wait until G-ROBT [P2902] and G-HRLI [V7497] had gone out of the door, because we had contracts for those and wanted to get them finished. It was coincidental that the Teichman aircraft was available for sale at the same time as we were looking to build one. We discussed whether a conversion was possible and whether there would be any benefit [in] that. The main benefit is that we can do it in a year’s period as opposed to a three-year period. With the way things move in the industry, everything moves very quickly, and it was decided that this was probably the best way to proceed.
“The very first discussions were centred around C of G [the centre of gravity]. Would it be possible to accommodate a second occupant in the back and still have the aircraft within established and approved safe limits? Would it be possible to move parts of the aircraft from where the passenger was going to sit into other places, in order to accommodate the passenger? That was the crux of the conversations.”
Another dilemma concerned the configuration to be adopted: Soviet, American or Persian? “We found a few photos of in-field conversions which were done, which managed to retain the Hurricane’s classic fuselage profile and canopy line very closely”, says Wenman. These were of a 350th FG machine. “When we saw those photos, we decided that was going to be our goal: making sure the very recognisable profile of the Hurricane was kept, as far as possible. I think we’ve managed to achieve that objective.”
The ex-Peter Teichman aircraft arrived at Elmsett on 19 January 2018. However, the conversion programme did not begin immediately. Rather, in Andrew Wenman’s words, “First we established the basic design requirements and settled on a second seat equipped with primary flight controls”. There followed “a preliminary design review” using full-size structural mock-ups. “We constructed an airworthy, single-seat forward fuselage frame, attached to an original centre-section [of another ex-Canadian Hurricane]. At this stage we brought in our CAA-approved design engineers. In addition to the primary steel and aluminium alloy structure, there was a wooden structure to consider as well, made before my time here, which allowed us to think about how the ‘doghouse’ would look and how it could be reconfigured.
“At the end of last year we built up the rear fuselage structure extending back to the fin post area, to help develop changes to the flexible control cable runs. At this point we started mocking-up parts in nonairworthy materials, running control cables, evaluating different types of seats and seating positions. We went through all of that on the mock-up. The live aircraft sat there for four or five months without being touched.”
Peter Johnson, the company chief engineer and project leader, says, “When you start the process, you stand back and prioritise the technical challenges. One of our principal design aims was to follow as closely as possible Hawker’s original design philosophy. The Hurricane is an extremely complex structure comprising thousands of individual parts. Space is always a premium commodity when re-engineering the structure. You very soon realise the space constraints are so severe that you’ll have to use a design approach used by other manufacturers.
“The other thing is that when you’re converting a live aircraft, you don’t want to make a mistake with the primary structure, because you could effectively stop it flying for the future. You have to be very sure. The design modifications we’ve come up with have been doubly complex because if you were starting from scratch, building the aeroplane, you could modify it in a certain, logical way, but if you’re converting an existing aircraft your accessibility is limited and you have to develop the mod in a way that is retrospectively suitable for existing single-seaters.
“We always knew, by reference to photographic evidence, that a two-seat Hurricane was viable. What we didn’t know was whether those aircraft were dual-control. We took the decision early on that we would try very hard to re-engineer the primary control system so it could be dual-control, the rationale being that the aeroplane would be ideally suited to provide members of the public with a flight in a Hurricane, but that it would also be very suitable for type conversion flying. As the job has gone on, we have overcome all the technical difficulties of dualling the control system. The only facilities we don’t have in the back are the undercarriage position selector and flap position selector. All other controls have been duplicated in the back cockpit.
“It’s a matter of density. The structure is very complex. Space is at a premium. To re-engineer control systems into areas that were intended never to have control systems requires you to reposition oil pipes, cooling pipes, and all those ancillary systems. It’s been quite complex. The original Hurricane was designed to meet a design code of practice called AVP970, a military document that pre-dated the Second World War. Now, that’s no longer appropriate, so we’re using a modern design code by agreement with the CAA. Any changes we make to the aircraft, be they structural changes or system changes, all have to comply with that modern design code, so we have firm guidance in that respect.
“The fuel system is unchanged, the cooling system is unchanged. On the oil system, the original single-seater has a viscosity valve, essentially a thermostatic system which allows the oil to warm quickly in the event of a ‘scramble’ — not strictly necessary in civilian use. We’ve retained all of that originality, but it has meant repositioning the valves and associated pipe-work.
“The 24-volt battery used to occupy the space now occupied by the rear seat, so we’ve put it into the starboard wing gun bay. It’s a very comfortable location for it strength-wise — it’s much lighter than the weight of the guns and the ammunition — but it does involve some quite extensive rewiring, and the adoption of some modern thinking in terms of safety. We’re having isolating relays right next to the battery, so with the power switched offall the long cable runs are safe. The avionics stay in the fuselage, but much further aft than they were”. For operation in the modern air traffic environment, a Mode S transponder and 8.33 radios are fitted, while another addition, vital for passenger flying, is an intercom between the cockpits.
“When we came to the primary structure”, Johnson reports, “we obviously removed parts of the structure that had to be removed, otherwise you couldn’t sit in it. We then had to make good the structural integrity of the modified structure by adding new members or bracing as required. That’s gone through detailed stressing. We used a modern technique called finite element analysis, and we’ve now completed that process, so structurally the aeroplane is as strong as it was before conversion.
“Finite element analysis is a very powerful computer program, whereby you model every structural member of the structure. You can then lock certain parts in space and apply aerodynamic loads to other extremities of the structure. You can get a mathematical and a visual representation of the ‘pain’ each member of the structure is experiencing. If you remove a member, you find that the load path changes, and other existing members are now taking more load than they were designed to take. In that case, you’ve got to do something about it — you’ve got to put some new structure in to redress the balance.
“The centre of gravity range — the forward limit to the aft limit — on the Hurricane is only 4in. That’s because the disposable load — all the ordnance, the fuel — sits on the centre of gravity in flight, so it doesn’t have to have a very wide envelope. The aircraft we’re converting is particularly suitable because it has a forward C of G to start with. It has a Merlin 29 engine, which is quite heavy, it has a long engine mount and a very heavy propeller. It’s one of the reasons we opted for this programme on that aeroplane. In wartime use, the bay that will be occupied by the second person was used to store batteries, accumulators, radio sets and Iffequipment, all of which was extremely heavy. Straight away, we knew we had a ‘budget’ of 150-160lb in there before we started.
“It’s been our intention — and to date we’ve been successful — to develop a two-seater that meets all the existing design and flight envelope limits, in terms of empty weight, maximum weight and C of G position. We’re not going to be looking for a write-down of performance or an increase in certified weight. We’ve been very careful with the canopy line because not only do you have structural integrity to consider, but also aerodynamic qualities. What we wanted to avoid was a protracted aerodynamic air test programme because we had essentially changed the form of the aeroplane. We have been successful in that. We have applied very good engineering practice to the canopy line and the method of operation.”
This also goes for the additional entry/exit door for the rear cockpit, situated on the starboard side. “The design drivers”, Johnson continues, “are that it has to be as large as possible for easy access, it has to be jettisonable with the canopy closed, it has to be simple to operate, and it has to provide sufficient rigidity to that side of the original dog-house structure. We did consider putting it on the left side, but that would compromise dualling the throttle mechanism, which is on the left for obvious reasons, and didn’t give great structural advantage, so we opted for the right-hand side.
“A benefit that wasn’t really perceived very early on is that all the pilot in command’s activity to the cockpit is via the left side, so we thought that having the passenger enter on the right would distance the passenger from the pilot, and would be operationally safer. It wouldn’t be a distraction for the pilot, so he could enter and exit the aircraft as normal while his groundcrew administer the aircraft on the other side.
“We anticipate we’re going to need specially designed staging once the aircraft enters service. It’s quite a high aircraft when you go up to it, so you would offer up the staging and members of the public will mount the stairs into the aircraft. It’s a fabric-covered aeroplane, so the primary covering is a little bit more vulnerable to damage than a Spitfire, for example. That’s another reason why we think dedicated staging would be appropriate.”
With all two-seat warbirds operating under its SSAC (Safety Standards Acknowledgement and Consent) regulations, the CAA is obviously keen to ensure occupant safety. “Luckily enough”, Andrew Wenman says, “with the Hurricane there’s space behind where the passenger’s head will be, and we’ve put a pylon in there that will act as roll-over protection for the passenger. The front windscreen has a metal bar in it, and will provide acceptable roll-over protection for the pilot”. Meanwhile, Peter Johnson adds, an extra wooden hoop behind the aft cockpit provides “a great increase in strength to that part of the structure.”
He continues, “We have tried all the way through the process to continue the Hawker design concept. It would have been quite easy to introduce extensive welded sub-assemblies that Hawker didn’t use. They used high-performance materials that didn’t take to welding very well. Where possible, we’ve followed that practice. Ergonomically, when you look into the cockpit, you’re looking into a Hawker aircraft, not something which is half-and-half. We tried at the start to engineer a conventional, original, Hurricane seat. That proved impossible — we just physically couldn’t get it in. We’ve opted for a simpler, lighter, aerobatic seat from a period biplane, which sits in there very well.”
As the aircraft nears completion in its new guise, engagement with the CAA will likewise heighten. “We haven’t had a huge amount of involvement from the CAA thus far”, says Wenman. “We have told them what we’re doing, and we’ve asked them, should we be able to get it signed offthrough the right channels and should it be within the limits of the original aircraft or the limits set by the design organisation who’ve stressed it and tested it, would they accept it? They said, assuming we’ve gone through the correct channels and had it signed offcorrectly, it shouldn’t be a problem. At the end, the stress engineers will submit the modification to the aircraft to the CAA. It will go through as a major modification. So, from the CAA point of view, that’s still all to come.”
When finished for Peter Teichman, the aircraft — registered G-HHII — was configured as a MkIIb ‘Hurribomber’ of No 174 Squadron, with serial BE505. “We don’t have a plan to change it”, Wenman states. “It’s a very recognisable aircraft as it is, and it was very active on the airshow scene before it came to us. So, we’re going to leave it as it is”. Nor has there been any disruption to the existing fabric during the work so far.
Where the two-seater will end up is, as yet, unknown. “The aircraft is for sale”, says Wenman, “for someone to purchase and operate themselves or to partner with an operator. We are in the business of restoring, as opposed to operating. Should it be finished and we don’t have a buyer, we will look to make an arrangement with an operator to have it out there flying on pleasure rides.”
It has been said that the availability of passenger flights has only enhanced the Spitfire’s already considerable appeal among the British public. The Hurricane has, of course, long played second fiddle, but that is now changing. Hawker Restorations’ exhaustive and innovative two-seater project is only the latest in its long list of projects that have helped bring this about.
