Why is a Hurricane so costly to restore.

Tubular rivets – http://silverbiplanes.com/SPECIFICATION_PDFS/ACA/CA142.PDF ?

The Hurricane was not built to be restored; lower cost restoration was never an original design factor. It was, however, cheap to build, using available materials. The Hawker patent for pin jointed, squared tube construction was originated by Fred Sigrist, with the first design using this method being the Heron of 1925 and the last being the Typhoon of 1945. So the same rack of tube and flat stainless plate could be used to construct a venerable biplane and a snarling monoplane. A versatile, low cost, fast, flexible construction method. So Sigrist created Hawker Lego and Camm put it together in interesting ways and Hawker Aircraft and other licence builders output tens of thousands of Harts, Furys, Hinds, Demons, Hurricanes, Henleys, Typhoons quickly, on time, and to budget. The same method allowed fast and simple repair with hand tools, surrounded by camels in a desert, or bullants and kangaroos.

It is a labour intensive method. It would be worth understanding the cost of a semi skilled artisan hour in 1939, in comparison to the cost of a highly skilled, licensed man hour today, and multiply the same fitting and riveting task by the comparative wage rate. That probably explains why it is expensive to restore a Hurricane today.

Original Hurricane wing spars were British Standard S88c, hardened nickel chromium alloy steel. This is not commercially available in strip form, although the same material is still used today in solid forgings in the European built airliner taking you on holiday, based on its characteristics of high strength, ductility and elasticity. Wonderful work has been done in the modern day to find steel strip substitutions, that work. Another expense factor is that it might take ten feet of material to make one foot of acceptable spar, given losses in establishing roll forming tool geometry, methods and spar material heat treatment before experience is gained. Somebody has to pay for the nine feet in the bin.

If a spar roll forming machine has 12 stations, and there are two rolls, top and bottom at every station, there are 24 rolls to machine, test, re-machine, case harden, at considerable cost for each tool. A billet of solid steel to make a tool is not cheap, and machining is not cheap. Then if you have two profiles in a wing spar, eg a boom and an enveloping boom, you need two sets of 24 rolls. Then if you have a web profile that is different, and horizontal stabilizer profile that has different booms and webs, you have perhaps a shelf of 100 roll forming tools at GBP 1,000 each, just to be able to start at restoring a Hurricane. Somebody has to pay for this up front. Of course all this material and tooling investment in 1940 was amortized over thousands of Hurricanes, while today, perhaps only a handful. A restorer also needs to make a profit, otherwise they become a divorced restorer, and cannot provide maintenance support from a locked hangar.

There is a memoir on Harry Hawker that deals, in the end, with his death by accident. ‘HG Hawker, Airman, his Life and his Work’, by Muriel Hawker, published, I believe in 1923. At the Aerial Derby on July 16th, 1921, Harry Hawker plunged to his death in a Nieuport Goshawk biplane. It was presumed a fire broke out while he was in the air. In the relevant chapter, and without comment, a telegram is reproduced : “Hawker, Ennadale, Hook Rd, Surbiton – Machine ready for flying Tuesday afternoon (21st) – FOLLAND.”

So the reason Folland may have left Glosters when Hawker Aircraft Ltd took over that firm might not have only been about design differences.

Creaking Door
Although originally the Hurricane spars were made from S88 which is now obsolete hence no longer available, an alternate route has already been accepted by the CAA. I understand an original Hawker Mod was found from the time they Navilsed the Hind to the Nimrod which changed the spar material to stainless steel S535. The mod helpfully declared S535 as fully alternative to S88 but the really good news for Hurricane restorers is that S535 is still available. Forming it into the Polygons is done in the hardened condition using a multi stage roll former (aka a battery former). Sure this is difficult but far from impossible.

Another route was found in the USA where JNE’s Hurricane AM274 restoration has used 4340 steel, formed by brake pressing in the annealed state and then heat treated/vertical quenched on a steel core support. JNE restoration effort is outstanding and shows what can be achieved by just one person with the right mix of skill, enthusiasm and tenacity, whilst on a limited budget, not to mention having to also work for a living.

Most of the steel tubes are available even if you have to premium select from T45 to get T50/55 or cold expand to create just the right dimensions. One type which is obsolete is T26, used in the tubular rivets I think, not sure what the replacement is but this is only a 20T soft steel, so there should be plenty to choose from- Anyone?

As far as I know Gloster’s metal fabrication system was inherited from The Steel Wing Company which they bought in 1927. There is an article on the details in Flight 18th April 1930. I believe I read somewhere that the Gauntlet, the origin of which dated back to 1928, was originally built using this system but was re-engineered to the Hawker system mid way through the production run.

I don’t think that I’ve ever seen any description of the Gloster method, as opposed to the Hawker system. Is it possible that the Gauntlet used this, in which case the Finnish example might be educational? Otherwise I don’t know of any existing Gloster airframe of the appropriate vintage.

Yes, the result of Camm’s intervention, as Graham said in post #10. Right or wrong it was instrumental in Folland’s decision to quit.

Here’s a copy of the original Hawker A. Standard for just one element of the main spar…
[ATTACH=CONFIG]252991[/ATTACH]

Funnily enough; the Gladiator used the same principle for her main spars.
(That’ll be J.A.Ms next long-term project from the remains we recovered from Norway.)

Fly, yes. Climb to altitude at speed, no. That was the key design parameter…

Yes, I agree…..but I only meant as a design modification for current display aircraft.

Of course you’d have to have the redesign certified by the CAA and that opens a whole different can of worms!

I agree with CD, to increase the spar thickness by a gauge (or more) would be insignificant in terms of weight gain, easily offset by removing gun internals (if fitted) and if not it’s already lighter; win,win

Fly, yes. Climb to altitude at speed, no. That was the key design parameter, and it proved barely enough in the Battle of Britain and indeed worse later in Burma where the climb rate was insufficient to reach the altitude of the Oscars. Turn as tightly, no. There were a few features where perhaps, with all the benefits of hindsight, the Hurricane could have been improved at the design stage (without making it another aircraft altogether), but increasing weight is not one of them.

This site has some nice photographs of a Hurricane centre-section spar under rebuild:

http://www.jneaircraft.com/am274/2009-2/

The irony is that you could easily build a much simpler, and stronger, wing-spar for the Hurricane if you weren’t bothered about adding a bit of weight; and considering that Hurricanes flew happily, later in the war, with four 20mm cannons and a 500lb bomb under each wing there is no doubt that a Hurricane could fly well with that extra weight!

I was trying to find an illustration of the Hurricane main spar when I found this that sums it up well:

http://hawkerrestorations.co.uk/a-complex-structure/

Although I don’t think even these guys are actually working to 0.0005 thousandths of an inch!

Possibly a modern equivalent steel specification could be found but then that replacement would need to be certified by the CAA and that is probably where the difficulty lies.

If I remember correctly the spars on the Hurricane are ‘dumbbell’ section with the hollow ‘flanges’ formed from thin high-tensile steel intricately folded into an octagon (or a decagon?); the beauty of the design was that the strength of the spar was derived from its shape rather than the quantity of steel that formed it.

I guess that getting a steel specification that had the required tensile-strength yet could be folded into the required shape, and was available in specific (very thin) thicknesses would be pretty difficult these days. The same would be true of the tubes required for the fuselage; they would need to be right specification material but then they would need to be (cold drawn?) into the right dimensions. On low production volumes that is going to be very expensive!

Post 63 in the P9374 thread:

http://forum.keypublishing.com/showthread.php?142058-Spitfire-P9374-heading-stateside!/page3&highlight=p9374

albeit brief, but hey, that’s how I roll! sometimes!

In the case of the Hurricane, there were two issues – recreating the tooling, and commissioning a run of appropriate steel. The original steel specs are now unobtainable, and the only way to get them is a mill run – if you can find a mill. You will recall that the steel industry in this country failed owing to its reliance on unusual specification steels – the chances of getting it made now is pretty remote, and would be vastly expensive. Once they run out, that will be it.

Individual aircraft companies had their own ways of doing things: Folland had converted Gloster to his own way of thinking on making structures just before HSA took over the company and Camm insisted on his methods. Double retraining time for Gloster draughtsmen.

Although most of the Spitfire was fairly conventional stressed-skin construction, the main spar was highly individual and (or so I gather) this was a major stumbling block for many years when it came to restoring damaged airframes. It required re-establishing the entire procedure from scratch. With regard to the Hurricane, as I understand it the main cost was that of recreating the tooling. As this has now been done, the manufacture of more parts may not be cheap but is at least feasible.

The manufacture of almost anything in the industrialised world is biased very heavily towards mass-production; the more complex the product and the greater the number that are produced the more the design will rely on manufacturing techniques involving great investment in specialised manufacturing tooling. This is especially true of a high-performance combat aircraft where strength and weight are critical (and the Hurricane was a very high-performance aircraft when it was introduced).

And there is nothing particularly special about the Hurricane in this respect; imagine trying to reproduce any part of your modern car without the ‘production’ tooling!

In 1925 Sidney Camm developed a form of metal construction, using cheaper and simpler jointed tubes, rather than the alternative welded structure. This system was employed from the Hawker Bi-planes, through the Hurricane to the Typhoon and Tempest.
The Sea Fury did not employ this system. Instead, it comprised individually manufactured frames joined by numerous stringers; the whole airframe being covered by stressed alloy skins riveted into position.

The fuselage tube joint plates are held together with either tubular rivets or nuts and bolts which are inserted though “Top Hat” ferrules which are machined from S80 high tensile stainless steel.
The ferrules sits on the stainless joint plates and partially project into the fuselage tubes and an internal aluminium distance spacer is used within the tube so that there is no gap between the top hat ferrules that are on either sides of the fuselage tube. A tubular-rivet or nut and bolt holds the whole lot together. The tubular rivet/nut and bolt choice is dependent on the stress requirements of the relevant joint.
The interference fit top-hat ferrules take the shear loads on the joint, with the tubular rivets/nuts and bolts only providing a clamping effect, preventing the ferrules from moving.
As Sidney Camm is purported to have said…
“If you have to weld it… then you’ve failed.”

This is the Typhoon set-up on our old lady…
[ATTACH=CONFIG]252942[/ATTACH]
(And this is just the forward engine/cockpit structure.)

If I remember correctly, one of the restoration firms managed to find/create a machine that did the tubing for Hurricanes. They only made a finite amount of the tubes due to A) the complexity, and B) the cost of producing them.

I would imagine that their stock of tubes is either tapped out or running short, and it would take a lot of money to produce a viable amount of parts.

Is this very complex construction typical of Hawker aircraft of the thirties? How about the Gladiator – as difficult to restore? How about all thirties UK warbirds also as difficult to restore?