It is maybe 2,000 years ago. Or anything up to 6,000, depending on which historian you read. You have a roof over your head, but oddly enough it still gets nippy in winter. And also you’re a tad unhappy about the way wolves, bears, and possibly His Majesty’s Revenue men can stroll in and gollop up your pigs, goats, and wife whenever they feel like it. You might not mind so much if they golloped your mother-in-law, but they seem to feel she might be a bit indigestible. 

So you build wattle walls. Split staves of wood – usually hazel, willow or ash – woven into a lattice. This is stronger than it looks, and keeps out the bears and the wolves, if not the Revenue geezers. But it still lets in the cold. So you get this great idea one day, and plaster cow-poo – which has stickiness properties you are more than familiar with first-hand – all over it to seal the gaps. The cow-poo promptly falls off, but being a Bill Gates of your time you then experiment with mixtures of soil, clay, sand, straw, horse-hair and more cow-poo. You have to tread these ingredients to mix them, which does not do a great deal for your foot-odour, but eventually you find a mix which sticks to the wall, stays stuck, and only takes a couple of months to dry out – to cure. And once cured, not only does it stop niffing and keep out the wind and the rain, but it also strengthens the wattle – not by much, but a bit.

You have invented wattle-and-daub.

Which ultimately leads us to BLOODHOUND. Unlikely as it may seem.

 

Torsion box, Pitts Special-wise

Fast-forward now to the 1980’s. I am flying Pitts Specials, which are biplanes with wood-framed wings, fabric covered. These wings are light, yes, but not in themselves inherently strong. In fact, if you even tried to take-off with just the basic wood and fabric wings they would quite certainly break off, which would leave you piloting a wingless fuselage and feeling a bit of a dork. The trick is that they are wire-braced internally with a criss-cross of wires (called drag and anti-drag wires) and also externally by another criss-cross running between the upper and lower wings from the centre-section to the wing-struts. (Called flying wires and landing wires). The net result is a sort of torsion box – whatever you do with it, jump on it from the top, jump on it from the bottom, try to twist it – the stresses are channelled to a wire or wires which are simply in tension. Being pulled. Wires are at their strongest being pulled. They are obviously useless in compression, bending, twisting, and are not all that keen on sheer loads – but give them a straight yank and they dig their toes in. Just four Pitts flying wires, slender, streamlined things that they are, would lift up a heavy lorry.

Which made the dear old Pitts very strong. It was formally licensed to operate to plus 6 G (positive G) and minus 3 G (negative G). Airworthiness law requires that there must be at least a 50% safety factor on top of that, so it was actually safe to +9 and –4.5 G. In fact it was even stronger than that, and several pilots around the world – my badself included – occasionally landed with a bit more than that registered on the recording G-meter. At which point we hastily pressed the button to reset it to zero, and closely examined the airframe.

So the Pitts was – and remains – one strong aeroplane. But an aeroplane with built-in snags. Being a biplane it suffers from interference-drag between the upper and lower wings. And it has wires and struts out there in the airflow which also add drag…

Aerodynamically, a monoplane – just one wing – is much more effective. Other manufacturers of aerobatic craft – Zlin of Czechosvakia, CAP of France, Yak of Russia – fielded monoplanes, which are, obviously, cantilever (un-braced) structures, with wings made of wood or an amalgam of aluminium-based alloys.

All suffered wing failures at various times. And the list of deceased pilots slowly crept up. While the Pitts Special – the ‘old’ biplane – soldiered on, proving that it’s easier and cheaper to make a biplane stronger.

 

Torsion box, carbon-fibre wise

Then along came carbon-fibre.

Which has now come to be the magic bullet in thousands and thousands of applications. You find it in major bridges. In military vehicles and equipment. And bicycles. And yachts. And in green energy generators, particularly wave and tidal. And in ejector seats. And especially in F1 cars. And of course in aircraft production…

In aerobatic aircraft, the first all-carbon-fibre wing was produced by the Russian Sukhoi factory in 1984. The second was the German-built Extra 300, which first flew in 1988. I bought and operated carbon Extras, and went to see my first one under construction.

It was kinda eerie, as if some weird alchemist in a pointed hat had suddenly overturned everything I’d ever known about wing construction. Instead of a wing-jig there was a female mould in which the underside wing-skins were laid-up with varying layers of carbon-fibre cloth, which then had resin painted onto the inside of it by hand. Then the whole thing, mould and all, was encased in a huge polythene bag, and the air sucked out of the bag so that the entire caboodle was pressurised by external atmospheric pressure (about 14 psi, or 1013 Mb on an average day), to make sure the product was firmly pressed into the mould. Said caboodle was then gingerly slid into a huge oven called an autoclave, and heated up for many hours to some 70 deg C to cure (liquefy and then harden) the resin. The same thing happened with the top wing-skins, and also with the double-box main spars. The spars especially made me blink. They had many carbon-fibre layers at the wing roots, certainly – but as the spars themselves tapered off towards the wing tips, so the layers gradually reduced until right at the tips the box-section walls were the thickness of, oh, maybe a playing card. And the hollow insides of the spars were filled with – foam. You could dig your fingernail into it…

I made a mental note to wear a parachute when I test-flew this aeroplane.

The spars were then glued to the bottom wing-skin, along with a remarkable scarcity of shape-holding wing ribs plus thin honeycomb layers on the wing-skin panels in between. Then finally more glue was applied to the leading edges, the trailing edges, the ribs and the spars. The top-wing skins were then positioned on top, the vacuum-bag process was repeated, and it was back to the autoclave.

It was just like a big Airfix kit. Of which I had glued together many in my childhood.

I wondered if there was any way I could wear two parachutes when I test-flew this aeroplane…

In the event I just wore one. The Extra was stressed to operate to + 10 G and –10 G. Not with a safety factor of 1.5, as was the Pitts Special, but of  2.3, because the airworthiness people were worried about how the structure might age. (How they came up with the odd figure of 2.3 I have no idea, but suspect it was the result of an arm-wrestling match down the pub). So the Extra would theoretically not break unless you pulled or pushed no less than 23 G – by which point any homo sapiens would not just be asleep but more like a puddle on the cockpit floor or up in the canopy. I made a point, on this test flight high-up in the sky, of pulling +11 G and pushing about –8, which was all I could manage.

Nothing moved. It was astonishing. In my flying world up until then it was a given that all wings flex to a degree. Next time you’re sitting in a 747, watch the wing tips during take-off. They flex through an arc of about two metres (more than six feet) as the weight comes off the wheels and onto the wings. And that’s at just 1G. In metal or fabric-covered aerobatic aeroplanes – which are far, far stronger than 747’s – you could usually see the wing-skins ‘pant’ slightly between the ribs at high G as the wings flexed just a tad.

In the Extra there was – nothing. Zilch. No tad of flexing. Not even a smidgeon. No panel panted. No movement. It was like flying some deep I-section steel girder which, by some manner of wizardry and mysterious incantations over bubbling cauldrons, had just lost 95% of its weight. (I later learnt that Extra wings do in fact flex a little – by a whole 4 cm at the tips at 10 G – nothing for an 8 metre wing-span).

Carbon-fibre.

Incredible stuff.

The front end of BLOODHOUND – the nose, the cockpit monocoque, and the jet intake – are all to be carbon-fibre.

Which is why I am here today, guest of Advanced Composites Group (ACG, a division of UMECO) in Heanor, Derbyshire. ACG are aces in carbon-fibre, and have recently signed up as Product Sponsor for BLOODHOUND to produce these slightly major items. I am here to find out more about carbon-fibre.

And as the most amiable Group Design Manager, Pete Watt, shows me around, it is rapidly coming home to me that these guys are wizards – although they have carefully hidden their pointed hats – and also that carbon-fibre came on apace shortly after I watched my first wing being laid-up 20 years ago. (Oddly enough the same aeroplane which Andy Green now flies to progress his advanced aerobatics). The principles remain the same of course – it’s more that not long afterwards the technology broadened into a thing called prepreg, and from there into so many other areas and sort of matured. We’ll get to that. Prepreg. Remember that.

 

Wattle and daub

All technologies have their particular slang. And one form of slang for carbon-fibre lay-ups is Wattle-and-Daub – a tipping of the wizard hat to where composites actually started.

Well…I can certainly say that no house-builder from 2,000 years ago would even begin to recognise any of the equipment in the ACG wattle-and-daub production. The first element, in several factory spaces, faintly resemble a series of cotton looms from the Industrial Revolution mated with printing presses from the same era. There are large machines bearing dozens of bobbins of flat carbon-fibre tapes of different widths. These tapes are called tows. And tows come in different widths – but not of much different thicknesses, a typical thickness in the finished product being about 0.3 mm. One third of a millimetre.

The narrowest of the tapes on the first machine I’m being introduced to is maybe 3 mm wide – so 3 mm x 0.3 mm.

Containing, depending on specification, anything up to 48,000 strands of carbon filament. Not a slip of the keyboard – up to forty eight thousand strands of carbon filament.

I just stare at it.

I could probably bite through it (shear load). But I could also probably swing from the ceiling on just this one tape (tensile load, like the old Pitts Special wires, but this single tape being 3 mm x 0.3 mm…)

These tapes are not cheap. And nor is gloop.

Gloop is what in my day we would have called the resin, and most folk still do. Medieval man would have called it daub, or maybe cow-poo. Which sinks into the tows – the gloop, not the cow-poo – during the curing process and bonds them together. I’ll come back to that. Only nowadays the non-slang word for Gloop is matrix, and there are at least sixty options of matrixes (strictly, matrices), depending on the properties required of the end result. I will not detail them all, for the good reason that I understand them on about the same level as I understand the Theory of Relativity, which is regrettably not a lot. But basically you the customer can choose the matrix you want (epoxy, bismaleimide, etc) for purpose. You may want very high tensile strength, very high stiffness, very high temperature operation, or any combination of the above. Well, can do – and part of the equation is very much picking the right gloop. Oh, and signing the right cheque, because some of the more esoteric gloops cost a very, very great deal more than cow-poo. Such as a million times more.

When I watched my wing-build the gloop was, if I remember aright, an epoxy hand-brushed onto the inside-side of the skin just before the vac-bag and autoclave business.

Since then – in fact quite quickly afterwards – the technology became more than a bit more high-tech. It is called ‘prepreg’ (meaning pre-impregnation before processing). Extra Fluegzerugbau will I’m sure be now using it.

Hence this Victorian loom/printing press which is very far from a Victorian loom/printing press.

Tapes of varying widths are being pulled off the bobbins and wound onto a drum which is the start of the roller-based section. To grossly over-simplify, a broad strip of brown paper – needless to say, very special brown paper – passes around these rollers and in the process passes through a trough of… well, gloop. The depth of the gloop film on the paper is most carefully controlled by – well, I’m not entirely sure what by, presumably a computer programme with a wizard hat on it – and then the carbon-fibre tows are rolled onto it. The whole strip then passes through a heating element which melts the gloop into the tows, then through some compression rollers like a mangle, then has the brown paper stripped off over another roller, has release-film rolled onto it top and bottom via more rollers – and Lo!

This is prepreg. It’s then wound onto yet another drum.

What I’ve described above happens to be a uni-directional cloth – all the carbon strands going one way. Which would only be really strong in that one way – in tension. Suitable for some applications, but most need to be somewhere between a bit more and a lot more than that. So instead of uni-directional you can have a woven lattice like a wattle fence – strong in two ways – and if you need more still then you can put several layers one on top of the other, with the carbon tows going every which way in the different layers. The basic prepreg process remains very similar, and once it’s done it’s ready to be used.

 

Cold climes

Except that you may not – probably won’t – be ready to use it today. Life is rarely that simple. So you have to store it – and you can’t just heave it onto a rack up against the wall, because at room temperature it will start to cure all by itself. Slowly – or not always slowly – but surely. So you have to store it in a very, very cold freezer. Just how cold is largely dictated by the properties of the particular gloop you’re using, but for certain specialist applications it can need to be as chilly as minus 18 deg C, which means a big-time cascade freezer. The time between defrosting the cloth and it becoming unusable at room temperature is called the ‘Out Life’ by the wizards, and it is very far from infinite. So, particularly with certain of the more esoteric gloops, you have to get your planning and timing right. Moreover the colder stored cloths become kinda difficult to ship out to a customer, since if you shove it behind the beef carcases in your average refrigerated lorry the temperature ain’t going to be anything remotely like low enough. So you have to solve that…

But when it’s finally laid up into a mould, pressurised, put into an autoclave, pressurised again to maybe seven times atmospheric and then heated to anything up to 180 deg C for anything up to a day – why, then it will cure as the wizards intended. The prepreg gloop will slowly flow everywhere, form a smooth surface on the mould, and solidify.

This will happen. There are no buts.

Except…

That for structures requiring great rigidity the best answer is to separate two carbon-fibre skins with a layer of honeycomb in between them. The wizards call this a sandwich. If you separate the upper and lower carbon-fibre skins of (say) a wing with a honeycomb core, then you can increase the bending resistance by up to 37 times. Sticking with our wing analogy, under positive G the bending forces are taken in tension by the lower wing skin. These are also resisted by the lay-up directions of the upper wing skin, but most importantly the skins are prevented from collapsing together by the honeycomb in between them.

A sort of 21st century version of the old Pitts Special torsion-box wing principle.

Well, well…

The BLOODHOUND monocoque and nose-cone are not in fact going to challenge ACG over-much material-wise, Pete Watt tells me. The skins will be of Intermediate Modulus – very high quality but not quite High Modulus. The honeycomb will be of very light aluminium – well, honeycomb – about 2 cm thick. With perhaps ten layers of prepreg on each side adding, combined, a whole 6 mm of thickness.

3 mm for each skin.

20 mm for each honeycomb.

Making a total skin-sandwich thickness of 26 mm.

I look at Pete for body-signs that he is kidding me.

He is not.

I say: “BLOODHOUND is going to need access panels in all sorts of places. How big a problem is that in monocoque construction?”

“It’s a problem. We can always strengthen the areas around access panels, but it always adds weight*.

“And stone-chip damage at 1,000 mph…?”

“We have some military requirements to stop at least low-velocity bullets. They tend to add weight”. Something in Pete’s expression suggests I should not pursue this line of inquiry.

Enough, for this visit.

I have learned a lot. Not mentioned in the above, I have also learned how moulds are created, and also how even the BLOODHOUND components will be created in two sections each and subsequently be ‘glued’ together.

Like an Airfix kit. Like an Extra wing.

And, why carbon-fibre is so berluddy expensive – it isn’t just the ingredients, the tows and the gloop, but the time-consuming processes and the minor costs of creating and operating big autoclaves…

We will re-visit this as the project progresses.

Wattle and daub has moved on…