powerandpassionTDP, yes, great photo ! It gets me thinkin’ that the spar design was kind of like an old school automobile leaf spring, the telescoping tubes sliding inside each other as the wing bent up and down. In this case each tube is stretching along its length, and a buckling tube face is opposed by a face in tension, creating ying yang serendipity. Somebody in China should now build the world’s tallest skyscraper using a telescoping shell central lift well as a structural spine, on the Spitfire spar principle.
In this case the shouldered bolts are acting in shear, to anchor, all tubes, as the external clamping force might not be reliably transferred through a number of wall thicknesses, in the nightmares of the designer, but I still think this is secondary ‘belts and braces’. The placement of a solid core implies the joint was meant to be ‘clamped up’, transmitting friction love across all tube faces, and the friction forces resulting are quite incredible. There is an idiot here, I mean Aussie, who has done some good broadcasting work recently testing scientific theories with his life. One example was bungee jumping with only an interleaved telephone book in the line, where the simple friction of the pages arrested his fall. Luckily there are a lot of Smiths in the book !
I still think the shoulder was meant to stop you over tightening, as the aluminium would squeeze out like toothpaste under the clamping force of a steel bolt, and, over time, reduce joint effectiveness. As the evidence of wings coming off in belly up landings past the joint indicates, it was a very effective joint, probably over engineered, as high speed monoplanes were all new in 1936. It looks like there was room for a straight through conventional bolt and larger nut. In earlier pin jointed structures a straight through bolt would be used with a ferrule – so the Spitfire bolt and its reaming kit is kind of the same thing.
I’m not trying to be a smart **** but I can’t slot this away until definite dimensional proof blows the theory out of the water that the cross sectional mass of the hollow shaft equals the cross sectional mass of the threaded section to allow even bolt stretch. Consequently more load put into the bolt, as distinct from a solid shaft bolt of the same metallurgy.
The nearest thing I can find to this hollow bolt in the detritus of my mind are hollow rivets, which were based on a great deal of 1930’s developed science on hollow tube performance. No rivet was primarily designed to act in shear, although it would have a degraded, secondary performance in shear, if the primary tension/friction function failed. I kind of think this hollow bolt came from that.
I don’t see hollow bolts out there today. It’s like stumbling onto a lost island and seeing some archaic proto human branch of the bolt family tree. It’s fascinating ! So somebody could build the highest skyscraper in the world using articulated joints connected by more ductile hollow bolts – and claim this as an amazing new invention in 2018!
More beer, wench !
powerandpassionAnon, the leather looks like part of a boot covering over hydraulic spool valve or lever assembly – I have seen these on Oxford and Mosquito cockpit spool valves to stop dirt getting in, also on emergency hydraulic handpumps. Much like the boot on your Aston Martin gear stick, or 2CV, which FFT drives.
The nav light housing looks 1940’s, not sure if there is an A prefix before the part number, but, if so, Hawker ? Henley? Miles Master I ?
powerandpassionTo be civilised, this conversation really needs a pub, with a publican that doesn’t like to sleep !
On the lack of torque data and bolts in shear – bolt stretch is how bolts work and bolts are not designed to take shear loads as their primary design function. Without getting the ouija board out and calling up Barnes Wallis I assume that the shouldered bolt acts as a STOP, in other words you can tighten up the nut as much as you want but once it hits the shoulder, that’s it. I am not too familiar with the ridiculous telescoping spar design of Heinkel elliptical wing designed Spitfire (just getting your beer frothed up), but I assume the shouldered bolt was designed to stop the chance of tube crush in the spar. If you kept on tightening it once it hit the shoulder, the bolt would eventually break at thinnest point, but I do not think the form was dictated by this function.
There would have to have been mechanical torque ratchets in Grandpa’s hands. There would have to have been a tightening sequence in a group of bolts. How all this translated to a field depot in the middle of the night in the middle of winter might have been quite different.
A bolt that was more elastic, that could store more energy, would maintain it’s clamping force as the stresses of flight impacted on the joint, like the waves of phosphorescence giddying through the translucent body of some disturbed deep sea jellyfish. (the beer is taking effect). It’s like squeezing a rubber ball in one hand, and a cricket ball in the other, then getting an eight year old to try and chop either of the balls out of your hands. The less elastic joint is easier to work apart.
The way to test this theory is to take a hollow bolt, and a solid bolt, and clamp two separate test pieces together, using the same torque, and measure the stretch in each bolt.
I actually think you could test this theory out with a CAD model on Solidworks, and run some analysis over it, where the metallurgy is the same.
Got some dimensions?
powerandpassionHere are the Hector engine bearers, looking for a Napier Dagger. They almost look like later Griffon engine bearers. Also a photograph of the Hector prototype, K2434, basically a Hart airframe with new engine bearers and the upper plane with no sweep back. The amount of oil streaks on the under fairing is interesting. The Dagger was Left Hand Tractor, and on the prop hub I can read Watts Z13/0/1 for the prop drawing. The only ‘off the shelf’ LHT fixed pitch props at the time would have been for British high power radials – perhaps it was borrowed from an early Pegasus application. I have dimensions for fixed pitch LHT props for Jupiter powered Bulldog and Wapiti, but like Goldilocks and her porridge, one is too small and the other is too large. I would be grateful if anybody could provide any information for the Watts Z13 prop, which would make it ‘just right.’
The Hector is like the dog from the pound, that should be put down, but it grows on you. It would be interesting to hear one in flight. I do have a conrod from a Dagger, and it is a surprising thing, like the little man that said Welcome to Fantasy Island. It is very short. The thing would pump up and down like a mini poodle reaching up for a biscuit. When you look at the RPM gauge for a Hector it goes up to 5,000 RPM, which is a lot for the 1930’s. The combination of open exhausts and high RPM and a fixed pitch prop would be a very interesting sound indeed.
So, who’s got a Dagger ?
powerandpassionHappy as a pig in ****e to root around in the dirt and find bits ! Also have a marine magnetometer which makes me happy as a goldfish in a pint ! I can’t think of anything less depressing than discovering buried treasure – the great fist pumping moment is when an unknown relic resolves as a known part. It’s an utterly accessible pastime for anybody who only owns a spade and a car that can hold a chunk of rusty armour plate on the back seat, which I understand a Maserati cannot. Nothing as pleasant as pulling up to a pub in an Australian country town with an oxidising chunk of WW2 fighter poking out the back window, and striking up a conversation with a curious local, that then leads to an old woolshed holding a crated Spitfire. All from rooting around in the dirt !
I reckon there’s a whole world of discoveries under water, and with side scan sonar and cheap cameras that go down with fishing rods it might be amazing what is found. Not depressing at all, no siree !
powerandpassionJules,
Is it possible to measure the outside diameter of the ‘holey bolts’ (there seem to be two sizes) and the inside diameter of the holes to figure out whether the below theory has merit :
“The bolt is designed to be stretched as it is torqued up. It might fail in tension at the shoulder, where the threaded, narrower dimension meets the body of the bolt. The difference in mass of a solid bolt body and the shoulder concentrates stress in this point. If the bolt is hollow, the stress is spread over the length of the bolt, as there is less difference in mass. The whole bolt stretches evenly, and deals with service stresses evenly. This is the same principle as the waisted conrod end cap bolt in the Merlin”
If the punt holds then the cross sectional area of the hollow shaft equals the cross sectional area of the threaded end. I would be interested to know how deep the hole goes too, and the length of the shaft and threaded section.
Understanding that these bolts are a humble carbon steel, they were designed to be quite ductile, which makes me think this is all about bolt stretch. Getting the torque, and stretch, right was critical. Maybe the original design was meant to fail if you over tightened it, in other words the narrow threaded section would fail. Apart from the space limitations of fitting a larger nut, I can’t see why they they wouldn’t have used a bolt of consistent diameter all the way through, as they do for modern structural bolting applications, assisted with washers that distort and signal correct torque. It’s a weird bolt. Where’s Barnes-Wallis when you need him !
powerandpassionThe Irish Hectors are like the young lasses in the movie ‘Picnic at Hanging Rock’, they just seemed to, accompanied by ghostly pan flute music, have walked off the face of the earth….
I can confirm that Hector engine bearers are in Australia. These are not corroded and their condition does not seem to be consistent with an airframe that was left in the grass over many an Irish winter. Either they were taken off and stored inside or they are not from that particular airframe – a picture of the airframe in the grass would confirm whether the engine bearers were intact. If it just shows a firewall, then perhaps they are. I will post some pictures on Monday.
My understanding, third or fourth hand, is that a Hector airframe was part of the donor material for Demon G-BTVE, which is not a bad thing. From the firewall back, a Hector airframe is a Hind airframe is a strong Demon I airframe, and the 3% nickel T50 tube is otherwise unobtainable, particularly in the diameters and wall thicknesses desired.
There can’t be any secret Irish project, as every joker sitting on a barstool in the Republic would know and talk about it.
There can’t be any secret US project, as Senator Edward Kennedy is no longer with us.
It is possible for there to be a secret Welsh project, down an old mine shaft, but the story would leek.
If the engine bearers are all that is left, then you may be looking at K8130-IAC 88.
I can take the bearers down to the Police Forensic labs here and pull out some dirt from a nook or craic, and get it analysed like they do on Silent Witness, if you go down to the sawmill and get a soil sample from there, and we might solve the mystery !
powerandpassionOnly if it is specified on the drawing as British Standard S2 material, is it possible to get a copy of the drawing specifying this, and a chunk of bolt, to do metallurgical analysis on ? I am punting it is nickel chromium alloy steel. What is the material specification for the bolt ? It may be a later NiCr composition….
powerandpassionI find these things fascinating. A hollow bolt would be there for a definite reason, and I am not so sure that weight or material saving would be the primary reason. I do not know the reason, but I would throw these guesses out, and would love to follow the story to a definite conclusion.
1. Interference fit.
A tight fit for a wing connection is good practice, evidenced by the use of drift to drive it out. The bolt may have been frozen, to aid insertion into a spar fitting at ambient temperature. As it heated up to ambient it would expand to an interference fit. A hollow core bolt may shrink further than a solid bolt. In reverse, the injection of a stream of readily available compressed gas, such as CO2, into the hollow, to freeze the bolt, might have allowed it to shrink and be removed more easily.
2. Protection against failure.
The bolt is designed to be stretched as it is torqued up. It might fail in tension at the shoulder, where the threaded, narrower dimension meets the body of the bolt. The difference in mass of a solid bolt body and the shoulder concentrates stress in this point. If the bolt is hollow, the stress is spread over the length of the bolt, as there is less difference in mass. The whole bolt stretches evenly, and deals with service stresses evenly. This is the same principle as the waisted conrod end cap bolt in the Merlin. However, for the spar joint, the connection or surface area meeting between the outside of the bolt and spar fitting is critical, so the waist becomes a hollow.
Something else is niggling me about even heat treatment, but this would be secondary.
I would be surprised to see stainless steel bolts in this application, given the propensity for work hardening due to cyclical service stresses, and electrolytic corrosion between aluminium spar, unless this was a Seafire type thing.
Most fascinating. I must light up my pipe and think about this some more. Well done Sir, a bloody good question !
powerandpassionJohn, great piece and great progress ! If only you were carrying an XRF to confirm the metallurgy ! Nice to see you getting access to bits and pieces. I don’t disagree that the system of building is straightforward. So was the Bristol system used in the Bulldog. I would counter by saying that Hawkers got ahead of them all by making the bits and pieces used in Hawker construction easier and cheaper to build, with the technology of the day. The sheer fecundity of the Hart biplane family bore this out, while the other constructor systems, for all their ingenuity, became manufacturing dead ends. There seems to be an incredible amount of machining in the Siskin bits. Lucky for you, CNC machining was developed 80 years later ! Might not be too hard to do this, if only we knew the metallurgy ! I reckon Siskin III tubes will be 3% nickel alloy T50, major wing joints & fittings nickel chromium alloy S2, minor fittings S1 and spar material a combination of DTD54a – S88. Easy ! Armstrong Whitworth spars are very similar to Hawker Hart type spars….
powerandpassionBrian, brilliant read on the seaweed, thank you !
powerandpassionDave, thank you, brilliant. Benny Hill would have liked AGS 209.
powerandpassionTubular rivets – http://silverbiplanes.com/SPECIFICATION_PDFS/ACA/CA142.PDF ?
powerandpassionThe 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.
powerandpassionBrian, how do you make balsa from seaweed, fascinating ! I do not know this story, where can I chase it up please.
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