Hi Spitfire guys;

Something interesting may be shaping up here. I have always assumed that bending and heat treating tubes to make the spars is out of the realm of homebuilt possibility. Maybe not.

I have called some heat treat and hydraulic bending companies around eastern Canada. So far, I have found that most will help if they can. I want to find the guys who have solid experience with treating metal. Aircraft metal shaping is an asset !

Here’s how I’ve broken down this process into steps:

1. Locate the right material. This would be either 1.75″ or 2″ square tubing of something like 7050 alloy (strong like 7075, better corrosion and exfoliation resistance). If I wish to use tubes within tubes like Mitchell’s design, I would need to find tight-fitting tubes of the right alloy down to a solid bar for the core. It would be an effort to find these. However, as some pictures in this thread showed, solid bar may suffice. Cost? God knows at this point.

2. Softening the metal. If I cannot buy this material in T4 condition as suggested, it must be annealed to be workable. This could be done at All Source Heat Treating in St. Catherines Ontario. (Wayne, 905 682-2289). Cost for this stage, for 5 pieces, approximately $200 to $300 Canadian. The first four pieces are to create the spar caps, and the fifth piece is for a certification tester.

3. Bending the spar caps in a hydraulic press. Wayne would then take them across the road to Hingston Metal Fabricators and hand them off to Randy Hingston, who would be aware that they had to be bent within a short period of time. He would use his V-dies and nudge them a bit at a time until he achieves the right bend. Randy cautioned me against bending them in too tight a radius, so he’s obviously aware that tearing on the tension side of the spar is a concern. I could build a set of simple wood cradles to act as go, no-go gauges to verify the angles. The top and bottom spar caps could be bent into slightly different angles, as on the original. Cost for this work, $200 to $300 in total.

4. Re-aging the metal. Back at All Source, the metal can be artificially aged back to the right temper. Wayne has substantial experience at this, and feels he can get them extremely close to the right specs, T6 or T7351 (does this temper exist in 7050??) Cost $200 to $300.

5. Verifying the metal temper. Companies such as BodyKote or Vac Aero can do this. They will test the metal and provide a certificate stating its specs. Cost???

So, I ask, can this be done? That would only account for the spar caps, but in RJ Mitchell’s world, that seemed like a pretty big piece of the puzzle. I would then add a rear web plate, spacers, etc., and proceed as he did.

Comments?

Tom.

QldSpitty;

That’s a great looking Spit. Are you sure it’s 70 % ? The reason I ask, is that the Mk26 from Australia is an 80%. Is this from somebody else? I’d like to keep this 70 percent, or 75 max. If there were a 70 percent Spit, all metal, I’d think about buying the plans and perhaps doing some mods to detach the wings as per original.

Creaking Door, thanks for thinking about this. I think I understand what you were getting at, and check my drawing below to see if we’re on the same page. This is a heavy plate with tubes or solid bar bolted to it to complete the spar caps to the tip. Is this what you had in mind? I didn’t show the web plate in the drawing, but add it in your mind.

There are some areas of concern, and I have circled the biggest one that I can think of. I don’t like the way the heavy plate suddenly changes from about 8 inches high to around 4.5 inches where it meets the two tubes. I still don’t think we have it solved, but I haven’t done anything to check how strong this might be.

With regards to your third post on the spar, I don’t mind lots of milling, but if I make the heavy plate go way out from the root, to mid span or so, that’s a lot of extra cost. That plate is quite expensive, so I’d like to keep the plate as short as I can. So far, it’s around 40″ long X about 10″ high x 1.5″ thick before milling. Cost is around $500 US per plate.

ZRX61, that’s a very interesting metal working site. I’ll have to go through it more, but the shrinking is the one that I went to right away, and saw some great work. One end of a flat plate is suddenly a half spherical dome. Incredible.

AVI, nice pictures of the spar. I am guessing they don’t detach? And they start the dihedral right at the center point? Just curious.

Well, thanks and bye for now. Tom.

Hi MkI;

Again, terrific detail about the fuel. I recall seeing a video about the Spitfire and Merlin, and how it needed tetra-ethyl lead to act as an anti-detonation agent. I didn’t know that it helps preserve exhaust valves too. The only thing I recall there, is that Rolls-Royce innovated (so the video claims) sodium in the exhaust valves to carry away the heat (presumably to the valve guide, then into the aluminum head casting, then coolant).

What do you mean by suction head? Do you mean the tank is pressurized to some small level to positively push fuel into the carb, or is that wrong?

Creaking Door, welcome to the thread. There seems to be some interest in this topic of spars and other parts for Spit replicas, so naturally I’m pleased I started it. I’m sure it’s been started before too, elsewhere on earth.

I truly like real-world examples, and yours is good. I can picture me grabbing a wrench and doing a twist test on square tubing, then on angle, and I can also see who’d be the winner. Terrific example.

The problem I’m facing is trying to come up with a spar concept that mimics but simplifies RJ Mitchell’s original design. I simply do not have the machinery to bend tubes and then get them back to a predictable level of temper or hardness to do their job properly. My way is to perhaps over-compensate, at the penalty of adding weight, by building a spar that acts the same but fabricates differently than the real McCoy. I can mill parts out, or worst case, have someone else do it. That service is findable and at least reasonably affordable.

I saw your part about the design not being stiff enough. You certainly could be right. I think it would be interesting to make small-scale models and just generally see what acts stiffer than others. This is something I could easily do at work. I could also go through the dress rehearsal of order-of-operations when it comes to machining and assembly, which bolts first, which rivets have to wait til last. I would want to get the design to a point where I think it’s as good as I can make it, then build a small model. I wouldn’t want to do this too often. Recall too, that the gross weight is way down from the original. The spars have to be strong, but not quite as beastly. I think the Spit was rated at +10, -5 g’s, wasn’t it? I wonder if there was a factor of safety above and beyond that.

So, I have yet another iteration of the spar concept. This mod deals with my concern for the way the heavy root chunk abruptly ends, and then you’re left with just web plates and channels. I want a smooth transition in cross-sectional thickness as you move from root to tip.

Notice how I have tried to use the step-down method of reducing the beefiness of the heavy root chunk as you move toward the wing tip. This is done both on the web area and the top and bottom flanges as I hope you can see. I’m a decent draftsman, but not an award winner.

Thanks again for the replies. By the way, this isn’t drudgery. I’m having fun, and as long as I don’t completely overstay my welcome, I intend to keep at it. Cheers,Tom.

Slightly revised Spit Replica Spar

Hello;

I have done a few mods to my replica Spit wing spar. The purpose here is to reduce any stress riser points (I’m a little concerned with where the thick stub mounting plate meets the angles) and also to add a middle web plate that was absent on the previous design.

Comments are welcome, especially construction ideas about how you make a Spit spar out of metal. Preferably a drawing of the concept, or at least a good description of your ideas.

Thanks, Tom.

Hi Phantom Phixer and MkI;

Actually the concept of buying plans for a wood Spit and simply using metal (if that’s at all simple ??) is of interest. Of course, that doesn’t solve such questions as the spar and LE skins, and there would still be a lot of new work involved, but the idea of having the aerodynamics solved is a good one. Keep the balance and weights roughly the same, and who can tell if it’s metal or wood?

MkI, yours is a 70% Spit. When you bought it, did you get the plans? Whose plans are they? I know it’s wood, but yours is 70% and Tally-Ho’s is 80%, both wood. I think I’d like to stick to 70 percent.

Cheers again, Tom.

Hi AVI and MkI;

Thanks again for the response, and Alex for the phone call. Sorry I couldn’t talk longer, I have to sort of act like I’m working sometimes.

So, I am paying attention to what you guys are saying, even though I am not prepared to finalize and engine decision yet. So far I’m 70% scale, metal, powered by ??? and willing to slightly alter the tail in area to be more effective. If your best guess at gross weight is the 1600+ lbs you have mentioned, then I really do have to throw some design skill into this, and not just over-beef it. Again, I have done no stress calc’s yet (nor could I) and all I’m really doing is working on an alternative concept to Mitchell’s bent spar tubes. I have no way of bending heavy spar tubes. I just want to mimic them.

So, as out of place as this is in the design order of things, here’s my third crack at it.

Any thoughts on this concept? It uses 4 angles, a much thinner web plate and only one beefy chunk of aluminum at the root end, which can be liberally pocketed to reduce weight where acceptable. I just wish to know if the angles are incorporated in a way that causes no stress risers or sudden changes in strength as you travel from root to tip. If needed, I could add a second web plate near the root to double the thickness, but it wouldn’t be full span, and no thicker than the web plate I’ve shown in this version.

Thanks, Tom.

I wasn’t aware that the Ozzie Spit is only 1800 pounds. I was thinking of Terry Wilshire’s at 2800 pounds gross.

This is a copy of what I just sent to a Spit enthusiast in the UK.

I am starting to think I’m making very fundamental errors in my spar design. I have a friend at work, Rob, who’s a lot younger than I am, but pretty crafty at aircraft design. He’s been involved in building for a good part of his life, including renovating a Champ. He mentioned that the web plate in a spar is not the critical issue, but that the spar caps are. This would be similar to an engineered joist used in a house. I have quite honestly forgotten all of my math and stress concepts from Aerospace school 25 years ago, and it’s really showing.

I did a second design and posted it on the same forum (post # 43). I had intended to simplify things by making the spar out of fewer parts. I just added angles on the back of the spar web, and removed the ones from the front, as the L/E and T/E skins butt together over the rear angle on the real Spit, and are fastened down with rivets. I thought I’d be smart and eliminate the front angles, and a percentage of the parts.

But my friend points out that having a heavy web plate is anything but elegant. The main beef should be in the spar caps, not the web. Alex had pointed this out in the forum. My reason for going with such a beefy web is that I felt it would be stronger. I also left off the front angles because I couldn’t figure out a way to joggle the angles around the parts in the front of the spar where the first two plates end, part way toward the tip. It’s hard to joggle an angle, I think.

So I added more thickness to the web plate to compensate. Perhaps this is simply wrong. I should focus on having both, but really add importance to the spar caps. In fact, as you can see on the original spar, there were lots of large holes in the web plate. I mean, really big holes, possibly for hand access. This, my friend Rob assures me, is OK, because the spar’s neutral axis is right down the middle of the web plate, assuming the spar is fairly equal, top and bottom.

I really have to think this one through, but that’s the whole point of doing the exercise I’m doing right now, and asking for help. The key things I want to maintain, are removable bolted on wings, much like the original at frame 5, and a beastly strong wing that can’t break under anything like normal circumstances. And metal construction until further notice. I just prefer it, although I know wood is perfectly dandy too.

Thanks again guys. Pointing out my errors is what I’m hoping for. Mistakes, and hearing about them, are OK at this stage.

Tom.

Hi MkI;

Thanks for chiming in. A fair bit of what you mentioned is covered by my reply to AVI, so I won’t duplicate at this point. I’ll try to PM you as well. Your project looks terrific.

I have a question about how the leading edge skin is attached to the spar, as well as the trailing edge skin. I think I know the answer, based on the visual evidence, but perhaps someone could confirm or correct.

I always wondered how the LE skin would attach to the front angle of the spar, and the TE skin would attach to the rear angle. It seems easy, as they can just be rivetted, since the angles on the spar stick out far enough. But, if the two skins butt up against each other (as shown by the Skin Butt Line), and are rivetted far back from their edges, wouldn’t this mean a big gap between the two rows of rivets, given that there would be a spar tube and web plate in between? I think that usually skins are rivetted fairly close to their cut edges, right?

As I previously have mentioned, I love pictures. The bigger and higher res the better. I found a nice website with extremely high-res Spitfire shots, presumably for model builders to glean details. Well, I’m a gleaner too. I’ve posted the picture below, with some mods.

Assume the spar tube is in red, and 2.5 inches wide. Assume the spar web is green, and mounted on the rear of the spar tubes, and that it has flanges on the top and bottom as indicated by its width. Assume the front angle is blue.

If all of this is correct then the LE skin is fastened to the front angle AND the rear spar web. The rear spar web flange also holds the TE skin.

Another observation is that the LE skin appears to be bolted into the front and rear attach locations, not rivetted. Is this true? You can see what appear to be slotted, countersunk bolts holding the LE skin (all of the blue circles). The TE skin is only rivetted (green circles) and there are more rivets for the nose ribs (red circles).

Can anyone confirm or correct these obsevrations? Was this construction typical of most Mk’s? I can understand wanting to really fasten the LE skin well, to the spar. It forms the D-cell, and withstands a lot of forces. But man, that’s a lot of screwin’ around. Pun intended.

So that might be another mystery solved. Any feedback?

Thanks, Tom.