LiteFighters: P-40/P-36 and beyond

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ZacYates

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Did any forum members catch up with Scott & co at AirVenture? I've seen the P-36 pop up in many friends' Facebook posts.
 

Bigshu

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Did any forum members catch up with Scott & co at AirVenture? I've seen the P-36 pop up in many friends' Facebook posts.
I talked to Sam for a while, saw Scott but he was tied up talking Verner motors with a guy. It's great to see the P36 in flight test. I almost pulled the trigger on a deposit, but I want to get a better idea how things will go with a P40, engine wise. Exciting to see the energy at their booth. I think they have a winning business model.
 

wsimpso1

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Did any forum members catch up with Scott & co at AirVenture? I've seen the P-36 pop up in many friends' Facebook posts.
I did. The bird looks FABULOUS in person. Eliot was there when I stopped in, and said it had no handling squawks, just cooling and sump venting. Cooling was fixed as displayed, and sump venting will get figured out too. Looks like it should go nicely in the market.

Billski
 

WWhunter

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I talked to Sam for a while, saw Scott but he was tied up talking Verner motors with a guy. It's great to see the P36 in flight test. I almost pulled the trigger on a deposit, but I want to get a better idea how things will go with a P40, engine wise. Exciting to see the energy at their booth. I think they have a winning business model.

I have also had a life long desire to fly a P-40 replica. Not sure it will happen but the Scalebird option is definitely on my radar. Life is short....not sure if I will have enough time on this earth to obtain all the planes I dream about! LOL
 

Bigshu

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I have also had a life long desire to fly a P-40 replica. Not sure it will happen but the Scalebird option is definitely on my radar. Life is short....not sure if I will have enough time on this earth to obtain all the planes I dream about! LOL
It doesn't take a lot of time...if you have a ton of money!
 

ScaleBirdsScott

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Well we've been back from Oshkosh a bit now, and finally starting to get caught up on things. Lots of cleaning and putting things in order. The P-36 is mostly back together, but we've got the oil system pulled apart while we wait for our welder to wrap up our changes to the system. For now adding a tall expansion tank to augment the sump tank. Then adding a long tube to our return line fitting so that it will not be a vent line. Based on talking with other Verner users with similar situations they've had luck with this approach. Sounds like Monday we'll have our expansion tank and modified fittings back. So next week at some point we should be ready to start running and see where it stands. I'd love to see it get off the ground soon!

In the meantime, I've been doing some work on actually getting to work in CAD world, and start implementing some of the upgrades we've been talking about over the years into the model. The primary goal of it right now is start to finalize the P40 cowling geometry and confirm from at least one engine maker that we have good layouts for radiators and such. Once I am fairly confident, I can start looking at how to carve up some foam and make some test plugs. In the meantime at least we can show some people what it'll look like with some full renders.

But cowling and cosmetics aside, are those upgrades I was talking about. One is the center section spar. I think the central spar needs a bit of attention in how it's made and how it mounts to the fuselage. The way we did it on the current prototype was fairly standard but the execution was such a pain that I can't sleep knowing I'd ever submit another soul to trying to build it. So I've been looking at how to make the main spar far less of a chore to build, and install.

Also we need to fully incorporate the retract mechanism, which all needs to be strong and bulletproof. Also the changes to the outer wing attach for a quick-removable wing and folding mechanism. Also need to take out some weight while we're at it cuz it's heavy with a lot of steel splice plates. And it needs to handle a full 6Gs as the prototype had a few places that limit us to 4. It's all minor stuff to address in isolation but still, we're packing a lot of stuff into an 8 ft span of wing so at this point I'm going more-or-less clean sheet on the spar, with lessons learned.

One way I'm thinking of tackling the center-spar situation, and I'd appreciate feedback on this notion, is just "cutting out all the middlemen" and machining the spars whole from solid blocks of aluminum. (Like this) One 44" chunk for left, one for right. Join the two complete halves, then set the fuselage on top. I like the idea of a fully machined spar just like the big boys do, in part because I just like it but also CAD seems to suggest it would indeed be lightweight and I can bypass a lot of problems related to holes and fasteners, and mounting points and spacers and stiffeners and shims by just controlling the whole thing in 3D vs relying on stacks of metal. Figure for such a critical but relatively short structure it would be a good way to pack a lot of strength into a very few parts. Plus, again, no rivets/bolts to worry about for keeping the spar caps intact.

Am I missing any major issues with this approach? Material cost, machine time, and expensive scrap seems the major downsides. But I would have almost the same machine time (plus the labor time) making and assembling everything from smaller pieces, and then the cost for the material is like $2-400 per piece of stock depending on alloy and source, which is not all that much in the grand scheme of things. And while we might scrap the occasional piece, the tolerances would actually be pretty forgiving. As long as we never machine it too thin, or get bad chatter on a finish pass, we can deal with the tolerance being in basically the .010 range which is basically "don't worry about it" territory.

Seems like a "if you have the means, go for it" thing to me. But then again, Van's Aircraft technically has the means, but I don't see them machining their spars from one piece of solid metal yet. I must imagine a lot of it comes down to "well we've always just riveted them and for what we're doing it's not worth changing."

There's a few other places where currently I have some structure made of 3-4 riveted parts that could instead be one machined part, and basically anywhere I can make that transition I want to study the pragmatism of that change.
 
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cvairwerks

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Scott: One item you are missing on the machining of a 44" piece, is material behavior when large amounts are removed. A pieces that size with significant material removal will tend to banana, and in unpredictable ways. I used to make some prototype parts for a medical device. We used 6061T6 for the blanks. About 60% of the time, the part would banana long before I finished roughing it our. We had to redesign it probably a coupe dozen times before we could get one that hadn't bowed more than .020" over it's 13" length.

We machined large bulkheads for the F-16 on a pair of huge mills...they could hold either 6 or 9 blanks and cut 3 at a time. We're talking about spindles with somewhere over 75 horsepower, to hog the blanks our. I don't know all the steps in the process, but I believe there was a couple rounds of stress relief, and maybe a round of additional heat treat too. It takes lots of hp to hog out material in a reasonable time frame and be cost effective. If you were only going to do one or two parts in total, you could get away with using a big knee mill, but production parts...no way, as machining time would kill you cost wise.


You would be much better off cost wise, if you redesigned the spar sections to be built up using flat stock and off the shelf extrusions, then riveting with a floor based squeezer, or shooting Huck bolts. Use match drill tools for hole patterns and dedicated assembly fixtures and the process will go faster than you think.
 

ScaleBirdsScott

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Thanks for the thoughts, raises a definite issue to think about. Stress relief is a big factor there, for sure, and I don't have a lot of experience with especially that amount of material removal in aluminum. I deal with it machining polymer parts a lot, even small parts, but relieving the part and re-fixturing tends to alleviate most issues there so I figured I'd scale that mentality a bit. For aluminum it's usually been more about removing unsupported spans; but stress is still there and I'd imagine it would creep up for something like a spar block.

My assumption earlier was it'd involve getting a plate that's a bit thicker than final size, 1/8 extra material thickness at a minimum. Then roughing the bulk of the material out on both sides leaving a good .050 or more all around. Then pulling it out of the machine to sit and relax for some amount of time, before putting it back in to face the 'back' side, and then use that as a datum for a final finishing pass on the 'pocket' side with sharp tools. Another option is adjust the design so that I'm machine both sides about evenly, more like an I beam vs a C beam, so that stress should be fairly balanced front and back. But I could still see the part twisting like Home Depot lumber if it's not controlled across the beam. Maybe rough the middle span then work the ends, then go back and finish the mid area and final work the ends last.

Worst case in my mind was needing to get heat treatments and annealing involved. That would be a bit of a bummer.

Of course at the same time, I'd be very curious how flat and true any kitplane spar is after being riveted up, which adds its own stresses. I distinctly recall that halfway through riveting our test wing spar, it had clearly turned into a ski, especially as the laminations thinned. We made some changes then and got something decent. I'd guess the good spars out there are within about .030 over a few feet, maybe better for the real meticulous builders. I think if I only need to get it flat within that margin, we can account for any variability just from jig assembly of the ribs/skins. The big thing would be that when machined it stays flat in the jig so that I don't slough off part of an .080 web down to .020, or get floor chatter that ruins the surface finish. After that if it twists, we can take it back out using fixtures at assembly.

Either way, what we have now for a spar is made from flat stock extrusions and sheet, and it's about 36 parts, 46 driven spar-cap rivets and about 28 bolts per side in order to build up each center-section spar half. So double that for the whole center spar. I think it goes reasonably fast, all things considered, and it's traditional. But it's nowhere near optimal.

If I simplify the current design using a few simple machined components vs just cut stock lengths, we might get down to 20-ish parts per side and save a few bolts and rivets; but not many. If I switch to the method I was originally planning to use going forward: where I have big 'ladder frames' hogged out of 3/8 plate on a waterjet/laser, then cleaned up on the mill as needed with clean edges and final holes, then use those frames to sandwich a web sheet... I get down to about 6 major parts per spar, which is definitely better. And there are examples of that, such as Vans and the like, working out. But at that point I just wonder if waterjetting plates and then drilling and riveting that to a sheet web is going to be any flatter at the end of the day than a machined spar if I spend some time letting the twist out. It certainly would save some fixture time over completely stick-building, but when it comes time to join we're doing the same song and dance.

Of course the outboard wing spars will be the same song and dance built-up construction regardless.
 
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galapoola

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Reasonably sure a wholly cast spar is a no go. Has anyone ever built up a hybrid? Was thinking riveted extruded angle or strips in key runs to sure up the cast part.
 

Bigshu

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Seems like a "if you have the means, go for it" thing to me. But then again, Van's Aircraft technically has the means, but I don't see them machining their spars from one piece of solid metal yet. I must imagine a lot of it comes down to "well we've always just riveted them and for what we're doing it's not worth changing."
Are there any Vans with retracts? They aren't going for the same target, so they can have a different approach. Make a prototype, test it against the original, if the test pilot says they're both good, go with what makes your life (and your builder's lives) easier. Or whichever one brings you joy! Lol...
 

ScaleBirdsScott

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Are there any Vans with retracts? They aren't going for the same target, so they can have a different approach. Make a prototype, test it against the original, if the test pilot says they're both good, go with what makes your life (and your builder's lives) easier. Or whichever one brings you joy! Lol...

True point that they have a fairly simple spar so no big need to change it all that much. It's all a bit apples/oranges anyway. I don't know if I'll wait for a test pilot to decide on the best spar to use, if all other things being equal it shouldn't matter all that much to how it flies. But certainly a production and load test would prove out the potential flaws.

Other issues I can see with a single-piece machined part vs a riveted laminate:
  1. If someone dings or damages that part at all during the build, you risk scrapping the whole stub section. There's no opportunity to swap one plate or piece out during the build.
  2. If a crack starts, there's no inherent 'stop' point since the entire part is homogenous. (Not that a crack is a good thing on any spar regardless)
  3. Such a part is not within the realm of the plans builder to create, nor most shops, and so down the line how hard would it be to replace such a part assuming production has moved on?
  4. Labor may be expensive, but its still not as bottlenecked as CNC machine time.
  5. It may not save all that much labor time afterall.
  6. It makes a part of what is supposed to look like a WWII era machine feel a bit too modern and aerospace-y.
What I don't know is whether there's something I'm missing about riveted/bolted laminate aluminum (either never fully knew or just forgot to think about) and that might be something like grain structures, flex, etc that can actually advantage a part built of many smaller pieces vs a super stiff single unit.
 

Hawk81A

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Scott, Watching with great interest. While I am FAR from being any kind of engineer, I find mechanical things interesting (especially if they are attached to a warbird). I know a P-40 restorer commented either in this thread, or another about the original spars in P-40s and a "stub spar" for part of the landing gear. While assembly is involved, I wouldn't think building up a spar should be all that difficult for a builder. I agree that in the long run, built up should be better than machined. Dennis
 

cvairwerks

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Scott: Should see if there are any Bearhawk builders close to you. Bob uses a build up spar on his designs and they use easy to obtain materials, other than one item. He's got some oddball length capstrips that can be hard to find the material for.
 
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Scott in regards to retracts the version of the P-36 with wheel spats is actually the best looking of the group? Why not just make it simple and have people build those versions?

As for the Spar I have bearhawk LSA plans and the spar is quite simple. Its similar to the Rv3 I am told. I drew it up in solidowrks long ago but sadly I lost the file with a hard drive crash. Also maybe a Thatcher CX-4 or CX-5 type spar woudl work as well since its a similar sized plane and weight. I bought CX-4 plans as well to study the design as a basis for a metal 1930's looking air racer I was interested in designing. If you want to borrow my set of Bearhawk LSA or CX4 plans to study met me know.

Marc
 

ScaleBirdsScott

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Scott in regards to retracts the version of the P-36 with wheel spats is actually the best looking of the group? Why not just make it simple and have people build those versions?

As for the Spar I have bearhawk LSA plans and the spar is quite simple. Its similar to the Rv3 I am told. I drew it up in solidowrks long ago but sadly I lost the file with a hard drive crash. Also maybe a Thatcher CX-4 or CX-5 type spar woudl work as well since its a similar sized plane and weight. I bought CX-4 plans as well to study the design as a basis for a metal 1930's looking air racer I was interested in designing. If you want to borrow my set of Bearhawk LSA or CX4 plans to study met me know.

Marc

Well, plenty of people want a retracts setup. Or think they do at least. Some like the fixed gear for various reasons, some like the look of the spats even better, as you do; but plenty want a P-40 Flying Tiger. And that calls for gear that can pull a disappearing act. ScaleBirds mission doesn't work without at least having the option.

Indeed, the CX-4 is an interesting reference point as it's a good simple example of what we might be going for in some sense. But for one it's rudimentary, favoring what one could do by hand. For another it's not aerobatic. The CX-4 spars especially for the outboard wing are pretty close to what we did for ours actually. Now the Panther has a slightly better approach in some ways, as they reduce some of the weight out of the design at points, and put more in where they need a stout spar, which they make from layered CNC-cut plates. Also the Sling seems very lightweight but stout, lots of smart ideas there, tho it's optimized for different things than our mission. Both of the latter designs are such that a homebuilder can't readily plans-build them, but that's not a bad thing for me.

To be sure, over the years I've been looking at everything I can get my hands on images or drawings of: Mustang II, CX-4, RV-3 thru RV-14 builds, Titan T-51, the Stewart Mustang, SPA Panther, the Flying Legend Tucano, Sonex and Onex, Hummelbird, Sling, Thorp, Bearhawks and Cessnas, Bonanzas, Pipers, Globe Swift, the Ercoupe, and probably more I'm forgetting. If there's a decent build log of it or other reference pictures, or drawings; I've tried to pull it up as a reference point on the matrix. Then also been looking at the warbirds; been pouring over the drawings and restoration photos of the P-51, Hellcat, P-40, Zero, Spitfire, P-47, etc etc.

My challenge mostly come down to there's a hundred ways to build what we need, based on just some combination of assumptions. What we have now is something a bit like a Mustang II or CX-4 where it's 2" x .25" thick strips sandwiched over a web, with some flanges in that sandwich to mount the skins (vs trying to bend the web itself) It works, but the problem is we currently have to make it very narrow to work with the fuselage we built. Currently we have a 'tunnel' through the fuselage that the spar stubs stick into, and then at the mid-point they're sandwiched by big steel plates to join the two halves. It works, but it's very tight quarters and takes a long time to make all those parts, drill everything exact, and then put it all together such that it fits. Plus the plates on the spar just to act as spacers for the bolts that pin the spar to the fuselage are no joke. So to install the center section you build a left half, a right half, and then stick them into the tunnel hanging in space, and have to crawl around inside this narrow space to install the big steel splice plates on either side of the thin spar with a bunch of bolts; and then go back and pin the spar to the fuselage. It's not fun. And it's not optimal.

So one option is do an overlapping spar. Another is build the center spar into the fuselage and just live with the fuselage and center being one big piece. (The Czech Zero replica does this to good effect)

Looking at it, the method I prefer (and originally wanted to do on the LiteFighter anyway) is to emulate the tried and true method of the original warbirds (P-40, P-51, F6F, etc) and some replicas such as the T-51, and have the fuselage just sit on top of the wing. Build the center section as a big torsion box, mount the seat to the top of the wing, and the fuse is just a cage around it. The spar can end up being a lot lighter because it's not the only part doing the carry-thru job anymore. And all-in-all it'll hopefully make assembly far easier at the end of the day.

Whether I make the spar from one chunk of metal, two, or built it from a dozen bent sheets and stiffeners, my main goal is just reduce manual labor. CNC time is cheap over time. People keep charging for their time, and rightly so.
 

Victor Bravo

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that might be something like grain structures, flex, etc that can actually advantage a part built of many smaller pieces vs a super stiff single unit.
^^^^ I think so ^^^^^

Doesn't the "rolling" process for aluminum sheet and plate also actually compress and align the grain along the major rolling axis... and doesn't this improve some of the mechanical properties measurably? I am unfortunately not qualified to know or quantify that, but it definitely "feels correct" to this old model builder.

I believe laminated wood delivers better mechanicals than slab-cut wood, and although a completely different material I believe the overall principle is similar.
 

cluttonfred

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Scott, whatever you and the team decide, there will always be lots of knuckleheads (including me!) who have opinions on the matter. Listen only enough to avoid the "oh [email protected]!" moments because someone actually raises a valid point that you didn't think of. Other than that, build it the way you think it should be and see if that works to attract customers. I think it it will and have my fingers crossed that it does.
 
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Well, plenty of people want a retracts setup. Or think they do at least. Some like the fixed gear for various reasons, some like the look of the spats even better, as you do; but plenty want a P-40 Flying Tiger. And that calls for gear that can pull a disappearing act. ScaleBirds mission doesn't work without at least having the option.

Indeed, the CX-4 is an interesting reference point as it's a good simple example of what we might be going for in some sense. But for one it's rudimentary, favoring what one could do by hand. For another it's not aerobatic. The CX-4 spars especially for the outboard wing are pretty close to what we did for ours actually. Now the Panther has a slightly better approach in some ways, as they reduce some of the weight out of the design at points, and put more in where they need a stout spar, which they make from layered CNC-cut plates. Also the Sling seems very lightweight but stout, lots of smart ideas there, tho it's optimized for different things than our mission. Both of the latter designs are such that a homebuilder can't readily plans-build them, but that's not a bad thing for me.

To be sure, over the years I've been looking at everything I can get my hands on images or drawings of: Mustang II, CX-4, RV-3 thru RV-14 builds, Titan T-51, the Stewart Mustang, SPA Panther, the Flying Legend Tucano, Sonex and Onex, Hummelbird, Sling, Thorp, Bearhawks and Cessnas, Bonanzas, Pipers, Globe Swift, the Ercoupe, and probably more I'm forgetting. If there's a decent build log of it or other reference pictures, or drawings; I've tried to pull it up as a reference point on the matrix. Then also been looking at the warbirds; been pouring over the drawings and restoration photos of the P-51, Hellcat, P-40, Zero, Spitfire, P-47, etc etc.

My challenge mostly come down to there's a hundred ways to build what we need, based on just some combination of assumptions. What we have now is something a bit like a Mustang II or CX-4 where it's 2" x .25" thick strips sandwiched over a web, with some flanges in that sandwich to mount the skins (vs trying to bend the web itself) It works, but the problem is we currently have to make it very narrow to work with the fuselage we built. Currently we have a 'tunnel' through the fuselage that the spar stubs stick into, and then at the mid-point they're sandwiched by big steel plates to join the two halves. It works, but it's very tight quarters and takes a long time to make all those parts, drill everything exact, and then put it all together such that it fits. Plus the plates on the spar just to act as spacers for the bolts that pin the spar to the fuselage are no joke. So to install the center section you build a left half, a right half, and then stick them into the tunnel hanging in space, and have to crawl around inside this narrow space to install the big steel splice plates on either side of the thin spar with a bunch of bolts; and then go back and pin the spar to the fuselage. It's not fun. And it's not optimal.

So one option is do an overlapping spar. Another is build the center spar into the fuselage and just live with the fuselage and center being one big piece. (The Czech Zero replica does this to good effect)

Looking at it, the method I prefer (and originally wanted to do on the LiteFighter anyway) is to emulate the tried and true method of the original warbirds (P-40, P-51, F6F, etc) and some replicas such as the T-51, and have the fuselage just sit on top of the wing. Build the center section as a big torsion box, mount the seat to the top of the wing, and the fuse is just a cage around it. The spar can end up being a lot lighter because it's not the only part doing the carry-thru job anymore. And all-in-all it'll hopefully make assembly far easier at the end of the day.

Whether I make the spar from one chunk of metal, two, or built it from a dozen bent sheets and stiffeners, my main goal is just reduce manual labor. CNC time is cheap over time. People keep charging for their time, and rightly so.
Thanks for the very informative post Scott. The P40 is my favorite WW2 fighter so I'm happy your heading in that direction with retracts. However if I were looking for a radial option and a P36 then the spatted version is the direction I'd want to go. Looking forward to how everything plays out.
 

Jay Kempf

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Sheet goods can also banana. But laminated assemblies bolted or riveted can straighten each other out. We work with the limitations of our own homebrew machines and processes all the time. Sometimes the part fits what you can do. As long as the instructions are clear and the method simple doesn't matter if it is a bit lengthy with a lot of fasteners. That sort of redundancy is safer in the end. If you bend a major component like a wing spar pass thru and it was an elaborate CNC part with multiple heat treat stages how is anyone ever going to repair it? Works for General Dynamics but for the kit maybe not appropriate for the long haul. Always seems attractive to make a piece of CNC jewelry.
 
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