Warbird Information Exchange

The Vintage Aviation Museum is excited to announce that we have found a company that can make new wing spars for the Desert Rat B-17E! This has been a major hurdle in the restoration of the Desert Rat and we have finally been able to overcome it. Now the Desert Rat will be able to get her wings finished and on her way to taking to the skies once again. The wing spars are a major structural component for the wings and the Desert Rat's original spars were badly damaged and needed replacing. Stay tuned for an upcoming fundraiser. The new spars are quite expensive so every dollar will certainly count and it will take a group effort by all of our supporters to make this happen. Who wants to see the Desert Rat back in the air?

Aren't the spars on the -17 basically rectangular extrusions plus gussets? Is the biggest part of the cost and difficulty getting the extruded sections?

I was in Tom Reilly's place in Douglas a while back and he was setting up to make some B-17 spars. I have a picture around here somewhere of a big 'ol cardboard box of gussets.

Not quite that simple. They're square tube, but the wall thickness changes from one end to the other. And it's all on the inside of the tube. They're 25 feet long so it's a manufacturing nightmare. The tooling to do this has been destroyed/sold to China depending on who you ask. So this really is a major breakthrough. Without these, Desert Rat would be stuck on the ground forever.

I had no idea those tubes are internally tapered. You're right, that's not an easy get.

Yes, the crew has been searching for a solution for a very long time. I personally tried to do my part and help find someone capable of doing this. Everybody pretty much told me the same thing "We can't do that, and good luck finding someone who can". I sent the drawings to a few dozen machine shops who all claimed they could take on any job. But this one stumped them all!!

T-shirts are great fund raisers. I'm wearing my Luck of the Irish t-shirt today...

So, how do you make an internally tapered tube? Enquiring minds and all. Acid milling process or???

Originally formed over a mandrel, I'm not sure what the new manufacturers plans are. I've heard chemical milling was discussed, but you'd need to be 100% positive that afterwards the acid was neutralized. One of the processes I looked into was called EDM, which, in a nutshell, is machining with a single stand of wire which is incredibly accurate. Unfortunately, there it doesn't work if the piece is over a certain size, so that ruled this out. I'm really curious to see how they're planning to do these.

Does something go inside the tube that requires it to be tapered internally so it will fit? If nothing goes "inside", why couldn't you just use a square tube of constant wall thickness, so the thickest wall requirement is met for strength, and live with the weight penalty?

Just curious.

Very interesting; this is new to me and I'd love to know how it's done. Aside from the weight penalty, I'd assume that deviating from the design of a major part such as this would bring all sorts of certification issues with the FAA? Maybe preclude carrying fare-paying passengers too?

It would make the outer portions stiffer, which could have all sorts of unforeseen consequences.

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Lest Hero-worship raise it's head and cloud our vision, remember that World War II was fought and won by the same sort of twenty-something punks we wouldn't let our daughters date.

The wings are designed to flex. So the thin tube wall on the outer portion allows this to happen. If the wing didn't flex, you'd start to get cracks forming and from there, possible failure.

Tapered chemical milling is a somewhat common process on aluminum aerospace components, both wing skins and extrusions. There is no issue neutralizing the solution. C-17 used a lot of parts processed in this manner.

They are building to the Boeing specifications for all these new parts including the spar tubes. Any deviation will demand extensive engineering studies and a whole new set of approval hurdles to end up with an airworthy B-17. What they are doing is not for the faint of heart or for the shallow of pockets.

The Champaign Lady effort also built new wing inner sections. They had to build new spars also but the old spar tubes they possessed were usable.

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Scott Thompson
Aero Vintage Books
http://www.aerovintage.com
WIX Subscriber Since July 2017

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Good to know, thank you!!