Warbird Information Exchange

So the spar cap tube material is 4130 steel?

menards: The outer spars appear to be constant internal taper parts, but most likely are formed with the tube restrained in an external die, and the taper formed by forcing a tapered plug into the tubing from one end. Like a closed set of forging dies, but of huge lengths. I think the one was over 300" when finished. Outers appeared to be 24ST with heat treat to 50 or 60ksi.

bdk: The pieces that I looked at were 4130 and made from a BAC tube number. Heat treat was to 100ksi, I'm assuming after forming.

Press wise, the inner ones were probably formed on a horizontal press with a stroke of about 8-10 feet, and probably in the range of several thousand tons. The outer ones, a press with a stroke of about 27 feet and again, several thousand tons. Big honkin presses, that probably no longer exist in the US anymore. And to top it off, most like heated ones to keep the dies hot enough and reduce the required tonnage.

Edit: After thinking for a few minutes, the long ones could be done in a hot toggle press, as the die would only have to move a relatively short distance to obtain full closure, but it would need to support the outer die's full length, and have the ability to rapidly remove the inner die. The short ones could be done by a progressive set of dies in a shorter press, but not knowing the internal taper or pattern, it's all speculation based on some production guesswork.

"The outer spars appear to be constant internal taper parts, but most likely are formed with the tube restrained in an external die, and the taper formed by forcing a tapered plug into the tubing from one end.... "

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John: This video might give you a better idea. It's forging pistons, but the same principle.

https://www.youtube.com/watch?v=Jr6qk5wCPEQ

Just to clarify, the spar chords...the square tubular upper and lower chords that are the primary structure of the spar assemblies, are an aluminum alloy. The wing attach terminals at both ends of the spar chords (for inner attach to the fuselage and outer attach to the outer wing sections) are steel. There are other steel parts in the spar assembly such as channels and some parts of the spar caps that separate the spar chords from the corrugated aluminum attached between the wing spars and wing skin, but the spar chords are aluminum. There are upper and lower spar chords for the forward and aft spars, for a total of four for each inboard wing panel or eight for the aircraft. Each of the spar chords of the inner wing sections are about 25 feet long. The problem addressed by the 2001 FAA AD was the joints between the aluminum spar chords and the steel spar terminal fittings at the inboard section of the wing. Internal spar chord corrosion, cracking around bolt holes, and bolt integrity were all addressed by the AD. (There are eight close-tolerance bolts that hold the two parts together.) Besides the inner upper and lower attach points on each spar assembly is a third attachment, a shear terminal. The fuselage side of that shear terminal attachment is what the EAA has publicly identified as the problem in April 2021 with Aluminum Overcast. Perhaps they have found more issues; if so, they are not sharing that information yet.

Unless someone has an inside track on the possible AD or what was found on Aluminum Overcast, the extent of such an AD as to what will be required for inspection and repair is pretty much speculation. In theory, though, it would not be in the same area as the 2001 AD.

This is a good description of the wing spar assembly from a B-17 technical order (01-20E-3):

"The two-spar wing construction used on this airplane consists of two main beams or trusses known as front and rear spars. These are separated by compression and former ribs at intervals along the spars, all connected to the spars, corrugations, and wing surfaces by riveting. The front and rear spars each include one upper and one lower square tubular chord, extending the entire length of each of the two wing panels. Tubular members are attached vertically and angularly to the chords with rivets and gussets to form "N" or "Howe type trusses out to station 30, and "Warren" type trusses outboard of station 30. At the top and bottom of each chord are suitable fillers and spar caps which serve to strengthen the wing, form an attaching flange for the skin, and give the necessary angularity to fit the contour. Attached to either end of each chord of the inboard panel, and to the inboard ends of the outboard panel, are the wing spar terminals. These are highly heat-treated parts requiring replacement if damaged. The union between body and wing, and between inboard and outboard wing panels, is accomplished through these terminals with the use of tapered pins.
Removal and replacement of tapered pin joints is a precision operation. Refer to T. O. No. 01-20EE-2, 01-20EF-2 or 01-20EG-2 for instructions dealing with tapered pin removal and replacement procedures. The body attachment also incorporates a shear connection midway between the two terminals."

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And, to further clarify, the carry through spar tubes that tie the left and right wing attach fittings together in the fuselage are made of 4130 steel and have a similar shape as the wing spars, but no internal taper.

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Rod Schneider
Ball Ground, Ga

Rod: If there's no internal taper or form change, that greatly simplifies making them. Still not a quick and easy task on the manufacturing end of things. Just takes buckets of dollars to make it happen.

The 4130 carry through tubes are not a problem. They span the interior of the bomb bay and tie the left and right wing fittings together. New ones have been made for various restoration projects. The problem is the aluminum spar tubes that run through the wings themselves. Those are the ones with the internal taper..........

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Rod Schneider
Ball Ground, Ga

Rod: Bummer that it’s those tubes. I had assumed it was the 4130 ones from the published info from AO. If it does end up being the tapered ones, that puts all the operators in a world of hurt. It can be fixed, but it’s going to be long and expensive.

We’ve got a pair of similar problems in the L-5 world....lift struts and engine cylinders. Both are getting harder and harder to find airworthy ones.