Warbird Information Exchange

Anyone have information on the BT-15 with the fiberglass aft fuselage that was built during WWII. I'm trying to gather info. For a possible future project. Thanks in advance.

http://www.airweb.faa.gov/Regulatory_an ... -2-571.pdf

"Note: Wood components may be interchanged with similar metal parts, provided one
wood wing is not used with one metal wing."

BT-13s also had integral fuel tanks in the metal wings (i.e. "wet wings"). Must have had metal tanks or bladders in the wooden wings.

There is a mention of wooden elevator tabs, but not wooden tailcones, but some may have had them. I know North American built some T-6s with wooden tailcones, but they are specifically excluded from use by the Type Certificate Data Sheet.

I've never heard of ANY primary structure made out of fiberglass in that era.

1. The wing center section on all BT-13/15 variants is the same except some minor variations on trailing edge design. The center section is the fuel tank structure; fuel is held in place by center section spars, bulkheads, and skins. The outer wing panels have nothing to do with the fuel system other than keeping it and the rest of the plane aloft while flying....

2. BT-13B's were made with wooden monococque (aft fuselage), empennage, and outer wing panels. These all bolt up to the fuselages and center sections with minor or no modifications. Aluminum monococque and outer wing panels bolt right up to former wooden-constructed aircraft. You cannot mix wooden empennage on aluminum monococque and vice versa. If you look in the BT parts books, all these wooden parts (and almost all the parts which attach to them, including control parts) have different parts numbers from the aluminum components.

3. I don't know the real reason for the ban on mixing wooden and metal wings. I would guess it is due to the varying airframe stress put on the attach angles, or a weight difference (although my understanding is they weigh within a few pounds of each other), or just a policy carried out by the manufacturer or Army Air Forces.

4. From memory, there were BT-13's which were made with extensive phenolic structure (similar to the Timm N2T all-phenolic airframe) and these were tested but abandoned. But later BT-13B's have phenolic skins on a number of places on the fuselage section (not on the monococque) such as the fuselage cockpit panels, access panels, and baggage doors. But these are not really critical airframe structure. I think these might be your "fiberglass" components.

5. As a BT-13B project owner, I deliberately sought out the aluminum components to complete my project. But along the way I got to see the wooden empennage and monococque. I have not seen the wood outer wing panels. Based on what I have seen, I would not in any way fear flying a BT-13B with original wooden components properly overhauled, rebuilt, or re-manufactured. There were failures of these components in WWII which led to the prohibition of all aerobatic operations in the aircraft, use primarily as a basic instrument trainer, and post-WWII AD's. But I think these failures were due to the way the aircraft were stored more so than the way they were flown (although I am aware I could be proven wrong on that).

6. I think the wooden tailed T-6's were Harvard IV's, which were not built on the T-6 type certificate.

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Below are a couple pictures of the information I have. They are from Aero Digest. It was defiantly fiberglass. It was also just a single experimental model. I'd just like to gather up more info if it is out there.
Forgotten Field do you happen to have any pictures of the attach points used on the wood structure. I'm wondering if they would have used the same style attach since the article doesn't say. On a side note I've also got a BT-13B project that I'm slowly plugging away on.

No photos. But the wood former where the attach angles for the monococque was located were pretty close to the dimensions of the original type aluminum former- definitely larger than the one on the left in the article. I'd also say that even by today's standards of carbon fiber construction, those attach angles on the left look pretty flimsy to me.

My understanding of why these didn't work out was due to the quality of the fibers. Phenolic was mostly made from rayon fibers. Rayon can be chemically de-stabilized and deteriorate, degrading the structural strength of the material. No comment on the fiberglass, except to say this definitely was new technology.

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Since there are some genuine experts here can someone tell me why there was a difference in the rivets used in some BT 13/15 wings? When I first worked in the industry a colleague related he worked on restoring a BT built up from parts and the aircraft's wings were each built differently. One used far more flush rivets that the other. Protruding head rivets are easier to install but wings usually want more flush rivets at least from the leading edge back to about the normal center of lift.

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6. I think the wooden tailed T-6's were Harvard IV's, which were not built on the T-6 type certificate.

Not so, my SNJ had a wooden tailcone and at first IRAN, it was replaced with a metal one. A lot of late AT-6D/SNJs were made with wood parts.

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The Harvard 4 was made in Canada in the early 1950s and there are no wooden structural components in the Illustrate Parts Manual.