Titanium Tunnel

[Archer39J]

In reading Eric's build manual he calls out .032" 5052 AL for the tunnel. I can't seem to find where the tunnel dimensions or material are called for on the plans but in searching the forums I see mention that the kit comes with a stainless tunnel: https://bearhawkforums.com/forum/too...et-vs-nutplate

It seems Zzz's tunnel is .018" thick 304, is this in fact what comes with the kit?

I'm considering making a titanium firewall and figure if I'm doing that I might as well do the tunnel while I'm at it since the cost is so low. Has anyone done that? Any reasons not to?

ETA: https://bearhawkforums.com/forum/bea...anium-firewall

and

https://bearhawkforums.com/forum/bearhawk-general-discussion-news/37426-psa-titanium-and-cadmium-embrittlement

[svyolo]

I thought about the titanium firewall as well. Titanium is pretty hard to work with. At least as bad as stainless steel to drill, and hard to bend. I am not sure bending the flange on the firewall is doable for the average joe, or below average one like me.

[Archer39J]

Yeah I'm not thrilled about trying to put a flange on the firewall, though I think it would be better than the aluminum brackets. Figured I'd give it a try, worst case I could use the brackets off the firewall that comes with the kit, or fab new ones.

[Glenn Patterson]

The 304 SS firewall works well & is not difficult. The process is identical to making a wing rib. Make a 3/4"plywood forming board to the shape with a backer that was clamped together to form the flange. Cut the firewall pattern with the flange allowance, clamp between the board & hammer. The flanges on our firewall build was fluted to square up the flanges on the curves. We put fire sealant from a caulking tube into a glue syringe then used the syringe to fill the flutes between the cowl & firewall. Go for the SS firewall and it will be over before you start.
Glenn

[Chris In Milwaukee]

Go for the SS firewall and it will be over before you start.
Glenn

Heck, if that's the case, I should make the whole plane out of SS!

[Archer39J]

My kit will come with a CRES firewall with the aluminum brackets/flange riveted to it. I'm going to be fabricating a titanium firewall from that design and would like to do the tunnel in Ti while I'm at it. I just can't find any info on the plans regarding tunnel dims or material and want to confirm the tunnel that comes with the kit is .018" 304 as per the linked post. I'm also interested in what folks think of doing the tunnel in Ti.

[Bcone1381]

Bob calls out .032 5052 H-32 aluminum for the four place Bearhawk tunnel in the July 1998 edition of Beartracks.

I know you are asking about the four place, but all I can give you information for the Patrol that I found.

Bob also specifies .032 5052-H32 aluminum for the tunnel in his Patrol Book. I see you are asking specifically about .018" 304 stainless steel for the tunnel. I don't have access to my Kit until Friday, but my sense is that .018 would be flimsy compared to what I think my tunnel is. From what I recall, my kit's is made of pretty significant material. I have not paid enough attention to it know if its stainless or AL, but if I were betting, I would think it might be .050 5052 like the rest of the aluminum formers.

[Chris In Milwaukee]

In the Bob Book, pp. 49-50, he discusses it, and references sheet 18 in the plans. No specific dimensions, but he does describe it as a challenging piece to build. Seems like it’s something you’d build in place once you have the FW mounted and you’re fabbing up the boot cowl.



I suspect that there’s enough variance between builders that you’d want to wait until you’re there. If building a kit, then the factory jigs would ensure things line up consistently.

[randyhall99]

Where do I find this Bob book?

[jaredyates]

The book comes with the plans. In the early years, the plans instead included reprints of the early Beartracks. At some point around the 800-900 range he consolidated those Beartracks into the book.

[svyolo]

I am going to build the tunnel as supplied with the kit, but I will weigh the panels. I think it can be done lighter as well, but I don't think any thin sheet metal will have the best result. The tunnel is mostly unsupported sheetmetal. It ends up being thicker than it needs to be just to keep from oil canning. etc. After it is built and flying, i will take a look at making cored carbon fiber panels, but it would have to save a significant amount of weight to make it worth it. Two very thin laminates with a 1/8 or 1/4 inch lightweight core, are shockingly stiff and light.
I will for sure make the instrument panel, all floor boards, and the cargo area sidewalls out of this. Flat cored sandwich panels are extremely quick and easy to make. If I use a foam core, it might even reduce exhaust noise a bit.

After I thought of doing this, I was looking at one of the Cub clones websites. They offered carbon sandwich panel floorboards as an option. Just the floorboards saved 8 lbs. That is on a skinny 2 place cub, a bear hawk floor has a lot more sq footage.

For simplicity and ease of fabrication, I was thinking of taking the supplied SS firewall, and cutting out 70% of the SS from the middle. Attach a .018 Titanium sheet to the remaining flange. 2/3's of the weight savings of an all titanium firewall, for a lot less work. Basically, I would be using the SS flange on the outside.

[Archer39J]

So what comes in the kit? In Zzz's post he says his tunnel is from "B.A"? If the tunnel would be too flimsy I would think you could get around that with beading or other features, I mostly design small sheet metal parts so I'll have to do some research on that.

Thats a fine idea about the carbon panels, 1/8 sounds quite thin but I've only seen fiberglass that thin up close. I'd be curious what's available for purchase vs the time and effort to lay up my own.

Yeah, I'm really liking that flange idea. Would save a bit of work and you wouldn't have to waste the supplied parts. An titanium-stainless-aluminum firewall... heh

[svyolo]

I have been looking for pre-made panels, but they are pretty expensive. For flat ones, it is super easy. Get a sheet or two of 1/2 inch or thicker 4X8 panels with some kind of smooth laminate on one side. Brand new, perfect ones. Either put mold release, or wax paper down. Or if you have something else that good, use that. Lay out one layer of biaxial cloth, impregnate it with normal room temperature, slow cure epoxy made for laminating. Remove ALL excess resin. Use a squeegee, and then roll paper towels over it to get rid of the resin. It is dead weight, and a lot of dead weight. Put the core material on top of that while it is still wet. Put more wax paper on the core, and then put another piece of plywood on top, with some weights, to smash it all down flat. That whole process would take me 15 minutes. I prefer to cut the panel to fit with just the core and 1 side laminated. It is easier than when it is finished. When it is the perfect shape, put the top layer of cloth and resin on, again removing excess.

The only place I am buying something premade is the face of the instrument panel (or anywhere else you want to leave bare carbon fiber). I will buy 1 really nice sheet of glassy smooth CF, core it, and laminate the back. A couple of coats of automotive clear coat for UV protection.

1/4 inch cored paneling is pretty stiff. I might do a little thicker for the from floorboard, as it will used the most. The thicker, the stiffer. Most coring is pretty light. Weight doesn't go up proportionately with thickness.

The tunnel panels would take some work. They need to be close to perfect shape due to their stiffness. I will probably build the tunnel out of AL according to plans first, and maybe revisit them later. Or maybe I will just enjoy flying the thing. Anyplace you know there will be fasteners, you would cut out the core, and put solid glass. I would probably just use chopped glass fibers with resin or layers of fiberglass tape. Either work, they just create a solid core, that doesn't compress with compression.

[svyolo]

I will also ask Bob his opinion of what I would want to do, if I wanted to make a CF tunnel. Floorboard definitely not required.

[Chris In Milwaukee]

I will also ask Bob his opinion of what I would want to do, if I wanted to make a CF tunnel. Floorboard definitely not required.

Does CF burn or melt? In my mind, I see this component as integral to the firewall and boot cowl. Since the aircraft's exhaust passes by it, and some mufflers nearly against it, it seems like a hot spot. Would the cabin's fire integrity be compromised with a CF tunnel?

[whee]

The former that aft portion of the tunnel attaches to isn't curved on the kit planes. It is flat with makes the tunnel very easy to make. A Ti tunnel and firewall would be pretty cool. The RV guys say to be very cautious with Ti because any tool marks are likely to crack.

It would be interesting to see how much weight is saved with CF floorboards. I talked to another builder about them but decided against them, for now. My Al floorboards are stacked in a pile in the garage; I'll try to weigh them today. If laminated boards weigh half what 0.032" 2424 weighs there might be 8lbs to save.

I hope you guys post pictures. I really enjoy this aspect of building and seeing how other builders make the planes their own.

[Archer39J]

I will also ask Bob his opinion of what I would want to do, if I wanted to make a CF tunnel. Floorboard definitely not required.

I'm not sure a CF tunnel would be best due to the heat as mentioned, especially with foam core, probably best to stick to metal for that area. Check out what heat did to this cowl: http://www.vansairforce.com/communit...d.php?t=150624 I would expect similar damage to a CF panel. Never mind the reaction to an in-flight fire, even aluminum would fare better.

For attaching the floors, rather than building up glass you could just pot a through insert. NAS1834* for example, NAS1834C* if you're going into carbon.