Model IV Firewall - Cowl

[Av8r3400]

I'm looking for some fabrication ideas from the experts on the forum. This may get a little long, so be warned...

I am not the original builder of the Mangy Fox. I am the re-bulder. During the rebuild I fixed and reworked a lot of different problems with the plane. One area where I just simply ran out of time was the firewall and cowling. Both of these items really needed to be redone from scratch, but the clock was running out on me and I had to get the plane finished. So here's my problem:

The firewall does not seal to my cowling. The fit is pretty terrible to be honest. I thought I could overcome this issue by using silicone baffling flap material to make a seal covering the difference. Well that didn't work worth a darn, so I'm back to the drawing board.

In a long enough timeline, I want to completely redo the firewall forward on this plane with a new design engine mount and cowling. Something following the design of the model 7SS type FWF. I have room in the weight and balance to do this, but I'm not to the point of doing it just yet, so this will just be phase two of the bandaid.


There is a crescent shaped gap between the top curved portion of the firewall and the cowl. I started making a female template today of the outside of the installed cowl in order to get the shape and curve correct (or at least closer). I plan on using this template to make a sheetmetal crescent to attach to the top of the firewall. To the edge of this crescent I would like to attach a flange so that a "bulb seal" like this can be added to really seal the firewall to the cowl.

I'd like some opinions on this idea from the experts here, if I'm on a good track or if there's a better way of accomplishing what I want to achieve.

Fire away with what ever brainstorms you may have!

[rv9ralph]

One method of duplicating the curve of the cowl, usually used when installing baffling, is to put paper clips on the vertical edge, in your case the top of the firewall, when you put the cowl on it will push the paper clips down to give you the curve of the inside of the cowl. See picture for Vans AirForce Forum. Hope this helps.
Ralph


IMG_0259.jpg

[Av8r3400]

THAT'S WHY I POSTED THIS HERE!!!

What an awesome idea! Thank you!

[rv9ralph]

Ok,
Now that we have the method to close the gap, how do you do the cowl so that you block the nasty heat on the front and your comfort on the back?
Suggestion, and this may not be a pretty way to do it. Put angles on both sides of the firewall. Split the top cowl at this point and with nut plates attach the firewall to windscreen portion to one angle. Attach the firewall forward portion of the split to the other angle, leaving a butt joint.

Thoughts?

Ralph

[Av8r3400]

My plan is to put a stainless sheet "gap filler" piece across the crescent shaped gap, then a flange at the outside of this filler piece which will hold the bulb seal.

Something like this:



Probably add a small fillet of high temp silicone in the corner.

[rv9ralph]

High temp RTV is only good to about 400 degrees F. Fireplace Caulk is good up 2500F, depending on the brand and type. It is cheaper that the specialized products that ACS sells and provides the same temperature resistance.

Ralph

[Av8r3400]

I've thought about putting a tall triangular bead of "sealant" on the flange and assembling the cowl (heavily waxed on the inside) to form-fit the two together as a seal.

(That bulb-seal is pretty pricy...)

At what temp will the fiberglass cowl start burning? Maybe 400º is good enough...

[avidflyer]

If you were to get some of that fireplace caulk, lay a bead on the cap strip on the firewall, put some packing tape on the underside of the cowl so the caulk doesn't stick to the cowl, and fasten the cowl down before the caulk sets up, I bet that would seal things up fairly well. I've got to do some sort of seal under my cowl yet as well, maybe that will be what I try. JImChuk

Sorry Larry, I should have read your last post before I posted mine.

[Dusty]

I have a crappy seal by the rudder pedals.My "to do"when time permits is to use very thin aluminium panel from the firewall back to the door opening effectively extending the firewall.Has anyone done this and can post photos or suggestions?
The problematic top cowl could be tackled in a similar way and be made removable for instrument /wiring work?

[HighWing]

Dusty,
I have been thinking about your idea for several days. I have built two Model IVs and try as I might, the firewall seals were not perfect on either one. I put 900 hours on the first one and about a hundred on the second one. I guess I am curious of what exactly you are wanting to correct or prevent by the modification you are suggesting. One of the things I really like about the earlier models is the ready access to the back side of the panel, the pedals and the brake master cylinders by simply removing a couple dozen or so Clecos. If you are concerned about the CO possibility, my suggestion would be to put a monitor on your panel for a glance or two while flying. As I mentioned 900 hours on the first one and much of that was alongside other IV owners and none of us ever felt an issue with that risk given the typical "ventilation" elsewhere around the cockpit.