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 (https://www.brownaircraft.com/product-p/ba-28-1.htm) 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


14446

[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 (http://www.skygeek.com/brown-aircraft-supply-ba28-1-red-bulb.html).

Something like this:

http://www.avidfoxflyers.com/uploads/monthly_2017_11/Photo0819.thumb.jpg.4c4d4bb28b27e16567267f818c650a 1a.jpg

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.

[Av8r3400]

I know and understand that there will never be a perfect seal on the firewall to cowl. However, the seal that I had was less than terrible. There was a noticeable leak of fumes and (cold Wisconsin) air into the cabin. Much more than was on my previous plane.

This is my fault. I did not spend the time I should have on the firewall and cowl. The original builder used expanding foam to seal the very poorly fit cowl. I chose not to rework the same cowl and firewall to fit properly. (...and I can't stand the idea of using great stuff to seal the gap!)

My idea is coming together and I'll get a photo or two of the finished product.


As I said in the original post, in a long enough time line, I will rework the entire firewall forward on this plane, just not right now.

[avidflyer]

In regard to Dusty's comment about extending the bottom of the firewall back, I did that on my Avid Flyer. Controls and floorboards were installed before I covered the plane, so I ran the fabric all the way to the front first of all, as well as up the sides in front of the doors part way. In addition, I put a piece of thin aluminum sheet on the bottom of the plane from the firewall back to just in front of the landing gear. (standard bungee gear) My firewall bends back at the bottom about an inch, and the aluminum goes above it and they are fastened together and through the floorboards. I glued a few 1/2" thick wood blocks to the underside of the floorboards so I could run some screws up through the aluminum and hold it in place. Bent the sides of the aluminum up 90 degrees and put a few screws into small wood blocks glued to the top side of the floorboards. That seals up the bottom quite well. Northern Mn is cold in the winter and anything stopping the wind is welcome. (we have 3" of snow now, and it was +2 F last night) Hope you were able to follow my explanation. JImChuk

[Dusty]

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.
Lowell
My main reason for trying to get a better seal is cold air.I have tried different seals but my lower cowling is now a little distorted from the pressure from these.
Additional panels must be easily removable for inspections ,as easy access is another great Kitfox feature I wouldn't want to lose.
The rest of the cabin is surprisingly draft free.

[Guy Buchanan]

My upper cowl is split. I have a flange on the upper firewall that contains nutplates for both he forward and aft half. The purpose was to be able to totally seal the back half from rain, preventing nasty electrical problems, while leaving the forward half relatively easy to remove. The back half is sealed using standard white bathroom sealant. (No need for fire barrier.)

The bottom cowl is still full length and to seal that to the firewall I had a bunch of stainless angle formed that has a rolled edge. I then attached these to both sides of the firewall such that by bending them I'd end up about 1/8" away from the cowl, all along the join. I stuffed the triangle gap between the two flanges with the 3M fire barrier moldable putty (https://www.3m.com/3M/en_US/company-us/all-3m-products/~/3M-Fire-Barrier-Moldable-Putty-Stix-MP?N=5002385+3293123899&rt=rud). It seals pretty well to the cowl. I run a line of 2" aluminum tape on the cowl where the seal is to get a better seal and to make it easier to clean. In case of a fire, the moldable putty chars and seals the engine compartment until the cowl burns.

[Av8r3400]

I like your idea of splitting the upper cowl into two pieces for weather sealing. Sounds like something to consider on my eventual front end rebuild.