Firewall for wood aircraft

The firewall bulkhead for wooden aircraft is usually plywood and must be protected by a metal firewall. It must also be inaulated from heat passing thru the metal firewall. Tony Bingelis in "Engine Installation Methods" suggests using a layer of fiberfrax ceramic insulation between the plywood bulkhead and the metal firewall, but does not indicate the thickness of fiberfrax to be used. Dave Prizio in sept 2012 Kitplanes suggests placing the metal firewall directly against the plywood bulkhead and placing 4 layers of 1/8 inch fiberfrax in front of the metal firewall and sandwiching that with a sheet of aluminum. I would like to know what other builders are doing in this regard and if there is a current standard to be met in firewall construction for wood aircraft. Randy McCoy

Honestly, if I were going for the engine side of the firewall, I would face it with cork, then Fiberfrax (or something similar) and then the metal. Cork is extremely fire resistant to the point that it actually provides some protection as it burns (much like tumescent paint). Another point that should be made is to cant the bottom of the firewall back at least 45 degrees to prevent it digging in during a forced landing or crash. This helps to minimize the impact forces.

The material on the engine side of the firewall must be resistant to oil and fuel and misused tools etc. Metal is good. Fiberfax is good. Wood and other natural materials like cork are not good.

Fiberfax has some temperature insulating qualities. It is not horribly expensive so you can buy a small amount and check it out. That's what I did.

Angling the firewall aft works against other structural requirements. Most small airplanes have the engine mounts bolt onto the lower longerons. Since the engine mount tubing can be viewed as an extension of the fuselage structure, and the lower tubes are usually angled upward to meet the engine, you get what you are talking about for free in most designs. So no need to do something different with the firewall and make that part of the airplane more complicated.

Best of luck,

Wes
N78PS

Quote:

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Originally Posted by mcstorch

I would like to know what other builders are doing in this regard and if there is a current standard to be met in firewall construction for wood aircraft. Randy McCoy

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What kind of "metal" are you planning on using for the firewall?

Having just finished the #1 bulkhead/ firewall on my wood project , this is something I have also been looking into. Basically, my question is how does one go about mounting items to the firewall once the fiberfax insulation is installed? If the insulation/fiberfrax is installed between the wood and the steel would the motor mount and other items mount on top of everything, will the insulation crush? In the kitplanes scheme the motor mount could be mounted first and the insulation/fiberfax cut around it but I imagine that would be difficult to do neatly and then there are all the smaller items such as the battery box, fuel pump, contactors, colator, etc. Can they be mounted on top of 1/2" of fiberfrax?
Thanks
David
http://websites.expercraft.com/dnorth1/

Mounting items to the firewall in such a case is problematic, but here are some thoughts. First, the engine mount should be attached to the firewall (and structure behind) before installing the Fiberfrax and any covering, The mounting needs to be solid. After cutting around the mount as best you can fill up the voids with a 2000 degree fire caulking like Flame Safe. For other items it is best to minimize the number of items that hang off the firewall. Often things like the gascolator can be supported off a lower engine mount tube, same for the oil cooler. If you simply must mount something to the firewall bolt through the firewall assembly using bushings to prevent crushing the Fiberfrax. Then relieve the wood or composite in the area where the bolt projects through the firewall to avoid direct contact with the bolt head. I suggest cutting out a 2" diameter hole in the wood or composite centered around the bolt head where it passes through the firewall. This helps protect the flammable structure from direct heat. It isn't perfect, but it is the best you can do.

Bolts through a composite firewall conduct heat. Not recommended.

So what do we want to put in the engine compartment? Voltage regulators and other electrical components will last much longer on the cabin side of the firewall. B&C Specialty urges installation on cool side of the firewal. My factory built Pitts Special has the gascolator on the cabin side of the firewall. The fuel boost pump is also on the cabin side of the firewall.

As noted above, you can hang oil coolers, and engine sensors from brackets clamped to engine mount tubes.

With some thought, you can have a clean firewall.

Best of luck,

Wes
N78PS