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Tunnel Insulation

jeffwhip

Well Known Member
I've been reading through many of the posts. Those of you who followed Dan Horton's method of insulating the firewall, how did you insulate the tunnel and/or belly? Did you insulate the belly on the outside?
 
I've been reading through many of the posts. Those of you who followed Dan Horton's method of insulating the firewall, how did you insulate the tunnel and/or belly? Did you insulate the belly on the outside?

I have a 24"x24" piece of stainless on the belly. I rolled the edges, placed a 1/16" fiberfrax blanket under it, sealed the edges with pro seal, and pop riveted it in place with stainless pop rivets.
 
I used fiberfrax and stainless foil in the tunnel. In over five years I've not had any issues. The tunnel is cool to the touch.
 
I have a 24"x24" piece of stainless on the belly. I rolled the edges, placed a 1/16" fiberfrax blanket under it, sealed the edges with pro seal, and pop riveted it in place with stainless pop rivets.

While Dan H has convinced me this is the only way to go for insulating the firewall, I would not do this for the belly. If the fire gets so bad that it is needed on the belly, the plane is already gone. The downside of pop riveting this sandwich to the belly is it will tend to collect all the oil and grime that tends to end up there.

I used this insulation (double sided and edges sealed with aluminum tape) in the bottom of the tunnel and never had a hot tunnel: http://www.aircraftspruce.com/pages/cs/insulation/insulator3.php

I used the same stuff inside the double walls in the cabin and the rest of the floor. If I had to do over again, I?d probably not do that as most noise comes from the cabin top.

Again - do what Dan H recommends on the firewall. Leave the inside of the firewall bare.

Carl
 
Belly Shield

I did install a belly shield too, using 0.010" SS sheet, which is plenty thick (i.e. it behaves like sheet stock, not foil). Using 1/16" Fiberfrax and securing with rivets and Fire Barrier 2000, the total assembly turned out to be fairly flat and well transitioned. I primed the aluminum skin underneath the belly shield too. See picture on my builder's log below.
http://www.mykitlog.com/users/display_log.php?user=mikrettig&project=803&category=10711&log=222385&row=99
There wasn't much pillowing at all, and I assume that, after final paint, it will all but disappear. After seeing pictures of the RV-10 that had a fire event, with the gapping hole through the belly into the tunnel, I figured it was good insurance. Additionally, shielding the firewall and belly should keep tunnel heat down too; an added bonus. My $0.02.
 
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While Dan H has convinced me this is the only way to go for insulating the firewall, I would not do this for the belly. If the fire gets so bad that it is needed on the belly, the plane is already gone.

No, it is not gone. We've already seen one example glide to a safe landing with a big hole melted through the belly skin. The issue is occupant smoke and fire exposure.

The downside of pop riveting this sandwich to the belly is it will tend to collect all the oil and grime that tends to end up there.

The edges are sealed, and oil and grime won't hurt stainless steel. If desired, add this panel after the aircraft is painted, or shoot an epoxy primer on the bare aluminum prior to attaching the panel. It adds an additional layer of corrosion protection in the event the proseal or 3M Firebarrier silicone perimeter seal is breached.

I used this insulation (double sided and edges sealed with aluminum tape) in the bottom of the tunnel and never had a hot tunnel: http://www.aircraftspruce.com/pages/cs/insulation/insulator3.php

The temperature of the belly skin is a function of energy-in less energy-out.

Insulating the inside of the tunnel reduces the fire exposure time and/or severity necessary to melt through the belly skin (into the tunnel, with its fuel lines, etc), because you have blocked one energy-out path.

Insulate the outer surface of the belly, and the aluminum skin has no significant energy-in path. And from the standpoint of comfort in normal operation, the structure never gets heated.

Adding worse to bad, the insulation you're recommending is cotton fiber. Here's the TDS:

http://www.aircraftspruce.com/catalog/pdf/09-06014tds.pdf

It's a home construction insulation not even remotely suitable for aircraft. Even if we ignore all questions related to smoke and flame, it is a corrosion risk...it holds water.

Look, the rule here is the same one your doctor lives by, primum non nocere...first do no harm. If you don't want a stainless belly overlay, fine, don't add it. But please, stop doing stuff that can hurt people.
 
Structural

I did install a belly shield too, using 0.010" SS sheet, which is plenty thick (i.e. it behaves like sheet stock, not foil). Using 1/16" Fiberfrax and securing with rivets and Fire Barrier 2000, the total assembly turned out to be fairly flat and well transitioned. I primed the aluminum skin underneath the belly shield too. See picture on my builder's log below.
http://www.mykitlog.com/users/display_log.php?user=mikrettig&project=803&category=10711&log=222385&row=99
There wasn't much pillowing at all, and I assume that, after final paint, it will all but disappear. After seeing pictures of the RV-10 that had a fire event, with the gapping hole through the belly into the tunnel, I figured it was good insurance. Additionally, shielding the firewall and belly should keep tunnel heat down too; an added bonus. My $0.02.

If I read your blog pictures correctly, the 1/16 Fibrerfrax is sandwiched between the two skins (SS and original belly aluminum) by the same rivets that attach the skins to the fuselage ribs.

Wouldn't having this layer of Fiberfrax in the assembly screw up the structural integrity of the skin to rib riveted joint?
 
If I read your blog pictures correctly, the 1/16 Fibrerfrax is sandwiched between the two skins (SS and original belly aluminum) by the same rivets that attach the skins to the fuselage ribs.

Wouldn't having this layer of Fiberfrax in the assembly screw up the structural integrity of the skin to rib riveted joint?

I don't know how either Mike or Kyle assembled theirs, but I would not use the rivets in the plans to ensure no compromise to structural integrity. I would off set rivets lines dedicated to the stainless plate.
 
I don't know how either Mike or Kyle assembled theirs, but I would not use the rivets in the plans to ensure no compromise to structural integrity. I would off set rivets lines dedicated to the stainless plate.

That's what I did. The primary structure is per plans. The pop rivets for the overlay are just there to attach the stainless sheet.
 
My belly shield does, in fact, use the rivets that attach the ribs to the bottom skin. I used rivets slightly longer than the ones called for by the plans, in order to get the proper diameter/height on the shop head.

Keep in mind, that 1) my belly shield is not the full length of the bottom skin, only the first 18 inches or so, 2) the Fiberfrax crushes a little at the point of attachment, leaving very little thickness at the rivet lines and 3) the SS sheet would add a bit of tensile capacity to the bottom skin.

It would probably have been better to use separate/extra rivets to secure the SS sheet/FF to the belly (I thought of that for my firewall), but, for my small belly shield, I didn't do it that way.
 
Dan,
Do you have any pictures of what that looks like?

Jeff

Here you go (I think)...

ouw.jpg
 
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If it was me, I'd want to be sure the underlying skin was carefully prepared and corrosion proofed before riveting a plate on. Once you install it inspection will become impossible, and there is a good chances of moisture getting in between the 2 skins and with the dissimilar metals it could be a problem down the road. It would be more work but maybe worthwhile if you're building to use screw/nutplates to intall the sandwich.
 
If it was me, I'd want to be sure the underlying skin was carefully prepared and corrosion proofed before riveting a plate on. Once you install it inspection will become impossible, and there is a good chances of moisture getting in between the 2 skins and with the dissimilar metals it could be a problem down the road. It would be more work but maybe worthwhile if you're building to use screw/nutplates to intall the sandwich.

Unless you start your -10 fuselage with this in mind, retrofitting nutplates under the floorboards will be a bear. That said, yes, do put a coat of epoxy or zinc chromate on the aluminum to protect it.
 
If I remember used 0.005" SS


23356872600_d5773d002e_b.jpg

I am going to order an SS sheet to do this on my 10. Do you recall what size sheet you used? Also, did you find the .005" sheet to be rigid enough for a good installation?

Thanks,

Larry
 
I thought about this when building my RV-10 but could not bear the thought of riveting an ugly/pillowing SS sheet to the underside of the nicely flat and finished belly. I opted to insulate under the floor and in the tunnel. Its easy to get carried away with the "what if (fill in the blank) should happen" issues. If I were that concerned about an engine compartment fire melting the belly skin, I would incorporate a SS skin in that area instead of the per-plans aluminum. Then again, how many of our certified GA aircraft have SS belly skins?
 
Then again, how many of our certified GA aircraft have SS belly skins?

Not many, but it ain't a bad idea and you get three for one - a cooler tunnel, less noise, and fire protection.

The great thing is that on an experimental, you get to make choices to fit your mission and the set of compromises which are most important to you.
 
I thought about this when building my RV-10 but could not bear the thought of riveting an ugly/pillowing SS sheet to the underside of the nicely flat and finished belly. I opted to insulate under the floor and in the tunnel. Its easy to get carried away with the "what if (fill in the blank) should happen" issues. If I were that concerned about an engine compartment fire melting the belly skin, I would incorporate a SS skin in that area instead of the per-plans aluminum. Then again, how many of our certified GA aircraft have SS belly skins?

Let's ignore all fire concerns, as you have, and merely consider comfort. By insulating inside the metal structure, you're allow the structure to heat. The structure conducts that energy very well (it's aluminum, after all), from component to component, by-passing insulation placed against the inside of the skin. It's a very inefficient method.

It's sorta like wearing your raincoat under your clothes.
 
And again I reiterate that we can get carried away with the ?what if?s? fixes. I?ve done my share I will admit but there comes to a point where I just want to make real progress. I decided to do the double SS firewall with fiberfraxed insulation sandwiched in between, along with SS fire stop pass throughs. Had to remove the mounted engine and the install of the double firewall is really snowballing into a time consuming ?fix?.
 
What stainless did you use? Aircraft Spruce sells 301, 302 & 304 and .016 seems to be the thinnest . (in a previous post I think you said .008 was the thickness)

I purchased a 24"x48" sheet from either Grainger or McMaster Carr. I don't remember which (I was shopping on delivered price). Off-hand, I don't remember which alloy I used, either. I used half of the sheet on the belly and half of the sheet on the bottom center of the firewall.

The 0.008 didn't pillow and was easy to cut with snips and drill with regular bits.
 
When you drill right down to it, the two causes of a hot tunnel are 1/ Two cabin heat scats blasting away diagonally on the firewall just behind the tunnel........and 2/. The exhaust pipes on exit out of the bottom cowl run very close to the bottom of the firewall and that?s just below the bottom of the tunnel. The RV10 is not really any different (apart from size) to all the other Vans models in that apart from these two features they do not normally catch fire or heat the cabin/firewall excessively. Sure if you really have concerns about fire then fit stainless and insulation everywhere. But to simply treat the cause, try fitting butterfly valves in the heater scats so the hot air can be exhausted down at the bottom of the cowl until needed, of course if you don?t normally fly in sub zero temps then doing away with the rear heat scat altogether halves the problem straight away. I?ve done both the above to my RV10. It?s also pretty normal to put a rounded fairing from the firewall about two inches up and down to the flat bar that most folk fit to attach the bottom of the bottom cowl. As the Lancair speed demons have demonstrated, rounding that exit area instead of leaving it sharp improves the Airflow out of the cowl reducing drag and improving speed. That round fairing is pretty easy to fit to a taildragger, a little harder and not so efficient on a tricycle RV10 cause you have to fit the fairings around the nose gear horizontal and allow for the the gear movement, but it?s doable. On my RV10 (250hrs) that fairing is made of aluminium and effectively does two things, smooths the exit Airflow, and it also insulates the bottom of the firewall and reduces any heating of the bottom of the tunnel. Interestingly even though the aluminium fairing runs very close to the exhaust it hasn?t been adversely affected by the heat. To answer the original question by Jeffwhip, Apart from putting some of the standard Vans supplied self adhesive heat shield on the firewall from the heater vents down as far as I could and only caus I had some left over from the cowl, then that?s all that?s been done on my aircraft, it?s been flown in some 40*c Australian heat and no tunnel heat problem. Cheers from Western Australia
 
..... It?s also pretty normal to put a rounded fairing from the firewall about two inches up and down to the flat bar that most folk fit to attach the bottom of the bottom cowl. ...

Can you give us pictures of this?
 
Can you give us pictures of this?

Gil, Greg is speaking of something like the standard exit radius on an -8. You've seen the idea adapted to other models.

The -8 has an exit ramp inset into the belly, so the inset has sidewalls. The stock ramp is aluminum. A stainless overlay for a 10 could be curled at the cowl end, very much like the ramp in the foreground (here upside down on the bench), a one piece SS panel with integral sidewalls for an -8.

1sd0n6.jpg
 
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Gil, Greg is speaking of something like the standard exit radius on an -8. You've seen the idea adapted to other models.

The -8 has an exit ramp inset into the belly, so the inset has sidewalls. The stock ramp is aluminum. A stainless overlay for a 10 could be curled at the cowl end, very much like the ramp in the foreground (here upside down on the bench), a one piece SS panel with integral sidewalls for an -8.

...

Thanks, but I was wondering about the work fitting around the -10 nose gear, which is different from the -6A I'm familiar with. I'm also guessing that the lower cowling "bar" referred to was for the alternate screw (not hinge) mounting, but it wasn't quite specific.
 
Thanks, but I was wondering about the work fitting around the -10 nose gear, which is different from the -6A I'm familiar with. I'm also guessing that the lower cowling "bar" referred to was for the alternate screw (not hinge) mounting, but it wasn't quite specific.

I'd bet it was for the bracket that ties the cowl to the bottom of the fuselage. None of the other models (except maybe the -14) have that.

There's a lot to work around down there on the -10.
 
I'd bet it was for the bracket that ties the cowl to the bottom of the fuselage. None of the other models (except maybe the -14) have that.

There's a lot to work around down there on the -10.

But, isn't that the U-620 and U-621 stuff on my -6A? Plans sheet 61.
 
I removed the heater cuff off of the right side. I have never missed it.
The scat from the left side connects to the SS diverter during winter and dumps overboard in the summer. That alone solves most of the problem. When it is in use it goes into a Y in the tunnel to provide heat fore and aft and it is plenty. In the summer I also use exhaust tip extenders since I have an A/C duct.
The fresh air that used to supply the right side cuff now goes direct to the SS firewall diverter and from there straight into the tunnel to ventilate it with cool fresh air. . In winter I have to close the valve in cruise because it gets cold to the touch. With this controllable setup, my tunnel and fuel plumbing stay ?room temperature? year round.
 
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