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Simple solution to hot tunnel

jeremiahd

Member
I have about 10 hours on my RV-10, and like a number of other builders, had an extremely hot tunnel. It was so hot that on my last flight a week ago, I couldn't even touch the side near my right leg after only about 10 minutes in the air. Since I have the fuel lines running through the tunnel, and my transponder's black box is located in it, I grounded the plane until I found a solution.

I came home and searched this website, and found that I wasn't alone. Two suggested solutions caught my eye: 1) replace the aluminum heater bypass valves that Vans provides with the stainless-steel valves made by Plane Innovations; and 2) add firewall insulation (or other thermal barrier) to keep engine heat and exhaust heat out of the tunnel area.

There seemed to be a lot of variations on the threads suggesting solution #2, and nothing seemed clear-cut or used by the majority. So I tried two heat barriers offered by a racing car supply company called "Heatshield Products" which is located here in San Diego. (I've used one of their products -- the "Lava Shield" -- to protect the fiberglass engine cowling from the exhaust pipes in my RV-6A, and that worked very well.) Their website is www.heatshieldproducts.com.

I ended ordering 12 square feet of "Lava Shield", and 12 square feet of "HP Sticky Shield", which are rated at 1200 F and 1100 F, continuous, respectively, and 2000 F intermittent for both products. (I probably only needed about 8 square feet.) The Lava Shield is cardboard-thin, easily cut with a scissors, and has a stick-on backing. The HP Sticky Shield is a little thicker (1/8 inch thick), but also is easily cut with scissors and has a stick-on backing. The stick-on backing makes installation easy.

I put the Lava Shield on the firewall side of the tunnel, and between the firewall and the new stainless-steel heater bypass valves. I also placed it on the floor of the tunnel. I placed the HP Sticky Shield on the cabin-side of the firewall, covering the bottom half inside the cabin. While I was at it, I put Lava Shield on the inside of the fiberglass engine cowling, where the exhaust pipes came close. It took me about a day to do all of this, since the plane is built, and it's a bit tough fitting my hands through all that engine stuff to replace valves and insert heat shield.

Well, I just flew her about two hours ago. It was a sunny, warm (80 F) day, and I climbed to 6000 ft to see what would happen. Eureka! The tunnel stayed cool to the touch, even when engine cylinder temps briefly touched 400 F.

So, my friends and fellow RV-10 drivers, I wish to report a successful fix to that tunnel problem, that can be retrofitted to your plane with just a day's work.

Jerry
 
Hot tunnel

Hi
Thinks how many times you will be using the heater in the plane,I disconnect the scats from the firewall valves ,I do have the Ss ones,redirect the hoses to the bottom .
Never a problem again
If I will decide to go north in winter ,it's 5 minutes to reconnect
Good luck
Hugo
 
Jerry,
Thanks for the wright up, I will defiantly look into doing this and thanks for posting the link.
Tim
 
It would be interesting yo see how these products hold up to Dan Horton's fire breathing dragon test rig.
 
Hi
Thinks how many times you will be using the heater in the plane,I disconnect the scats from the firewall valves ,I do have the Ss ones,redirect the hoses to the bottom .
Never a problem again
If I will decide to go north in winter ,it's 5 minutes to reconnect
Good luck
Hugo

Just a thought - do you risk getting firewall forward fumes or CO into the cockpit? Don't know how good the seal is on the heater box, even the after market ones.......
 
A little square of UL181 foil tape should cover those two holes nicely along with a CO detector.

I used 1/8" fiberfrax with good results too. I leave both of mine connected with no overheating. Probably could get by with one connected to a wye fwf. Being at 38 Deg latitude and enjoying flying high on most xc flights, we usually use heat on most flights. Good to assist defrosting on those cooler humid mornings.
 
My solution was a little different

I am about 1/3 along on my RV-10 and have also read all the stories about the hot tunnel. I have put a good amount of insulation on the firewall hoping this would help. I also insulated the tunnel itself to reduce interior noise (not good for a hot tunnel).

Anyway, I ran an air intake from using a 1" tube from the passenger NACA duct over to the front of the tunnel with a open/close valve. Also, made a fiberglass exhaust duct which looks like a mini hood scoop (1 x 3") but mounted in reverse and installed near the rear of the tunnel as an exhaust vent.

I didn't post this because I figure there would be too many nays..

Oh, I also plan on putting a temperature thermocouple in the tunnel to monitor the temperature.
 
We have used, and have sold, Heatshield Products for several years with good results. They make good stuff at a very reasonable price. I particularly like their HeatShield Mat.

It works well on the engine side of the firewall and also works well to reflect exhaust heat away from the cowling. Easy to cut and install too. A bead of high temp silicone or ProSeal will make a very tidy installation on the cowling too.

For Mike Starkey, no, this stuff will not hold up to Dan Horton's fire breathing dragon set up. It isn't intended to be fireproof. It's intended to insulate and reflect heat.

I use this on the engine side of my firewall(s) but would have no qualms about putting it in the tunnel of an RV-10, although there are undoubtedly many ways to insulate that area.

I am building an RV-6 currently and the firewall will get the usual layer of HeatShield mat to help keep the heat out of the cockpit. The floorboards will also get some insulation, but i haven't made up my mind which product will go there yet.

Heat, sound, paint, and trim products
 
Jerry,
I have a similar set up in my 10 and absolutely no tunnel heat issues.
I fly in shorts most of summer as does my wife and passengers and most everyone comes in contact with the side walls of the tunnel and it is just a little warm.
Like you, I have used the aftermarket stainless steel heater valves and sandwiched fiberfrax insulation between the heater box and the fire wall, then sealed with Pro Seal.
On the inside is a piece of fire wall heat shield against the firewall as well as on the floor. I do have a double floor in the tunnel to accommodate the insulation.

I do have a reverse 1" NACA that I was going to install if needed but it is definitely not needed.
Seems that the hot tunnel issue has pretty much disappeared and I am not sure exactly what has changed since the early models had tunnel heat problems.

My installation was not designed to be fire proof just insulation against excessive heat.
 
Two tiny points...

The "fire-breathing" test setup is merely the FAA standard, nothing radical at all.

As demonstrated by an unfortunate RV-10 owner right here, the floor will probably burn through below the tunnel in the case of a fuel-fed fire, igniting a poor insulation choice. Choose carefully.
 
Two tiny points...

The "fire-breathing" test setup is merely the FAA standard, nothing radical at all.

As demonstrated by an unfortunate RV-10 owner right here, the floor will probably burn through below the tunnel in the case of a fuel-fed fire, igniting a poor insulation choice. Choose carefully.

And melting our brake lines.
 
I don't expect an answer and I am sure we could all learn something from that accident .
Would like to know what happen and lead to the fire in the first place.

As demonstrated by an unfortunate RV-10 owner right here, the floor will probably burn through below the tunnel in the case of a fuel-fed fire, igniting a poor insulation choice. Choose carefully.
 
Search the forum, it's in here.

I like Jack's solution; not only does it positively cool the tunnel but also vents it in case a leak puts fumes in there.
 
I used fibre frax on the cabin side of the firewall and the floors, including the tunnel, two layers of 1/8". I also installed the stainless heat valves. Everything else is stock. My tunnel is cool to the touch. I believe that the newer exhaust system with the outlet extension pointed downward, is probably one of the reasons the tunnel issue has gone away.
 
I am about 1/3 along on my RV-10 and have also read all the stories about the hot tunnel. I have put a good amount of insulation on the firewall hoping this would help. I also insulated the tunnel itself to reduce interior noise (not good for a hot tunnel).

Anyway, I ran an air intake from using a 1" tube from the passenger NACA duct over to the front of the tunnel with a open/close valve. Also, made a fiberglass exhaust duct which looks like a mini hood scoop (1 x 3") but mounted in reverse and installed near the rear of the tunnel as an exhaust vent.

I didn't post this because I figure there would be too many nays..

Oh, I also plan on putting a temperature thermocouple in the tunnel to monitor the temperature.


How did your experiment work out?
 
I did nothing to my RV-10 other than put a firewall blanked on the engine side of the firewall... i have not noticed that my tunnel is hot.
 
Regardless of product used, consider the difference between insulating outside the structure vs insulating inside the structure.

Insulating outside (notably the engine side of the firewall, and the outer surface of the belly skin) slows heat transfer to the entire airframe.

Insulating inside allows the airframe to heat, while attempting to slow energy transfer from the hot airframe parts to components in the human space. It may slow radiant and convective transfer, but it does nothing for the metal-to-metal conductive path.

When someone talks about insulating inside the tunnel, I get a mental picture of them wearing their raincoat inside their clothes.

That's comfort insulation. Engine fire is a different subject. Assume a fuel or oil fed fire. If you insulate the inside of the tunnel floor without external surface shielding, I can pretty much guarantee you'll melt through the belly skin immediately aft of the firewall. Simple enough; temperature rise of the aluminum skin is a function of energy-in less energy-out. Insulating inside the skin blocks the energy-out path.
 
Regardless of product used, consider the difference between insulating outside the structure vs insulating inside the structure.

Insulating outside (notably the engine side of the firewall, and the outer surface of the belly skin) slows heat transfer to the entire airframe.

Insulating inside allows the airframe to heat, while attempting to slow energy transfer from the hot airframe parts to components in the human space. It may slow radiant and convective transfer, but it does nothing for the metal-to-metal conductive path.

When someone talks about insulating inside the tunnel, I get a mental picture of them wearing their raincoat inside their clothes.

That's comfort insulation. Engine fire is a different subject. Assume a fuel or oil fed fire. If you insulate the inside of the tunnel floor without external surface shielding, I can pretty much guarantee you'll melt through the belly skin immediately aft of the firewall. Simple enough; temperature rise of the aluminum skin is a function of energy-in less energy-out. Insulating inside the skin blocks the energy-out path.

I'm familiar with techniques for insulating the firewall, but how do you insulate the outer surface of the belly skin?
 
My Belly insulation

Hello, Just to show you people, my solution to the hot tunnel issue,
After 100 hours flying with the problem,I riveted an stainless steel skin to the belly with a fiberfax sheet between with amazing results
Here you are some pictures
Jorge
Buenos Aires
 

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Hello, Just to show you people, my solution to the hot tunnel issue,
After 100 hours flying with the problem,I riveted an stainless steel skin to the belly with a fiberfax sheet between with amazing results
Here you are some pictures
Jorge
Buenos Aires

That's the right idea.

Jorge, can you put some numbers to "amazing results"?
 
hot tunnel- my solution

That's the right idea.

Jorge, can you put some numbers to "amazing results"?

Dan, first, let me tell that my solution was inspered in yours, so thanks.

Well, honestly, I didn´t measure the temperature before adding the isolation
nor after, but the difference is really very, very notorious. I can say at less 20
Celsius degrees less ( around 68 Farenheit less),
It seems that almost all hot air flowing from the engine and the exhaust pipes, not ¨attack¨ the first 20 inches of cabin floor and that made the difference.
The fibrefax I put is 1/8"
In addition, I wrapped the exhaust pipes with a isolation tape
regards
Jorge
 
I installed a high flow fan at the front of the top tunnel cover. When the tunnel gets a little toasty, I turn it on. The air flows out the many openings in the back. Not perfect, but works well enough to be comfortable.
 
I had originally considered this, but skipped it in favor of getting in the air. I hadn't noticed any real warmth from my tunnel until a couple of long x countries this summer. I simply felt there was more warmth coming from my leg area than there should be. Tunnel is definately not hot, but seems a bit warmer than ambient.

Are others seeing this as well? I am guessing that this approach would eliminate that.

My F/W has insulation and also have some insulation (non flamable or at least non-toxic, mineral insulation) on the floor of the tunnel.

Larry
 
RV-10 Fiberfax for Tunnel Heat

Is there a reason to use stainless steel rather than aluminum on the belly? Is aluminum ok? To me the firewall stainless steel makes sense. I like the solution but I'm not clear on which metal.

Thanks
Dave Syvertson
 
Is there a reason to use stainless steel rather than aluminum on the belly? Is aluminum ok?

Dave, it depends on your goal. If your only desire is comfort insulation, then an aluminum cover over a fiberfrax insulator would work just fine.

However, aluminum melts around 1100F, so it's not going to hold up in the event of an engine compartment fire. Not guessing; we're already seen aluminum belly skins melted through. A stainless cover sheet will serve the comfort role as well as the fire protection role.

If I were to do an RV-10, I'd install a titanium cover sheet..performs like stainless at less weight.
 
On my RV-3B, my feet are on the belly skin. I used 1/16" Fiberfrax on the outside belly, with a .020 titanium overlay. You can read about it on my VAF blog, posts around #365 through #373. There will be a few extraneous topics in there, too, sorry about that.

I chose the .020 titanium because like stainless, it's a material that the FAA allows for a firewall on certified planes. I thought that perhaps it would be less personally annoying to work on than stainless, and using Grade 2, it turned out to be a pleasure to work with. I bought it from McMaster. They might not have a size suitable for your plane - my RV-3B is tiny by comparison.

It's density is about 56% of stainless.

I used .063 aluminum stand-offs where I wanted to have rivets through the titanium, so that I wouldn't crush the Fiberfrax. You'll see that in the blog.

Dave
 
Think the best solution to the hot tunnel is to “dump” the heated heatmuff air overboard before it starts blasting away at the firewall, the crazy thing is that the heating of the firewall and tunnel is worst when the Wx is hot and cabin heating isn’t needed. If dumping the hot air at the muff with a valve from Spruce is not a liked option, then reverting to the previous Vans cabin heat control (RV6 era) where at least the unwanted hot air was exhausted vertically down the firewall minimising the firewall heating would be a big improvement because the hot air could get away quicker. The current cabin heat control by Vans or the aftermarket stainless steel ones maximise the heating effect on the firewall (and tunnel) cause they direct the hot air AT the firewall at approx 45* as we all know. When both heater options are used (front & rear) both blasting away then it’s pretty well assured that the firewall/tunnel area is going to get hot. Another option, is to site the cabin heater control at a different position on the firewall, NOT in front of the tunnel. The last option is good for front occupants, hot air easily piped with scat to pilot & copilot sides, a little harder for the rear passenger heat ....... but hey .... it is Experimental Aviation. Cheers from Western Australia.
 
I used 1/16" thick FiberFrax on the belly, sandwiched under 0.005" thick stainless steel sheet. For the firewall, I intend to use 1/8" thick Fiberfrax under the same stainless steel sheet. I did not think about Titanium at the time of install. Stainless steel rivets are what I used and they are "extra," not a replacement for any structural rivets called for. For the structure, the regular aluminum solid rivets called for were used. In order to hold the sandwich onto the fire wall, additional 1/8" holes were drilled in a regular patterned grid, so that SS rivets can be used there. Since the aluminum structure is now behind the FF//SS sandwich, it is thermally protected to a greater extent than stock. Hope this helps you.
 
I used 1/16" thick FiberFrax on the belly, sandwiched under 0.005" thick stainless steel sheet. For the firewall, I intend to use 1/8" thick Fiberfrax under the same stainless steel sheet. I did not think about Titanium at the time of install.
...


Mike, Thanks for the reply; I appreciate the information on what you used!

Anyone else use titanium sheet over the firewall fiberfrax?
 
..has anyone used stainless rivets on the firewall or is the consensus that the stainless/titanium sheet protects aluminum rivets well enough?

Don't know about consensus, but measurement says the standard firewall aluminum angle extrusions stay cool enough to remain structural with a 2000F test patch going on the forward side.

Wasn't able to measure an individual rivet ;)
 
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I’m interested in knowing if anyone can tell me why some folks have this hot tunnel issue and others dont. I built my plane according to plans. Thunderbolt engine, standard configuration. I have not noticed that my tunnel is hot.
 
So when you say built to plans, you used the Van's Heater boxes, hooked up both Muffs, and did no firewall insulation of anysort on the engine side or between the Heater boxes and the firewall?

I’m interested in knowing if anyone can tell me why some folks have this hot tunnel issue and others dont. I built my plane according to plans. Thunderbolt engine, standard configuration. I have not noticed that my tunnel is hot.
 
Fiberfrax Sandwich

I believe the fiberfrax / SS sandwich was recommended as added fire protection, not to simply solve a problem with a hot tunnel. There was an RV-10 that had a fire in the engine compartment and the fire burned through the floor under the tunnel and through the firewall at the point where the tunnel is. Since there are fuel lines that run through there, a fire that could/would do this a concern. Added additional thermal shielding on the firewall and at the exhaust exit (belly) is one means of mitigating this risk. However, as it has been written many times, whatever you do to insulate the firewall, do it on the engine compartment side of the firewall, not the cabin interior side or the inside of the tunnel. Hope this helps explain. There's really two issues. It just so happens that providing additional fire protection also helps solve the "hot tunnel" issue.
 
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