The Famous RV-10 "Hot Tunnel" issue

I just wanted to write this up to provide some background info on the issue of excess heat in the tunnel area, and combine a bit of the info that's currently going around.  Thanks for some of this info needs to go to many various people, but especially Dean and Bob from Nebraska, and Scott Schmidt who I stole some of these photos from, and Randy for a couple of the ideas he originally gave me.

First, the issue:  Some builders are seeing some very high temperatures in their tunnel area.  Rumor has it that it may have even led to a vapor lock issue, as the tunnel is the area where all the fuel lines pass through.  It is not an issue for every builder, but this may be for a couple of reasons... A) The earliest builders had the AWI exhaust, not the Vetterman.  and B) At least one of the earliest builders, Randy #40006, had a blast tube on his fuel pump that may have helped cool the heater box area.   It is also not an issue for every builder with the newer exhaust, as some of us have tried to take some logical steps to prevent the problem in the first place.  Those will be listed below.

Van's did come up with a Hot Tunnel Service Bulletin on this issue, that should be read to help get some good ideas as well.

Theories:

Here's a few photos of the exhaust and how it exits the cowl near the firewall.  You can get an idea of why some of the radiant heat concepts came about.  The exhaust tailpipes pass fairly close to the firewall, and the gasses wouldn't be too awful far from the skin.  It is my opinion only though, that the gas temperature coming out of the tailpipes, being close to the lower fuse skin, is probably not one of the biggest factors.  I may be wrong, and some of the ceramic ideas presented below may or may not help...or could even possibly make the situation worse from the exiting temperature perspective.

   

Here is a photo that shows the layout of the heater control boxes on the firewall. When the valves are shut, hot air will be pumped into the valve, but blow out the center area between the valves.



Possible solutions:

Here I'll try to give some pointers for possible solutions.  Remember that all of them may have some merit.  Many of them may also have drawbacks.  Since this is free info, it's up to you to decide.



Bob's great drawing of this concept


Dean's great photos of his modification
   

Here's Dean's original Post:  "I wanted to pass on a suggestion for those looking to keep their RV 10 TUNNEL cool. Take your Black & Decker Wet vac and hook the hose to the blower/discharge air. Then place the hose over one of the heat vents on the firewall simulating hot air blowing into the cabin. Close the vents and see if air is still entering the cabin with the vents shut and the wet vac blowing air into the vent on the firewall side. What we discovered was alot of air was entering the OPPOSITE vent when both were closed. The air was coming in under the hinge on the vent door. When I placed a small aluminum divider between the air vents on the firewall the air ceased leaking into the cabin tunnel. After playing with a few DEFLECTOR designs, I used the inside mounting bolt on the vent box to hold a deflector in place and then also put an 1/8 inch deflector inside the vent box so air could not sneak under the front of the vent door. This should solve the problem with HOT air entering the Tunnel with the vents CLOSED. There still may be a Radiant heat problem but insulation on the firewall and tunnel floor and silicone between the firewall and vent box helps for Radiant heat.         Dean        40449"


A photo of my blast tube


A photo of my gasketed control boxes



Additional Possible Solution Components:

Different Exhaust:  Some have suggested that either the AWI or the Forsling exhaust might at least provide a different situation.  I don't personally see these as great "solutions" to the problem, because I don't personally see the problem as that big.  That said, if you're buying an engine and NOT going with the whole FWF kit, you may want to explore the Forsling option.  I know very little about it.

Ceramic Coatings on part of the exhaust:  Scott Schmidt did ceramic coating on his exhaust, and from what I understand, it can significantly reduce inside-cowl temperatures.   Note that I'm absolutely no expert on this.  It appears that it helps keep the heat within the exhaust pipes better.  I can see this being both a help and a problem.   It could help, because it could lower the general inside-cowl temp.  It could hurt because now the exit temp of the air, right by the skin, could be raised.   Probably in the end, it could be both good and bad...nobody yet knows.  We'll just have to see. One concept I thought of though is just possibly ceramic coating the lower extension from the heat muffs rearward.  At least that would cut the radiant heat from the tailpipes by the firewall.  If you do ceramic coat your exhaust from cylinder to tailpipe, and the exit heat is an issue, you may try exhaust extension tubes.  This one one of the possible "solutions" discussed early on in the problems history.

False floor added to tunnel:  I can see this one being kind of nice to do, at least for the first 12" of tunnel.  I don't know that it will be worth the effort or not, but the only downside I can see is the time invested, which may not be too bad, oh, and for those nit-pickers, the extra couple of ounces.  Basically, you'd just add another false floor above the tunnel floor.  This will require you to re-mount your fuel pump, and fuel flow sender (if it's mounted there), and may cause you to have to re-work some of your fuel valve stuff, depending on what you did.  If it helps the problem though, then it may be nice.  We'll have to wait to hear reports from those who have flown with this mod.

Gills on the bottom of the tunnel or in various cowl areas:  Seeing some photos of the "gills" on the cowl, I am almost inclined to think that this idea wouldn't be such a bad one, if I were still building and could easily do the mod.  You could increase the airflow exiting the cowl, and that might help. I personally don't like the idea of gills or vents on the tunnel floor area though.  I don't think the problem is that bad, and I'd rather keep that area weather-tight if possible.

Vent fresh air through the tunnel:  Some have thought of pumping air from a NACA vent into the tunnel.  Then you'd have to have an exit area for it too.  I don't like this idea unless you use inside-cabin air, that would be dry.  I'd rather not expose the tunnel to the elements.

Y-Tube bypass of part of the heat muff air, and vent it to the heat boxes:  I may have to re-read the SB, but I'm thinking the concept is to split the feed INTO the heat muff, and have some of the air pass through the muff (to help keep it from getting too hot, and also allow hot air), and have the rest bypass the muff and be used as blast cooling for the air control boxes.   This idea, if you can easily get the Y-Tubes, I think woudl be EXCELLENT.  The RV-10 has PLENTY of heat in the winter, even in real cold weather.  If you could Y-Tube it off and use some of that air to cool the heater boxes, I think you'd be doing just great.  Just make sure you leave enough air flowing through the muff to keep it cool.  It could also be that just disconnecting the outlet from the air boxes, and redirecting it down and out of the cowl, would be an OK idea in the summer.  I'd rather have a year-round fix though if possible.  The Y-Tube sounds great, and then if you need to shut off the air in the winter to the blast cooling, you could just pull out the Y and put in a straight union.

This isn't meant to be complete, to bash anyones ideas, or to promote my own as "expert".  I just want to give some background on the problem, and potential fixes, both tested and untested, for the builders.  If you have other ideas, I'll be happy to incorporate them here too.  I personally don't have a tunnel heat "problem", as I mentioned above.  My "Fix" was the tunnel/firewall insulation, the heat box gasket, and the blast tube on the heat boxes. Those are all fairly cheap, fairly painless, and pretty easy to do.  It is perfect, maybe not.  I may actually do the Y-tube thing too, just to save myself the lost pressure from my baffle blast tube.  Finding the Y's may be a challenge, but I'm sure I can find them.  Ideally, I'd have a 2" inlet, a 2" outlet, and an additional 1.25" outlet.  That would be idea as best I can imagine.

Hope this helps.