Hot Tunnels

[attackpilot]

Call me dumb, but what is the tunnel section that everyone is refering to. I am assuming it is near the bottom of the engine compartment near the firewall, is this correct?

As far as reducing the temperature around the exhaust section, has anyone thought about thermal barrier coatings? I would think that it would help some. Also, the heater box could be set off from the firewall with metal stand-offs. Sort of like someone else mentioned. Finally, it seems like Vetterman could angle the exhaust outlet down a few more degrees to give some more space between it and the fuselage.

Now for a far fetched idea I would like to throw out there. The way the
AH-64 Apache exhaust/transmission/engine is cooled by venturi effect. The primary exhaust nozzle lays inside a larger nozzle assembly. The hot gases from the ehaust nozzle creates a venturi effect and draws air in to the larger nozzle where it mixes with ambient air to cool it from around 700 C to less than 200 C. Granted turbine engines are essentially air pumps, so the volume of air that it pushes is way more than a piston engine, but I think this could be adapted to the RV-10 fairly easily. Any thoughts?

Joe Hutchison
RV-10 Tail Kit

[brules]

thermal barrier coatings would help but would add weight and cost. this problem IMHO could be designed out of the aircraft without additional penalty. the airflow inside the lower cowling and across the belly skin just aft of the firewall should be investigated to see if the heat is not being allowed to escape from the engine compartment properly. some items i would look at are the slot opening for the nosegear travel. it could be pressurizing the area in question and maybe the louvers should become cowl flaps instead. the air outlet ramp is blocked with numerous items restricting that area and the heat blast from the cabin heat valve boxes certainly should be dealt with. as an example the cherokee six aircraft from pipers lineup has a tunnel that runs from the firewall to the tailcone and i don't remember that aircraft having this issue as it also was powered with a lyc 540.

[brenthg]

We are in the process of trying to cool down the HOT TUNNEL issue in 331JH. We made a 4" x 10" cutout in the bottom of the tunnel, just aft of the front seats. We're designing a custom louver to cover the hole, similar to the gills on the bottom of the cowl. As soon as it's complete, we'll fly and see if this vents the hot air out of the tunnel.

Will get back here and report our experiment.....hence, that's why we call them Experimental Aircraft, huh?

Posting for Jim Headberg - RV-10 N331JH
Brent Headberg
West Palm Beach, FL

[Mike S]

brenthg-----have you already provided a fresh air source to the tunnel, or you just planning to suck out cabin air with the bottom vent??

I have been watching this thread with carecully, and find some interesting things going on.

The tunnel is hot------not the floorboards. The floorboards are double layered with an air gap between.

There have been cases or engine stoppage in flight, attributed to possable vapor lock---------the fuel lines, and pump, valve, and sometimes gascolator etc reside in the tunnel.

After lots of discussion with my RV mentor up the street, I have come to the following conclusion/plan for my -10.

1. false floor in tunnel, one or two inches up.

2. insulate the top side of the false floor.

3. duct fresh air in at the front of area under false floor.

4. exit gills as per above post on a/c bottom.

5. mount all fuel system componets above false floor.

6. if needed, heat shield mounted to exterior of a/c with standoffs for
cooling air, and exhaust deflection.

Mike

[glenmthompson]

Hi Mike, I am glad somebody is beginning to listen my recommendations and observations. If your 10 was mine..You and I would have identical tunnel sections. Maybe Vans will take heed!.
Here is our latest update. We changed the erl and filter and improved the fit of the carb inlet seal, lest we were pressurizing the lower cowl (# 5 was still running up to 460 on most takeoffs, but would cool well even at high cruise power settings. AN washers on aft shroud is my plan upon next cowl removal) Note, the #5 now hits 440, and cools quicker (90 deg day too!) after the following was done. see below....
We took the right cabin fresh air tube that was on the right side of the plenum, and blocked it off with high quality al. duct tape. Flew with Jim on 20 June...92 degs here in Wellington that day!
It appears we are almost at square one again ( but remember it WAS a really hot day Jim pointed out) as far as the tunnel is concerned. The top of the tunnel was very warm, just "cool" enought to hold your hand there without discomfort. As the hand is moved down the side toward the floor, one is unable to hold the hand on the side of the tunnel once you reach about 4" below the top of the tunnel. I believe at this point, the temp here is above 140 F, MUCH hotter as you go lower towards floor.
Please remember at this point, Jim has one cabin heat fresh air scat tube blowing into the tunnel for fresh air. Hence why he wants to try to let some air out through the proposed louvers on the bottom...Can you say more volume via louvers, equals more heat lost with the single proposed and VERY necessary scat tube? So maybe we can have cabin heat, with the other tube in the future?
****Also please note the following...The tunnel at the front near the firewall, is just as hot (using the ouch touch method with my hand) as the tunnel near the fwd spar. It is just barely cooler aft of the front seats next to the pax feet. Lending more proof to my theory the heat is radiating/convecting from exhaust/cooling air escaping from the cowl.
*****Gills, IMHO, the gills HAVE helped, since without the gills and just one scat tube entering the tunnel, in prev. testing, the top of the tunnel was scalding hot!!!! And on these previous flights, the temp was no where near 90 deg! So IMHO, the gills help,.... significantly? hhhmmmm, no, but noticably? YESSSSSSS!!!!!!!!!!!!!!
************TAS update 1500 feet, we had 29" and I held 2400RPM, (sorry, higher rpm, MAKES THIS PLANE SLOWER!!!!! FF was an enormous 24 +++ gph. We acheived 214 MPH TAS, STILL HIGHER THAN VANS PUBLISHED NUMBER. The CG was pretty far fwd as compared to the trip we took, prev. mentioned Can you say S*** Hot! ?
Come on guys/gals get off your *** and order this awsome plane!!!!
Glen

[w1curtis]

Van's has published a service letter on the tunnel temps and it pretty much echos what Mike S has stated above.

http://www.vansaircraft.com/pdf/tunnel_temp.pdf

[glenmthompson]

Brent installed the 4x10 louvers today in the bottom of the tunnel, aft of the spar. It apppears all that has been done is to now make the aft tunnel as scalding as the front, by apparently drawing the blistering air to the back. Top of tunnel warm to touch, down sides 4 ", too hot to touch. Please note that the cabin heat blast tube has just been removed and blows mainly toward the top of the tunnel, straight aft, causing IMHO the top to stay tolerably cool, while the bottom of the tunnel remain VERY hot. All is hotter towards firewall.
Right now only 1 cabin heat blast tube blowing into tunnel as per Vans SB (We did this many months ago). Gills installed in sides of cowling. Louvers installed in floor aft of main spar in tunnel area under elev. push tube. Vans bandaid is not the solution ladies and gentlemen.
**
**Please re. my original idea of a false floor. Also note that with BOTH cabin heat fresh air tubes blowing into the tunnel, all is acceptably cool.
This is such a sweet flying example of how a 10 should fly,.... even faster now than Vans says it should be.
Gawd, this tunnel is annoying on this 89 deg fl. day.
*
* Signed....Discouraged and giving up until we pull the cowling again and try to further insulate the firewall on the engine side, per jim's suggestion.
Glen's further $.02 for current builders. Leave the tunnel accessable until somebody can figure out an ACCEPTABLE, NON BANDAID fix.
Some people are poo pooing this issue, but lemme tell ya, it is a P R O B L E M !!!
********
On a brighter side guys, We all L O V E this plane, I am just still whining about this tunnel heat issue.
***** I now pose a question to qualified metalurgists and/or engineers. Lets say the spar carry through on some planes, in the tunnel reaches, say oh 150-160 deg. F virtually on every flight. Very possibly higher at the very bottom of the spar where it touches the bottom skin What will this do to the temper/strength of the spar over the long run? Especially for those that ignore the heat in there and just insulate and carpet their tunnels?
Glen

[glenmthompson]

I just thought of a little demonstration for those non believers out there. Next time you fly, take off your shoes and put your bare feet up against the firewall. Then, take your scalded toes and further burn them by placing them on the floor directly on top where the exhaust/cowling air blows across the bottom of the plane. Those who have insulated their floors, already know how blistering a hot it gets. An RV6 I test flew, literally melted the heals of my sandals, until we insulated significantly the floor.
Now, imagine all of this heat entering a "tunnel area". Hence "The" problem.
I don't create the problems, I just furiously point them out and search for acceptable answers.
Guys, when we get some of these things fixed, I predict the 10 wil be THE plane to have and even non experimental types, will just have to have one.
IMHO this is SOOOO much more plane than the Columbia, and it could be certified and built so much cheaper. But lets get the really annoying things fixed and fixed well!
Glen

[attackpilot]

Ok, now that I know that it is the actual tunnel that gets extremely hot in the RV-10, I would like to suggest my plan to deal with it and see what you guys/gals think:

1. Extend the exhaust by a few inches and have it point more down than the prototype. Yes, it may cost a knot or two, but it should help.

2. Have exhaust ceramic coated to contain the heat.

3. Have the cabin heat selector on standoffs or some other insulating material (any suggestions?).

3. Spray on a ceramic insulating paint on the cabin side of the firewall, bottom sub-floor, bottom of floor where your feet are, and both inside and outside of the tunnel (I plan on having carpet going up to the top of the tunnel)

4. Firesleeve all the fuel lines inside the cabin area to help reduce chance of vapor lock.

5. Install the extra louvers in the cowling.

6. If required, I think I will run a duct to the tunnel with an exit toward the back so that fresh air can flow through it.

I have also thought of somehow placing the cabin heat selector on the heat muff itself to help reduce the conductive heat problem, but that might be a big headache.

Yes it will add weight (maybe 10 lbs max) and make it a knot or two slower, but I think it will be worth it in the end.

I would appreciate any insight and further ideas.

Joe Hutchison
RV-10 Tail Kit

[glenmthompson]

We have done all U mentioned, except ceramic coating, which IMHO is not worth the cost, since it will not help the heat significantly... Any more ideas?
Glen