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Tunnel heat SCAT tubing size

Bill Boyd

Well Known Member
I've been advised to finish plumbing the tunnel heart SCAT runs while access is still open (canopy top and instrument panel area still off/open).

Someone here mentioned using 1.5" SCAT in the tunnel to increase working room on other things that share the tunnel interior. This would apparently involve obtaining or fabricating some 2" to 1.5" reducers for the SCAT.

Those who've passed this point already: is this really worth the effort?
 
Someone here mentioned using 1.5" SCAT in the tunnel to increase working room on other things that share the tunnel interior. This would apparently involve obtaining or fabricating some 2" to 1.5" reducers for the SCAT.
Those who've passed this point already: is this really worth the effort?

Be aware that reducing the diameter of tubing reduces the volume by a MUCH higher ratio. i.e. Most of the area of a circle is around the circumference.
 
Be aware that reducing the diameter of tubing reduces the volume by a MUCH higher ratio. i.e. Most of the area of a circle is around the circumference.
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Yeah. Poiseuille's Law. That was on the MedCAT's 40 years ago. Laminar fluid flow resistance varies inversely with the tube radius to the 4th power :eek:

The VAF post that mentioned scaling down the SCAT to 1.5" said there was still plenty of hot air to heat the cabin on the -10. I was suspicious this was a solution in search of a problem, and the replies here would suggest that is so.

Thanks, all. You're great! Have a blessed day; smash some rivets and/or some bugs at altitude.
 
The problem was substantially improved circa 2009, when there was a design change in the location of the fuel valve.
 
Aha.

Well I hope the problem was not resurrected with the use of the Andair simplex valve, which I have installed. Haven't tried running the SCAT yet; trying to anticipate the next few weeks in the shop and what I'll run in to.
 
Well I hope the problem was not resurrected with the use of the Andair simplex valve, which I have installed. Haven't tried running the SCAT yet; trying to anticipate the next few weeks in the shop and what I'll run in to.

There were a couple of issues that the early builders experienced.

The mount bracket was moved for the fuel valve to accommodate better routing of the scat tube. The would only be an issue if you have a really old fuselage kit.

The other issue was the tunnel walls getting extremely hot. There were even some minor skin burns. The reports of this started diminishing around the time I started to build. My tunnel is lined with fiberfrax. My tunnel is cool to the touch.

There is also no need to worry about the efficiency of heat in a RV-10. I might open mine about 20%, but only when in gets to be under 20F. I've never used the heat fully open.
 
Thank you, Bob

Always appreciate hearing form you.

I plan to leave well enough alone. Fuse kit is circa 2010.

Also plan to put fiberfrax on firewall and between S.S. heat selector boxes and firewall which I hope will mitigate hot tunnel issues. Without A/C installed, I'd prefer the tunnel not even be warm :eek:

Planning to extend fiberfrax and stainless foil onto the belly in the exhaust plume area to help protect there from an engine fire. Will not insulate the tunnel floor on the inside to lessen theoretical risk of burn-through of belly skin from fire as happened once already. I also figure fuel lines in tunnel will run cooler if it's not insulated all the way around. Maybe, maybe not. Since I hope to run mogas, it's a concern.

I've toyed with the idea of bleeding some air from the pax side forward NACA vent and forcing it via small blast tube into the tunnel near the front, with a small exit scoop in the belly at the back of the tunnel, to circulate cool air in there. But- complexity, and added drag...
 
\

Yeah. Poiseuille's Law. That was on the MedCAT's 40 years ago. Laminar fluid flow resistance varies inversely with the tube radius to the 4th power :eek:

The VAF post that mentioned scaling down the SCAT to 1.5" said there was still plenty of hot air to heat the cabin on the -10. I was suspicious this was a solution in search of a problem, and the replies here would suggest that is so.

Thanks, all. You're great! Have a blessed day; smash some rivets and/or some bugs at altitude.

Not only did I reduce the rear heat to 1.5" SCAT hose, I also added a 3/4" restictor orifice on the back of the baffles to reduce the air going to the heat muffs. I did this as even with the 1.5" SCAT hose in the back there was still way more heat than I need, and that is with the front heat always off. And yes - the 1.5" hose makes the tunnel easier to work with.

Remember - all those hot days you are flying you are stealing air off the top of the engine just to dump it (after heated) right behind your fuel pump (cabin heat valves are shut) - this reduces the differential pressure for engine cooling air . If you don't need the heat on your coldest day, let the engine have the extra air for cooling on your hottest day.

Side note - I also added a heat deflector over the top of the cabin heat control valves so that this hot air is deflected down to the cowl exit instead of at the engine.

Carl
 
So it was you ;-)

Good thoughts, Carl.

Are you of the opinion that a single heat muff is more than adequate for the -10 in the coldest conditions seen in the lower 48? I've heard that a few times here as well.

Appreciate all the input from those who've gone before.
 
Carl,

I'd like to know your answer about the single heat muff and also would like to see pics of your work that allowed the heat dumped when the valves are closed (i.e. channels the heat down and out). Thanks.
 
Carl,

I'd like to know your answer about the single heat muff and also would like to see pics of your work that allowed the heat dumped when the valves are closed (i.e. channels the heat down and out). Thanks.

I recommend doing both heat muffs. They come with the exhaust and the control valve hole is already in the firewall - and you might have colder conditions than I.

On the cabin heat deflector, I used a piece of this: http://www.aircraftspruce.com/catalog/appages/koolmat.php?clickkey=183585

The Koolmat goes between the cabin heat control valves and the firewall (hold cut out) and extends over the top of the control valves and then drapes down over the front of the valves. This way when the valves are closed the dumped hot air bounces off the Koolmat and down to the cowl exit. Since the Koolmat can be moved you still have access if needed. The Koolmat stays where it is as the engine mount keeps it tucked in.

Carl
 
I have just a single heat muff installed, feeding rear outlets. Pennsylvania based. In three years of all season flying, we were cold just once on a flight back home from Florida in winter, but the OAT was -22 F. Under 99% of your flying conditions, one heat muff is more than adequate. If you think about it, that extra heat muff is robbing engine cooling air and dumping heated air it onto the firewall next to the fuel pump. There are so many things wrong with that picture.......
 
Yes, I simply removed the heat shroud from one of the mufflers, so it is cooled by lower cowl air. The other works as normal. Fabricated a steel cover plate to bolt to the firewall to cover the hole where the valve would go, and used fiberfax sandwiched between the plate and the firewall, well sealed around the edges with high temp RTV, as described by others. Seems to have worked out for the first 300 hours.
 
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I have just a single heat muff installed, feeding rear outlets. Pennsylvania based. In three years of all season flying, we were cold just once on a flight back home from Florida in winter, but the OAT was -22 F. Under 99% of your flying conditions, one heat muff is more than adequate. If you think about it, that extra heat muff is robbing engine cooling air and dumping heated air it onto the firewall next to the fuel pump. There are so many things wrong with that picture.......

So you don't plumb any hot air to come out of the front outlets at all?

I'm wondering if the best compromise, keep-all-options-open scenario wouldn't be to install dual heat muffs plumbed per plans, but with the 3/4" restrictor plates on both baffle-to-muff tubes to reduce standby / wasted airflow when the heat is off.
 
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yeah

I was thinking a Tee inside the tunnel to feed front and back from a lone heat muff (weight savings) but a "Y" would work, too.
 
Maybe a silly question but would it be better to take the muff off one side versus the other ? Being that there is a rear baffle scat input and a front input?
 
As I previously mentioned, recommend keeping both heat muffs but adding a restrictor on the baffle air take off. I have a 3/4" hole in a 2" round piece of .032" aluminum on each baffle flange, held in with aluminum tape. The SCAT hose slides over this. This was a trial and error deal - the 3/4" hole provides me more than enough heat. For your northern folks you might need a little bigger hole.

I suspect it is easier to adjust the restrictor hole than going back to reinstall the second heat muff, control valve and hose.

Carl
 
I have an issue with spacing in the tunnel. I currently have the stock tubing and two AC hoses on standoffs crossing the spar. It turned out nice and snug.

IMG_0077.jpg


Fast forward a year and I'm working on my avionics installation and wiring routing. My problem is I plan to put my headset jacks into my throttle quadrant/arm rest deal from Aerosport. This requires me to run about an inch bundle of shielded cables down the tunnel and up about 6 inches or so rear of the spar. I'm not particularly comfortable shoving those into the remaining space... concerned about chaffing on everything else there.

Reducing my scat tube to 1 inch over the 2 inch stock tube should give me enough space to get the bundle by. I just wanted to check my logic in that getting two 2" to 1." reducers and a length of 1" tubing will do the trick?

Also not worried about not having enough heat either. Flying my buddy's RV-10 I've used the heat twice, for about 20 seconds each time.
 
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Reducer

I used the reducers. The 2" SCAT would kind of fit but it was rubbing against the fuel valve extension and parts of the tunnel wall. It was also a tight fit where it passed over the spar web. The reduction to 1.5" made everything fit just fine.

I'm not flying yet but from most comments, I don't think heat in the cabin will be a problem...
 
Yah now that you mention it, it does squeeze a bit around the fuel valve. I'll try the 1" vs 1.5" and see how that goes. Probably will make a few 1" routing brackets to keep it all in order as well.
 
Headset cables

I have an issue with spacing in the tunnel. I currently have the stock tubing and two AC hoses on standoffs crossing the spar. It turned out nice and snug.

IMG_0077.jpg


Fast forward a year and I'm working on my avionics installation and wiring routing. My problem is I plan to put my headset jacks into my throttle quadrant/arm rest deal from Aerosport. This requires me to run about an inch bundle of shielded cables down the tunnel and up about 6 inches or so rear of the spar. I'm not particularly comfortable shoving those into the remaining space... concerned about chaffing on everything else there.

Reducing my scat tube to 1 inch over the 2 inch stock tube should give me enough space to get the bundle by. I just wanted to check my logic in that getting two 2" to 1." reducers and a length of 1" tubing will do the trick?

Also not worried about not having enough heat either. Flying my buddy's RV-10 I've used the heat twice, for about 20 seconds each time.

I'm running my headset cables on top of the tunnel cover to the armrest location. It will be hidden by the aerosport center panel drop down and the armrest itself. Im leaveing a little slack in them so i can set the armrest off to the side for maintenance access to tunnel if needed. Maybe you can do the same.
 
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