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  #1  
Old 08-24-2017, 07:50 AM
Bill Boyd's Avatar
Bill Boyd Bill Boyd is offline
 
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Location: Landing field "12VA"
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Default Tunnel heat SCAT tubing size

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?
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RV-10 - N130YD reserved - under construction

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  #2  
Old 08-24-2017, 07:54 AM
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Auburntsts Auburntsts is offline
 
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I didn't have any issues with the stock tubing--seems like a solution in search of a problem to me.
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  #3  
Old 08-24-2017, 08:23 AM
jwilbur jwilbur is offline
 
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I also had no problems just following the plans.
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  #4  
Old 08-24-2017, 08:32 AM
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Location: Dallas area
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Quote:
Originally Posted by Bill Boyd View Post
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.
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  #5  
Old 08-24-2017, 08:57 AM
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Quote:
Originally Posted by Mel View Post
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.
\

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

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.
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RV-10 - N130YD reserved - under construction

donating monthly to the VAF - thanks, Doug
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  #6  
Old 08-24-2017, 10:00 AM
BobTurner BobTurner is offline
 
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The problem was substantially improved circa 2009, when there was a design change in the location of the fuel valve.
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  #7  
Old 08-24-2017, 10:18 AM
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Bill Boyd Bill Boyd is offline
 
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Default 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.
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RV-10 - N130YD reserved - under construction

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  #8  
Old 08-24-2017, 10:30 AM
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rleffler rleffler is offline
 
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Quote:
Originally Posted by Bill Boyd View Post
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.
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  #9  
Old 08-24-2017, 10:46 AM
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Bill Boyd Bill Boyd is offline
 
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Default 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

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...
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RV-10 - N130YD reserved - under construction

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  #10  
Old 08-24-2017, 10:51 AM
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Carl Froehlich Carl Froehlich is offline
 
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Default

Quote:
Originally Posted by Bill Boyd View Post
\

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

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
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