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Cabin heat dumping under cowl

J

Jettison

Member
Not a very good descriptor, I know. Let me try to explain.

We are doing the first condition inspection on my RV-7 since I bought it already flying. My mechanic does not love the fact that when the cabin heat is not in use for the cabin, the hot air dumps into the space in front of the firewall, about half way up. He is concerned that this is a source of more hot air into this section that we would prefer be cooler.

Is this a problem? If so, is there a fix?
 
The air dumping onto the firewall is not hot, it’s warm. If it’s not too hot to be used for cabin heat then it’s not a problem for it to be vented to the firewall. Is it ideal for the summer, no, but not really a problem. You could attach a 30 to 45 deg bent alum below the exit to deflect it off the firewall
 
How is that any different than the hot air coming off the exhaust pipes? It’s the same heat, much cooler at that point when it uses cold air across exhaust and then down a long scat tube.
 
Jettison said:
Not a very good descriptor, I know. Let me try to explain.

We are doing the first condition inspection on my RV-7 since I bought it already flying. My mechanic does not love the fact that when the cabin heat is not in use for the cabin, the hot air dumps into the space in front of the firewall, about half way up. He is concerned that this is a source of more hot air into this section that we would prefer be cooler.

Is this a problem? If so, is there a fix?
Click to expand...

One data point is that this is the same as what is done on lots of different standard certification airplane models that your mechanic may have also worked on?
 
I would worry a bit about the experience of your A&P in particular on experimental airplanes. As others have said there are alone 10k+ RVs flying like that. I am surprised that is what he is concerned about.

Oliver
 
Scat tube attached to the divider box outlet and have the end near the outlet of the cowl. Has worked well for 34 years, your results might be different.
 
Jettison said:
Not a very good descriptor, I know. Let me try to explain.

We are doing the first condition inspection on my RV-7 since I bought it already flying. My mechanic does not love the fact that when the cabin heat is not in use for the cabin, the hot air dumps into the space in front of the firewall, about half way up. He is concerned that this is a source of more hot air into this section that we would prefer be cooler.

Is this a problem? If so, is there a fix?
Click to expand...

Maybe you could ask him why he feels this is a problem first. Ask why the air in that area needs to be cooler. Following this basic logic, it would be even worse to send that hot air into the cabin.

There are a least a few square feet of exhaust pipe surface area snaking all around the lower cowl area. The heat sheds off this pipe creating hot air that goes right past the area in question. The lower cowl is simply a hot and unpleasant environment no matter how you cut it. It is my speculation that the area with the heater valves is going to be at the same temperature whether they are there or not. If anything, they are cooler because the heater valves have more air movement via the scat connecting them to the higher upper cowl pressure.
 
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lr172 said:
Maybe you could ask him why he feels this is a problem first. Ask why the air in that area needs to be cooler.
Click to expand...

Plus one on this. Lots of bright ideas out there that just need to be articulated in order to get past our preconceived notions. Of course, this idea might just be silly once fully described.
 
On both RV-10 builds I did not like the cabin heat (when not in use) being dumped right at the firewall. This configuration provided a bad path for this dumped hot air to bounce off the firewall directly toward the mechanical fuel pump.

I did two things:
- Installed a 3/4” restrictor on the rear baffle SCAT hose flanges. The RV-10 has excess cabin heat so why steal cooling air off the engine just to dump it and increase the back pressure in the lower cowl area?
- Between the firewall and cabin heat flapper valves I installed a sheet of KoolMat: https://www.aircraftspruce.com/catalog/appages/koolmat.php
The mat goes between the flapper valves then up over the top of the flapper valves and drapes over the front of the valves. The drape redirects the dumped cabin heat air toward the cowl exit. I also believe the layer of mat insulates the tunnel from the firewall heat created by cabin heat being dumped.

Side note - the new owner of the first RV-10 needed to increase the size of the 3/4” restrictors for very cold flying conditions (but he is a Florida boy…).

Carl
 
There is a huge amount of high velocity air being evacuated from the bottom of the cowl. I suspect the heat added to this air flow from the diverted cabin heat is barely enough to wiggle the needle.
 
Thank you all for your responses. I mentioned to the mechanic that it seems to me that there is a lot of hot air coming off the exhaust into the cowl anyway, I wouldn’t think it would make much difference.

I don’t think this likely rises to the level of great importance, but I did since find (I think) a part from Vans that is an enclosed box (Vent DL-01 or Vent DL-01SS) which houses the door and three openings, one for warm air into the box, one for air into the cabin, one for diverted warm air to connect to a scat tube to be vented overboard.
 
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Hot Air

My remote oil cooler is mounted on the left lower area of the firewall and the heater box outlet flow is pointed in that direction. I bent up a simple little box out of thin stainless to deflect this air straight down toward the cowl exit.

Don Broussard
RV9 Rebuild in Progress
57 Pacer
 

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All I can say it this is a ??solution?? that is looking for a problem that does not exist.
I have burnt my hands on the exhaust of a number of engines. go figure, never burnt my shoes in the plane.
WOW, My luck varies Fixit
 
[email protected] said:
All I can say it this is a ??solution?? that is looking for a problem that does not exist.
I have burnt my hands on the exhaust of a number of engines. go figure, never burnt my shoes in the plane.
WOW, My luck varies Fixit
Click to expand...

Hmmm, on my previous RV4 with an O360 on a warm summer day running 75% power for any length of time my feet would cook. Certainly uncomfortably hot.

Tim
 
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