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Heater Vent Valve

tgmillso

Well Known Member
Sponsor
Hi All,

I was vacantly staring at my firewall today waiting for the last of the required parts before I lift the engine in place when I realized that there was potentially a massive heat dump starting me right in the face.
With the heater vent valve closed, the air is simply ducted into the back of the engine bay, where you have temperature sensitive items such as a lithium battery, magnetos or electronic ignition, not to mention that on hot days all you are doing is heating the firewall, which is actually the last thing you want to be doing. What will probably make things worse is that I have an outlet duct/ramp planned to improve exist flow and ensure adequate engine cooling on hot days, but the downside of this will be that this air against the firewall will have minimal air circulation apart from the heater duct pumping a scorching hot blast into the area.
I'm thinking that a possible solution is riveting a 2" diameter stub to the outlet of the heater bypass, then ducting this air through a scat tube into the exit airflow before it leaves the cowling.
I continually read on this forum about equipment overheating at the back of the engine or mounted near the firewall. Ashley Miller's RV-10 has firewall insulation, but you will see in the uninsulated sections (engine mount bolts etc) the temperatures are still hitting 60 degrees (see pictures in the link below).
https://photos.google.com/share/AF1...?key=Y3p5MnhtM1hGeHBKSlR1cS1PQXV0MVZXcWVPandn
Am I way off the reservation here, or is this something that should be addressed whilst I have access and the possibility to do so?
Tom.

2ur9zzn.jpg
 
That's exactly what I did...I just routed the hot air to the exhaust exit area. I can't swear that it makes a difference, but it makes me feel better :D
 
Thanks Mark. I think I will give it a crack. I found the flanges on Aircraft Spruce for $7. They're only aluminum, however this doesn't really matter in this case. I'll give it a run with the exit duct on, then run it with the duct off and will report back with the temperature differentials. I payed way too much for this EarthX battery to be unnecessarily slow roasting it.
Tom.
 
Thanks Mark. I think I will give it a crack. I found the flanges on Aircraft Spruce for $7. They're only aluminum, however this doesn't really matter in this case. I'll give it a run with the exit duct on, then run it with the duct off and will report back with the temperature differentials. I payed way too much for this EarthX battery to be unnecessarily slow roasting it.
Tom.

FWIW I?ve never had any issues with temperatures in the cowl in over 700hrs of all sorts of environmental conditions around the country. I became mildly paranoid about my PMAG when I installed if at 350h and put a RTD sensor on it linked to my EFIS.
It rarely goes over 110F in flight. However heat soaking on shutdown sees it rise rapidly. IMHO that?s going to be the main issue for your battery. In flight temps on the firewall (at least in the 7) are a non issue.
Also, the actual amount of heat transferring into the heater air from the heat muffs isn?t that great at cruise speeds either. A common RV complaint is not enough heat.

Cheers
 
This is one of the big problems with the RV-10 cabin heat valves location. I used a piece of this between the valves and the firewall (holes cut out for the valves), leaving it long on the top so that it draped over the valves. This way the dumped air was directed away from the engine driven fuel pump and down toward the cowl exit. http://www.aircraftspruce.com/catalog/appages/koolmat.php?clickkey=55063

Also considering the heat muffs in an RV-10 are way more than needed, I put a 3/4? office on each baffle heat muff SCAT tube flange (a piece of aluminum cut to fit the edge of the flange, 3/4? hole drill in it, held in place with some aluminum tape on the edge, and the SCAT hose slid over the top).

This way on the hot day you are not stealing all that air from the top of the engine just to dump it as well as decrease the differential pressure for engine cooling air, and there is still more cabin heat than I?ll ever use.

Carl
 
Hi Richard. Thanks for your real world experience with this. There's nothing quite like having some solid data.
Carl, you bring up a good point about wasted cooling air running through this tubing. I had originally planned on blocking off the outlet to create dead air in the SCAT tubing, then I thought that doing so would cause adverse heat buildup in the vacinity of the muff and also mean that the heater vent could only be fully open or closed, as there would be no bypass to split the flow into. A cover on the inlet side would make more sense, but I just didn't want another Bowden cable or servo if I could get away from it.
My concern is that with this exit air ramp that I create a bunch of dead air in front of the firewall, and even Emag Air warn about this in their installation notes. I'd ditch the exit ramp, but I've been told by some experienced builders that if you want to be operating in the heat of summer here, then it's a worthwhile investment, not to mention the fact that I've already riveted the attachment flanges for it to the firewall. Perhaps I'll start with blast tubes onto the EarthX battery and the PMags and monitor the temperatures and see how things progress.

Tom.
 
SNIP......
Perhaps I'll start with blast tubes onto the EarthX battery and the PMags and monitor the temperatures and see how things progress.

Tom.

A blast tube is required for each pMag.

I?m not a fan of mounting any battery on the engine side of the firewall as it is bad environment for what has become a critial flight component. For that matter a single battery IFR airplane exceeds my risk tolerance. So - you are on your own for that.

Carl
 
Fully appreciate your concern about the battery operating environment. I'm still thinking of rebuilding my battery holder so that it is an insulated vessel with blast tubing. This should also stop the heat soaking on the ramp, as the core temperature will be super low and it will take time for the engine bay heat to soak through after shutdown. Right now I just have 16 thou radiation shields on standoffs around the stock EarthX battery box.
Tom.
 
Waste heat muff air dumping into the engine compartment has be a standard part of light aircraft design for 60+ years.

The volume of hot air exiting the heat valve is small when compared to the amount of hot air being dumped through the cowl after passing through the cyl. fins.
It is critical that there always be airflow through the heat muff(s) to prevent the muff(s) and the exhaust system an that area from getting over heated.

In my opinion there is little to be gained (other than weight , cost, and build complexity) by adding a duct to out the heat valve bypass air overboard.

It is good practice to avoid that air dumping directly on heat sensitive items.
 
Waste heat muff air dumping into the engine compartment has be a standard part of light aircraft design for 60+ years. SNIP

Yep - you are correct. It is also reasonable to point out that wasted cooling air translates to needless engine cooling drag across the operating envelope. I am one of those ?get the most speed out of every drop of fuel? guys so I worry about such things.

Carl
 
Yep - you are correct. It is also reasonable to point out that wasted cooling air translates to needless engine cooling drag across the operating envelope.

Carl

I agree it adds some drag but I wouldn't call it needless. If you are going to heat the cabin with a system that uses heat muffs, you have to keep the muffs cool.
If you use something to direct the waste cooling air outboard you are controlling where it goes but not likely having much influence on the drag that is produced.
 
It is critical that there always be airflow through the heat muff(s) to prevent the muff(s) and the exhaust system an that area from getting over heated.

Empirical data from multiple installations, including mine, have shown that this is not always the case. My aircraft uses a slide valve from Tom Berge, as does Tom's aircraft as do quite a few other aircraft I know of. Almost 900hrs and no issues to date on mine, and others have experienced similar reliability.
 
Am I way off the reservation here, or is this something that should be addressed whilst I have access and the possibility to do so?
Tom.

Yes I think you are worried about airflow that just isn't going to happen. Normally the heated air travels through your firewall valve from the the engine compartment in to the cockpit since the engine area (firewall forward) is at a higher pressure than the cockpit. Air always moves from high to low pressure regions.

Since that valve just opens the engine area in to the same engine area, there won't be airflow from the heat muff to the valve. The two areas are at approximately the same pressure.

Andrew Anunson
 
Empirical data from multiple installations, including mine, have shown that this is not always the case. My aircraft uses a slide valve from Tom Berge, as does Tom's aircraft as do quite a few other aircraft I know of. Almost 900hrs and no issues to date on mine, and others have experienced similar reliability.

I think the danger with this type of information is that there is no specifics provided.
We can all agree that the surface temp of the pipe adjacent to the cyl head is quite different than what it is where the pipe exits the cowl.
So, depending on where a muff is located along the length of the pipe could have a major effect on the actual temp when the air is static in the system.
 
Yes I think you are worried about airflow that just isn't going to happen. Normally the heated air travels through your firewall valve from the the engine compartment in to the cockpit since the engine area (firewall forward) is at a higher pressure than the cockpit. Air always moves from high to low pressure regions.

Since that valve just opens the engine area in to the same engine area, there won't be airflow from the heat muff to the valve. The two areas are at approximately the same pressure.

Andrew Anunson

DanH's might with the closed cowl flap, but others? (yes, I know you don't have one) Are you sure for the lower cowl, data? Sounds like an exhaust contamination problem.
 
Yeah. Darn shame eh?

Yes it is.

I chalk it up as one of the engineering trade-offs we make balancing benefit against simplicity.

Similar to the decision of putting retractable gear on an RV. The performance benefit would be small so the cost and complexity is not worth it.

But I am open to ideas for a great performing heat source that is guaranteed to work for any installation, but requires no constant system flow.
 
Yes I think you are worried about airflow that just isn't going to happen. Normally the heated air travels through your firewall valve from the the engine compartment in to the cockpit since the engine area (firewall forward) is at a higher pressure than the cockpit. Air always moves from high to low pressure regions.

Since that valve just opens the engine area in to the same engine area, there won't be airflow from the heat muff to the valve. The two areas are at approximately the same pressure.

Andrew Anunson

Not true Andrew

The traditional system has the heater air sourced from the high pressure side (top of engine) of the cooling plenum.
Air moves through the system when the valve is in the off position, they same as it does through the engine cyl.
 
I agree it adds some drag but I wouldn't call it needless. If you are going to heat the cabin with a system that uses heat muffs, you have to keep the muffs cool. SNIP

Scott,

While some constant heat muff airflow is nice to prolong the life of the attached SCAT hose, I do not believe the heat muff itself is going to melt without cooling air. The heat muffs I use are made of Stainless Steel, just like the exhaust pipes. For that matter since I believe no one mounts heat muff at the cylinder exhaust port, the exhaust pipe temperature at the heat muff is much lower than at the exhaust port.

So as all things go, there are extremes that bound the problem. No heatmuff flow is probable not good for the SCAT hose. Too much heatmuff flow is a waste of engine cooling air - and for that matter not a great way to heat the cabin. Heat to the cabin is a function of pounds mass of air and temperature. The more airflow, the less temperature rise across the heat muff. Those interested students out there can take the data then do the equations to find the airflow that transfers the most heat to the cabin.

As I previously posted, a 3/4? orifice to reduce the heat muff airflow in the RV-10 still provides far more cabin heat that I?ll ever use. For the RV-8A single heat muff install, I increased the heat transfer area in the muff to get the added cabin heat - as well as reduce the airflow though the muff (less pounds mass of air but higher temp rise across the muff). 900+ hours in the RV-8A install and all is well. I?ll do the same for the new RV-8 project.

Carl
 
Scott,

While some constant heat muff airflow is nice to prolong the life of the attached SCAT hose, I do not believe the heat muff itself is going to melt without cooling air. The heat muffs I use are made of Stainless Steel, just like the exhaust pipes. For that matter since I believe no one mounts heat muff at the cylinder exhaust port, the exhaust pipe temperature at the heat muff is much lower than at the exhaust port.

So as all things go, there are extremes that bound the problem. No heatmuff flow is probable not good for the SCAT hose. Too much heatmuff flow is a waste of engine cooling air - and for that matter not a great way to heat the cabin. Heat to the cabin is a function of pounds mass of air and temperature. The more airflow, the less temperature rise across the heat muff. Those interested students out there can take the data then do the equations to find the airflow that transfers the most heat to the cabin.

As I previously posted, a 3/4? orifice to reduce the heat muff airflow in the RV-10 still provides far more cabin heat that I?ll ever use. For the RV-8A single heat muff install, I increased the heat transfer area in the muff to get the added cabin heat - as well as reduce the airflow though the muff (less pounds mass of air but higher temp rise across the muff). 900+ hours in the RV-8A install and all is well. I?ll do the same for the new RV-8 project.

Carl

I have little concern for the SCAT hose and not much for the muff because there is generally airflow over the exterior surfaces.
For the same reason that Larry Vetterman has always told people to not use insulating wrap on his exhaust systems, I am most concerned for excessive temp of the pipe itself, being insulated by the muff and static air on the inside.
Particularly on a muff mounted near the fwd (hottest) end of the pipe.
 
Scott?s point is it?s important to allow cooling air flow over the exhaust pipe which is enclosed by the heat muff.
 
Scott?s point is it?s important to allow cooling air flow over the exhaust pipe which is enclosed by the heat muff.

I think his point is that it depends on where the muff is. The muffs on the lower portion of the Vetterman crossover exhaust are fine after almost 900 hours as are the pipes. This is on an IO360, fwiw. The vans exhaust with the muff up front might be a different thing given its proximity to the exhaust ports.
 
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