Data point regarding Cabin Heat output versus mixture setting

[Shadrach]

Given the discussion in a previous thread regarding heat output, I decided to grab a few IR readings from my main heat duct in level flight at 4500’.  I took several max heat readings at each mixture setting so I could see the change in heat output between mixture setting  at 2500RPM and 100ROP (~1350°) and 25LOP (1425°). I took the highest reading I could get.
OAT was 45° Baro 30.45. Cowl flaps were closed as we were way above their operation speed of 150MIAS.

100ROP (~1350°)  generated a max output of 192.5°

25LOP  (~1425°) generated a max heater output of 198.4°

Not a huge delta. I would never run unblended max heat as a practical matter. Both settings are way above what I would use in practice.  For a noramally aspirated 4cyl, it appears that small changes in power/mixture settings have little effect on heat output. On the upshot, I now know I could heat water to brew tea inflight.

[carusoam]

Awesome details Ross!

The long Bodies got a modified butterfly valve for the cold air… it is drilled with a few holes to always allow some cold air to pass by…

This keeps a fresh supply of cold air coming in from the outside…

The additional cold air probably lowers the hottest temp by some amount….

 

My M20C didn’t run out of heat until it was winter at 12k’….  That was mostly because the energy produced was lower, AND air leaks in the cabin were too large, and the OAT was close to zero….

 

LBs have their main air supply near the floor at the center console… nylon carpet begins to melt at 215°C, so there is a bit of a safety margin there…. 
 

Now is a good time to fix those air leaks…

-a-

[Ragsf15e]

11 hours ago, Shadrach said:

Given the discussion in a previous thread regarding heat output, I decided to grab a few IR readings from my main heat duct in level flight at 4500’.  I took several max heat readings at each mixture setting so I could see the change in heat output between mixture setting  at 2500RPM and 100ROP (~1350°) and 25LOP (1425°). I took the highest reading I could get.
OAT was 45° Baro 30.45. Cowl flaps were closed as we way above their operation speed of 150MIAS.

100ROP (~1350°)  generated a max output of 192.5°

25LOP  (~1425°) generated a max heater output of 198.0°

Not a huge delta. I would never run unblended max heat as a practical matter. Both settings are way above what I would use in practice.  For a noramally aspirated 4cyl, it appears that small changes in power/mixture settings have little effect on heat output. On the upshot, I now know  I could heat water to brew tea  inflight.

I flew yesterday in Washington, 7500’ at -11c (12f).  I wish I had your ir temp to know, but full unmixed heat was just warm.  I was wearing a thick jacket to stay comfortable.  I’m guessing the OAT says a lot about the end result.

[Hank]

40 minutes ago, Ragsf15e said:

I flew yesterday in Washington, 7500’ at -11c (12f).  I wish I had your ir temp to know, but full unmixed heat was just warm.  I was wearing a thick jacket to stay comfortable.  I’m guessing the OAT says a lot about the end result.

Absolutely! The heater muffler simply adds thermal energy to the air coming into the cowl. Shadrach heated the air from 45° to 195° at full heat, so yours would be cooler but still warm. You also have to consider the continuous airflow through the cabin will have cold outside air push warm air out the back. And of course there is heat loss through the fuselage walls and windows.

The obvious questions are how well sealed is your heat system? Does it lose warm air between the heat cuff on your exhaust and the cabin wall? Is there leakage of warm out from the heater cuff instead of going into the tubing? And how leaky is your fuselage? Some people have reported that the back seat heater hoses are broken, holey or even missing. 

Just a few things to check.

[Bolter]

On my J, I can quickly feel the heat difference when I transition from LOP to ROP.  I have to fiddle with the cool air knob to get back to a comfy mixed setting.  Never measured it directly, but at the vent output, the difference is more than 6F.  That could be differences between J and F heaters, or maybe the metal temperature of the duct you are measuring does not change as much as the air temperature swing.  

[Shadrach]

On 12/16/2021 at 12:17 PM, Bolter said:

On my J, I can quickly feel the heat difference when I transition from LOP to ROP.  I have to fiddle with the cool air knob to get back to a comfy mixed setting.  Never measured it directly, but at the vent output, the difference is more than 6F.  That could be differences between J and F heaters, or maybe the metal temperature of the duct you are measuring does not change as much as the air temperature swing.  

You could be correct that the duct louvers are holding some heat but I don’t think it’s significant.  I waited several minutes after setting the mixture to get a reading. The louvers are aluminum and the volume of airflow through the vent is considerable. The first reading I took was at ~100LOP (1350°, about as lean as I could run with acceptable engine smoothness at 4500’).  The highest output temp I could attain was 181.5°. I didn’t bother to include this in my original post because it’s not a very practical mixture setting. 

What I can derpduce from my test is that heat output is affected by both raw EGT and power setting.  All other things being equal, 25° LOP is certainly a lower power setting than 100°ROP, yet it yielded better heater output.  However, when comparing the same EGT (1350°) on each side of peak 100° ROP yields considerably more heat output than 100° LOP.

 

[Shadrach]

On 12/16/2021 at 11:43 AM, Hank said:

Absolutely! The heater muffler simply adds thermal energy to the air coming into the cowl. Shadrach heated the air from 45° to 195° at full heat, so yours would be cooler but still warm. You also have to consider the continuous airflow through the cabin will have cold outside air push warm air out the back. And of course there is heat loss through the fuselage walls and windows.

The obvious questions are how well sealed is your heat system? Does it lose warm air between the heat cuff on your exhaust and the cabin wall? Is there leakage of warm out from the heater cuff instead of going into the tubing? And how leaky is your fuselage? Some people have reported that the back seat heater hoses are broken, holey or even missing. 

Just a few things to check.

We must also consider the amount of air moving through the heat circuit not just temperature at the outlet. Yesterday was a High pressure day with surface temps below standard. At 4500 I was indicating 148Kts. At say 10,500, the the mass of the air over the exchanger is going to be quite a bit less, so temp alone does not tell the whole story when comparing one altitude to another.

[Bolter]

6 hours ago, Shadrach said:

You could be correct that the duct louvers are holding some heat but I don’t think it’s significant.  I waited several minutes after setting the mixture to get a reading. The louvers are aluminum and the volume of airflow through the vent is considerable. The first reading I took was at ~100LOP (1350°, about as lean as I could run with acceptable engine smoothness at 4500’).  The highest output temp I could attain was 181.5°. I didn’t bother to include us in my original post because it’s not a very practical mixture setting. 
 

What I can derive from my test is that heat output is affected by both raw EGT and power setting.  All other things being equal, 25° LOP is certainly a lower power setting than 100°ROP, yet it yielded better heater output.  However, when comparing the same EGT (1350°) on each side of peak 100° ROP yields considerably more heat output than 100° LOP.

 

I was meaning that there may be a notable difference in air temp versus the duct (metal) temp.  The IR thermometer is seeing surface temps, not the actual air temp.  

-dan

[Shadrach]

1 hour ago, Bolter said:

I was meaning that there may be a notable difference in air temp versus the duct (metal) temp.  The IR thermometer is seeing surface temps, not the actual air temp.  

-dan

Fair point. I think it’s pretty close to as high as is safely possible. It’s already hot enough to burn skin, much hotter and I would expect deterioration of the surrounding interior trim.

[carusoam]

430°F to melt ABS….  Turn it liquid for molding purposes…

But, it does start to soften and lose the shape that was frozen in it during forming…. At a much lower temp…

It can go through its glass transition temp of 220°F

Warm it enough, it will try to go through stress relief…. And take on the flat sheet shape it was before being formed….  
 

polymer memory experiments….

The long chain molecules stretch out when being thermoformed into their final shapes….

Just add heat… you can watch the parts start to wiggle as the long chains relax the locked in stress…
 

Best regards,

-a-

[bmcconnaha]

thats quite a heater.  my J has a decent heater, but nothing like that 

[Ragsf15e]

I’m going flying this Wednesday, forecast high is 9 F on the ground.  Ill try to remember to bring a thermometer to test my heat, but I’m gonna dress warm just in case..,

[Ragsf15e]

It was about 15 degrees F on the ground today.  -22C at 9500’ (that’s real cold).  Heater output was about 75F no matter what I did (ROP or LOP), but my temp gage was admittedly low tech.  Beautiful day to fly but really glad I had lots of layers on!  My heater isn’t perfect, but it’s actually aligned/working pretty good with the butterfly valve.  A tiny bit of cold air leaks in the vent with both heat and vent off.  
 

She climbed like a homesick angel with just me and 30 gallons of fuel at that temp!

[Shadrach]

1 hour ago, Ragsf15e said:

It was about 15 degrees F on the ground today.  -22C at 9500’ (that’s real cold).  Heater output was about 75F no matter what I did (ROP or LOP), but my temp gage was admittedly low tech.  Beautiful day to fly but really glad I had lots of layers on!  My heater isn’t perfect, but it’s actually aligned/working pretty good with the butterfly valve.  A tiny bit of cold air leaks in the vent with both heat and vent off.  
 

She climbed like a homesick angel with just me and 30 gallons of fuel at that temp!

Beautiful bird, panel and scenery! I think your heater circuit needs some attention. We have ground temps in the teens here as well and I can say without question that heater output remains strong. I have no recollection of ever leaving it in the unblended position  beyond the first few minutes of flight.

Edited December 29, 2021 by Shadrach

[Ragsf15e]

15 minutes ago, Shadrach said:

Beautiful bird, panel and scenery! I think your heater circuit needs some attention. We have ground temps in the teens here as well and I can say without question that heater output remains strong. I have no recollection of ever leaving it in the unblended position  beyond the first few minutes of flight.

I agree, I’m gonna open up the air exchange box next month during annual.  I’ve adjusted it before and it’s pretty easy but never perfectly sealed.  Is yours the butterfly valve type?  Mine is a ‘68.

[Shadrach]

20 hours ago, Ragsf15e said:

I agree, I’m gonna open up the air exchange box next month during annual.  I’ve adjusted it before and it’s pretty easy but never perfectly sealed.  Is yours the butterfly valve type?  Mine is a ‘68.

I believe my 67 is also a butterfly valve. I have never had it apart nor needed to adjust it. The hoses have obviously been replace many times.  I would also check the outer casing of the heat exchanger. Ours may not be completely air tight but you’d never know it to look at it. 

[carusoam]

For reference…

65s got lots of updates… but, cabin air controls were still slide gates…

Butterflys are much nicer… slides need more attention than just annual lubricating….

-a-

[Shadrach]

Circling back around on this at colder temps. OAT does obviously affect output, but not as much as I thought (may be of interest to @Ragsf15e). At OATs in the single digits, output was still very robust. All of the settings felt quite hot and were more than adequate to keep me comfortable while wearing a T-shirt and lightweight fleece.

OAT 7°F

 

25LOP Max heater output 172.7°

100ROP Max heater output 157.2° 

Peak Max heater output 174°.

My little 200hp bird really tries to overachieve in these chilly temps despite a governor that’s holding 50rpm shy of redline.

 

[Ragsf15e]

56 minutes ago, Shadrach said:

Circling back around on this at colder temps. OAT does obviously affect output, but not as much as I thought (may be of interest to @Ragsf15e). At OATs in the single digits, output was still very robust. All of the settings felt quite hot and were more than adequate to keep me comfortable while wearing a T-shirt and lightweight fleece.

OAT 7°F

 

25LOP Max heater output 172.7°

100ROP Max heater output 157.2° 

Peak Max heater output 174°.

My little 200hp bird really tries to overachieve in these chilly temps despite a governor that’s holding 50rpm shy of redline.

 

If you could convert some of this heat into thrust, you’d be 10 knots faster!  (And need a heavy coat).

[Shadrach]

12 hours ago, Ragsf15e said:

If you could convert some of this heat into thrust, you’d be 10 knots faster!  (And need a heavy coat).

She was doing just fine.  Calculated TAS of 156kts at 100ROP. Plane was light (~2100lbs) and it was cold. Possible my ASI is a touch optimistic, can’t say for sure.

 

 

 

 

Edited January 12, 2022 by Shadrach

[Ragsf15e]

56 minutes ago, Shadrach said:

She was doing just fine.  Calculated TAS of 156kts at 100ROP. Plane was light (~2100lns) and it was cold. Possible my ASI is a touch optimistic, can’t say for sure.

 

 

 

 

Yeah that’s a good speed!  Cold for 6500’ too.