Carb Heat Effectiveness and RPM Drop?

[Ed_Wischmeyer]

On my RV-9A (O-320), there is no noticeable RPM drop when I apply carb heat. On the other hand, the one time I had carb ice in the air, flying in and out of cumulus clouds on a summer day at maybe 6,000 feet or so, carburetor heat cleared it up immediately.

What's the body of evidence on RV carb heat? Has anybody ever measured the temperature increase and compared it with factory built airplanes? Any anecdotal evidence?

 

[dbaflyer]

Not sure the theory but my O-320 RV9A drops 40-50 RPM when tested during run ups

 

[Freemasm]

The hotter the day, the lower the drop obviously. A summer day in FL would barely show any sometimes.

If there is concern, don’t waste your time with secondary indications. Install a carb temp probe and indicator and actually know your systems effectiveness.

 

[RV6_flyer]

Carb heat on our RV does not meet any temperature rise or RPM requirements unless you used a carb heat system from a certificated aircraft. Each one will be different and one should not expect to see the same RPM drop that you have on a certificated aircraft. IF you have a constant speed prop like I do, you will not see any RPM drop with carb heat. You may or may not see a MAP change on the ground with carb heat on. I do see a MAP drop in cruise flight with carb heat on.

Looked for an FAA requirement that I saw 30 years ago about a requirement for carb heat to have a 100 degree F temperature rise on certificated aircraft but cannot find it. I have CAT so I know that the carb heat on my RV does NOT raise the temperature that much.

 

[Dugaru]

On my RV-9A (O-320), there is no noticeable RPM drop when I apply carb heat.

Same with my O-320 -9A. The only thing I ever noticed with carb heat was when the knob wouldn’t operate - turned out the hinge for the carb heat door was broken. The original design of that area was not Vans greatest moment.

 

[riobison]

Same with my O-320 -9A. The only thing I ever noticed with carb heat was when the knob wouldn’t operate - turned out the hinge for the carb heat door was broken. The original design of that area was not Vans greatest moment.

I have an O320 in my 9A and there is no heat muff. It draws the warm air from in the cowel. I see a 14 deg rise in temp when I apply my carb heat. I bought the plane this way and it has been this way since 2012 and 550 hrs. Will it work when needed? That is a good question and one that I should address this winter while I'm not flying.

Has anyone else seen this and have any actual hands on experience?

Thanks
Tim

 

[Scott Hersha]

It might depend on what type carb heat muff you have on your exhaust pipe(s). I’ve had carbs on 5 of my 6 RV’s and 4 of those 5 carbed birds had the original Vans half open tube strapped to one exhaust pipe on the Vetterman exhaust system. Those ones showed no RPM drop during run up when checking carb heat. Two of those had a carb temperature probe, one on a GRT EFIS/EIS system and one on a Garmin system. Neither of those two with carb temp indications worked. Carb temp didn’t change when checking carb heat. In fact, they usually indicated a negative temperature, even in the summer when it was 90* outside and the airplane was sitting on the ground running. However, my new airplane, an RV6, has the newer carb heat/cabin heat muff (shown below), and my carb heat check on run up shows a 60-80 RPM drop, so it is definitely doing something. I don’t have a carb temp probe on this one because I just haven’t had good luck with them. That being said, I’ve never had carb ice on an RV, and I use carb heat when in potential icing conditions, so even the original open tube CH pipes are probably working. Normal updraft Lycomings don’t have as much propensity for carburetor ice as some other engines, but it does happen, so built per plans and operated IAW Lycoming recommendations you should be safe.

 

[riobison]

It might depend on what type carb heat muff you have on your exhaust pipe(s). I’ve had carbs on 5 of my 6 RV’s and 4 of those 5 carbed birds had the original Vans half open tube strapped to one exhaust pipe on the Vetterman exhaust system. Those ones showed no RPM drop during run up when checking carb heat. Two of those had a carb temperature probe, one on a GRT EFIS/EIS system and one on a Garmin system. Neither of those two with carb temp indications worked. Carb temp didn’t change when checking carb heat. In fact, they usually indicated a negative temperature, even in the summer when it was 90* outside and the airplane was sitting on the ground running. However, my new airplane, an RV6, has the newer carb heat/cabin heat muff (shown below), and my carb heat check on run up shows a 60-80 RPM drop, so it is definitely doing something. I don’t have a carb temp probe on this one because I just haven’t had good luck with them. That being said, I’ve never had carb ice on an RV, and I use carb heat when in potential icing conditions, so even the original open tube CH pipes are probably working. Normal updraft Lycomings don’t have as much propensity for carburetor ice as some other engines, but it does happen, so built per plans and operated IAW Lycoming recommendations you should be safe.

There is no heat muff, there is no hose. I open the carb heat and it draws warm air from under the cowl.

 

[Scott Hersha]

There is no heat muff, there is no hose. I open the carb heat and it draws warm air from under the cowl.

That might work, but if it does, I would think that you don’t even need any carb heat. The air temp in the forward part of your lower cowl area isn’t that warm - air from above is moving toward the exit with only cylinder fin heat from the cylinder(s). If that’s the case then exhaust carb heat air isn’t really necessary on a Lycoming. Lycoming didn’t design it that way in any certified airplane. I’ve only used carb heat in flight during icing conditions, which has been practically never. I would rather never say never, and in the event of a possible carb icing event, I’d like to have some form of heat that will help me resolve the situation. Not sure 200* cowl area heat will help much after circulating through your airbox and filter. My exhaust pipes are probably 1200* or maybe more. But……. possibly with a Lycoming - carb mounted on the warm oil pan - doesn’t need any carb heat. Your choice. There isn’t much evidence on carb ice failures on Lycoming powered RV’s.

 

[riobison]

That might work, but if it does, I would think that you don’t even need any carb heat. The air temp in the forward part of your lower cowl area isn’t that warm - air from above is moving toward the exit with only cylinder fin heat from the cylinder(s). If that’s the case then exhaust carb heat air isn’t really necessary on a Lycoming. Lycoming didn’t design it that way in any certified airplane. I’ve only used carb heat in flight during icing conditions, which has been practically never. I would rather never say never, and in the event of a possible carb icing event, I’d like to have some form of heat that will help me resolve the situation. Not sure 200* cowl area heat will help much after circulating through your airbox and filter. My exhaust pipes are probably 1200* or maybe more. But……. possibly with a Lycoming - carb mounted on the warm oil pan - doesn’t need any carb heat. Your choice. There isn’t much evidence on carb ice failures on Lycoming powered RV’s.

I agree Id hate to find out that it doesn't work when I need it the most. Only 3 or 4 times have I had carb ice and its been on the ground and discovered during my run up and never in this plane. And never in flight at least that I have known about.

In the Vans AC, does anyone know what they are doing? Is it with a heat muff or with out? We had some ice forming on the ground with Mike in the Vans RV9A. Turning the heat on, ran a little rougher and cleared up. High humidity morning.

Anyone with a Heat Muff, how much of a temp climb do you see when applying the carb heat?

For me it was a 14 deg rise over a minute or 2 while in flight.

Tim

 

[rockwoodrv9]

I have a temp sensor on my carb. I have flown in some pretty cold temps and while in flight I have never been below 110 degrees F in flight and 89 degrees after landing in 12 degree temp. I do get a slight RPM drop but not significant when pull the knob. I watch the temp gage and if it starts going up when I pull it, that tells me it is working. I have not seen how much temp it will go up - just that it is going up. Maybe I am doing it wrong?

On my check list for landing, I have check carb temp. The temp has never been low enough to pull it on. Am I crazy to think if the carb is 100+ degrees that I dont need to worry about carb ice? I have an O-320.

 

[Scott Hersha]

I have a temp sensor on my carb. I have flown in some pretty cold temps and while in flight I have never been below 110 degrees F in flight and 89 degrees after landing in 12 degree temp. I do get a slight RPM drop but not significant when pull the knob. I watch the temp gage and if it starts going up when I pull it, that tells me it is working. I have not seen how much temp it will go up - just that it is going up. Maybe I am doing it wrong?

On my check list for landing, I have check carb temp. The temp has never been low enough to pull it on. Am I crazy to think if the carb is 100+ degrees that I dont need to worry about carb ice? I have an O-320.

My opinion - if your indicated carb air temp is 100+ on a cold day, I wouldn’t trust it. It should be reading a colder than OAT because of evaporative cooling in your carburetor throat. The fact that you’re getting an RPM drop when checking carb heat tells me it must be supplying some carburetor heat - that’s good. Tells me your carb heat is working, but maybe your carb temp probe isn’t working properly. If you are flying or landing in a possible carb ice environment, you should use carb heat in my opinion. What can it hurt? The air through your engine is still being filtered. If you go around and forget to remove carb heat, unless you are in a high DA environment with obstacles, it probably won’t matter, but follow your checklist when you catch up.

 

[rockwoodrv9]

My opinion - if your indicated carb air temp is 100+ on a cold day, I wouldn’t trust it. It should be reading a colder than OAT because of evaporative cooling in your carburetor throat. The fact that you’re getting an RPM drop when checking carb heat tells me it must be supplying some carburetor heat - that’s good. Tells me your carb heat is working, but maybe your carb temp probe isn’t working properly. If you are flying or landing in a possible carb ice environment, you should use carb heat in my opinion. What can it hurt? The air through your engine is still being filtered. If you go around and forget to remove carb heat, unless you are in a high DA environment with obstacles, it probably won’t matter, but follow your checklist when you catch up.

Thanks Scott. I am a reasonably new pilot with about 300 hours and have no ego on my knowledge or ability! I wondered about the temp sensor so I used a pretty sophisticated instant read therm with the cowl off and both were within a few degrees. With the oil sump on top of carb and all the exhaust pipes around it, it is a pretty hot part of the engine.

As you said, there is no harm in running carb heat on landing so I will give it a shot next time I go up and record the temps. It is carb ice season here most of the year. The blue tube coming off the heat muff is where the carb heat comes from on my plane. Not sure if it is the same on others. I have a non typical exhaust so maybe that helps keeping the carb warm. It sure doesnt help much with the cockpit heat!!

 

[rvbuilder2002]

I agree Id hate to find out that it doesn't work when I need it the most. Only 3 or 4 times have I had carb ice and its been on the ground and discovered during my run up and never in this plane. And never in flight at least that I have known about.

In the Vans AC, does anyone know what they are doing? Is it with a heat muff or with out? We had some ice forming on the ground with Mike in the Vans RV9A. Turning the heat on, ran a little rougher and cleared up. High humidity morning.

Anyone with a Heat Muff, how much of a temp climb do you see when applying the carb heat?

For me it was a 14 deg rise over a minute or 2 while in flight.

Tim

The RV-9A demonstrator has the Van's produced exhaust system with the large heat muff on both of the cross over pipes.

I have the original engineering prototype of this system on my O-360 RV-6A. It produces about a 50 RPM drop at run-up RPM.
I don't know what the temp rise is but it has quickly cleared the light icing that occurred on the ground maybe twice in the ten tears or so that the system has been installed.
I have developed a muscle memory such that I use it on every landing regardless of what the conditions are.

 

[bardample]

If you want to check your Carby temp gauge accuracy,look at the carby temp on your EFIS prior to start up.
If CHTs EGTs OAT and Carby temps are all within a couple of degrees , you should be able to believe your in flight reading, even if your engine doesnt run rough

 

[Dan 57]

My 2ct...
The RPM drop will be heavily dependant upon which type of muff is installed. The one installed on my baby pictured below is certainly more a carb warmer, or alternate air. The RPM drop is insignificant at around 10-20. In view of this and the text below, the carb heat operation is only checked at every 50h inspection.

Beware, the observations as related below apply specifically to my airplane as equipped with said muff and the standard Van's FAB.
I've now flown my -6.9i for more than 1'500hrs and encountered light, and medium airframe icing. A single encounter with what was probably severe icing had the engine sputter after a short while. Pulling the carb "heat" knob and the engine returned to normal ops immediately, so the cause was probably an iced clogged snout, and pulling that knob restored air supply to the carb/engine.
Other than that, I've never had any sign of carb ice during operations, so basically never use carb heat during flight or ground ops.

 

[penguin]

I flew in North Texas for 5 years with a carbed O-320 and had carb ice perhaps once or twice. I fitted a carb temp probe which said the only time the sarb was cold was after starting until the oil warmed. After that the carb temp was always well above freezing. But the atmosphere was dry and we were very rarely in carb ice territory. The was fortunate as the standard Vans carb heater is not at all effective.
When I moved back to England I fitted a 6" long heat muff that was certainly better. But not as good as certified installations. Carb temp gauge continued to say carb was warm enough most of the time and I only ever had any icing on start up on a damp morning usually on grass airfields. After I fitted an EFIS I don't recall getting a carb temp warning while flying.
My conclusion was a carb temp sensor/gauge was invaluable. A reasonable carb heater is very useful. In a maritime climate it is not possible to avoid carb icing conditions all the time where it may be in the middle of the US. Therefore where I flew it was important to have an reliable way to melt the ice. If the carb temp dropped toward 32 degrees I pulled the carb heat knob often for 30s at a time (at least).

 

[riobison]

I flew in North Texas for 5 years with a carbed O-320 and had carb ice perhaps once or twice. I fitted a carb temp probe which said the only time the sarb was cold was after starting until the oil warmed. After that the carb temp was always well above freezing. But the atmosphere was dry and we were very rarely in carb ice territory. The was fortunate as the standard Vans carb heater is not at all effective.

I'm not sure I would agree on Vans Carb Heater not being effective. I would agree it may be questionable based on what we have experienced from Certified AC and my self included when I initially started flying a Vans.

I do know the Vans AC with the Heat Muff and carb heat on we get an average drop during mag check of around 20 RPM. I did question that initially. But I know from experience that if I had carb ice forming on the ground that this system does work fine for melting the ice. I dont think Ive heard of a Vans AC with this set up having a major problem in flight with actual Carb Ice.

What I don't know for sure is how effective drawing only under cowl air is with no heat muff. Yes there is a 14 deg temp rise and a slight drop but in real practice I don't know.

I will probably install the heat muff out of an abundance of caution.

Tim

 

[Finley Atherton]

Years ago there was a thread on carb temp probes and the general consensus at the time was that many probes measured the temp of the metal body of the carb rather than the carb air. This was my experience with my original probe. My current probe is a JPI K type which does seem to accurately measure carb air temp.
Finley Atherton
RV 9A

 

[rvbuilder2002]

Years ago there was a thread on carb temp probes and the general consensus at the time was that many probes measured the temp of the metal body of the carb rather than the carb air. This was my experience with my original probe. My current probe is a JPI K type which does seem to accurately measure carb air temp.
Finley Atherton
RV 9A

This is largely true, but there is another factor that causes error in the reading.
If you look at the typical carburetor temperature probe, the majority of the mass of the probe is the nut portion that is used to turn it into the port on the carburetor.
This nut portion of the sensor is exposed to the ambient air in the area just forward of the carburetor. Considering the conductivity of the brass that the sensor is usually made of, this means that to some degree the sensor is reading the temperature of the air outside of the carburetor.
This issue is the same reason that it was found years ago that mounting an OAT probe anywhere around the area of the cockpit caused major errors because the main body of the sensor would be exposed to the interior cockpit air.

My opinion is that if you are using a carburetor temperature indicator with the typical probe, you should assume that the temperature is always colder than what is indicated.

 

[rockwoodrv9]

I preheat my engine to about 100 degrees before I fly most of the year here in Michigan. This is the carb temp gage for a flight I did yesterday. It was between 0 and -12 degrees outside on my flight. I checked at startup and applying the carb heat dropped my RPM by about 20 and my temp went up several degrees until I pushed it back in. The two drops in temp on the graph are when I made landings and cut power. I am wondering if my carb heat is leaking hot air all the time? I will check that out and make sure it fits tight.

Scott, do you have an idea on how far off the temp could be where it is critical? I agree the sensor is probably close but not measuring exactly the temp where you can get ice. I cant see any disadvantage to using carb heat on ice days so I will modify my checklist from check temp to include carb heat. Thanks

 

[Vern]

I preheat my engine to about 100 degrees before I fly most of the year here in Michigan. This is the carb temp gage for a flight I did yesterday. It was between 0 and -12 degrees outside on my flight. I checked at startup and applying the carb heat dropped my RPM by about 20 and my temp went up several degrees until I pushed it back in. The two drops in temp on the graph are when I made landings and cut power.

Scott, do you have an idea on how far off the temp could be where it is critical? I agree the sensor is probably close but not measuring exactly the temp where you can get ice. I cant see any disadvantage to using carb heat on ice days so I will modify my checklist from check temp to include carb heat. Thanks

View attachment 56074

Years ago there was a thread on carb temp probes and the general consensus at the time was that many probes measured the temp of the metal body of the carb rather than the carb air. This was my experience with my original probe. My current probe is a JPI K type which does seem to accurately measure carb air temp.
Finley Atherton
RV 9A

This is largely true, but there is another factor that causes error in the reading.
If you look at the typical carburetor temperature probe, the majority of the mass of the probe is the nut portion that is used to turn it into the port on the carburetor.
This nut portion of the sensor is exposed to the ambient air in the area just forward of the carburetor. Considering the conductivity of the brass that the sensor is usually made of, this means that to some degree the sensor is reading the temperature of the air outside of the carburetor.
This issue is the same reason that it was found years ago that mounting an OAT probe anywhere around the area of the cockpit caused major errors because the main body of the sensor would be exposed to the interior cockpit air.

My opinion is that if you are using a carburetor temperature indicator with the typical probe, you should assume that the temperature is always colder than what is indicated.

 

[rvbuilder2002]

Scott, do you have an idea on how far off the temp could be where it is critical? I agree the sensor is probably close but not measuring exactly the temp where you can get ice. I cant see any disadvantage to using carb heat on ice days so I will modify my checklist from check temp to include carb heat. Thanks

No I don’t, but years ago when it was a common practice (until word spread to not to) to install OAT probes through the fuselage side of the cockpit, errors of 15 degrees or more were typical, depending on what the temp. delta was between inside and outside the cockpit.

 

[pa38112]

RV6-A with FP O-360-A-1-A and Sam James Cowl. When I got the plane there was 0 RPM drop on the ground, and I am pretty sure the ram air would have overcome the carb-heat air. I ended up re-working the air box so the ram air would be sealed, and I installed a heat muff VS the simple shroud that the builder had used. I also installed a carb temp probe. I now see about a 70 degree F rise when I select carb heat.

 

[Scott Hersha]

I preheat my engine to about 100 degrees before I fly most of the year here in Michigan. This is the carb temp gage for a flight I did yesterday. It was between 0 and -12 degrees outside on my flight. I checked at startup and applying the carb heat dropped my RPM by about 20 and my temp went up several degrees until I pushed it back in. The two drops in temp on the graph are when I made landings and cut power. I am wondering if my carb heat is leaking hot air all the time? I will check that out and make sure it fits tight.

Scott, do you have an idea on how far off the temp could be where it is critical? I agree the sensor is probably close but not measuring exactly the temp where you can get ice. I cant see any disadvantage to using carb heat on ice days so I will modify my checklist from check temp to include carb heat. Thanks

View attachment 56074

I don’t have any empirical evidence concerning indicated carb air temp and when to expect carb ice. Those conditions can vary widely. I’ve flown airplanes that were much more susceptible to carburetor ice than any of my RV’s. Most Continental powered and carbureted Cessnas I’ve flown, and the Beech 18 would make ice pretty quickly, and none of those airplanes had a carb temp probe. The RV’s don’t seem to be as susceptible, but they are not immune. Carb ice can and does happen above freezing in moist conditions. Temperature and moisture level makes the difference. During freezing weather, the decision is obvious. When flying in “cool” weather with high humidity, that also could be an area of attention, especially at low power settings and rich mixtures - like when landing. I wish I could trust the carb temp probes I’ve had, and how to know what that info is telling me, but I don’t. Atmospheric conditions has a lot to do with it, so I use caution when in a possible icing environment, and use my fully functioning carb heat system.

 

[Freemasm]

Anyone know the Static Pressure profile through the carb? Even better for different (lower) throttle settings. Even with good instrumentation, would be nice to know the actual temp to be concerned with.

 

[avrojockey]

FWIW I have the original carb heat shroud on a single crossover pipe. I get a ~30 RPM drop with carb heat during run-up at 1800 RPM. My carb heat door is not sealed with baffle material sandwich like plans now show...it's just the aluminum door fitted fairly well to FAB. I have never had carb ice in 400 hrs of owning this ship so I've never had it on in flight