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What do you guys use for Carb Heat??

C

ccryan

Active Member
Hey guys, I'd love your opinions/input on this one;

I've just finished my second RV. My first was an RV-4 with an O-320, and for carb heat I installed the standard Van's "trap door" that only allows ambient cowl air into the intake. I was surprised to find no RPM drop when I selected the carb head "on". I thought the ambient air in the cowl would be warmer. I've done this even after a thorough warm up and in flight, never seeing the slightest hint of a drop. I've flown about 5 other RV's with the same setup, (3's, 8's, and 6's), and have never seen a drop in those either.

My question for you guys is, do you think that the trap door setup for carb heat is effective enough to actually melt any carb ice?
 
I use fuel injection for my carb heat - sorry I'm just jokin' around.

Seriously, I have only seen what production aircraft us for carb heat, which is an exhaust muff with scat hose going to the airbox. I'm sure someone on here can give a more comprehensive answer with pictures, etc.
 
My O-320 will show about 10 rpm drop, but it shows an increase in carb temperature, enough to keep ice away and probably melt most anything. It does show a decrease in rpm at cruise, probably due to lack of ram air effect.

Bob
 
First off, I go by the Piper/Lycoming rule..........which is to use carb heat, only when you think it's necessary. No applying carb heat for every landing like Cessna did when using Continental engines. Lycoming carbs are a bit pre-warmed by the oil sump that it's attached to.

The carb does get some additional heat heat from the fitting that attaches to the exhaust, in addition to the ambiant air.

As I remember, an off field landing in Spanish Fork in an RV6A was due to carb icing..........according to the NTSB. In effect, the 2" scat pipe didn't have sufficiant area, for enough air flow to the carb. That's why you want to use the flange, that has open sides, for extra air. Some builders have covered the sides up. This 6A didn't have the additional flange side airflow, as it was a much older kit.

Other than that, I haven't found the Van's carb heat issue to be much of a problem in the mountain west. We just have to stay home because of airframe "icing" on those cold crappy days, where the clouds cover half the mountains. :D

L.Adamson --- RV6A
 
Carb Heat

I had Vans standard carb heat system with a Vetterman cross over exhaust and got the same result with an 0-320...almost no RPM drop at any time. My tach needle just barely moved at all when heat was applied but if you focused on the tach needle I did get about one needle width drop. Recently. I installed Rick Robbin's carb heat muff which now gives me a small but definitely perceptible movement of the needle on the tach, maybe 20 RPM drop but I am still using Vans Carb heat valve as designed in the FAB kit. I had expected to see a larger RPM drop but that was not the case. I haven't flown the RV6 much since installing the new muff because I came up with bronze particles in my oil filter last inspection so as soon as the weather warms I will be doing engine work meaning I have bigger fish to fry than the carb heat. When the aircraft is once again airworthy, I expect I will reduce the air gap, at least partially, in the carb heat air valve in attempt to get a larger RPM drop.

Dick DeCramer
RV6 N500DD 375 hrs.
RV8 fuselage structure complete
Northfield, MN
 
L.Adamson said:
As I remember, an off field landing in Spanish Fork in an RV6A was due to carb icing..........according to the NTSB. In effect, the 2" scat pipe didn't have sufficiant area, for enough air flow to the carb. That's why you want to use the flange, that has open sides, for extra air. Some builders have covered the sides up. This 6A didn't have the additional flange side airflow, as it was a much older kit.



L.Adamson --- RV6A[/QUOT

I'm not sure what flange your referring to? I'm not aware of an installation that has a side flange.
Click to expand...
 
ccryan said:
L.Adamson said:
I'm not sure what flange your referring to? I'm not aware of an installation that has a side flange.
Click to expand...

It's just the standard U'shape flange, that the lower end of the scat tube is clamped to. It allows additional air in from the sides. It's been this way for quite a number of years. Some builders have covered the sides, figuring that the carb heat will be higher. I say no, as we need that additional airflow.

http://www.vansaircraft.com/cgi-bin...756&browse=engines&product=carbheat-connector

L.Adamson --- RV6A
Click to expand...
 
Carb Heat on O-360 A1A

I see 20 RPM drop with carb heat on my RV-6 O-360 A1A. I check it at 1800 RPM after my mag check. I have not had any issue with icing to need it yet. A couple of times, when conditions seemed right for icing, I have pulled it with no change to indicate any icing taking place.
 
Digital Tach

I have a digital tach, in addition to the standard mechanical one. The carb heat drop is barely noticeable on the mech tach, and is difficult to hear/feel. But the digital tach consistently shows a 25-40 rpm drop.

I do the test at 1700 rpm, on an O-320-E2D, RV-6A, with the current carb heat kit from Van's. The Tube that straps to the exhaust pipe has the extra slot in it.

Hope this helps.
 
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U Connector with added flange

"I'm not sure what flange your referring to? I'm not aware of an installation that has a side flange."

Here is what I have done. I do get a drop of 55 rpm during run up. I also use the Wicks carb heat box.

2mr9xj6.jpg

219q6pt.jpg

23krocp.jpg
 
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I used a heat muff

get a pretty good RPM drop at run-up, about 50 rpm. Not saying I needed to, but I feel better knowing that if ice develops inadvertantly, I have a good chance to do something about it.

Randy
8A 0-360 A2A
 
Mine came with a standard Van's "stovepipe" that attached to the crossover exhaust. I could see no drop in RPM with that setup, and the previous owner suggested that it probably wasn't necessary anyway.

I replaced it with a Robbins Wings carb heat muff, around the front crossover pipe but drawing air from between the two pipes. I now get a consistent 50rpm drop from my runup at 1700. I have also seen carb ice on two occasions... Once on the ground and once in the air. Oh, and my bracket at the air box has the sides open too.
 
I have the standard Vans carb heat kit, home made valve and heat cuff on my O-360. My side vents on the flange are open. I had an extra temp probe so I installed it down stream of the carb heat valve. This way I can measure incoming air temp with the valve closed and open. There is a big difference, typically 20 degree F rise in temp with carb heat ON.
 
I didn't see this thread until after I posted the following over on the thread about the engine replacement on my RV-9. It really should be in this thread.

N941WR said:
On my second flight I tried the carb heat (Also checked it prior to flight) and found it quickly raised the carb temp by 10 degrees F.

I don't know if this is a result of the FAB I created to match with the Sam James cowl or the heat muff I used.

My suspicion is it has more to do with the heat muff than the FAB.
Click to expand...

My old engine had the "stock" Van's juice can clamped to the cross over tube with scat tubing running down to the carb heat valve on a ?stock? FAB. I would see maybe two or three degrees increase in carb temp and even then it would take some time to register. With the heat muff from Wick?s installed, the heat would start to climb immediately.

I've been told that one of the problems with blocking off the valve so the heat has no place to go with the valve closed is that the heat can "cook" the valve, that's why the cabin heat dumps the air when it is closed.

The carb heat may be different because you don't have ram air packing up the heated air. I don't really know, just reporting what I've been told.
 
I have standard FAB

carb heat without anything connected to it..just air from inside the cowl. Today when I opened my carb heat full open I gained only 1deg F in carb temp. Guess I need to plumb this to a heat muff now...
 
Jeff Vaughan said:
"I'm not sure what flange your referring to? I'm not aware of an installation that has a side flange."

Here is what I have done.

2mr9xj6.jpg
Click to expand...

I think it might be a mistake to block off the sides of the flange. The spaces are part of the design, and are there for a reason. Not 100% sure what the reason is, but likely has to do with preventing heat buildup and insuring airflow to the carb somehow.
 
PCHunt said:
I think it might be a mistake to block off the sides of the flange. The spaces are part of the design, and are there for a reason. Not 100% sure what the reason is, but likely has to do with preventing heat buildup and insuring airflow to the carb somehow.
Click to expand...
The air flow is rather restricted at the heat Cuff, just guessing it is about 1/2 the area of the tube. The side vent holes are probably needed to get adequate intake air.
 
Thanks for all the info guys. I just felt like the standard trap door opening to the heated cowl air seemed inadequate. I like the idea of using the front of the crossover pipe.
 
In regards to the open sides on the hose fitting attached to the FAB.......it's for air circulation to cool the hose, when not in use.............and as far as I'm concerned, to allow more air while carb heat is in use.

This is from the accident report, I mentioned earlier. It's been years, but my memory say's that older models didn't have the open sides.


"The Van's Aircraft construction and operating manual states that one method of building the
carburetor heat system is to 'run a 2-inch air hose from a heat muff and position it to feed into the alternative air inlet of the
carb[uretor] air box,' which is the method by which the pilot constructed the airplane. According to the inspector, the 2-inch hose is
not large enough to adequately supply enough heat to the carburetor to sufficiently melt the ice."

L.Adamson --- RV6A
 
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