Carb Heat

I would be interested to know builders views on carb heat in VANS aircraft?

VANS standard (FWF kits, their demonstrators) do not provide for significant carb heat other than the little door which shuts at the front of the FAB and draws air from around the lower cowl. I quizzed Gus on this and he indicated carb icing was not a problem for them. Here in the UK most folk add some scat so the air is drawn from a muff around an exhaust pipe. (I built a 9 to VANS plans - no muff - and never had a problem). It was suggested to me that because the engine is cowled so tightly, the air in the lower cowl was keeping the carb body warm and hence icing is not a problem.

Anyone got a good understanding of what is going on? I need to make a decision as to what I shall do on my -4.

NB I am not talking about the issue of people flying into snow and jamming the FAB full and VANS magnetic door.

Thanks

Since most of my training was in Pipers with Lycomings, I went with the camp that does NOT use carb heat as a standard part of the landing checklist.

Back in those days, many Cessnas used Continentals that do not have the carb bolted directly to the warm oil pan, as does the Lyc. See explanation in link below.

I do have carb heat on the 6A, and feel it would be better than nothing, should the need arise. Living in a high altitude area, I figure leaving carb heat on during a go-around is worse than using it just because of habit's and old checklist's.

Van's has also done away with the magnetic trap door, and replaced it with a manual reset sliding plate, that uses a standard carb heat cable to the cockpit. I did replace the magnetic door with the new setup, just in case a bird hits that air intake dead center.

http://www.sacskyranch.com/crbice.htm

Lycomings can and will get carb ice first flight of the day before the oil is warm and the conditions are right (50 F, high humidity).

John

Hot flashes , I'm melting

Steve Sampson said:

I would be interested to know builders views on carb heat in VANS aircraft?

VANS standard (FWF kits, their demonstrators) do not provide for significant carb heat other than the little door which shuts at the front of the FAB and draws air from around the lower cowl. I quizzed Gus on this and he indicated carb icing was not a problem for them. Here in the UK most folk add some scat so the air is drawn from a muff around an exhaust pipe. (I built a 9 to VANS plans - no muff - and never had a problem). It was suggested to me that because the engine is cowled so tightly, the air in the lower cowl was keeping the carb body warm and hence icing is not a problem.

NB I am not talking about the issue of people flying into snow and jamming the FAB full and VANS magnetic door.

Thanks

Click to expand...

Steve, I think you have the big picture. Consider a dedicated SCAT and heat muff for carb heat.

Van's Carb heat does provide some heat, may be not as much as factory planes, but it does provide something if you use it proactively (early and often). The flap just opening in the cowl is obviously not as good as having a dedicated heat muff / scat to the flap.

Here are two ideas:
http://www.wicksaircraft.com/catalog/product_detail.php/pid=4494~subid=1253/index.html

http://www.wicksaircraft.com/catalog/product_detail.php/pid=4733~subid=1343/index.html

I think Van's stove pipe (link below), which got a mention in the NTSB accident report I also refrence (below), is marginal or of dubious improvement in my opinon. I would prefer a full heat muff. Also in my case my exhaust Pipes don't cross over in the front of engine.

http://www.vansaircraft.com/cgi-bin...-336-720&browse=engines&product=carbheat-muff (I don't personally recommend "The stove Pipe")

The above "EA CARB HEAT MUFF" may be a bit better than just a flap opening into the cowl area, but clearly by inspection the two heat muffs I recommend are better at providing more heat.

A quote from the NTSB report says: "With respect to carburetor heat muffs, a section of the builders manual states that each builder is responsible for evaluating his/her own installation based on the likelihood of carburetor ice in his/her intended operating environment." In the factual report the NTSB identifies the heat muff as the "P/N EA CARB HEAT MUFF" per Van's 2003 accessory catalog, aka the stove pipe.

With out flight test and a Carb Air Temp gauge (CAT) you don't know how effective your heat system is. I don't know of anyone who has done comparative evaluation of differnt heat muff installations. I never used the Vans EA Heat muff (stove pipe), so I don't know how effective it is. I am not bashing or saying it does NOT work in some capacity, only you should evaluate it and consider something more substantial. My opinion is if you're going to put a heat muff on, get the best you can. I doubt the whole installation, using the larger Muffs weights more than a pound.

The other piece of the puzzle is Van's bracket for the FAB, holds teh 2" dia hose over the "FLAP". It's also a compromise, but it's effective. It allows air to bleed out when not in use. The down side is extra heat may leak onto the carb when not needed, causing a small loss of power (HP) due to fuel heating, but that's conjector. On the plus side it's like a little preventive carb heat all the time. Even factory carb heat systems leak. It's just the nature of the beast. I think it's a pretty good solution. I guess you could add another "VALVE" and cable control to positively turn the heat ON and OFF from the Muff, but that's more weight, cost and complexity. You would than have to pull two controls to turn the carb heat on and off.

http://www.vansaircraft.com/cgi-bin...720&browse=engines&product=carbheat-connector

What do you have to lose by adding the heat muff? A little cost and weight. UK is pretty wet and suspect it's as good a place as any to get carb ice, but that's up to you. I flew my RV-4 for years in a wet environment with just the flap opening into the cowl, but I also got carb ice (read on).




STUFF and CARB HEAT HAPPENS - NTSB REPORT
The key is being aware if the conditions are ripe and not flying at real low power for extended periods. If the engine is real hot (making power) carb ice is truly unlikely. However an idle decent from cruise altitude, not using carb heat (what little we have) is asking to produce carb ice. Once the heat is gone, carb heat is too little too late. Not even a factory planes produce enough carb heat at idle or after the engine is choked off.

Carb ice accidents with RV's are rare, but I know of one for sure. It occurred in Georgia (humid part of the country), the temps and humidity where right, and he did a low powered decent from altitude. Here's the accident report. There are some telling comments in the synopsis:

http://www.ntsb.gov/ntsb/brief.asp?ev_id=20030110X00044&key=1

Here is the full factual report:

http://www.ntsb.gov/ntsb/GenPDF.asp?id=MIA03LA035&rpt=fa

I know someone will notice the 3 degree spread, temp 11C, dew 9C, and they'll think he should have known carb ice was likely. Well how many listen to ATIS/AWOS and note temp/dew point and consider their carb heat operations? Probably a good idea. I know I don't always think of the temp spread. I recall about 12-15 accidents are attributed to carb ice every year in the USA. Of course some accidents are not definitively determined as carb ice since the evidence melts.




OPPERATIONAL PHILOSOPHY - KEEP IT HOT
Keep the fire stoked with coal, keep power up, keep it warm, and if you suspect you might get carb ice and need carb heat put it on early and leave it on. Don't let the whole engine compartment cool.

Of course you have to throttle back to land. I not big on doing lots of touch-N-goes (probably because I did one million as a CFI), but I rarely land with carb heat on. I might check or apply carb heat entering the pattern or on the down wind, as a preventive measure, to melt ice that might have formed. If I suspect conditions are ripe, I'll leave it on for a minute and than turn it off, say abeam the numbers.

It's unlikely but if your engine chokes off from ice and dies, it's too late. Fortunately as you where told it's not likely with a Lycoming in a tight RV cowl, BUT it's not impossible if careless, extended low power and right conditions.

My RV-4 got carb ice a few times, always on the ground, as Yukon said taxi out first flight. I would do a run up and sure enough the engine RPM would go down than UP after leaving the Carb heat on for a period of time, a sure sign carb ice had melted. I did live in a high humidity area at the time. All I had was the "FLAP" so it was good enough during run up to clear it up. Not very scientific but my observation.

If you're in part of the UK that's real wet and want to fly lots of T/G's or low power, consider the SCAT tube to bring dedicated heat to the little flap, we call carb heat. I had the same quandary as you and many RV'ers on what to do. My exhaust pipes do not cross over in front of the engine, so to bring more heat to the "FLAP" I plan on the SCAT/MUFF set up on my new RV-7 project. I will have carb temp and may play with differnt configurations to come up with the best solution for me. I'll try it with just the FLAP and compare to the heat muff. I am careful about adding weight and cost, but than safety is priceless.

It's a decision that you need to make. I just advocate not being cavalier about it, which clearly you are not, but I know some folks don't even put the carb heat flap in at all. They're under the impression IT CAN'T HAPPEN to them and they will never get carb ice.

Heat Muffs

Steve,

I would recommend the Robbins heat muff of which he has a new slightly modified version. It fits on the Vetterman cross over exhaust pipes and has a 2 inch scat tube connected to a flange mounted on the front top of the air filter box.

You can contact Rick Robbins at [email protected]

Barry

Thanks for all the replies, especially your long one George. In case is anyone is any doubt I certainly believe in carb ice. I get it all the time in my O-320/Supercub. I am just doubtful that the it is a real problem in the RV because of the very close cowl. I think the carb body is kept unusually warm wrt say my S'cub. However since I wouldnt bet on a single number on a roulette wheel I think I will put the Robbins muff on. Its not the cost but the weight I object to George. The temp rise with the VANS system was about 3deg F on my -9 by the way.

Finally George, you said "If you're in part of the UK that's real wet...". Can you tell me where the dry bit is? Cheers!

When I'm throttled back flying formation or other relatively low power flying, I have noticed carb ice (O-320). Maybe 5-6 occasions in 830+ hours. I have the vetterman style carb heat "half" muff. Works fine. Only used when needed.

The filter's the problem

I don't think it's possible to get much improvement over the stove pipe when you've got a filter between the heat and the carb.

I've got an after-market cub 6" carb-heat muff on the cross-over tube and I've blocked the side-flow around the flapper valve. I have a carb temp guage. During run-up, there is no perceptible rise with carb heat on. In flight, I see about 20 degrees. I guess it might be enough to help if I used it before encountering ice, but I doubt that it be much good once the sputtering starts. Fortuanately, like others have said, Lyc-equiped RV's are not very ice-prone anyway. I've personally never seen my carb temp gauge register anything close to zero C, even during the winter in Tulsa before I moved to Houston.

I've seen other posts where people have said that they got a 75 degree rise after switching to a standard heat muff, but it's a mystery to me how they did it.

I have a Grand Rapids EIS 4000 with optional carb temp. I'm not sure at what temp to get worried and use carb heat. Is it safe to assume that if the carb temp, as measured by the probe and displayed on the EIS (assuming, of course that the reading is accurate), is above freezing you can't be making ice? Actually, I've never noticed it showing much less than 70 degrees.

I have the Vans stove pipe, and I test it on runup with my carb temp guage, and the temps rise very quickly. Even this morning starting at 80 degrees F.

I rarely us it, however, being here in GA.

I supose I would not be nearly as comfortablle that it works without the temp guage, as rpm drop is negligable

Aerosport O-360-Superior Parts

cheers!