van's carb heat

[vlittle]

Interesting discussion on Van's beer-can carb heat muff.

The Van's set up is not truly carb heat, it's induction heat.

As posted earlier, it's supposed to minimize the build up of frozen precipitation on the air filter.

A true carb heat system would bypass the air filter and let hot air directly into the carburetor intake. Certified A/C do this, and hence the warning about applying carb heat in dusty conditions (unfiltered air into carb).

It may be hard to get a 90 degree rise with Van's set-up because the Air Filter has some thermal mass and acts as a bit of a heat sink.

If someone out there wants to experiment with a true carb heat system, it may be worth looking at the induction bypass system as a starting point, and routing hot air to it and controlling the magnetic flapper valve with a cable. This will take some effort because the cowling fit is tight under the FAB.

The worst case scenario is both induction ice and carb ice simultaneously. I doubt that any induction heat system would could melt a plug of filter ice and carb ice quickly. This would call for a large heat muff.

On a final note: the design of the Van's system lends itself to ground taxiing with carb heat 'hot'... the air is both filtered and drawn from inside the cowling, which can be cleaner in dusty conditions. Lean the engine for taxiing (good practice at all times to keep the plugs clean).

Vern Little
RV-9A

[mtclay]

To Jon Baker, jclark, and others.

Carb ice can and does happen with very high ambient temperatures and at low altitudes!!

I am a retired Air Force pilot who flew the Cessna O-1 Bird Dog (L19) in Viet Nam as a Forward Air Controller (FAC). I don't remember the engine designation, but it was a 213 HP Continental.

I experienced sudden engine failure due to carb ice on two occasions with the OAT in the mid 80's to low 90'sF and humidities near 100%. Once was at around 1500'AGL and again around 800-1000'AGL. On both occasions rapid application of carb heat resulted in restarts within 15-20 seconds.

My personal observation was that the key seemed to be the near juxtaposition of the OAT and the dew point. The supersaturated air racing through the venturi can freeze in a heartbeat. In the Mekong Delta region of South Viet Nam as the temperature rose each day you could almost count on the sudden formation of a very thin, but solid (maybe 200-300' thick) overcast deck. It usually formed at around 800' then slowly rose in altitude to around 2000' where it began breaking up into cumulus puffies that continued to rise and build throughout the day. The life span of the overcast deck was usually 30-45 minutes. This behavior clearly indicates overlapping temps and dew points.

I even had engine coughing and sputtering on the ground while taxiing a few times that was cured with application of carb heat. Maybe that particular Continental engine setup (not always the same airplane) was more prone to developing carb ice.

At any rate, I quickly became a believer in carb ice forming at unexpected times and in high OAT conditions.

Mike Clay

[jonbakerok]

Originally Posted by mtclay

Carb ice can and does happen with very high ambient temperatures and at low altitudes!!

I experienced sudden engine failure due to carb ice on two occasions with the OAT in the mid 80's to low 90'sF and humidities near 100%. Once was at around 1500'AGL and again around 800-1000'AGL. On both occasions rapid application of carb heat resulted in restarts within 15-20 seconds.

Mike Clay

No argument there.. carb ice definately happens at unexpected times.

My point is that an anecdote telling me that you had carb ice once when it warm outside, or a "rule" that tells me to "watch" for carb ice when it's 86 degrees and 50% humidity, are basically worthless. You might as well just say "Be careful -- sometimes carberators ice up for no apparent reason". (Or for that matter -- "Be carefull, sometimes fuel injectors vapor lock for no apparent reason").

I just think it would be much more helpful if at sometime during the last 100 years, somebody had bothered to do some real science and figure out why hundreds of carburated planes are able to fly around Houston nearly every day of the year in the exact conditions you describe without a problem -- and what was different about the one in a thousand that had a problem.

If it was simply humidity plus temperature, then a given engine would always ice when the humidity and temperature reached a specific, predictable point.

Now THAT would be a useful scientific fact to know.

[Jeff Vaughan]

My instructor taught me that carb ice was possible whenever there was moisture present. Dew, mist, clouds, humidity etc. This is the rule I was taught.

Jeff Vaughan

[Build9A]

Since were on this carb ice thread.....Is there an increased chance of carb ice using MOGAS? thanks, Jack

[Barry]

I have often wondered about the possibility of carburettor ice with my RV. To the best of my knowledge I have never experienced carburettor ice with the RV and I have flown it in many different climatic conditions including IMC.

I have about 670 hours on it over the last 6 years. The plane has the Robbins (sp?) heat muff on the cross over tube which routes the air via a scat tube to the filtered air box with my LycO360A1A engine.

I also have a RMI engine monitor which has a carburetor temperature probe. The display shows carburettor temperatures up to +19 deg. C.

It is very rare that the carburettor temperature drops to +10 C, even with freezing OAT. With the Lycoming the carburettor is bolted directly to the sump, Continental engines are different and this may be why they are more prone to carburettor icing.

As the oil temperature in the sump is in the region of 90 C this heat is conducted directly to the carburettor and is probably the reason for seeing high carburettor temperatures.

For example today I was out flying today, the OAT was +19C and the carburettor temperature display was pegged at its maximum of +19 C.

Even during the engine run up prior to take off the carburettor temperature was pegged at its maximum of +19 C

I would be interested in hearing from anyone else you has a carburettor temperature probe so see what their reading are.

Some one talked about using an Ice Man carburettor ice detector. They mentioned that during ground operation they so often had ice warnings that they turned the system off. As this system works with an optical detector instead of providing a temperature output is it possible that condensation is forming in the carburettor and this is being erroneously detected as ice?

Barry RV6A

[kentb]

I am building a RV9A with IO-320 engine. I will be installing the induction alt. air into the filter and the other door below the TB if the filter gets clogged. Both doors will be operated by pull cables.

Question: Should I try to route extra heat off the crossover to the induction alt. air door?

Kent

[gmcjetpilot]

Originally Posted by kentb

I am building a RV9A with IO-320 engine. Kent

No carb ice with fuel injection. Yes an alternate air is good but you don't need the heat. George

I agree Van's "carb heat" is marginal from a part 23 certification standard. The saving grace is the Lycoming is less likely (LESS NOT UNLIKELY) to get carb ice. The tight cowl may even enhance this characteristic.

I will be blunt. You are crazy to build your plane without carb heat, unless you plan on only flying in Southern Arizona in the summer VFR.

Yes carb ice happens and claims a dozen or more planes a year. Since the evidence melts there are many more cases no doubt not quite explained but due to carb ice.

Google carb ice. Lots of good info. George

[hngrflyr]

About a week ago, I started my RV-6 on a cold damp morning in Oregon. We had light rain showers and the temperature was below 40 degrees F. My run-up before departure revealed that I'd picked up carb ice while taxiing out. The Van's "beer can" heat muff provided enough heat to remove the ice in about a minute. I would say it appears to be effective, at least in this climate.

Bob Severns

[gmcjetpilot]

Originally Posted by hngrflyr

About a week ago, I started my RV-6 on a cold damp morning in Oregon. We had light rain showers and the temperature was below 40 degrees F. My run-up before departure revealed that I'd picked up carb ice while taxiing out. The Van's "beer can" heat muff provided enough heat to remove the ice in about a minute. I would say it appears to be effective, at least in this climate.

Bob Severns

Bob I flew in the North West for 15 years and know what you mean. It is good to know you had enough heat on the ground. Taxi is a prime place for carb ice. I guess the real measure would be in-flight, which I would guess would be sufficient if caught early. I guess the watch word would be if at all suspect, carb heat on often and early. After the flame is blown out (carb chokes the engine) you may not have heat to melt the ice.

Putting my CFI cap on for one minute, my pet peeve:

(after a long wet taxi) During run-up check the carb ice by leaving it on for more than a fraction of second. Leave it on an note the RPM drop (fixed) or MAP drop and than wait to see of the RPM/MAP rise while the heat is still on. Than after the run up note any change. If idle RPM or MAP is higher after the run-up (and carb ice) you most likely burned ice off.

Hat off If you are in mild conditions, HOT/DRY, than hightened caution may not be as warrennted, but the range of humidity and temps that are condusive to ice are pretty wide.

George