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kitfoxfugit(at)yahoo.com
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 Posted: Mon Dec 03, 2007 10:59 pm Post subject: Carb Ice |
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For 400 hours I flew a Model 4, 912 Rotax, with the
air cleaners mounted on the carbs near the firewall.
The carb heat box had never been used in the 400 hours
before I got the aircraft, but it was in a desert
environment. Carb ice never seemed to be a problem
most of the time.
During cloudy conditions last summer, temps in the
40's (F), scattered rain showers, I encountered carb
icing from time to time. RPM's decay a little.
Suddenly, the engine stumbles briefly as it ingests
the ice apparently shaken loose by the vibration. The
engine resumes smooth operation at the original RPM.
The cycle repeats periodically until conditions
change.
I have been told that carb ice can cause this engine
to quit at temps in the high 20's to low 30's. Now,
we have installed the carb heat box that came with the
kit but have not tried it yet as the plane is
mothballed. It now has a bigger air cleaner taking in
cooler air, so there is a hope for more power, as
well.
John Allen, 4 Speedster, 912 Rotax, O70
Never miss a thing. Make Yahoo your home page.
http://www.yahoo.com/r/hs
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wingnut
Joined: 11 Jan 2006
Posts: 356
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| Posted: Tue Dec 04, 2007 7:19 pm Post subject: Re: Carb Ice |
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Well that settles it then. The 912UL without carb heat is not immune to ice. Thanks for sharing.
| Quote: | During cloudy conditions last summer, temps in the
40's (F), scattered rain showers, I encountered carb
icing from time to time. RPM's decay a little.
Suddenly, the engine stumbles briefly as it ingests
the ice apparently shaken loose by the vibration. The
engine resumes smooth operation at the original RPM.
The cycle repeats periodically until conditions
change. |
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_________________
Luis Rodriguez
Model IV 1200
Rotax 912UL
Flying Weekly
Laurens, SC (34A) |
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lcfitt(at)sbcglobal.net
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| Posted: Tue Dec 04, 2007 10:13 pm Post subject: Carb Ice |
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This is an interesting thread. Once again it is a rehash of numerous
discussions over the years. It is a fact that early in the evolution of the
912 in the Model IV all kits came with the carb heat muff and box. It
didn't take long for almost all builders and flyers to eliminate the
installation of the box or removing it from flying aircraft if it was
previously installed. None of the airplanes I am familiar with have the
carb heat provision. This includes the three guys with Rans aircraft that
will fly with us in the group flights from time to time - aggregate time
about 6,000 hours in every condition you can think of except frank IMC. It
just has not been an issue for too many years and too many hours in too many
airplanes. It is also a fact that everyone that removed the carb heat set-up
reported increased power output from the engine due to reducing the
induction air restrictions into the carburetor.
I liked Louis Rodrigues' comment: "I agree that anecdotal evidence is
useless. A few people saying that they've never had carb ice doesn't prove
much at all no matter how many hours they have flown. However, if we can say
that no one on this list who is running without carb heat has ever had carb
ice; That's a whole different ball game."
I don't know if he meant it this way, but my guess is that if the whole 912
series powered fleet (Series 7 excepted - see below) was polled, (and there
are tons of pilots flying our airplanes that are not on the list - none of
my buddies are) we would find that most, maybe up to 95% don't have the carb
heat box installed with the Rotax 912 series engines, and check the reports,
if it was an issue, there would be more talk - lots more talk - on the
problem than what we bring up here. I did read the one report by Louis
reporting icing, but given his experience, having the engine injest a bit of
ice and then purr along afterward is not enough for me to worry even a tiny
bit about not having the system installed.
This from an old post by John McBean - Jan 2004:
Maybe this will help.... The earlier Kitfox's, pre Series 7, had filters
on the intake of the carbs.... Unless the optional carb heat box was
installed. Then both carbs were tied to the carb heat box and the filter was
on the intake of the box. I do not believe that many carb heat boxes were
installed and all that I have spoke with that used them said when removed
saw a increase in performance.
The Series 7 is a horse of a different color. For the first time the Kitfox
is utilizing the Rotax induction plenum. The plenum ties both carbs
together and has two ports on it. One port is for fresh air. The Series 7
install has you route a Scat tube from the port down to the NACA scoop in
the cowling and into a filter that has been mounted outside the cowling in
the NACA. The other port is designed for carb heat. SS does not currently
have a kit to utilize this feature. It is designed to use a pull cable to
flip a butterfly shutting down the fresh air and opening the heat. One
could very easily adapt it.
On my aircraft, I chose to install the air filter directly to the induction
plenum and not run the Scat tubing. With the filter inside the cowling I
have not noticed a decrease in performance. I also safety wired the
butterfly so that it could not shut the filtered air off. The Series 7
install is much different then the Series 6.
Hope this helps some.
Blue Skies!!
John & Debra McBean
Maybe it's time for another poll. How many 912 pilots have the carb heat
box and who does not. And carb ice experience in both instances.
Lowell
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pwmac(at)sisna.com
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| Posted: Wed Dec 05, 2007 7:43 am Post subject: Carb Ice |
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One the reasons for not using the carb heat from the Skystar kit was
the use of scat for air flow. The guys using the kit were down on
performance compared to using the K&N air cleaners.
I was at Osh one year and discussed this issue with one of the racer
guy crew chief and he pulled the cowl and showed me ho to make ducts
without using scat. What they did was make custom fiberglass ducts
with rubber boots where movement was encountered. Not a complex on
his engine since it only had one EFI intake compared to the Rotax
which has to be split to get to the two carbs.
His process was to use 4" diameter swimming pool snakes then he fed
soft copper tubing thru the center hole and then bent to fit. Then he
used fiberglass to cover it. After cure he just pulled out the copper
& removed the foam. The result, he said, was a super smooth inside
surface with little pressure drop.
The intake still had the carb heat box but when open to the ram air
he said the performance went up. An interesting fix.
Now days for the M7 which uses the air intake designed for The 912
engine there is only one intake duct from the carb heat box to the
engine intake which slightly simplifies the fix.
Regards, Paul
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