Date: Tue, 31 Jul 2012 09:58:07 -0500
Reply-To: JRodgers <jrodgers113@GMAIL.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: JRodgers <jrodgers113@GMAIL.COM>
Subject: Re: Carburettor Set Weber 44 IDF VW Wasserboxer 2,1
In-Reply-To: <03b901cd6f1e$21ddd2e0$659978a0$@net>
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Airplanes - piston types and jets - have combustion air heaters to
pre-heat the air to remove/prevent ice formation inside the throat of
the carb/turbine. A short story about this kind of ice.
Once in Alaska, while on a trip as co-pilot in a Skyvan, was at 12,000
ft over the Cook Inlet and coming home to Anchorage. Had been cleared to
the Anchorage VORTAC for a hold outside Anchorage prior to landing and
began decent. We were in the tops of clouds and began picking ice, and
it would get worse as we descended through it to below the freezing
level. The captain reached over and flipped both smile heaters switches
to the solenoid valves that routed heat from the turbine compressor
sections into the stainless plenum around the air intake of the
engines. Those plenums were coated with ice. The instant that ice melted
that slug of water went directly into the engine and snuffed out the
flame in both engines. At 12,000 feet both engines had quit. We suddenly
became a very heavy glider with a pee-poor glide ratio. WE were out of
7000 feet and nearing 6000 and descending fast before we managed to get
the engines started again. Scared the bejeebers out of us. Everything
was OK after that, except for the seat of our pants and the cones in the
captains and co-pilots seats. An official change in operating procedures
followed, along with some pilot training - Only one smile heat switch at
a time was to be turned on to eliminate dual engine failure.
Carb ice works the same way. Turn on a carb heater on a piston aircraft
- the engine will run rough until it swallows all that water from the
melting ice. Pilots learn to tickle the carb heat, so the ice goes away
slowly, otherwise the engine may quit.
Same goes on these carbs. But with no Carb air heaters, they can be a
major PITA in wet, cold air. Ice can build in the throat of the carb and
simply choke off the combustion air.
It's all about Bernoulli's Principle. Carb has a venturi, venturi's drop
the pressure, increase the air flow speed and drop the temperature. If
weather is cool and moist, heck -even warm and moist - if the temp in
the throat of the carb goes below freezing, whatever moisture is in the
air will begin to plate out as ice in the carburetor. Thus the need for
a carb air heater of some sort.
John
On 7/31/2012 8:12 AM, Tom Hargrave wrote:
> Frost on the outside = frost & ice on the inside and this causes serious
> issues with carburetor performance.
>
> Thanks, Tom Hargrave
> www.stir-plate.com
> www.towercooler.com
> www.kegkits.com
> www.grow-sun.com
>
>
>
> -----Original Message-----
> From: Vanagon Mailing List [mailto:vanagon@GERRY.VANAGON.COM] On Behalf Of
> Hector Zapata
> Sent: Tuesday, July 31, 2012 4:26 AM
> To: vanagon@GERRY.VANAGON.COM
> Subject: Re: Carburettor Set Weber 44 IDF VW Wasserboxer 2,1
>
> On 7/30/12, JRodgers <jrodgers113@gmail.com> wrote:
>> These carbs don't have carb air heaters, and are subject to icing when
>> conditions are right. Makes operation in nasty weather that much more
>> miserable.
>>
>> John
>
> I thought that was considered good for the mixture. Thirty-something years
> ago I met a mechanic who was very proud of the way he adjusted carburators,
> they frosted at idle, 80º weather...
>
> -----
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