jeanphilippeneel(at)yahoo
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 Posted: Mon Dec 13, 2010 10:15 am Post subject: Re : Icing: 912S versus 912 |
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Rob and Barry,
Many thanks for your answers.
I well know the process that lead a carb to icing. It's usefull to remember that water can be provided by air and by gas itself and the alcohol effect.
I also know that if I worry about icing , there are a lot of solutions to prevent from: hot water circulation on the carb body, hot air admission .
In fact my question was : Could some body explain why a Rotax 912S should be more exposed to icing than a 912?
Jean-Philippe Neel
F-PSLH Kit N°273
Grenoble France
De : FLYaDIVE <flyadive(at)gmail.com>
Ă€ : rotaxengines-list(at)matronics.com
Envoyé le : Lun 13 décembre 2010, 15h 06min 44s
Objet : Re: Icing: 912S versus 912
Excellent point Rob:
But I think you have taken it one step further than the Gaggle need to disseminate.
Everything said is true but there is one ingredient missing....WATER.
No mater how great the isentropic action is ice will not form unless the temperature drops to and below the freezing point of the liquid. The freezing point of Gas is around -95 Deg F (-70 C) and water is +32 F (0 C). So the isentropic action upon gas is not an issue. You did say it but I do not know how well it was received. Ice forms from WATER. The temperature in the carb drops due to a drop in temperature due to the venturi action. Simply put: The speed of the mass of air going through the carb is increased by the carb's venturi. This increase in speed reduces the pressure in the air flow. The reduction in pressure lowers the temperature. Tests have shown that with an OAT of 70 F (+21.1 C) you can get ICE in the carb.
Taking it one step further the question of: Where does the WATER come from has to be addressed? It is present in the AIR and in small quantities in the gas. Even if you sum the tank you will not see the water in the gas. Water gets into gas through condensation and absorption. Anyone using straight Ethanol Gas? If so then due to the Alcohol you have water, as alcohol is Hygroscopic. <-- meaning it adsorbs water.
I recall some obscure FAA Reg. requiring that Carb Heat must raise the temperature 100 Degrees F above ambient. I have NEVER heard of that being enforced.
I would guarantee that there is not a single RV 6 that could reach that temp.
Barry
On Sat, Dec 11, 2010 at 6:31 PM, Rob Housman <rob(at)hyperion-ef.com (rob(at)hyperion-ef.com)> wrote:
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The cooling (isentropic throttling) that makes carb icing happen is a function of the amount of fuel vaporized so with all other things being equal (same atmospheric conditions, engine manifold, carbs, RPM, etc.) you can expect the somewhat greater fuel flow (more power requires more fuel) to cause more cooling and hence a greater propensity to make ice. Obviously the 912 and 912S are “different” engines but those differences are internal and downstream from the carbs. The 914 is less prone to carb ice because the air compressed by the turbo is warmer than the intake air. Thermodynamics rules!
Best regards,
Rob Housman
Irvine, California
Europa XS
S/N A070
Rotax 914
Airframe complete
Avionics soon
From: owner-rotaxengines-list-server(at)matronics.com (owner-rotaxengines-list-server(at)matronics.com) [mailto:owner-rotaxengines-list-server(at)matronics.com (owner-rotaxengines-list-server(at)matronics.com)] On Behalf Of NEEL Jean Philippe
Sent: Saturday, December 11, 2010 12:35 PM
To: rotaxengines-list(at)matronics.com (rotaxengines-list(at)matronics.com); europa-list(at)matronics.com (europa-list(at)matronics.com)
Subject: Icing: 912S versus 912
Hi all,
I flown my europa with a 912 80hp during 1200h/11years.It's a mark one but with XS engine mounting and cowlings.Majority of flights took place here in the French Alps as well in summer as in winter. I never had icing issue although I had no carb preheater .The only caution I had ,when descending ,was to keep 4500/4800 rpm . Never in idle position.
In the beggining of that year I change my 912 for a new 912S 100hp. That new engine has now about 125h. Yesterday I had for the first time a rough running during few seconds.Outer Air Temp was negative at every level and the issue occurs at the end of a descent 4500/4800rpm from 8000ft to 1500ft. There was about 8°C of difference at ground level betwen OAT and dew point.
Several time I read on Rotax and Europa web site discussions about icing issues and I kept in mind that 912S should be more sensitive to icing than 912.
As I know:
- Carbs are the same ,except needle diam (2.72 versus 2.70 on S) and ,may be, main fuel jet and diffuser (in accordance with needle) .Does those difference could explain more sensitivity to icing?
- Compression ratio and bore are increased on S but stroke is the same . In my opinion these difference have not thing to do with icing issue.
So here is my question : Could some body explain why a 912S should be more exposed to icing than a 912?
Jean-Philippe Neel
F-PSLH Kit N°273
Grenoble France
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