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View Full Version : The Practical effect of Temp in a carb venturi


tayportflyer
31st Mar 2010, 19:30
Out of interest what kind of impact would the venturi have on airflow temp in a light single?, just curious in regards to carb icing.

Thanks:D
TPF

Bullethead
31st Mar 2010, 20:14
G'day TPF,

You get a pressure drop in the carby venturi which cause a temperature drop also which is why you can get carby ice at ambient temperatures above freezing.

Regards,
BH.

welliewanger
1st Apr 2010, 02:53
I can't remember which engine manufacturer it was, but I read in a technical document by them that all their carburettor based engines could be subject to icing at up to 32C.

Don't forget that warmer air can hold more water, so on a warm day carb ice can build faster than a cold day. I read an AAIB report about a Jabiru which flew through the cloud from a cooling tower at a power station. This was very warm but very humid air. Even with full carb heat it still crashed.

Carb heat has to be able to raise the carb air temp by 50C. This suggests that the carb ice temperature range is pretty broad.

I've got some graphs if you'd like to see them.

Dan Winterland
1st Apr 2010, 02:58
The figures I was quoted once were about ten degrees cooling for the venturi effect and twenty for the cooling effect of fuel evaporation. Total 30 degrees C. So there is no day in the UK where carb icing won't be a factor.

Graybeard
1st Apr 2010, 04:31
..what kind of impact would the venturi have on airflow temp in a light single?

That is well answered above. Also pertinent is what impact the airflow temp has on the venturi. Carb ice forms on the downwind end of the venturi and disturbs its aerodynamics, causing the airflow to draw less fuel, to the point of starving the engine. Carb heat and full rich are the correct responses. A few cc of ice will make you sweat - when the fan stops.

Legend has it that carb icing occurs at partial throttle. Not always.

The only time I encountered carb ice in the 150 Franklin engine in my Bellanca Cruisair, was full throttle, leaned, at 5,500 feet over the former atomic power plant near Portland, Oregon. The icing remained almost to landing some 20 miles away.

The only time I encountered carb ice in the later 165 Franklin with larger carb, in the same Bellanca, several years later, was at 1,500 feet, near full throttle on climbout from KSAN. The rough running took me by such surprise that icing didn't occur to me. In fact, I was rehearsing my mayday call for return when I remembered what an ATC guy had told me over coffee. They have an emergency checklist in the tower cab, with basic instructions like full mixture, fullest tank, carb heat(!)...

GB

zulucharlietango
1st Apr 2010, 05:38
High rel. humidity helps too, my Aviation kindergarten days of trying to get over a hill in Chonburi Thailand, 2 up in a Rotax 503 (2 stroke twin) powered microlight, 10k h/w (going backwards) full throttle for about 3 mins before I decided it was useless, the little Rotax started to cough and wheeze, I cycled the throttle a few times and headed back, landed and immediately looked at the exposed engine and carbs to see the remains of ice melting off the venturi area, just downstream of the throttle plates, ice cold to the touch too, and it was a humid, clear, 34 degree day in the tropics, lesson learned.

asyncio
1st Apr 2010, 09:54
I can't remember which engine manufacturer it was, but I read in a technical document by them that all their carburettor based engines could be subject to icing at up to 32C.
There is the CAA safety sense document http://www.caa.co.uk/docs/33/ga_srg_09webSSL14October.pdf which has a diagram showing the same thing.

Also, as pointed out by Graybeard, where an engine can suffer from carb icing even at full power.