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  #1  
Old 01-05-2018, 08:13 AM
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DonFromTX DonFromTX is offline
 
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Default Intake Air Option

I was lurking around a friends Rotax powered plane yesterday, and noticed this intake manifold treatment. It always seemed a bit wrong to feed our engine heated air to burn. Has anyone ever played with alternate intake designs?
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  #2  
Old 01-05-2018, 09:10 AM
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Everyone always talks about heated air feeding the RV-12 Rotax. I’d be interested if someone has taken actual carb intake air temps while in flight.

I’ll bet there’s plenty of ram air through the single fresh air hole on the right side of the spinner especially when coupled with the large venture-type exit at the bottom of the cowling. I'm guessing that the carbs see near ambiant air temp in cruise flight.
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  #3  
Old 01-05-2018, 10:35 AM
rv7charlie rv7charlie is offline
 
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Are those constant velocity carbs? I wonder how they react to a pressure recovery air feed (common on Lyc powered RVs, etc). Just speculation, but that might be why they are fed from 'ambient' air in the cowl.
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  #4  
Old 01-05-2018, 11:06 AM
rvbuilder2002 rvbuilder2002 is offline
 
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Quote:
Originally Posted by rv7charlie View Post
Are those constant velocity carbs? I wonder how they react to a pressure recovery air feed (common on Lyc powered RVs, etc). Just speculation, but that might be why they are fed from 'ambient' air in the cowl.
Apparently it works fine as long as the carb. vent tubes are at the same air pressure as the inlet of the carb.
That is why the carb vent hoses are connected to the induction airbox on the turbocharged 914 or when the optional Rotax induction airbox is used on the 912.

I have mentioned previously....... during the development of the RV-12, testing was done with an airbox fed by outside air. The performance difference was determined to be small enough that it didn't justify the cost and complexity of making that part of the design
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  #5  
Old 01-05-2018, 12:32 PM
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On my other RV12 I made sure I had cool induction air - and it lets out heat when parked as well!
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  #6  
Old 01-05-2018, 01:25 PM
DHeal DHeal is offline
 
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It has also been suggested that the somewhat heated cowling air breathed by our RV-12 carbs serves to reduce the potential for carb ice. I'm not sure how much this helps, but IMHO it can't hurt in most situations. If an unheated ambient air source were to be used, a supplemental carb heat system (water, electric or exhaust pipe muff) might be appropriate.

ps -- For a real educational moment regarding how cold carbs can get while operating, just feel the temperature of the carb after its been running awhile - perhaps after a carb sync check. It is noticeably colder than the adjacent intake manifold. The carb/manifold rubber connector effectively isolates the carb from the engine's heat.
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  #7  
Old 01-05-2018, 06:33 PM
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Quote:
Originally Posted by DHeal View Post
ps -- For a real educational moment regarding how cold carbs can get while operating, just feel the temperature of the carb after its been running awhile - perhaps after a carb sync check. It is noticeably colder than the adjacent intake manifold. The carb/manifold rubber connector effectively isolates the carb from the engine's heat.
The J3-Cub I owned for more than twenty years had an updraft carb on Continental A65-8 engine. You could view the entire carb through the open cowling. I would land on a hot summer day and the carb would be dripping wet (literally) from condensation - cold carb / humid air.
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80 hrs Flying Aeronca Chief 11AC
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