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03-30-2017, 03:46 AM
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Join Date: Sep 2015
Location: UK
Posts: 11
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Carb heat - what temp rise should I see?
For the grimy UK weather, I just fitted a carb heat jacket to my RV-6 (this jacket: https://www.vansaircraft.com/cgi-bin...abin-heat-muff)
I took the air supply for the carb heat from the baffle above cylinder 4, low down on the baffle to get some heat from the flow over the cylinder.
I also have a Dynon with carb heat sensor I just fitted.
At cruise power, I saw a 9 deg F rise in carb temperature when selecting carb heat. Now, that does not strike me as very effective.
I've had it suggested I should restrict the flow of air into the carb heat inlet, but this wont work - the rate of flow of air down there is more or less fixed by the engine and what it is drawing.
What sort of temp rise should I be seeing? Anyone faced this challenge?
JP
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03-30-2017, 09:33 AM
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Join Date: Mar 2013
Location: Flathead Lake Montana - 8S1
Posts: 334
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FAA certification standards require 90*F carb heat heat rise for normally aspirated engines when the ambient air is 30*F and clear of moisture. My experience is this delta is tough to to realize on many of the homebuilts i've been associated with. Far be it for me to advise on what is acceptable or not, but when I've been able to obtain 75-80F delta I'm a happy camper. The best I've seen was a O-320 D3G equipped RV-4 I helped a friend with - he was able to reliably achieve over 100*F temp rise with his carb heat system.
__________________
'83 LongEZ - N888EZ 3,671 hours
'89 LimoEZ - N26EZ 686 hours (sold)
A couple of Glasairs and a Lancair 320...
Next: a RV14 need something to build
Last edited by deek : 03-30-2017 at 09:38 AM.
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03-30-2017, 09:56 AM
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Join Date: Sep 2015
Location: UK
Posts: 11
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Quote:
Originally Posted by deek
FAA certification standards require 90*F carb heat heat rise for normally aspirated engines when the ambient air is 30*F and clear of moisture. My experience is this delta is tough to to realize on many of the homebuilts i've been associated with. Far be it for me to advise on what is acceptable or not, but when I've been able to obtain 75-80F delta I'm a happy camper. The best I've seen was a O-320 D3G equipped RV-4 I helped a friend with - he was able to reliably achieve over 100*F temp rise with his carb heat system.
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Wow thanks. Well, at a 9deg F rise, I'm a ways short of what I need.
What have you used for carb heat, deek? Not the Vans jacket here then?
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03-30-2017, 09:58 AM
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Join Date: Apr 2008
Location: Lake Havasu City AZ
Posts: 2,393
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carb heat
If you have a typical crossover system on a Lycoming with induction on bottom of sump, adequate heat can be obtained with a hose or solid duct CLOSE to the crossover directly above the airbox. In 40 years I have never had carb ice with a Lycoming when operating in VMC conditions. IMC in freezing temperatures is a different scenario. So it depends on whether you want to fly IFR as well as other weather considerations.
Best solution of all is fuel injection.
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03-30-2017, 10:27 AM
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Join Date: Mar 2008
Location: Clarksboro, NJ
Posts: 829
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20F to 30F is about all you will get. I have always questioned how the FAA measures their 90F. I am not sure I believe certified planes actually achieve that in flight. Maybe sitting on the ground with a lower throttle setting...
I would not take the air from above the cylinder. The engine pulls in the air it needs, so you do not need any ram air. I would get your air from the lower side of the engine. You can also put some stainless steal scouring pads inside that heat muff to improve the heat transfer from the exhaust pipe to the air. Check how well your air box seals off the cold air supply when you select carb heat. It does not take much leak-by to cool off your warm air.
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03-30-2017, 10:36 AM
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Join Date: Sep 2015
Location: UK
Posts: 11
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Quote:
Originally Posted by pa38112
20F to 30F is about all you will get. I have always questioned how the FAA measures their 90F. I am not sure I believe certified planes actually achieve that in flight. Maybe sitting on the ground with a lower throttle setting...
I would not take the air from above the cylinder. The engine pulls in the air it needs, so you do not need any ram air. I would get your air from the lower side of the engine. You can also put some stainless steal scouring pads inside that heat muff to improve the heat transfer from the exhaust pipe to the air. Check how well your air box seals off the cold air supply when you select carb heat. It does not take much leak-by to cool off your warm air.
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Thank you - great advice. I'll give it a go. I'll make those changes and report back - but just wondering if that Vans muff is up to the job or I need something better.
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03-30-2017, 10:40 AM
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Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 5,298
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I had the robbin's wing small muff on the cross-over pipe on my O-320. I was able to consistently get a 40 RPM drop during run-up. The key was to seal up the flange on the FAB, so that the vacuum is pulling air through the muff and not through the open area of the flange. The flange included in the Van's FAB kit is for a forced air system and most RV implementations use a pull through system.
While I never measured it at higher power settings, I am sure the temp rise was higher, as the EGT's would have been much higher than at the 1800 RPM run-up level.
Larry
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N64LR - RV-6A / IO-320, Flying as of 8/2015
N11LR - RV-10, Flying as of 12/2019
Last edited by lr172 : 03-31-2017 at 08:11 AM.
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03-30-2017, 03:00 PM
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Join Date: Jan 2005
Location: SC
Posts: 12,887
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Keep an eye on your temperature when you slow the plane down and configure it for landing.
I noticed that with a Lycoming powered RV (OK, my RV), the carb temperature rises when the air through the cowl slows down.
__________________
Bill R.
RV-9 (Yes, it's a dragon tail)
O-360 w/ dual P-mags
Build the plane you want, not the plane others want you to build!
SC86 - Easley, SC
www.repucci.com/bill/baf.html
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03-30-2017, 03:02 PM
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Join Date: Apr 2011
Location: Cincinnati, OH
Posts: 2,791
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Quote:
Originally Posted by RV8JD
I have on a few occasions, in the wet, cool Pacific NW. Others should be careful with this statement.
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I've needed it in the past. You want to make sure it works.
__________________
Steve Melton
Cincinnati, OH
RV-9A, Tip-up, Superior O-320, roller lifters, 160HP, WW 200RV, dual impulse slick mags, oil pressure = 65 psi, EGT = 1300F, flight hours = 800+ for all
Simplicity is the art in design.
My Artwork is freely given and published and cannot be patented.
www.rvplasticparts.com
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03-30-2017, 04:13 PM
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Join Date: Jun 2006
Location: San Jose, CA
Posts: 1,125
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How long should it take to come up the required amount of degrees? Should you see this temperature rise in the few seconds that it takes for the carb heat check during run up?
I have the Robbins Wings heat muff and a Sam James cowl. During run up, I do see a slight decrese in rpm and a very slow increase in carb temperature. I've never run it long enough to see just how high the temperature gets.
__________________
Kelly Johnson
San Jose, CA
RV-9A
Pink slip issued: 5/7/12
First flight: 5/28/12, Memorial Day.
Phase I Complete: 8/18/12!
2020 donation: complete
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