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  #1  
Old 05-18-2014, 08:01 PM
Paul 5r4 Paul 5r4 is offline
 
Join Date: Oct 2006
Location: Foley, Al
Posts: 632
Default High CHT's resolved and what worked.

Hi Friends...
History. I've been flying the RV 7A for two years. On climb out the number two CHT would always hit 400 before I reached 1000 feet! I had learned to live with it doing a kind of step climb thing.... level off/thottle back/ cool/then climb some more or enter a shallow cruise climb. I had tried the usual suggestions tight baffles etc. without success. Here's what worked and worked GREAT with several pics.

You MUST/HAVE TO/WITHOUT FAIL.... get airflow beneath the forward part of the number two cylinder. I cut a hole then built a channel for air to get beneath that cylinder. Success! Now I can climb to around 4000 feet before I hit the upper 390? range. Around 4000 feet, the temperature levels off then starts dropping because of the cooler OAT's. Having such great success with the number two cylinder I decided to attack the number three CHT the same way. It didn't get quite as hot as number two but it was pretty close. Yesterday just for fun I took off and climbed at 120 kts all the way to 10,500. Incidentally, I was heading into the wind during by climb to 10,500 and my groundspeed had dropped to 99 mph! When I turned around to head home I was doing 240 mph across the ground! Wish I could do that every time I fly!

Straight up... If a rather sloppy RTV application offends you, better bail here.
Otherwise, here are the pis of the CHT fix. (I had reached the point that I was getting ready to start cutting the cowl to increase exit airflow. Happy now I had not done that yet!!!)

You can visualize the airflow to the fins

Number two cylinder from the bottom of the baffle

Behind number three cylinder. Note minimal clearance for airflow


Hole cut. I almost cut the wrong section of the baffle

From the back

Front/inside of baffle

Looking down through newly created airflow channel

From the back behind number three cylinder
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Foley, Alabama
N729PG..... 500+ hrs
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pilotforfun2001@yahoo.com
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Last edited by Paul 5r4 : 05-18-2014 at 08:26 PM.
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  #2  
Old 05-18-2014, 09:53 PM
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DanH DanH is offline
 
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Location: 08A
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http://www.vansairforce.com/communit...ad.php?t=37835
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  #3  
Old 05-18-2014, 09:55 PM
BobTurner BobTurner is offline
 
Join Date: Dec 2011
Location: Livermore, CA
Posts: 7,859
Default Right answer, wrong physics

Quote:
Originally Posted by Paul 5r4 View Post
Around 4000 feet, the temperature levels off then starts dropping because of the cooler OAT's.
The CHTs start dropping because the engine is putting out less power. True, as you go up, OATs drop - but so does air density. Air density dominates. If the engine could put out constant power you would see CHTs rise with altitude.
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  #4  
Old 05-18-2014, 10:52 PM
Paul 5r4 Paul 5r4 is offline
 
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Location: Foley, Al
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Dan, yours is a perfect and elegant solution. I wish I would have done mine that way. Sure looks better. End result is the same though and I'm thankful it worked.

Bob, Yes the engine produces less power with increasing altitude.... and truthfully I didn't think of that. With that said, as I previously mentioned I couldn't get to 1000 feet without hitting 400? and the temperature was still heading well north of that 400 mark as was cylinder no. three. I successfully brought my cylinder head tips down to normal levels in the first 4000 feet of climb... I'll let physics take care of the rest and thank you for helping me understand better. :-)
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N729PG..... 500+ hrs
RV 7A, Lycoming 0 320 D1A, Sensenich FP propeller
pilotforfun2001@yahoo.com
VAF supporter $$$
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  #5  
Old 05-19-2014, 08:17 AM
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DanH DanH is offline
 
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Quote:
Originally Posted by BobTurner View Post
The CHTs start dropping because the engine is putting out less power. True, as you go up, OATs drop - but so does air density. Air density dominates. If the engine could put out constant power you would see CHTs rise with altitude.
Air density is indeed dropping, which is the same as saying that available cooling mass flow is also dropping.
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  #6  
Old 05-19-2014, 10:16 AM
Sig600 Sig600 is offline
 
Join Date: Nov 2005
Location: KRTS
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Quote:
Originally Posted by BobTurner View Post
The CHTs start dropping because the engine is putting out less power. True, as you go up, OATs drop - but so does air density. Air density dominates. If the engine could put out constant power you would see CHTs rise with altitude.
Also true. CHT's can be the limiting factor in a turbo charged airplane going high. You get the same internal pressures and heat as sea level, with much less cooling mass.
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  #7  
Old 05-19-2014, 04:02 PM
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Titan-Xpert Titan-Xpert is offline
 
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The question I have for you is, are you sure you have changed the cylinder temperatures or just the temperature indication? You basically are pushing cooler air directly at the temp probes this can cause a change in indicated temperature, but it might not change actual cylinder temperature.
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  #8  
Old 05-19-2014, 08:53 PM
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RV10inOz RV10inOz is offline
 
Join Date: May 2008
Location: Brisbane Qld. Aust.
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I will support the post above. My thoughts exactly.

Unless you take 6 probes placed all around the cylinder head and retest it you can be creating egg shaped Vs round.

This was done for the GAMI STC's for cooling on Bonanza cylinders, and nothing happens as you think it might, so hard data done with some proper analysis is the only way to know.
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  #9  
Old 05-19-2014, 10:07 PM
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DanH DanH is offline
 
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Quote:
Originally Posted by Titan-Xpert View Post
The question I have for you is, are you sure you have changed the cylinder temperatures or just the temperature indication? You basically are pushing cooler air directly at the temp probes this can cause a change in indicated temperature, but it might not change actual cylinder temperature.
Interesting supposition. I would reason that if the new airflow isn't changing cylinder head temperature, then it is certainly not harming anything.

Of course, if the airflow is having a effect on the cylinder head, how is it different from the existing airflow to the same area of cyls 1 and 4?
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Last edited by DanH : 05-19-2014 at 10:24 PM.
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  #10  
Old 05-20-2014, 08:35 AM
aerhed aerhed is offline
 
Join Date: Dec 2008
Location: Big Sandy, WY
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No biggie. The manufacturers have used that trick for years. "What do you mean the temps too high to certify? Oh well, put a blast tube on it. "
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