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  #81  
Old 03-29-2019, 10:31 AM
lr172 lr172 is offline
 
Join Date: Oct 2013
Location: Schaumburg, IL
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Default

Quote:
Originally Posted by Monkey View Post
The intake side of the head doesn't need as much cooling air flow as the exhaust due to the incoming air being cool. That's why the fins are shorter on the intake side but I think it needs more that it gets when the baffles are up tight against the head. Probably needs to be the same clearance as between the two cylinders. I wonder if too much cooling air going around the cold side would be a bad thing as in making the cylinder have a 'cool' spot or the head not being a more uniform temp all the way around? I guess the head needs several CHT probes around it to see what is really going on.
For the fins that wrap all the way around the head (all but the last two fins under Dan's black tape), they are dissipating heat from the crown of the combustion chamber. This crown/combustion chamber top has a fairly even temperature across it. The intake seats are pulling less heat from the valve, but is not significant in the grand scheme of the heads heat dissipation. Yes, they intake chamber area is cooler than the exhaust, but the bulk of the fins that are being dealt with here carry just as much heat away as their neighbors on the exhaust side.

Larry
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Last edited by lr172 : 03-29-2019 at 10:34 AM.
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  #82  
Old 03-29-2019, 09:34 PM
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sglynn sglynn is offline
 
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Location: Anacortes, WA
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Default flying result of denting the aft baffle

Okay, I hammered a dent in the rear baffle to let a little air behind #3 cylinder. I kept the dent only on the top part of the cylinder where the fins go flush with the cylinder. Today's test showed it below temp of the other 3 cylinder. Others were about 325 and #3 was 315F. At first I thought it was too low but after awhile (30 minutes) it came up to match the others. I might plug the slot a little if it continues to be lower than the other three. Cylinders 1,2, 4 are all about the same. So making the dent to open up air flow definitely lowered the temperature. And it doesn't take much space. !/8" max.
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  #83  
Old 03-30-2019, 07:49 AM
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DanH DanH is offline
 
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Quote:
Originally Posted by sglynn View Post
Okay, I hammered a dent in the rear baffle to let a little air behind #3 cylinder. I kept the dent only on the top part of the cylinder where the fins go flush with the cylinder. Today's test showed it below temp of the other 3 cylinder. Others were about 325 and #3 was 315F. At first I thought it was too low but after awhile (30 minutes) it came up to match the others. I might plug the slot a little if it continues to be lower than the other three. Cylinders 1,2, 4 are all about the same. So making the dent to open up air flow definitely lowered the temperature. And it doesn't take much space. !/8" max.
Nice. Steve, those temperatures are quite low, which suggests some headroom for the more difficult cooling regimes, and the possibility of exit throttling for cruise.

"Difficult" usually means low dynamic pressure and high AOA. What do you get (1) in extended full power climb at some typical speed, and (2) when you slow down to 90 knots and hold it there in level flight for 20~30 minutes?

BTW, I'm never made a big effort to even out four CHT indications. Within practical limits, they are really four independent engines, and the order of warmest to coolest tends to change with different flight conditions.
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  #84  
Old 04-02-2019, 09:37 AM
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sglynn sglynn is offline
 
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Originally Posted by DanH View Post
Nice. Steve, those temperatures are quite low, which suggests some headroom for the more difficult cooling regimes, and the possibility of exit throttling for cruise.

"Difficult" usually means low dynamic pressure and high AOA. What do you get (1) in extended full power climb at some typical speed, and (2) when you slow down to 90 knots and hold it there in level flight for 20~30 minutes?

BTW, I'm never made a big effort to even out four CHT indications. Within practical limits, they are really four independent engines, and the order of warmest to coolest tends to change with different flight conditions.
Per your question:
Yesterday 25x25 climb; OAT=55F, Alt=4k. The CHTs = 404, 399, 380, 392.
Cruise 23x23 CHTs= 354, 352, 342, 343.
At idle after flying awhile CHTs = 342, 330, 342, 335.
Haven't done 90Kts test.

I have one high EGT (also #3) it runs about 200F higher. I just read it may be non-firing plug. I'll check that.
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  #85  
Old 01-31-2020, 04:04 PM
Jonnyo Jonnyo is offline
 
Join Date: May 2018
Location: Fairbanks
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Default Naca Cylinder Cooling Report

Quote:
Originally Posted by DanH View Post
The best (i.e. most recent) data for air cooled engine design is probably found with the SAE. I'm not a member and not curious enough to spend non-member money for SAE books.

Heat transfer from metal to air is governed by temperature delta. The air entering a full wrap baffle is at ambient temperature, so heat transfer from the hot metal is high. Further around the cylinder the air is now much warmer, and heat transfer is lower. The result is less and less cooling as the air proceeds further around the cylinder.

There's an early NACA work illustrating the issue. See Table 1 and Fig 2:

http://ntrs.nasa.gov/archive/nasa/ca...2009013404.pdf

It is interesting to note that our stock baffle scheme is something like Fig 2a; small exit radius, close entrance width, no exit duct...the worst configuration tested in terms of uneven cooling and energy loss.

To be fair the deformation of the cylinder from perfectly round to some unknown shape may be minimal. I don't know. It should also be noted that there is a difference between cooling cylinders and cooling cylinder heads. Obviously the fin area is shifted toward the exhaust side, and aluminum has a far higher ability to transfer heat by conduction as compared the steel barrels.

Like you I'm tempted to try full wraps. I have hesitated because of economics, not lack of curiosity; IO390 cylinders are very expensive. I've mostly concentrated on extreme sealing and a bit of airflow control to the regions left lacking. I do have partial wraps on the barrels between cylinders, thus each cylinder has symmetrical airflow.

Some of the EZ bunch have done radical cooling experiments. You might look there.



Wow ! Thanks for taking the time to post the NACA report. Real data is sweet, as opposed to what " your friend heard at Joe's hanger". I printed out the data spread sheet and highlighted the best numbers. I will try and post that tomorrow.

Thanks again,

Jonny o
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  #86  
Old 01-31-2020, 04:43 PM
PilotjohnS PilotjohnS is offline
 
Join Date: Jan 2016
Location: Southwest
Posts: 2,163
Default Cht balance

People seem to want to balance cht’s between cylinders, but isnt it more important to balance temps across individual cylinders? For example, banging on the baffles on cy 3 lower the temps, but it still seems to me the part of the cylinder next to cy 1 still has much more air flow than the part that is next to the baffle. Wouldnt one wNt to have the same amount of airflow on all sides of a cylinder?
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  #87  
Old 09-05-2020, 09:16 PM
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rv6n6r rv6n6r is offline
 
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Default

When I had my baffles off for some other work recently I came across this thread. I made the suggested modifications to my #2 and 3 baffles (using the riveted on bypass method), and also modified (lowered) the dam in front of #1, and came away with 15-20 degree cooler CHTs on those cylinders. Which might have been expected given the comments on the thread but still I was surprised that such a simple mod could make such a big difference. Thanks Dan and the VAF community!
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  #88  
Old 09-05-2020, 09:35 PM
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1001001 1001001 is offline
 
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Quote:
Originally Posted by DanH View Post

There's an early NACA work illustrating the issue. See Table 1 and Fig 2:

http://ntrs.nasa.gov/archive/nasa/ca...2009013404.pdf
This link to the report is broken. Does anyone have the title, author, document id, etc. of this report so I can look it up?


Oh hey, maybe this is it: https://ntrs.nasa.gov/citations/20090014186

Last edited by 1001001 : 09-05-2020 at 09:36 PM. Reason: found a link?
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  #89  
Old 09-06-2020, 08:10 AM
BillL BillL is offline
 
Join Date: Sep 2007
Location: Central IL
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Quote:
Originally Posted by 1001001 View Post
This link to the report is broken. Does anyone have the title, author, document id, etc. of this report so I can look it up?


Oh hey, maybe this is it: https://ntrs.nasa.gov/citations/20090014186
You can always look it up and download. also, check Dans website, he posts some file for download.

If you really can not find it, give me an EMAIL and I will forward a copy.
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  #90  
Old 03-18-2022, 05:06 PM
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wirejock wirejock is offline
 
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Location: Estes Park, CO
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Default Baffle Mod

Resurrecting this thread.
Mine is an IO-360-M1B. Exactly like this one. Photo.
Notice the area in red. That's basically where the cut out will land. I can't get the cut out hole exactly over the problem area without busting edge distances.
Click image for larger version

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I started the relief area. Notice the pointer. That corresponds to the spot on the cylinder where fins are flat. I haven't cut the hole yet but I don't see how I can get much closer without busting edge distances. Shim is .063" and chamfered to fit flush into the bend. Cover plate bend is designed to fit flush and close any leaks.
I don't see any way to get closer to the flat area. Any ideas?
Click image for larger version

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