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  #11  
Old 11-18-2019, 11:29 AM
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gmcjetpilot gmcjetpilot is offline
 
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Quote:
Originally Posted by DanH View Post
Nope.

Mass flow (cooling air in) is fundamentally determined by exit area, not inlet area. The measure of a cooling inlet is how well it harvests available dynamic pressure. There is no tailoring of inlet size for a particular Lycoming engine. Inlet area can be large or small, depending on the choice of internal or external diffusion.
Well you put the word fundamentally. Yes exit area may be primary but inlet area has to be sized for the engine as well as exit area; there are rules of thumb of inlet/exit ratios. Inlet area is a factor.
Example you will reach a max exit air area where the inlet will limit. To make my point if you use two 4" round inlets and go to 2" you will have less cooling. Absurd example (way roo small inlet area) but inlet and outlet ratios count. Mass flow does count as well as pressure. Many cooling issues are due to poor airflow into the cowl. I bought inlet rings from Dave Anders and modified my stock cowl; he had a larger size for the 200HP vs 180HP. It makes sense a larger engine does need more cooling. Not saying exit does not drive cooling but there are limits based on mass flow and pressure in.
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Last edited by gmcjetpilot : 11-18-2019 at 11:49 AM.
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  #12  
Old 11-18-2019, 09:26 PM
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Quote:
Originally Posted by gmcjetpilot View Post
Yes exit area may be primary but inlet area has to be sized for the engine as well as exit area; there are rules of thumb of inlet/exit ratios.
George, do some reading, please.

http://www.n91cz.net/Interesting_Tec...dingBasics.pdf

Now consider...for any given engine, a designer may select a low velocity ratio inlet, or a high velocity ratio inlet with appropriate internal diffusion. The resulting upper cowl pressure can be the same, thus either choice would have the same exit area for the same mass flow....but the inlet/exit area ratios would be quite different.
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  #13  
Old 11-20-2019, 08:55 AM
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Quote:
Originally Posted by DanH View Post
George, do some reading, please.

http://www.n91cz.net/Interesting_Tec...dingBasics.pdf

Now consider...for any given engine, a designer may select a low velocity ratio inlet, or a high velocity ratio inlet with appropriate internal diffusion. The resulting upper cowl pressure can be the same, thus either choice would have the same exit area for the same mass flow....but the inlet/exit area ratios would be quite different.
Dan, I have been following your experimentation for over a decade and know you have a passion for this subject. I very much have enjoyed it. We both have read the NASA Rpt 3405 (1981). After decades of research and some experimentation on my RV (no where near as much as you have done), leads me to my point.

Exit is very important and as Chris Zavanson says in the link your provided (I have his web page book marked for a long time), EXIT is the throttle. That is a good way to look at it and why engine cooling cowl flaps have been used almost from the beginning of powered flight, but really took on new meaning in the 30's and 40's during WWII. There really is nothing new under the sun, just variations in the theme. Once jets took over research into cooling of piston engines stopped except for the one off NASA/Ms State Report (which left some loose ends). People like you, Dave Anders, LoPresti and many others have taken the concepts further.

My subtle point which I have come to understand is the cooling of Horz opposed piston engines is a system. In Chris's article he spends a lot of time describing how important the INLET is. That is my point. From the INLET to the EXIT and all points between are important not just the EXIT. The devil is in 100's of details and the system has to be "balanced". As you point out there is Vi/Vo ratios, pressures, mass flow are all in play. Making a system that adequately cools over a wide range of Op conditions with less/least drag is not easy given real physical limitations. "Adequate" cooling is also subjective. That is what makes it a fun topic. There is no one solution. Compromises in drag is needed to get that "adequate" cooling at times. The range of RV inlet/exit area ratios I have seen are all over, from 70% to 200% with reported success. The air has to flow into cowl and exit cowl efficiently, while not creating spillage or interference drag with free air-stream. However some cooling drag is unavoidable since air is bumping around inside the cowl.

Bottom line most don't care about the details as you and I do. The common issue posted on VAF is "My RV has high CHT, OT", or "My RV is slow or does". Three are so many things that can cause high Temp indications or a slow RV including cooling drag. Why some have no issues and others seem to have chronic issues is a fun puzzle. However stuffing an IO-390 into a cowl with 24 sq-in total inlet area (may) be a problem regardless of exit area. I think you have larger inlets on your RV and believe your engine is over cooled.
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Last edited by gmcjetpilot : 11-21-2019 at 12:32 PM.
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  #14  
Old 11-20-2019, 12:13 PM
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Quote:
Originally Posted by gmcjetpilot View Post
I think you have larger inlets on your RV and believe your engine is over cooled.
Total 56.52 sq inches. No, it's not over-cooled. Can you tell us why?
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  #15  
Old 11-21-2019, 12:43 PM
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Originally Posted by DanH View Post
Total 56.52 sq inches. No, it's not over-cooled. Can you tell us why?
I was reading your post.... in thread: http://www.vansairforce.com/communit...ad.php?t=63952

Post #18:
http://www.vansairforce.com/communit...0&postcount=18

".... I'm over-cooling a 390 with about 40 sq in and plan to go smaller. Must be witchcraft"

Your Post #4 you make the same point about how exit is the main factor:
http://www.vansairforce.com/communit...65&postcount=4

What Chris Z does not say is INLET is unimportant. Regardless of how big or efficient your exit is, if your inlet is blocked off you engine will over heat. Again my point is it all has to work together and be balanced. Only so much you can do with theory. Craftsmanship and empirical experimentation is needed to optimize. I would not discount the inlets. We have less flexibility with fixed inlets due to the geometry of our SE tractor engine. The exit we have much more room to play with and control. I have seen people put shutters in the inlets. The Russian radials use a shutter to block inlet air.
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  #16  
Old 11-21-2019, 03:20 PM
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Quote:
Originally Posted by gmcjetpilot View Post
I was reading your post....

Post #18:
http://www.vansairforce.com/communit...0&postcount=18

".... I'm over-cooling a 390 with about 40 sq in and plan to go smaller. Must be witchcraft"
56.52 inlet area, 40 sq inches exit area, a ratio of 1/.76, roughly the reverse of the inlet/exit ratio claims of old...which were usually expressed by someone who insisted the exit must be 20% larger "because heated air expands".

The 40 sq in exit was fixed, one of three fabricated to empirically explore appropriate exit area for all conditions. The current exit is variable, with area anything between 30 and 48 inches.

Quote:
What Chris Z does not say is INLET is unimportant.
The thread is about adding exit area to a cowl which won't cool. I'm running about half the stock exit area to cool a 390 in cruise. Yeah, I think the inlets are important.

The inlets are responsible for pressure. Pressure drives mass. How much mass is determined by the exit throttle.

Quote:
Regardless of how big or efficient your exit is, if your inlet is blocked off you engine will over heat.
Kinda describes the path taken by a whole lot of folks....not blocked, but a medium to high Vi/Vo inlet with poor pressure recovery, thus a giant exit to compensate.
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  #17  
Old 03-07-2020, 05:08 PM
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Default High CHT #3

Hey Can someone go over the baffling remark again in more detail? I have a fairly high #3 CHT relative to the other three(IO-375) do I need to increase baffling on the #3 or decrease baffling.

Same Peak EGT when leaning across #3 and #4 cylinders. Also, #2 is running a little low relative to all the others with same peak EGT across #1 and #2. (they are a little behind the #3 and #4, however, I have some new fuel nozzles on the way so should smooth that out.)
Not really worried about the EGT's just some extra information.

Thanks

Andrew
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  #18  
Old 03-07-2020, 05:56 PM
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Why don't you just get that Anti Splat louver, that you can open or close at will. I wish I had it. Permanent louvers were installed in my plane (without my knowledge), and now my oil temp runs too cold in winter.
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  #19  
Old 03-07-2020, 07:02 PM
Northernliving Northernliving is offline
 
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Originally Posted by donaziza View Post
Why don't you just get that Anti Splat louver, that you can open or close at will. I wish I had it. Permanent louvers were installed in my plane (without my knowledge), and now my oil temp runs too cold in winter.
Install an oil cooler shutter and that problem will go away.
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  #20  
Old 03-07-2020, 08:18 PM
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Quote:
Originally Posted by Northernliving View Post
Install an oil cooler shutter and that problem will go away.
Already tried that. Darn thing wouldn't fit. Only thing I've been able to do is partially put (metallic) tape over my oil cooler in winter, and off when it starts getting hot outside.
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