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  #1  
Old 12-04-2017, 11:31 AM
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Onewinglo Onewinglo is offline
 
Join Date: Apr 2016
Location: Hammond, LA
Posts: 18
Default Overcooling the RV-8? (Warning Long Post)

Guys I suspect we are overcooling our RV-8 engine and suffering with excess cooling drag in the process. We built and installed a nearly leak free plenum.
The James Cowling exit area appears to be huge in comparison to the inlet area. The 2 round inlets total 33.6 square inches and the exit measures 84.2 square inches. We plan to reduce the exit area during the condition inspection in January, but the question is how much? Is there an inlet/exit area ratio that we can use as a starting point?
I did a test flight to collect a little data as follows:

RV-8 CHT and Oil Temp Test

Climbed out at 100-110 mph WOT to 8000' DA, Surface OAT 23c. Full fuel with 1 pilot at 170 lbs.
Max CHT as #4, 3, 2, 1 (oil cooler is behind #4)
355, 319, 292, 286
Max oil temp 168

Level at 8000' DA, 2500 RPM, OAT 13c, leaned to 7.5 gph resulted in:
Max CHT as #4, 3, 2, 1
297, 273, 278, 272
Max oil temp 168
Indicated/true AS in mph 166/187

WOT at 8000' DA, 2850 RPM, same lean position, 11.4 gph:
Max CHT as #4, 3, 2, 1
333, 306, 318, 311
Max oil temp 170
Indicated/true AS in mph 192/216

150 hours on the engine / airframe; First flight - January of 2017
Superior O360XP with 9:1 pistons and Phase 3 camshaft, Slick mags; Vetterman 4 into 2 exhaust, no mufflers
James cowling; Cato 3 blade composite FP prop; Grove airfoil gear; Wheel Pants and all fairings in place
Bottom line, I have never seen a CHT > 360 at any time, even the heat of a Louisiana summer.
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Old 12-04-2017, 11:43 AM
Robin8er Robin8er is offline
 
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Location: Oahu HI
Posts: 173
Default

I seem to recall that you want approximately a 1:1.1 entrance to exit area ratio (exit area slightly bigger). I cannot recall where I read that.
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  #3  
Old 12-04-2017, 11:55 AM
krwalsh krwalsh is offline
 
Join Date: Nov 2009
Location: San Francisco, CA
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Default The Shrinking Exit

Have you read through this thread?

http://www.vansairforce.com/communit...ad.php?t=68241
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Old 12-04-2017, 12:14 PM
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ColoRv ColoRv is offline
 
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Default

Nearly the same as my numbers, angle valve, 10:1, tight plenum. I have been considering a reduction in cooling air as well.
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  #5  
Old 12-04-2017, 12:50 PM
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Onewinglo Onewinglo is offline
 
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Location: Hammond, LA
Posts: 18
Default

Thanks for the ratio Robin8er.

Kevin I did read The Shrinking Exit some time back. I'll revisit the thread.
Thanks for the reminder.
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  #6  
Old 12-04-2017, 12:56 PM
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DanH DanH is offline
 
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There is no valid inlet- exit ratio.
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  #7  
Old 12-04-2017, 01:09 PM
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rv6ejguy rv6ejguy is offline
 
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Location: Calgary, Canada
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Default

Use a cockpit adjustable cowl flap and make your life easier with less time spent reshaping cowling exits multiple times. A fixed exit will always be a compromise between high power climb and cruise conditions.
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Old 12-05-2017, 12:53 AM
gasman gasman is offline
 
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Location: Sonoma County
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" Is there an inlet/exit area ratio that we can use as a starting point? "

Contact Dave Anders, he figured this all out over 18 years ago. http://www.vansairforce.com/communit...ad.php?t=26615
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  #9  
Old 12-05-2017, 06:42 AM
BillL BillL is offline
 
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Location: Central IL
Posts: 4,007
Default

First, how are you measuring to get 84 in^2? My measurements are way less than that. http://www.vansairforce.com/communit...6&postcount=30

Regardless of the numbers You might want to experiment to ensure you are managing (restricting) the air across the engine and cooler. i.e. begin to reduce the flow across heads, barrels, and through the cooler. I have been warned to go slower in doing this on a parallel valve as the fin heat x-fer efficiency is less than the angle valve. Heat tape can be used temporarily.

What are your exit gaps for the baffles under the head and barrels? Are they equal? 1" and 2.25"? The lower gap seal between cylinders had huge gap leaks on mine. These won't help cooling, but will help when managing the airflow.

The IO SJ cowl opening"height" is about 1 inch less than the "O" cowl. The "O" has the lower floor to accommodate the carb and inlet. I measure ~59 in^2 on the O (same as Vans stock) and ~48 in^2 on the IO SJ. Your cowl may vary.

edit: The first thing DanH will say (in addition to "forget about the ratios") is measure the upper and lower plenum pressures. He has dimensions for the piccolo tubes in the shrinking exit thread.
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Last edited by BillL : 12-05-2017 at 10:30 AM.
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  #10  
Old 12-05-2017, 08:23 AM
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DanH DanH is offline
 
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JP, is the 84 sq in figure a typo? To use a popular phrase, that is huge. Stock RV-8 is about 60 sq in if I recall correctly.

Recall I modified the cowl during construction with nutplates so the exit section could be swapped, then built three fixed exit panels. Any fixed exit will indeed be a compromise, but swappable panels allow changing with the seasons, and offer a lot of practical observation toward any future change. There's no great difference in the fabrication work required to make a removable exit panel as compared to modifying the cowl itself, and additional panels are way less work after the first.

Ultimately I built a 4th exit panel, reducing fixed area even further, with an additional 18 sq in of variable area. The fixed exit is a slot equal to the width and depth of the RV-8's recess at a point 4" aft of the firewall. Nothing but a single bump for the single tailpipe extends below the plane of the belly skin.

The variable exit design is unlikely to add much form drag. When open, half the additional area is a recess, and the other half doesn't protrude out into the freestream very much, or depend on generation of low pressure in its wake. There is a drag increase when open, but it's small, and to be expected due to additional, slower mass flow.

Having done both, there is no way I'd go back to a fixed exit on an aircraft with the speed range of an RV.

There is no valid inlet-exit area ratio because a designer may generate the same upper deck static pressure with either internal or external diffusion inlets. Equal pressure would require the same exit area for the same mass flow. Internal diffusion inlets will be small while external diffusion inlets will be large, so any notion of a "correct" ratio is goes right out the window.
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