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Overcooling the RV-8? (Warning ? Long Post)

Onewinglo

Active Member
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.
 
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.
 
Nearly the same as my numbers, angle valve, 10:1, tight plenum. I have been considering a reduction in cooling air as well.
 
Thanks for the ratio Robin8er.

Kevin I did read The Shrinking Exit some time back. I'll revisit the thread.
Thanks for the reminder.
 
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.
 
First, how are you measuring to get 84 in^2? My measurements are way less than that. http://www.vansairforce.com/community/showpost.php?p=832396&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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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.
 
Thanks for the reply guys. Yes 84 inches is correct. The opening measures 13.750" wide (at the middle of the trapezoid) and 6.125" high.
My partner and I were looking at how we could cut and raise the bottom about 1.125" (due to the exhaust) but that leaves us with a 68" opening - still very big. I love the idea of different size openings as Dan made but I don't think we are up for that.
Currently we are thinking we can raise the bottom an inch or so and extend the cowling aft a few inches to take advantage of the ramp to further close the exit area.
Hopefully my photo of the exit area will display.
Thanks again for the help!
KLIxjfuaOH2skVZ62

https://photos.app.goo.gl/KLIxjfuaOH2skVZ62
 
Dave Anders

Dave Anders has done a series of articles for Kitplanes over the last few months. I'm not sure how detailed the cowl info is but his outlet is substantially smaller than inlet. The articles do cover the importance of the proper transition from the inlets to the main plenum. I think the current Kitplanes is his final article of the series.
There is also an article about Dave's presentation years ago, I think to a Sacramento area EAA Chapter.
Dave has a highly modified IO 360(angle valve) with modified Hartzell CS in an RV4. I believe it is the fastest RV4 on the planet.
 
Out of curosity, are subtracting the area of the exhaust pipes blocking the exit?
 
Axel, no I did not subtract the area of the exhaust - good point. Makes the exit area a little less Huge.

jrs14855, Thanks for the note about the Dave Anders articles in Kitplanes. I found the second article and will read it tonight.
 
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