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  #1  
Old 04-10-2018, 10:21 AM
Onewinglo's Avatar
Onewinglo Onewinglo is offline
 
Join Date: Apr 2016
Location: Hammond, LA
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Default Tuft Testing RV-8 Sub-Cowl

My partner and I felt we were overcooling our O-360 XP so he designed and built a sub-cowl to reduce the exit opening and reduce cooling drag. We expect the reduction in exit opening will increase the CHT and oil temp. Testing has been limited due to weather and schedule but we have noticed a very modest increase in CHT and oil temp. I tufted the belly around the exit and recorded the flight to get an idea of how the sub-cowl disturbed the airflow. The test was at 3000' and 160 - 170 mph. A link to a short video is attached.
RV-8 with custom plenum and Sam James Cowl
Carbureted 360XP with Mags
FP Cato 3 bladed prop
https://youtu.be/qlX7UDfP8Ko
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  #2  
Old 04-10-2018, 10:51 AM
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That didn't work so well. Darn shame...it's pretty glass work.
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  #3  
Old 04-10-2018, 11:05 AM
Aluminum Aluminum is offline
 
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Neat, what camera was used to record this?

You should round the sharp edge aft of the exhaust pipes to >1" radius and add a little (0.25" - 0.5") spoiler on the opposite (fore) edge of the opening to separate the ambient stream so it rejoins the cowl exit air with minimal turbulence (can't tuft test this part, trial and error or CFD).
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  #4  
Old 04-10-2018, 11:28 AM
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Onewinglo Onewinglo is offline
 
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Default So Far, So Good

Quote:
Originally Posted by DanH View Post
That didn't work so well. Darn shame...it's pretty glass work.
Its better than we expected so we are calling it a success, so far. I'm interested in CHTs and oil temp in the heat of the summer AND any noticeable change in performance flying high and fast.
This a lot of fun!
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  #5  
Old 04-10-2018, 11:36 AM
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Onewinglo Onewinglo is offline
 
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Quote:
Originally Posted by Aluminum View Post
Neat, what camera was used to record this?

You should round the sharp edge aft of the exhaust pipes to >1" radius and add a little (0.25" - 0.5") spoiler on the opposite (fore) edge of the opening to separate the ambient stream so it rejoins the cowl exit air with minimal turbulence (can't tuft test this part, trial and error or CFD).
Thanks for the pointers Dan V.
The camera is a GoPro Hero 5. The mount was a Cessna strut mount attached to the gear leg with a double ball and socket arm. The ball and socket was fine at 160 mph but slipped at 200 mph.
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  #6  
Old 04-10-2018, 11:57 AM
scsmith scsmith is offline
 
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The ramp surface of your sub cowl is separated because the angle is too steep, so there is too much adverse pressure gradient.

I trust that the leading edge of your sub cowl blends smoothly into the round lip at the front of the stock cooling channel? If not, then the flow is separating off of a sharp corner. If yes, then it is an adverse gradient problem as stated above.

This is just one of the reasons why it is far better to throttle the exit by making the actual cowl fair into to a smaller opening rather than add a bulge to the belly. The other reason of course is that the frontal area is reduced at the same time.
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  #7  
Old 04-10-2018, 12:01 PM
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Bob Martin Bob Martin is offline
 
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Default nice work

I agree with Dan, nice glass work.
I was tired after doing my James cowling....
Bravo to you guys continuing on...
Are the inlets stock? Interior baffeling Vans?
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  #8  
Old 04-10-2018, 03:25 PM
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Onewinglo Onewinglo is offline
 
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Quote:
Originally Posted by scsmith View Post
The ramp surface of your sub cowl is separated because the angle is too steep, so there is too much adverse pressure gradient.

I trust that the leading edge of your sub cowl blends smoothly into the round lip at the front of the stock cooling channel? If not, then the flow is separating off of a sharp corner. If yes, then it is an adverse gradient problem as stated above.

This is just one of the reasons why it is far better to throttle the exit by making the actual cowl fair into to a smaller opening rather than add a bulge to the belly. The other reason of course is that the frontal area is reduced at the same time.
Thanks for the pointers Steve. Yes the leading edge has a large radius that spans up to the ramp (cooling channel) above. Therefore, it must be the adverse gradient you pointed out. Thanks for the education, I need all I can get!
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  #9  
Old 04-10-2018, 03:43 PM
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Onewinglo Onewinglo is offline
 
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Quote:
Originally Posted by Bob Martin View Post
I agree with Dan, nice glass work.
I was tired after doing my James cowling....
Bravo to you guys continuing on...
Are the inlets stock? Interior baffeling Vans?
The inlets are the standard round inlets that come on the James cowling. We built a custom plenum that cools very well.
Thanks for the comments.
JP
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  #10  
Old 04-10-2018, 05:16 PM
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DanH DanH is offline
 
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Instead of after-fairing the exit chute, you can elect to remove it, and replace the whole thing with a flush panel, which reduces frontal area, and leaves an exit the width and height of the belly ramp.

I built progressively smaller exits to determine what exit area was really necessary. This one is the last in the series, extended four inches rearward to make the exit smaller due to ramp taper. I could keep the IO-390 under 380F CHT with this size, 1.625" high, but oil would push 215 if run hard. It would not have worked for slow flight and long taxi (like into HBC at OSH), as I would not have been able to keep oil under the limits. With a minimum flyable exit area established, I added a variable exit panel, the long term goal. Note the panel center-pivots, so half the additional exit adds no frontal area when open. It's good for about 16 sq in. Lower cowl internal pressure rises significantly when closed, meaning velocity rises in the primary exit.

The stuff in the first photo is a coaxial pitot-static probe (left) and an exit air temperature probe (right) hidden from the pipe behind an insulated heat shield to block radiant energy from the hot pipe.







Early test flight. The internal pressures are high enough that the first iteration would not fully close the door due to panel bulging. However, watch the tufts when the door closes at 120 knots, both behind the door and behind the primary exit.

https://youtu.be/nA5PY7PYBsU

Adding some structural ribs stiffened the panel enough to allow full door closure: https://youtu.be/aIBXAE2Ezn4
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Last edited by DanH : 04-11-2018 at 05:10 AM.
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