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  #11  
Old 04-10-2018, 07:17 PM
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Steve Melton Steve Melton is offline
 
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what about a small plate to cover the center, between the pipes?
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RV-9A, Tip-up, Superior O-320, roller lifters, 160HP, WW 200RV, dual impulse slick mags, oil pressure = 65 psi, EGT = 1300F, flight hours = 600+ for all

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  #12  
Old 04-10-2018, 07:24 PM
Kyle Boatright Kyle Boatright is online now
 
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Would VG's help at the inflection point?
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  #13  
Old 04-10-2018, 07:27 PM
Aluminum Aluminum is offline
 
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Quote:
Originally Posted by Kyle Boatright View Post
Would VG's help at the inflection point?
I'd bet $1 that they would reduce net cooling flow. Would be interesting to stick a pitot or two in the opening and burn some avgas in the name of science.
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  #14  
Old 04-10-2018, 07:32 PM
BillL BillL is offline
 
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Go to the vetterman website and look at Larrys work on his afterbody with twin exhaust pipes. The got 4 mph or so IIRC. RV7A, but should apply.

My exit velocity on my SJ (7) is so low that the lower cowl pressure is nearly equal to static pressure up to 170 kts. I just have other issues now to complete phase I, so appreciate your project and sharing.

Note the air wants to come back to the centerline, so a narrowing of the afterbody should help.
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  #15  
Old 04-11-2018, 12:02 PM
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Quote:
Originally Posted by DanH View Post
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.
Dan, have you tested the 'variable exit panel' (aka cowl flap?) during hot weather and/or long ground runs like at OSH to confirm that it provides adequate cooling under those conditions? Or is that testing part of what you refer to by 'long term goal'? I've considered reducing the exit area on my RV-14 to try to pick up some speed (by all accounts it has more cooling than needed), but I'd rather not mess with a cowl flap and prefer to find a fixed exit area that provides an acceptable compromise in all operating conditions...
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  #16  
Old 04-11-2018, 12:40 PM
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bk1bennett bk1bennett is offline
 
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Quote:
Originally Posted by Onewinglo View Post
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.
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https://youtu.be/qlX7UDfP8Ko
I too am an aero engineer and I agree with Steve Smith's take that your aft-facing surface is too steep to keep the flow attached. However, based on my experience, you have done a pretty good job of keeping the flow-separation damage to a minimum. The separation bubble is not very big and is not very strong. You could easily do much worse!

Unfortunately, anything you do to add frontal area requires a lot of length to smooth it out downstream to avoid flow separation. The fuselage is a good example. You rapidly increase frontal area, then you spend about 10 ft or more fairing it back to a small cone to keep drag low.

Did you substantially increase frontal area in the forward cowl? I am a bit uncertain on how extensive your modification is to the outer moldline of the RV. It seems like you modified two cowl parts ... is that correct?
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  #17  
Old 04-11-2018, 03:52 PM
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Onewinglo Onewinglo is offline
 
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Default No frontal area increase

Quote:
Originally Posted by bk1bennett View Post
I too am an aero engineer and I agree with Steve Smith's take that your aft-facing surface is too steep to keep the flow attached. However, based on my experience, you have done a pretty good job of keeping the flow-separation damage to a minimum. The separation bubble is not very big and is not very strong. You could easily do much worse!

Unfortunately, anything you do to add frontal area requires a lot of length to smooth it out downstream to avoid flow separation. The fuselage is a good example. You rapidly increase frontal area, then you spend about 10 ft or more fairing it back to a small cone to keep drag low.

Did you substantially increase frontal area in the forward cowl? I am a bit uncertain on how extensive your modification is to the outer moldline of the RV. It seems like you modified two cowl parts ... is that correct?
No we did not change the frontal area of the lower cowl. We simply added the sub cowl and changed/reduced the exit in an effort or reduce cooling drag. The link below has a couple images that will help in my explanation.
Thanks for the comments!

https://photos.app.goo.gl/wUgNCDJEEkjXS5A32
https://photos.app.goo.gl/GRXanzkgtVp85Y223
https://photos.app.goo.gl/PiLQ3QxUtDN8iQIz2
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  #18  
Old 04-11-2018, 04:03 PM
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Default Inflection Point?

Quote:
Originally Posted by Kyle Boatright View Post
Would VG's help at the inflection point?
Kyle, I hate to display my ignorance, but is the inflection point on the following edge of the exit opening?
Thanks!
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  #19  
Old 04-11-2018, 05:17 PM
Kyle Boatright Kyle Boatright is online now
 
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Quote:
Originally Posted by Onewinglo View Post
Kyle, I hate to display my ignorance, but is the inflection point on the following edge of the exit opening?
Thanks!
I may have used an inappropriate term, but what I was referring to is the sharp radius I *think* I see where your new exit ramp turns parallel with the direction of flight.
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  #20  
Old 04-11-2018, 07:06 PM
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DanH DanH is offline
 
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Quote:
Originally Posted by czechsix View Post
Dan, have you tested the 'variable exit panel' (aka cowl flap?) during hot weather and/or long ground runs like at OSH to confirm that it provides adequate cooling under those conditions
Yes, it has seen all conditions, including OSH.

We can talk after S&F if you like.
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