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  #21  
Old 04-12-2018, 08:51 AM
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bk1bennett bk1bennett is offline
 
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Quote:
Originally Posted by Onewinglo View Post
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

Personally, I think you did fantastic work. I would wager that if you did a tuft test of the original design, you would find significant improvement. The part you added is probably light, and you may well have improved performance (drag reduction v. weight addition) in addition to getting to a better temperature for operation of the engine. Hard to tell without understanding the "baseline." If it were me, I would repeat the tuft test with the original configuration. Thanks for posting this experiment!
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  #22  
Old 04-12-2018, 04:09 PM
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Onewinglo Onewinglo is offline
 
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Default Baseline

Quote:
Originally Posted by bk1bennett View Post
Personally, I think you did fantastic work. I would wager that if you did a tuft test of the original design, you would find significant improvement. The part you added is probably light, and you may well have improved performance (drag reduction v. weight addition) in addition to getting to a better temperature for operation of the engine. Hard to tell without understanding the "baseline." If it were me, I would repeat the tuft test with the original configuration. Thanks for posting this experiment!
Brian,
Baseline is a good idea. I had not considered that. Our sub-cowl is easy to remove so I may take it off, tuft and record a flight.
Thanks!
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  #23  
Old 04-16-2018, 12:12 PM
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Onewinglo Onewinglo is offline
 
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Default Baseline Tuft Testing (W/O Sub Cowl)

I removed our SubCowl, tufted the belly and recorded the results. The exit flow from the Sam James cowl was pretty good. Better than we expected. Looking closely at the sub cowl we can see where the radius at the rear lip is too steep, or sharp. We may cut this area out and reglass with a larger radius.
Yep, Experimental aviation is fun.

https://youtu.be/eGloXIDuYTM

https://photos.app.goo.gl/XKJVgeca0T3qQXgW2
https://photos.app.goo.gl/j4BWvWKsmvDqsfrv1
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  #24  
Old 04-16-2018, 12:40 PM
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Hard to be sure, but it looks like the bottom surface just to the right of the exhaust stacks is curving up a bit where the air exits. This would tend to force the exit airflow back up toward the pipes, and create a bit of turbulence.

I think a small filler piece inside the lower surface with a smooth airfoil shape on its upper surface would direct the airflow downwards and smooth out the overall airflow. See the right spoon drawing below, imagine it is rotated clockwise 90*, and the inside of your lower cowl fairing.

Coanda effect. A world of info written by the late Paul Lipps.

You can look up his posts here. http://www.vansairforce.com/community/member.php?u=5053

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Last edited by Mike S : 04-16-2018 at 12:42 PM.
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  #25  
Old 04-17-2018, 06:19 AM
BillL BillL is offline
 
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What is the basic size (height width) of your cowl exit, both counting and not counting the chute under the fuse? My 7 SJ exit height (IO cowl) is 1" lower than the stock Vans and the SJ carb cowl. I was worried it might be too much, but now having measured the upper and lower cowl pressures, found the the lower was about equal to static pressure. The oil streaks (repaired now) on the fuse are like your tufting indicates, some retro flow just around the left/right edges.

If you measure pressures, upper/lower, we can share.

Thanks for posting your results!
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  #26  
Old 04-17-2018, 09:33 AM
David Paule David Paule is offline
 
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Since one of your goals is to reduce cooling drag, has there been any performance change with and without it?

Dave
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  #27  
Old 04-17-2018, 11:39 AM
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Onewinglo Onewinglo is offline
 
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Quote:
Originally Posted by Mike S View Post


Hard to be sure, but it looks like the bottom surface just to the right of the exhaust stacks is curving up a bit where the air exits. This would tend to force the exit airflow back up toward the pipes, and create a bit of turbulence.

I think a small filler piece inside the lower surface with a smooth airfoil shape on its upper surface would direct the airflow downwards and smooth out the overall airflow. See the right spoon drawing below, imagine it is rotated clockwise 90*, and the inside of your lower cowl fairing.

Coanda effect. A world of info written by the late Paul Lipps.

You can look up his posts here. http://www.vansairforce.com/community/member.php?u=5053

Mike, you are correct in that the bottom surface does curve up a bit. I see how that could disturb air flow. Thanks for the Coanda effect concept. We may incorporate the shape if we modify this thing.
JP
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  #28  
Old 04-17-2018, 12:39 PM
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Onewinglo Onewinglo is offline
 
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Quote:
Originally Posted by David Paule View Post
Since one of your goals is to reduce cooling drag, has there been any performance change with and without it?

Dave

Dave, we have not flown enough to get any real performance data yet. I'll call it "In Progress" and I'll report back when we have something.
We have noticed a modest increase in CHT and maybe oil temp, but we have lots of margin with our temperatures.
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  #29  
Old 04-18-2018, 08:33 AM
Tom Martin Tom Martin is offline
 
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Over the last 20 years or so I have spent a lot of time on cooling exits, always trying for a balance between cooling and speed.

Lessons learned

1. Not all engines, even those with the same designations, cool the same
2. Until you get the cowling inlets flowing properly, gradually smoothing the inlet area of the plenum and plugging all holes, there is not much use playing around with the outlet
3. Spinner gap seals help the system work properly, and yes the cooling system is just that, from inlets to outlet, a system.
4. the goal at the outlet is to get the air exiting the cowling to align with the outside air.

Number 4 is where I see a problem with the sub cowling outlet in this thread.
the outlet actually forces the air down at an angle to the relative airflow surrounding the aircraft. This will add drag and likely negate any benefits of the part. I would suggest cutting the "bump" out of the part and glassing in the outlet sides so that they are parallel to the airflow. This will get your outlet air travelling in the same direction as the relative airstream. If this works you can then reduce the outlet size to get the outlet required for hot day climbs, or to add a cowl flap of some kind so that you can manually adjust cooling in the air.
You will know if you have things right if the oil from the breather runs straight down the belly and remains attached to the fuselage the entire way.
I have found that extending the bottom of the cowling exit at least two inches aft of the firewall greatly aids in getting the outlet air flowing in the proper direction.
The exit air is pushed out of the cowling due to the differential pressures within the cowling. I have no proof, but I feel that if you get your outlet air flow in the right direction, the outside air may even help to pull the air out.

I have a cowl flap on my aircraft but seldom use it now that have the system flowing properly. If you live in the deep south then you will likely need some adjustment if you are looking for maximum speed/cooling.
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Last edited by Tom Martin : 04-18-2018 at 08:38 AM.
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  #30  
Old 04-18-2018, 09:31 AM
Marc Bourget Marc Bourget is offline
 
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Tom said: " I have no proof, but I feel that if you get your outlet air flow in the right direction, the outside air may even help to pull the air out."

It doesn't, as there's nothing between the molecules to "pull" the exit air out.

Try John Thorp's article from the December 1963 Sport Aviation article on "Cowling and Cooling" will help! (I'll have to learn how to add attachments!)

Till then, the EAA S/A archives will have to suffice!
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