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Old 04-18-2018, 09:41 AM
Tom Martin Tom Martin is offline
Join Date: Jan 2005
Location: Ontario, Canada
Posts: 1,402

If I may rephrase; a vacuum could be created with the passage of the faster moving airstream, relative to the slower air mass exiting from the cowling which could be at a lower pressure then the passing airstream.
When I did my differential air pressure tests I did not test the relative pressure exiting the cowling relative to the airstream around the bottom of the fuselage. I will refrain, in the future, from using speculation.
Tom Martin RV1 pilot 4.6hours!
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EVO F1 Rocket 1000 hours,
2010 SARL Rocket 100 race, average speed of 238.6 knots/274.6mph
RV4, RV7, RV10, two HRIIs and five F1 Rockets
RV14 Tail dragger flying #153

Fairlea Field
St.Thomas, Ontario Canada, CYQS

Last edited by Tom Martin : 04-18-2018 at 09:55 AM.
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Old 04-20-2018, 05:57 PM
Onewinglo's Avatar
Onewinglo Onewinglo is offline
Join Date: Apr 2016
Location: Hammond, LA
Posts: 39

Originally Posted by Tom Martin View Post
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.
Thanks for sharing your experience on this issue. I'm sure you are correct about the exit air should be aligned with the slipstream. After tuft testing and pondering everyone's suggestions the concept is getting much clearer. Our current plans are to fly for a while and evaluate a modification later.
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