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fiberglass oil cooler shroud shape opinions please

dwranda

Well Known Member
Making my first attempt at a fiberglass oil cooler shroud. Can I get some opinions of the shape and if the consensus is that it will work OK? I still have some detail work to do on it then I will use some putty to make it smooth before using it as a mold for the fiberglass.
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Basic Shape seems reasonable to me. What size duct are you using off the baffles? 3 1/2 inch D is bare minimum, 4 inch better. If you can figure a way of doing it, a rounded transition into the duct at the baffles will have significantly better airflow than a sharp edge.

Erich
 
One more thought, just in case. Make sure your scat hose or whatever you are using for the duct will actually bend around the angle between the back of the baffles and the oil cooler plenum in the space you have available. The space necessary to accommodate that turn was a lot bigger than I imagined when I did mine.

Regards

E
 
Hi Dave, its looking good so far! I would make one suggestion though. Keep the physical duct as short as possible. Once that duct is in there you will find it hard to reach behind the engine to do things like check the PMags or wipe the oil off of things. If you keep the duct short you can simply remove the scat hose and have good access without removing the duct.

Darin
 
If you shorten your transition duct a little bit, and/or have a little bit less of a turn, you might be able to get it into position where it is a single segment of a circular arc to go from the baffle attach to your transition duct. If you can do that, then you can use Scott McDaniels' trick of using an old 6.00x6 inner tube as the duct. Much smoother inside than Scat, much cheaper, and easier to install/remove for access to otehr things.
 
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A little late to the party, and Steve should be able to give much better advice.

But according to Kuchemann & Weber,
https://books.google.com/books/about/Aerodynamics_of_Propulsion.html?id=aRwIAQAAIAAJ
the shape along the last few percentage points of the length of the diffuser are counter-intuitive. They show a 'streamline' diffuser looking somewhat like the bell of a trumpet, where the duct reaches maximum area where it touches the face of the heat exchanger. (Apologies to Steve; redneck engineering terms.) They show the commonly occurring transitions, that go to full area at some distance away from the face, as having poor flow around the perimeter of the exchanger (making the effective area significantly smaller than actual). (edit: This should say, 'poor pressure recovery'.) The book details actual pressure measurements in the two duct shapes. Here's the 1st link I could find to a comparison of 'normal' vs 'streamline' profiles; it's from a different author, but copies the relevant profiles and their pressure gradients from K&W. The drawings are on the 4th page of the document:
https://www.google.com/url?sa=i&source=images&cd=&ved=2ahUKEwj4uojF06jiAhUGmuAKHeovC7wQjRx6BAgBEAQ&url=%2Furl%3Fsa%3Di%26source%3Dimages%26cd%3D%26ved%3D%26url%3Dhttps%253A%252F%252Fwww.researchgate.net%252Fprofile%252FMarco_Pellegrini6%252Fpublication%252F253649721_The_bonus_of_aircraft_piston_engines_an_update_of_the_Meredith_effect%252Flinks%252F0046351fa125b4ad8a000000%252FThe-bonus-of-aircraft-piston-engines-an-update-of-the-Meredith-effect.pdf%26psig%3DAOvVaw2O55ms8IKsYHM75h-csxI5%26ust%3D1558390988386237&psig=AOvVaw2O55ms8IKsYHM75h-csxI5&ust=1558390988386237

Just a fwiw...

As Steve indicated, SCAT is very draggy; I really like the inner tube idea for a duct. Hmmm; I wonder if that would work instead of wetsuit material, for the James cowl inlet connectors...

I think it would be an interesting exercise for you to use Dan Horton's silicone impregnated fiberglass for the whole duct. Slick on the inside, flexible, relatively low temp environment to allow using inexpensive readily available silicone; what's not to like? And then you could tell those lazy folks (me) how it works. :)

Charlie
 
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A little late to the party, and Steve should be able to give much better advice.

But according to Kuchemann & Weber,
https://books.google.com/books/about/Aerodynamics_of_Propulsion.html?id=aRwIAQAAIAAJ
the shape along the last few percentage points of the length of the diffuser are counter-intuitive. They show a 'streamline' diffuser looking somewhat like the bell of a trumpet, where the duct reaches maximum area where it touches the face of the heat exchanger. (Apologies to Steve; redneck engineering terms.) They show the commonly occurring transitions, that go to full area at some distance away from the face, as having poor flow around the perimeter of the exchanger (making the effective area significantly smaller than actual). (edit: This should say, 'poor pressure recovery'.) The book details actual pressure measurements in the two duct shapes. Here's the 1st link I could find to a comparison of 'normal' vs 'streamline' profiles; it's from a different author, but copies the relevant profiles and their pressure gradients from K&W. The drawings are on the 4th page of the document:
https://www.google.com/url?sa=i&source=images&cd=&ved=2ahUKEwj4uojF06jiAhUGmuAKHeovC7wQjRx6BAgBEAQ&url=%2Furl%3Fsa%3Di%26source%3Dimages%26cd%3D%26ved%3D%26url%3Dhttps%253A%252F%252Fwww.researchgate.net%252Fprofile%252FMarco_Pellegrini6%252Fpublication%252F253649721_The_bonus_of_aircraft_piston_engines_an_update_of_the_Meredith_effect%252Flinks%252F0046351fa125b4ad8a000000%252FThe-bonus-of-aircraft-piston-engines-an-update-of-the-Meredith-effect.pdf%26psig%3DAOvVaw2O55ms8IKsYHM75h-csxI5%26ust%3D1558390988386237&psig=AOvVaw2O55ms8IKsYHM75h-csxI5&ust=1558390988386237

Just a fwiw...

As Steve indicated, SCAT is very draggy; I really like the inner tube idea for a duct. Hmmm; I wonder if that would work instead of wetsuit material, for the James cowl inlet connectors...

I think it would be an interesting exercise for you to use Dan Horton's silicone impregnated fiberglass for the whole duct. Slick on the inside, flexible, relatively low temp environment to allow using inexpensive readily available silicone; what's not to like? And then you could tell those lazy folks (me) how it works. :)

Charlie

Charlie is correct here on all counts. However, the velocity in the 4" tube is fairly low already, so losses are small, and diffuser shape is not especially important. You already have near pitot pressure, fairly slow flow. So the details of the shape are not especially important. Just try to avoid really sharp turns and sharp edges.

And Charlie, thanks for the citations. I had not seen the paper by the Italians. Good data on wide-angle difusers is actually very hard to find.
 
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