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  #91  
Old 06-21-2011, 05:20 AM
flyvans.com flyvans.com is offline
 
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hi dan,

thanks for the info...
i'm thinking about blocking some of the exit area initially without actually changing the outer shape of the cowling... (major mod)
just to see what it changes.
in theory this should accelerate the outflow air and lessen the turbulence below the belly skin.

the oil cooler by the way is the default one from van's and we're even partly blocking it off to get approx 190F in 80?F oat weather

regards, bernie
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  #92  
Old 06-21-2011, 10:36 AM
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dessertire dessertire is offline
 
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Dan, this is a bit late, but just now seeing it... what a beautiful creation!
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  #93  
Old 06-21-2011, 02:41 PM
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DanH DanH is offline
 
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Quote:
Originally Posted by flyvans.com View Post
i'm thinking about blocking some of the exit area initially without actually changing the outer shape of the cowling... (major mod) just to see what it changes. In theory this should accelerate the outflow air and lessen the turbulence below the belly skin.
I have some reservations Bernie. Simply blocking exit area may or may not accelerate exit air at the enlarged ramp (it may accelerate air from the louvers or may push more air out around the propshaft opening), and will probably increase turbulence.

Want to do an easy experiment tailor made for your video camera? A lot of us think the -7's 90 degree corner at the lower firewall / belly skin intersection is a problem. Add a simple sheet metal radius, large as practical.

Quote:
the oil cooler by the way is the default one from van's and we're even partly blocking it off to get approx 190F in 80°F oat weather
With both louvers and the enlarged exit ramp you have a HUGE total exit area. Perfect example of "just increase mass flow". No surprise about blocking the cooler.
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  #94  
Old 06-21-2011, 04:39 PM
flyvans.com flyvans.com is offline
 
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i did not remove the cam mount yet... ;-)

the curved sheet metal is definitely an interesting idea, i've already thought about that as well, kind of rv-8ish... the problem will be fitting it as the area is pretty tight if i remember correctly / close to the exhaust. and it should be only semi permanent initially.

we did not actually intend to increase mass flow a gogo, it's more just a sum of fitting the louvers as per a number of recommendations and only then finding out about / fitting the muffler system.
well, for the breakin phase this certainly didn't hurt the engine and the airplane is still plenty fast.
now it's just a matter of unlocking the potential.
cowl is standard van's and i'd say the front inlet seals are tighter than average. (including blocking off the sides of the ramps in the top cowl etc...)


only problem is i'm having too much fun flying, and not enough time to tinker much at the moment.

cu bernie
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  #95  
Old 07-05-2011, 07:58 AM
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With the store closed for the 4th I had some time to build a new cooler exit duct. The original was two fiberglass transitions and a 4" SCEET hose, and I suspect rather restrictive:



So, start by shaping foam:





Prep it and do the layups:





Then remove the foam...rough duct ready for some sanding and general cleanup:



Hopefully back in the air by next weekend for test. I may need something similar for the entry duct from the plenum. Even SCEET (lined hose) is pretty bumpy on the inside
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  #96  
Old 07-05-2011, 09:12 AM
David-aviator David-aviator is offline
 
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Quote:
Originally Posted by DanH View Post
Cooling in an absolute sense is easy.....just increase mass flow. Temps will drop and the airplane will get slower.

To get 180-195F I install the "large" exit panel, which is about 77% of a stock exit.....but I don't want to go that slow.
I agree with this statement completely, based on personal experience - no science, no engineering data - just experience.

The Van's cowl for the RV-7 has a exit to inlet ratio of about 107.69%.

My cowl, with a 5x16 exhaust exit area and 2 Bonanza side vents has a ratio of 219.51%

I also have a remote SW oil cooler with 4" scat duct from #4 aft baffle. It does not have the nice exit flow device as per Dan, but simply dumps its air into the lower engine area just above the exit. I've flown twice this past week with the OAT in the low 90's and oil temp ran at 168F! Yes, I have checked the sensor in hot water and I can get warmer oil temperature by closing the air flow butter fly valve.

The point here being, good cooling is the result of mass air flow through the cowl.

OK, does the 219% exit ratio introduce a lot of drag? Very little if any. I've done several WOT runs at 75% at altitude and the machine moves along right at 200 mph, about the same as Vans RV-7A numbers with a 180 HP engine.

Could be though, the extra drag is offset by the extra HP from BPE. The engine did crank out 187 HP on the dyno.
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  #97  
Old 07-05-2011, 10:28 AM
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Quote:
I also have a remote SW oil cooler with 4" scat duct from #4 aft baffle. It does not have the nice exit flow device as per Dan, but simply dumps its air into the lower engine area just above the exit. I've flown twice this past week with the OAT in the low 90's and oil temp ran at 168F!
I believe it. Understand why I need the exit duct and you don't...my lower cowl volume is pressurized, while yours is near static due to the huge exit area. I must duct to low pressure at the cowl exit.

Quote:
OK, does the 219% exit ratio introduce a lot of drag? Very little if any.
Don't kid yourself.
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  #98  
Old 07-05-2011, 02:26 PM
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RV8RIVETER RV8RIVETER is offline
 
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Dan beat me to it.

To tweak it a little. Good cooling is efficient mass air flow thru the cylinder fins. The major driver of that is pressure differential and making the air go where you want it to.
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  #99  
Old 07-05-2011, 02:57 PM
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pierre smith pierre smith is offline
 
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Default Say, Dan....

I'd like to build an exit duct for my -10's oil cooler as well. In case you don't know, they're mounted on a firewall box that's angled about 30 deg downward, with the air coming in the top.

How do I determine the exit area? The cooler is fed by a 4" duct off the back of # 6 cylinder.

Thanks,
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  #100  
Old 07-05-2011, 06:52 PM
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Quote:
Originally Posted by pierre smith View Post
I'd like to build an exit duct for my -10's oil cooler as well. In case you don't know, they're mounted on a firewall box that's angled about 30 deg downward, with the air coming in the top.
How do I determine the exit area? The cooler is fed by a 4" duct off the back of # 6 cylinder.
I'm not so sure you can accurately calculate the exit area for a particular mass flow. Pressure differential drives flow, friction reduces flow. There are equations for a theoretical perfect duct.....but we build imperfect things. It's hard (for me anyway) to define the friction due to the duct wall, bends, etc. Maybe one of the bright guys can chime in here?

The one I built this weekend is more-or-less a constant section area equal to the cooler face area, roughly 25 sq in. The previous setup took the cooler outflow and squeezed it back into a 4" tube (12.5 sq in), ran it around a right angle bend with a bumpy minor radius, then into another fiberglass transition whose outlet was partially blocked by the exhaust pipe. Surely the new one will flow more.

Pierre, the first thing to do is measure pressures. Run one leg of a manometer to the upper plenum. Run the other to a few different locations; the cooler outlet face, a spot near the cowl exit (where your duct might end), and perhaps just outside the cowl exit. Make a few flights and record the differential pressures at your usual climb speed and at cruise. A cooler exit duct would be a waste of time if there is no significant difference between the three locations.
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