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  #21  
Old 05-07-2014, 06:31 PM
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DanH DanH is offline
 
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Quote:
Originally Posted by jj_jetmech View Post
Its my understanding that the James Cowl without a plenum would not yield the full benefit or performance gain.
A plenum cover is a sealing device. It eliminates the leakage of traditional rubber flap seals, which can be significant. NASA Report CR3405 suggests that in typical GA cooling systems with flap seals, as much as 55% of the air passing through the cowl bypasses the engine cylinder fins entirely. Put another way, bypass air (leakage) contributes little to cooling while being responsible for as much as 55% of the cooling drag slowing the old rental dog.

Yes, it is possible to install a really good set of flap seals and have far less than 55% leakage. The average RV install does much better. I don't think it is possible to equal a plenum lid.

Ok, so ditch the flap seals, install a lid over the engine, and couple the enclosed plenum volume to the intakes. We can assume bypass leakage is reduced to zero, but that doesn't mean the system is now flowing 55% less total cooling mass. There may indeed be some reduction in mass flow, but for the most part we merely re-routed the air, forcing it to take a path between fins where it does some good by picking up heat.

Here's the point; adding the plenum lid does not by itself reduce cooling drag. To do that we must reduce total mass flow and/or reduce momentum loss. The lid made the system more efficient, so we can, by some additional change, reduce total mass and still meet the engine's cooling requirement.

Quote:
Reducing overall inlet area and shape is only half the equation its also how the air is controlled after entering the inlet? You could use a plenum on a Vans cowl but wouldn't you just exacerbate the aerodynamic issues and problems with a rectangle inlet versus a round inlet?
We can obtain an identical conversion of dynamic pressure to static pressure with either a small area inlet or a large area inlet. It is more challenging with the small area inlet, as interior ducting detail becomes important. The large area inlet is non-critical; it needs nothing inside the opening (look at an Acclaim or a Cessna TTx). Either way, the inlet choice has little to do with controlling mass flow. The exit area is the system throttle.

Quote:
First do you think the SJ cowl and plenum for a 7 is a good option to the Vans cowl when considering possible performance gains?
As others have noted, speed gain directly attributable to the James cowl is rarely quantified with precision.

As for cooling parameters, two partners and I made quite a few accurate measurements of my custom setup, a Vans cowl with a variable exit, and a shorty James cowl. We agreed not to discuss details at this time, as any such effort suffers from a classic small sample problem. I can tell you a stock Vans RV-8 cowl inlet performs very well.

Quote:
Also why does a Vans top cowl, in some cases, bulge or pillow when seen from the cockpit in fast cruise flight?
Internal pressure is higher than external pressure. At 165 knots TAS and 5000 feet, on a standard day, the available dynamic pressure is 79 lbs per sq foot. By measurement, my own cowl converts (ballpark) 75% of it to increased static pressure in the upper plenum....let's call it 60 lbs/sqft, the pressure that drives mass through the system. It drops maybe half that pressure across the engine fins (depends mostly on fin spacing, baffle fit, and exit size), leaving in this example 30 lbs per sq foot to inflate the cowl like a balloon. Mostly we see it in bulges along the firewall seam, and a bulging oil door. This lower plenum pressure is highly desirable, as it can be used to increase exit velocity.
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  #22  
Old 05-07-2014, 07:11 PM
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Dan, I love the way you explain things, so.......the SJ cowl with plenum should be faster with better cooling, right? I hope so, I have one in my living room.....
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  #23  
Old 05-07-2014, 07:56 PM
Norman CYYJ Norman CYYJ is offline
 
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I believe that a properly built Vans plenum system will work just as well as the the other plenum systems. I think that the perceived speed increase is directly proportionate to the added cost factor.
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  #24  
Old 05-07-2014, 09:39 PM
Sig600 Sig600 is offline
 
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Dan,
After some more thorough familiarization with the performance of my new steed, I'm undecided now. It (the plenum) may be doing quite well after all.

Today at 10.5k
28F
20" at WOT
2400rpm
Leaned to 9.0 gph
175 knots TAS
hottest CHT was 330, coolest was 295
Oil I was struggling to get above 175, had the shutter on the cooler fully closed.

I'll try and get some screen grabs of the skyview.

http://i61.tinypic.com/nye0ci.jpg
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Last edited by Sig600 : 05-07-2014 at 09:46 PM.
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  #25  
Old 05-07-2014, 10:51 PM
Wayne Gillispie Wayne Gillispie is offline
 
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The bulging top cowl...

When we install a .093" ss hinge pin made for a .120" hinge, there is .027" of our bulge. Then add wear.
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  #26  
Old 05-08-2014, 06:06 AM
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Quote:
Originally Posted by Sig600 View Post
Dan,
After some more thorough familiarization with the performance of my new steed, I'm undecided now. It (the plenum) may be doing quite well after all.
Those cruise numbers say you would benefit from some method of closing down the exit area after you're established at altitude. No surprise; too much cooling in cruise is true of any fixed exit area large enough to cool the engine at full power and low airspeed, i.e. in climb.

Don't change anything right now. Report back after the atmosphere gets hot in your part of the world. Let's see a screen grab near the top of a 10K climb. Fly some practical profile like pitch for 120 knots IAS, WOT/2700/full rich for the first 2000 feet, then 120 IAS, WOT/2500 while leaning for best power the rest of the way up.

Looked at the plenum lid photo. Pretty thing, with good edge sealing and a reasonable shape aft of the inlet. I assume you have some simple flap seals attached to the cowl inlet "spigot", overlapping the sheet metal and carbon?
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Last edited by DanH : 05-08-2014 at 06:23 AM.
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  #27  
Old 05-08-2014, 06:21 AM
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Quote:
Originally Posted by jj_jetmech View Post
Reducing overall inlet area and shape is only half the equation its also how the air is controlled after entering the inlet?
Jason, specific to your question (and consideration of a typical James cowl), follow the link below and read a good article from Chris Zavatson. Pay particular attention to the three inlet cross section drawings on page 2, and read the section titled "Inlet Size and Efficiency" four or five times.

http://www.n91cz.net/Interesting_Tec...dingBasics.pdf

Chris elected to build high Vi/Vo inlets, a reasonable choice for his Lancair. The Lancair's prop extension provides the physical length (between the plane of the propeller and the front of the cylinders) to install the necessary internal diffusers. I chose low Vi/Vo inlets, as I wanted to run a BA Hartzell on a 390. A prop extension would have pushed the CG too far forward on an RV-8. The best choice for each revolved around physical factors, not cooling, at least not directly. We both get good pressure recovery.

Chris touches on the primary downside to a high Vi/Vo scheme in his article; it is easy to go wrong with design aft of the inlet plane. Look at the shapes Chris used, then contemplate the physical arrangements seen elsewhere. It is difficult to keep high velocity airflow attached to the wall of a duct with a sudden small radius into a volume, a sharp lip, a ballooned flex coupling, or anything else that might trip it into turbulent flow. Energy wasted in turbulence isn't converted to plenum static pressure.
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Last edited by DanH : 05-08-2014 at 12:38 PM.
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  #28  
Old 05-08-2014, 10:56 AM
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vfrazier vfrazier is offline
 
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Just to clarify, I did say that my friend's plane was at least 10 knots faster, probably more. If I find an exact number, I'll post again. Best I can do right now, but it is enough that I am doing the same... SJ cowl and PR pants.
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  #29  
Old 05-08-2014, 12:01 PM
jj_jetmech jj_jetmech is offline
 
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Quote:
Originally Posted by DanH View Post
Jason, specific to your question (and consideration of a typical James cowl), follow the link below and read a good article from Chris Zavatson. Pay particular attention to the three inlet cross section drawings on page 2, and read the section titled "Inlet Size and Efficiency" four or five times.

http://www.n91cz.net/Interesting_Tec...dingBasics.pdf
Hi Dan,

I appreciate your detail in answering my questions and providing this link, great article, very helpful.

It all about Mr Bernoulli again...

So Basicallly the more efficient your cowl and plenum combination are the smaller you can make your inlets further reducing cooling drag and go faster. The CHT and oil temp will ultimately tell you how well your doing or did with your setup. It's clear the Vans cowl could work just as well as any with a plenum with a proper transition from the inlet to the plenum. Having said that I think the other cowl gets me closer to what I want out of the box, the round inlet rings will make it simpler to modify if desired or needed.

As for now I gotta get back to riveting my fuse. After all I can't have high oil temps until I get this thing off the saw horses.
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  #30  
Old 05-08-2014, 12:49 PM
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Quote:
Originally Posted by jj_jetmech View Post
So Basicallly the more efficient your cowl and plenum combination are the smaller you can make your inlets further reducing cooling drag and go faster.
No.

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