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  #51  
Old 05-12-2014, 07:04 AM
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Jerry Fischer Jerry Fischer is offline
 
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Default And while you are at it...

A cheap and easy trick is to remove the fin casting "bumps" that are present in some of the fin design. I learned this trick from an old IA/A&P friend when building a simple O-235. Just be careful not to remove anything other than the rough casting as the illustration shows. It certainly helps reduce turbulent airflow as air passes between the fins.(And improves cooling along the way).

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  #52  
Old 05-12-2014, 09:21 AM
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az_gila az_gila is offline
 
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Quote:
Originally Posted by Jerry Fischer View Post
A cheap and easy trick is to remove the fin casting "bumps" that are present in some of the fin design. I learned this trick from an old IA/A&P friend when building a simple O-235. Just be careful not to remove anything other than the rough casting as the illustration shows. It certainly helps reduce turbulent airflow as air passes between the fins.(And improves cooling along the way).

....
If you look at the machined side of your #1 cylinder fins, you can see these casting "bumps" -



On the opposite side of the cylinder with deeper fins, they can reduce the airflow between the fins.

A needle file in a drill works well for removal.
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  #53  
Old 05-12-2014, 01:16 PM
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DanH DanH is offline
 
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Quote:
Originally Posted by Jerry Fischer View Post
It certainly helps reduce turbulent airflow as air passes between the fins.
(And improves cooling along the way).
Funny you should mention that.

Can't go wrong removing casting flash that seems to block airflow. However, turbulent flow
is generally considered to be highly desirable in the case of a heat exchanger. The goal is to thin
the boundary layer (an insulator) and thus increase heat transfer, allowing the use of less mass to
carry away the same quantity of heat.

Consider the design of a cooling system for an airplane with a wide speed range...you know, total
performance, where the designer tries to makes intelligent trade-offs so no part of the operating
range is crippled. One might select a large, external diffusion, low Vi/Vo inlet, and couple it with
variable exit area.

In low velocity, high power flight like WOT climb, the slow inlet wouldn't be as sensitive to high AOA,
and thus maintain good upper cowl static pressure. With the outlet wide open, the lower cowl pressure
would be low, meaning a large pressure delta exists across the cooling fins. That increases velocity
between the fins, pushing the Reynolds number toward turbulent flow and thus increasing heat transfer.
Lots of mass, high heat transfer...good recipe for cooling at WOT. Sure, high mass, turbulent flow,
and slow exit velocity drives up cooling drag, but in this flight regime drag isn't a great concern.

Cooling demand is proportional to power. In cruise, we might need only 65% as much cooling as we needed
at WOT down low. So, we close the outlet, which does three things. First, the mass flow is reduced; air
that can't enter the inlet streamlines itself around the nose, hopefully without separation. Second, lower
cowl pressure is increased, slowing flow through the fins, which transitions toward laminar; heat transfer is
reduced, as is momentum loss due to drag. Third, exit velocity is increased; no surprise, given smaller area
and increased lower cowl pressure.

Alas, such a cowl design might suffer some external inlet separation in level flight at top speed. It might be
kinda blunt-nosed. Maybe it would only exceed VNE by 3 or 4 knots in level flight, autopilot on, despite
having enough motor (and cooling) to do a steady 1800 FPM two-up on the sort of 80F afternoon we had
here yesterday. Such is the nature of compromise. It spends most of its time at cruise power, and climbs
to some altitude for every flight; those seemed like the best conditions to maximize, as compared to a top
speed run made five or six times a year.

Are you starting to see why it really is all about the exit?
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Last edited by DanH : 05-12-2014 at 04:47 PM.
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  #54  
Old 05-12-2014, 02:22 PM
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But wouldn't these be cool (arh) on those round inlets?
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  #55  
Old 05-12-2014, 09:37 PM
AV8AZ AV8AZ is offline
 
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Quote:
Originally Posted by DanH View Post
In cruise, we might need only 65% as much cooling as we needed at WOT down low. So, we close the outlet...
No disagreement with this erudite post. I would add the reminder of the variability of air density that is inversely proportionate to altitude.

I previous owned an Ez. The canard community is forever plagued with cooling issues. 90% of the efforts to resolve the issue have been on the intake side or baffling with varying degrees of success. The few that work on the exit side seem to more routinely achieve success much earlier in the trial-and-error process.

Tyson
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  #56  
Old 05-13-2014, 07:09 AM
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Low Pass Low Pass is offline
 
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Quote:
Originally Posted by Norman CYYJ View Post
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.
HA!! Definitely a consideration when evaluating the overall value of the various products.

But Dan H and Vetterman are the only ones capturing the best value on speed/cooling by regulating the outlet stream based upon my casual observations here. Simply throwing a plenum type cowling at an airplane doesn't necessarily guarantee anything. Dan, I need to find out where I can nominate you for an honorary mechanical engineering degree.

Last edited by Low Pass : 05-13-2014 at 07:13 AM.
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  #57  
Old 05-13-2014, 09:04 AM
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Quote:
Originally Posted by MarkW View Post
But wouldn't these be cool (arh) on those round inlets?
I realize Mark is kidding, but for the benefit of those who might miss the joke, the technical answer is no.

Cooling Drag = Mass x Loss of Momentum

Closing an intake louver would reduce mass (good), but it would also slow the exit velocity (bad).
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  #58  
Old 05-13-2014, 10:30 AM
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rv7boy rv7boy is offline
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Quote:
Originally Posted by Low Pass View Post
...<SNIP>... Dan, I need to find out where I can nominate you for an honorary mechanical engineering degree.
I would second the nomination. I have a BSME and an MSME with emphasis on Fluid Mechanics and I'm constantly amazed at Dan's understanding and application of air flow and pressure losses as they relate to our little airplanes. Oh, and I didn't mention his understanding of vibrations, rotating machinery, balancing, moments of inertia, and such stuff as that.

I keep wondering if he's not a retired college professor who has taken on a new identity...sorta like Clark Kent!
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  #59  
Old 05-13-2014, 10:55 AM
David Paule David Paule is offline
 
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Quote:
Originally Posted by Low Pass View Post
H....Simply throwing a plenum type cowling at an airplane doesn't necessarily guarantee anything....
A plenum only does three things. It's primary purpose is to seal the top of the engine. Its secondary purpose is to provide good airflow into the volume so that it doesn't stall.

We'd also like it to give evenly distributed air pressure across the engine but for that its volume must be reasonably large. Since we usually can't achieve the large volume, it seems that some internal flow control, controlling the flow across the cylinders, might help in those cases where the cylinders have uneven cooling.

Dave
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  #60  
Old 05-13-2014, 06:14 PM
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Bryan, Don...thank you. Education and recreation, right?
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