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Why a plenum?

Plummit

Well Known Member
What if anything is the advantage of having an engine plenum vs the Vans cowl baffling? It seems they both do the same thing (or at least are supposed to) which is cool the engine and oil. Is one better?

-Marc
 
The primary benefit is that it is much easier to assure that it actually does its job. Lots of people will insist that their baffles are 'tight' when they have missed some large flow-path altogether, or, in most cases, just leaks from poor fit to complex contours. It is difficult to get a good seal with baffles around the complex shapes across the front of the engine and transition to the intakes.

The disadvantages of the plenum are that it is more build time, must be removed to check spark plugs, inspect fuel injection lines, etc., and requires a flexible transition duct to a connection on the cowl intake.
 
Having done both (and baffles on the latest project), plenum are a little more up front work but after reworking this project’s baffles to get a good seal I wish I did another plenum.

The hardest part in doing a plenum (assuming you buy one pre-made from James) is the connection between the plenum and the engine air inlets on the cowl. If using a James cowl this is already figured out for you. If using a Van’s cowl you will have some cowl glass work to do.

Pick your poison,
Carl
 
I know the theory is sound but does anyone have any real data to show that plenums really make you go faster or cool better on an RV?
Practically every customer of mine that has a plenum struggles with higher CHTs than my stock cowl.
 
I suggest doing some searches for "plenum" on this forum, as I recall seeing a number of posts from people with before / after data, the gist of which seems to be that yeah, you can get improvements if you put some real engineering into it including careful cowl modifications & measuring airflow, etc. (or buy a setup from someone who has and has proven it). But just adding a plenum without carefully designed inlet and outlet mods to the cowl may not buy you much or even anything over a well-sealed baffle system.

That CAFE RV-4 plenum modification had a lot of experienced engineering behind it and the article doesn't really pull out how much performance / cooling improvement is due to it compared with the many other extensive modifications? Maybe someone who knows more can weigh in on that.

I toyed with the idea for a while before deciding I'd rather spend my time and money on flying and avgas. :)
 
If using a Van’s cowl you will have some cowl glass work to do.

Not necessarily. I built transitions (to aluminum plenum) out of aluminum. I don’t have cooling problems. Somewhat odd though is cylinders 3 & 4 run about 15-20deg warmer than cylinders 1 & 2.

Bevan
 
I know the theory is sound but does anyone have any real data to show that plenums really make you go faster or cool better on an RV?
Practically every customer of mine that has a plenum struggles with higher CHTs than my stock cowl.

It means they have lousy pressure recovery, i.e. a poor inlet duct system. The plenum lid further aft is just a way to achieve zero leakage, and as such cannot cause a cooling problem. In fairness, yes, I realize in intake ducts and the lid are often one piece, but lids with well designed ducts work fine.

The attached figure is from CR3405. The two lines represent cowl mass flow as it should be based on test cell data, vs inflight measurement as installed. About 55% of the air passing through the cowl was not passing between fins, i.e. leaking past the typical crappy GA rubber seals. I would like to think the average RV builder installs better seals. However, my walkabouts, peering into intakes like some upskirt pervert, tell me a lot of them are leaky.


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I built my plenum back in 2011 when James was about the only option. I wanted something simpler and built a cover using the standard aluminum sides and strips of baffling seal material as flaps to bridge the gap between the standard cowl and the plenum. No science here sense I have no data of running before. I built the plenum before first start. I have no hot anything issues and often have to watch the minimum temps so as not to run it to cold. Summer cruise CHTs run 320 to 375 ish. Climbing hard during the summer I might see #4 reach 405. Winter is never a heat problem!

Will add photos in a minute
 
It might be worth pointing out that the Lyc cooling spec, depending on the engine typically installed in a RV, requires a Delta P of 5.5 or 6.5 " H2O.

That should be a baseline target as . . if the airflow is properly controlled, you should always achieve/have adequate cooling.

Note, this spec is silent as to cooling drag and, as pointed out by DanH, the "devil" is in the execution of the details.

Extra cooling drag will hurt your pocket book, poor cooling hurts your engine and, thus your pocket book in amounts much larger than increased fuel cost.
 
I know the theory is sound but does anyone have any real data to show that plenums really make you go faster or cool better on an RV?
Practically every customer of mine that has a plenum struggles with higher CHTs than my stock cowl.

A plenum lid is just one aspect of making all inlet air go through cooling fins. A builder can construct the most exquisite, air tight plenum lid known to man, then throw it all away with a poor seal to the case, leaky inter cylinder baffles or countless other leak paths. Details matter, and a plenum lid is just an easy way to solve the big air leak potential at the top of the engine. RV's have far more inlet area than they need, so "poor cooling" is almost certainly a result of poor building, poor attention to detail, or poor engine tuning (like too much timing).

Consider this: I have a plenum on my Rocket and I'm cooling very well with 38 total inches of area (less total inlet area than on my RV-8). This engine made 330+ HP on the dyno, I have backed that up by soundly beating the "fast" Rocket in our group in head to head sustained climb tests, and I live in an environment that is the equivalent of the surface of the sun.

Nothing fancy here - just a simple flat wrap of 0.016 aluminum with piano hinges around the edge for a lid. It's the details that make the difference: fanatical attention to eliminating every leak path, substantially retarded timing at high MP, bypass ducts on #2 and #5 cylinders, and attention to smooth cowl outflow.
 

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Michael-

Very nice looking.

Can you provide more detail / pics of how your top cowl interfaces with the inlets? I'm not sure I've seen anything like that before, it looks very interesting.

Thanks
Thomas
 
A plenum lid is just one aspect of making all inlet air go through cooling fins. A builder can construct the most exquisite, air tight plenum lid known to man, then throw it all away with a poor seal to the case, leaky inter cylinder baffles or countless other leak paths. Details matter, and a plenum lid is just an easy way to solve the big air leak potential at the top of the engine. RV's have far more inlet area than they need, so "poor cooling" is almost certainly a result of poor building, poor attention to detail, or poor engine tuning (like too much timing).

Consider this: I have a plenum on my Rocket and I'm cooling very well with 38 total inches of area (less total inlet area than on my RV-8). This engine made 330+ HP on the dyno, I have backed that up by soundly beating the "fast" Rocket in our group in head to head sustained climb tests, and I live in an environment that is the equivalent of the surface of the sun.

Nothing fancy here - just a simple flat wrap of 0.016 aluminum with piano hinges around the edge for a lid. It's the details that make the difference: fanatical attention to eliminating every leak path, substantially retarded timing at high MP, bypass ducts on #2 and #5 cylinders, and attention to smooth cowl outflow.

Nice inlet shape. generous radius like that minimizes spillage drag.
How do you seal your inlets to the cowl and accommodate engine movement? That is the sticky point I always struggle with deciding how to do. DanH made urethane ducts, but they seem a little bit vulnerable to water.
 
Plenum

I made a plenum for my 390 powered 7. Molded it off the top cowl, then carved foam for the inlets. Lots of glass and carbon.

I spent tons of time making the fixed baffle pieces for as perfect of fit as possible. Didn't seal them to the case but will after reassembly after paint.

I did my 40 hour fly off during very hot temps in AZ. Cylinder temps and oil temps were lower than I was suspecting I would have. Oil cooler is for a RV10 and is firewall mounted. I'm hoping for better temps after sealing.

The plenum was a lot of work but I'm happy with the end results.
 
Nothing fancy here - just a simple flat wrap of 0.016 aluminum with piano hinges around the edge for a lid. It's the details that make the difference: fanatical attention to eliminating every leak path, substantially retarded timing at high MP, bypass ducts on #2 and #5 cylinders, and attention to smooth cowl outflow.

Michael, how are you sealing the dip-stick tube? It looks like it is entirely under the plenum lid, but I don't believe you have to pull the top cowl and the plenum to check the oil.

-Marc
 
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Michael, how are you sealing the dip-stick tube? It looks like it is entirely under the plenum lid, but I don't believe you have to pull the top cowl and the plenum to check the oil.

-Marc

Hi Marc. I thought long and hard about the dipstick tube and ultimately came up with a simple sliding door just to get me by until I came up with a better solution. So far its working great. Open the cowl door, slide open the shutter, and the dipstick is right there. A simple music wire lock keeps the slider closed in flight.

Mr. Smith - the inlets are fixed to the engine and articulate out in front of the cowl. I stole this idea from Burt and his Boomerang. Unlike his application, mine will have a compression seal of foam to stop the leakage out the sides. What I have here is very rough and I'm really considering just turning a set of rings on my lathe and doing the Dave Anders/Sam James thing.
 

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A plenum lid is just one aspect of making all inlet air go through cooling fins. A builder can construct the most exquisite, air tight plenum lid known to man, then throw it all away with a poor seal to the case, leaky inter cylinder baffles or countless other leak paths. Details matter, and a plenum lid is just an easy way to solve the big air leak potential at the top of the engine. RV's have far more inlet area than they need, so "poor cooling" is almost certainly a result of poor building, poor attention to detail, or poor engine tuning (like too much timing).

Consider this: I have a plenum on my Rocket and I'm cooling very well with 38 total inches of area (less total inlet area than on my RV-8). This engine made 330+ HP on the dyno, I have backed that up by soundly beating the "fast" Rocket in our group in head to head sustained climb tests, and I live in an environment that is the equivalent of the surface of the sun.

Nothing fancy here - just a simple flat wrap of 0.016 aluminum with piano hinges around the edge for a lid. It's the details that make the difference: fanatical attention to eliminating every leak path, substantially retarded timing at high MP, bypass ducts on #2 and #5 cylinders, and attention to smooth cowl outflow.

Michael, lots of good information as always, but just one thing. That Catalina in the background... how sad...
 
The Cat is the last remaining evidence of a lifetime of Pontiac ownership. GTO's, Trans Am's, Formula 400's... I've owned dozens. Can't let this one go. There's a pumped 455 and a Muncie 4 speed waiting to go in. No time... Airplanes now
 
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Consider this: I have a plenum on my Rocket and I'm cooling very well with 38 total inches of area (less total inlet area than on my RV-8).

5" diameter (well, ok, 4.9") isn't real small. Not that it really matters. The inlets can be large, or small, or very small, depending on design approach, i.e. internal or external diffusion. Mass flow is controlled the exit area, not the inlet. The inlet is responsible for delivering the mass without wasting any of the available total pressure.

Love the Boomerang inlets! Get some before and after upper plenum pressure measurements if you decide to switch to fixed rings and condoms.
 
5" diameter (well, ok, 4.9") isn't real small. Not that it really matters. The inlets can be large, or small, or very small, depending on design approach, i.e. internal or external diffusion. Mass flow is controlled the exit area, not the inlet. The inlet is responsible for delivering the mass without wasting any of the available total pressure.

Love the Boomerang inlets! Get some before and after upper plenum pressure measurements if you decide to switch to fixed rings and condoms.

Will do. The fixed inlets are my attempt at a "perfect" divergent duct for internal diffusion (keeping in mind this is a sport airplane, not a Reno racer). I have them pushed way forward and with them fixed relative to the prop, I can get them frighteningly close to the prop TE without fear of contact. The fixed inlets do make it easy to make a perfect seal and zero transition losses, but in reality that transition is just moved to the cowl OML and any discontinuity shows up as drag. And right now that drag is substantial. I have a lot of work to clean that up. Im working on molds for a new cowl section that will (hopefully) clean up the inlet/cowl transition and get me some cruise speed back. I think there is 5+ knots hiding in the mess that I have now.

You mentioned exhaust outflow. Just for discussion sake, my cowl outlet is a duct fixed at 27 inches area. But its also augmented with the exhaust flow - my pipes end entirely within the cowl. Cools great but the outflow is so hot that the floor is too hot to touch with a bare hand all the way back to the rear passenger seat. My plan is to replace the existing duct with a pair of stainless tubes (like a T-34) to get the heat out of the boundary layer on the bottom of the fuselage.

So win some, lose some. My airplane has a ton of power and I can keep it cool in some brutally hot flying conditions, but its still too slow in cruise and my shoes melt on the floor when I fly. More experimenting to come - wouldnt be any fun if it worked perfectly the first time, right?
 

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Good stuff Mike. Let me know if you want to gather pressures.

A question relating to the exhaust augmentation...does it cool well with the aircraft in level flight near stall speed? You know, like trying to hang in formation with a very slow friend, a little flap and moderate power to hang it on the prop? I suspect it does.
 
The Cat is the last remaining evidence of a lifetime of Pontiac ownership. GTO's, Trans Am's, Formula 400's... I've owned dozens. Can't let this one go. There's a pumped 455 and a Muncie 4 speed waiting to go in. No time... Airplanes now

Goddamit here I am looking for a PBY or avid amphib...
 
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