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Tip: All metal plenum

carguy614

carguy614

Well Known Member
Hey folks,
Been reading about the composite layup plenum tops that seem to be a hot topic here lately. Very cool!! My building partner Paul Salerno took on this sub-project to build a full plenum for my 9A with O-320. I now have 60 hours on the plane, and I can report good performance so far. This thing was a lot of work. I have cooler front cylinders, and I am waiting for some hot weather to install the always required deflectors on the front to warm them up a little. Oil temps are outstanding, and I am unable to get over 190 no matter how hard I climb. Hard climbs are yielding max 410 CHT on the rear cylinders during last years hot weather, after the engine began to break in real good. Fronts are about 370 CHT in the climb, and run about 50 degrees cooler at cruise, so I have a bit of balancing to work out yet. I have no speed data to provide, other than my plane gets published numbers. I have louvers on the bottom of the cowl. So far I am very happy, except when I need to gain access to the plugs.
Access panels for the plugs may come soon, and for goodness sakes, don't put as many screws on the lid!!!!

Regards,
Chris
http://img12.imageshack.us/img12/3400/plenum1.jpg
http://img15.imageshack.us/img15/6391/plenum2.jpg
http://img24.imageshack.us/img24/493/plenum3.jpg
http://img21.imageshack.us/img21/9205/plenum4.jpg
 
Same

I have just about the same plenum, and it works very well too. If you'll put a washer on the screw at the back side of the #3 cyl where it screws into the head it will solve your #3 temp.

-d-
 
Outstanding job! Looks like a bit easier to do than glass. I wish I saw your plenum couple weeks ago. What are under those bulging spots on the top?
 
Very cool (in more ways than one, obviously). Do you use any baffling or seal material around the inlets at the front?

greg
 
Metal Plenum Idea

Isn't insomnia great. Woke up at 3AM with an idea.

After reading about plenums (got to stop reading forums before going to bed), I came up with an idea and want to test the waters by asking the brain trust out there.

I would like to convert from the Van's design to a metal plenum. Currently, my temperature spread on my cylinders at cruise is 3 to 4 degrees spread so I am reluctant to mess with it but that darn sign in the back that says experiemental also means intelligent idiot without an ongoing building project.

To make the conversion, why not run a 3/4" angle around the baffle on the inside so you have a lip to lay the top on. Use a 3/4 piece angle for a center support. Build the top using 2 halves. For the inlets, form an oval top that is screwed in place and provides the front support for the top. All connections would use placenuts.

Step 1 - measure for supports and cardboard for top template. Cut out
Step 2 - drill and cleco supports - check top template.
Step 3 - Install platenuts on side of support side and cut out top.
Step 4 - Temp install side support, match drill top to support, and remove
Step 5 - Install platenuts on side support top
Step 6 - Install side supports
Step 7 - Form the oval opening from aluminum and install
Step 8 - Screw top down
Step 9 - Measure, remove flap seal and trim baffle top as needed and file smooth.

Plenum done.
Cost - time only.

Any thougths about this conversion?
 
seals and bumps

Those funky bumps are an epoxy resin layup to form up some extra space for the B nuts on the plug wires. I looked high and low for an angled plug termination of some kind, to no avail. Actually it was easy, and it works well. I do have the washer installed behind number 3. That was a lot of help cooling down that hot corner. As for the cowl to plenum seal interface, Airseal is attached to the cowl inlet flange, and flips into the inlet for the seal at the bottom. On the top of the cowl, it bends back, and seals over the highly angled top surface of the plenum. I used a double thickness airseal on the top to make sure that the ram air didn't blow the seal up and away from the plenum, but the truth is, I really don't seem to need it. I thought the interface to the cowl inlets was going to be a tough hurdle, but it was easy to engineer..

Good luck,
Chris
 
I've only got weight conscious lately................because my dieting isn't working well enough... :)

So; how much weight do these plenums add?

My engine cooling seems to work well so far, with Van's standard design; but I'm only measuring cylinder #3.

L.Adamson --- RV6A
 
Webb said:
Isn't insomnia great. Woke up at 3AM with an idea.

After reading about plenums (got to stop reading forums before going to bed), I came up with an idea and want to test the waters by asking the brain trust out there.

I would like to convert from the Van's design to a metal plenum. Currently, my temperature spread on my cylinders at cruise is 3 to 4 degrees spread so I am reluctant to mess with it but that darn sign in the back that says experiemental also means intelligent idiot without an ongoing building project.

To make the conversion, why not run a 3/4" angle around the baffle on the inside so you have a lip to lay the top on. Use a 3/4 piece angle for a center support. Build the top using 2 halves. For the inlets, form an oval top that is screwed in place and provides the front support for the top. All connections would use placenuts.

Step 1 - measure for supports and cardboard for top template. Cut out
Step 2 - drill and cleco supports - check top template.
Step 3 - Install platenuts on side of support side and cut out top.
Step 4 - Temp install side support, match drill top to support, and remove
Step 5 - Install platenuts on side support top
Step 6 - Install side supports
Step 7 - Form the oval opening from aluminum and install
Step 8 - Screw top down
Step 9 - Measure, remove flap seal and trim baffle top as needed and file smooth.

Plenum done.
Cost - time only.

Any thoughts about this conversion?
Click to expand...

I know of installations similar to your idea and they cooled worse than the standard baffling system.
I think the major reason is that you can't make the same shape in metal that you have with the top of the cowl. You end up having the plenum ceiling lower to the cyl than it is when the top of the cowl is the top of the plenum.
When the top of the plenum is lowered, it gets closer all of the cyl. Cyl #1 is the farthest fwd, and with the top of the plenum sloping down towards the air inlet this constricts the space for air to flow through.
Case in point...this installation http://img12.imageshack.us/img12/3400/plenum1.jpg (which looks very nice by the way) has bumps in the top to clear the top spark plug B nuts. This is a clear indicator that the top of teh plenum is rather close to teh top of teh cyl if you looked through from the inlet. My guess is that this system will not cool as well as teh standard baffling installation would have.This doesn't happen with the standard baffling because the inlet ramp curves up to the top of the cowl, which provides a larger area space for air to flow above the cyl.
The primary goal of our type of cooling system is for the inlets to be feeding a plenum area that is growing in cross section to decelerate the air as it enters the cowl. The standard baffling does a good job of this if installed correctly, with the inlet ramps providing a relatively smooth transition to a large plenum cavity.

Before the flame throwers com out...I am well aware that their are closed plenum systems that cool well. I am aware of the benefits (and down sides) of a fully sealed cooling plenum. Many people do it for speed increase. The only person I am aware of that ever did any before and after testing to be able to prove what the speed difference was, is Dave Anders. Going on memory, I think the difference was 3 or 4 MPH. Keep in mind this was on an RV-4 that was already doing over 230 MPH. The speed increase on most of our airplanes would likely be less.

I'm not telling anyone not to do it...just suggesting that their is more to it (to do it properly) than just putting a cover on the top of your baffling.
 
Last edited:
Scott makes a lot of good points. I was concerned about the relatively sharp transition at the inlets on my setup. However, the proof has been in the pudding, as my setup has cooled very well. I have flown with my setup since day one, so I have nothing to compare it too, but I suspect that a properly built standard baffle system would do as well.

There is one more advantage to a plenum system that is usually not mentioned in these discussions- it makes getting your cowl on and off much easier, because you don't have to wrestle the airseal fabric into position, or worry about the fabric folding over the wrong way. I've seen some standard setups where this is a two-person job.
 
sprucemoose said:
There is one more advantage to a plenum system that is usually not mentioned in these discussions- it makes getting your cowl on and off much easier, because you don't have to wrestle the airseal fabric into position, or worry about the fabric folding over the wrong way. I've seen some standard setups where this is a two-person job.
Click to expand...

How hard is it to remove the plenum itself?
 
Not hard at all. The metal top is held on by #8 screws (I switched to Torx years ago and never looked back.) I can unscrew just the outer section, which is hinged to the middle, for access to the plugs. At annual time I remove the whole thing for better access to the top of the engine to check bolt torque. Taking the whole thing on or off is a 10 minute job, maybe longer depending on how many people stop by to distract me while I'm doing it.
 
The only reason I'm putting a plenum on my rocket is to reduce the stress and vibration put on the hinges and screws. After eight years of flying and dealing with broken hinges, rivets coming loose, cracks in the fiberglass, etc. anything you can do to minimize the contact area of the engine to the cowling will help the longevity of the cowling.

The only thing I don't like to see on a plenum is where it takes removing 100 screws to get to a spark plug. Dzus fasteners or camlocs, and a lot fewer of them, would make maintenance a lot easier.
 
I thought about using camlocks or similar, instead of screws and nutplates. In my analysis there are two big disadvantages to camlocks- 1. $$$ 2. Camlocks don't clamp together with sufficient force to prevent the parts from wearing against each other. Something to consider.
 
Baffle cracks

One of the reasons I went with a full cover was to strengthen the baffles and oil cooler mounting area. This stabilizes the whole affair, and I believe it will help prevent baffle cracks, and the stress crack troubles we have seen with the oil cooler mounts. Time will tell. One of the reasons we used so many screws was to provide a stable assembly. Might just have over done that a bit, but I am not sure that camlocks would have suited that purpose in terms of providing stability to the entire plenum. As for cooling, seems to work OK, although I agree with the previous poster as to there being much more to a plenum than adding a top to the baffles. In the spirit of experimental aviation, this was simply a test to determine the viability of the installation. I have no idea if it makes me faster, but I do know that it cools OK. There is also no cowl puffing along the top fastener line, and it keeps the stress down on the cowl. It is a pain to remove all of the fasteners to service the plugs. Takes about an hour to r&r. I am getting too much air over the front cylinders, and in the winter the temp spread was 50 degrees. I will experiment with the little 1 inch ramps on the inlet, and post my results.
I was able to weigh the thing, and subtract the weight of that air seal I don't need, and the weight penalty works out to just a tad over 1 lb.
Do it again? Probably not. Truth is, we all know air seal over the baffles works OK. It was a lot of work, and is a maintenance issue to some degree.
Laying out the one piece top was a piece of cake.
It's a neat idea, and was a cool project, but I have mixed emotions about the pros and cons.
Clear as mud??


Regards,
Chris
 
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