How to build a plenum

I am 50/50 on building a plenum but would like to hear ideas or see pictures of others (build sites). I know the benifits but time to build will be the deciding factor.
I have checked the archives and looked for others sites but have come up with limited resources.
I assume I will add angles to the Vans baffles and then make a plenum top out of fiberglass using the cowl top as a mold.
I wonder if I will need to use a vacuum bag and what types of resin for the heat.
My biggest puzzle is how to tie in the stock cowl inlets.

Couple of random thoughts:

1. Almost everything I do is in a mold and bagged, but you don't "need" to vacuum bag parts like this. That said, the process is fairly easy once you have the stuff. These parts are non structural, so much of the laminating epoxies should work fine with these temps. I use West Systems a bunch.

2. What do you hope to gain with a plenum? If you intend to use the possible increased efficiency (yes, possible, but not assured) so that you can then restrict the cooling air for a net drag reduction, thats one thing, but the stock Vans cowl works quite well if you are just looking for cooling. The standard baffles on the other hand, leave a lot to be desired.

I am only talking about replacing the standard baffles and not the cowl. As a first time plane builder you have to look at the standard baffle and say " what the heck". All I have seen show leaks on the bottom side of the top cowl. I may not see improved drag reduction but at least there would not be added drag becuae of leaks.
Less pressure on the top cowl meaning less pressure on the top hinge line.
Less vibration as the seal rubs on the top cowl.
All in all it seem the plenum follows "Best Practices" if it doesn't add too much build time.

Don't forget to allow space for the plenum, which is attached to the engine, to move around inside the cowling, which is attached to the airframe.

Dave

OK, so it sounds like your goal is "adequate cooling"... This can be easily met with the standard open baffle setup without the significant downsides of plenum fabrication and ongoing access issues.

IMHO, spend your time on tweaking the detail parts of the standard baffles and you will be way ahead in the long run. There are plenty of individual things to do that result in big gains in cooling effectiveness.

I would guess a first time builder with limited glass experience will take 40 hrs to fabricate a proper fitting well sealed plenum. If you are handy fabricating glass parts you can shave the time down, but it will not be a trivial thing to do.

About right

hydroguy2 said:

I would guess a first time builder with limited glass experience will take 40 hrs to fabricate a proper fitting well sealed plenum. If you are handy fabricating glass parts you can shave the time down, but it will not be a trivial thing to do.

Click to expand...

That's around what I spent (first time). Vac, bags, tape etc takes some time to learn. On the positive side the inside of the cowl can be used to make the part. Wax, lay in the glass etc and vac down for a few hours to make a blank for the top

Why not build the riveted aluminum version?

Bevan

I have no problem with the 40 hours if I can get a good result and good performance. My original question was more "how".
I was hoping to hear from a few that built a plenum in the past on a few tips.
Maybe a build site with a plenum build.
JD, I read your thread on your plenum build but there has not been a follow up with results. What you did that worked and what didn't.
After reading you thread I believe I can do the layout with a vacuum bag using a sandwich layer. I kinda wonder about molding side flanges to add strength and mounting support. I am yet to see a inlet for a plenum using the stock cowl. This seems like it would be a difficult portion.

Mark there is a good post by Pete Howell here http://www.vansairforce.com/community/showthread.php?t=35428&highlight=Pete+plenum and following discussion. I built my plenum based on Pete's example. Enjoyed the process. Would I do it again? Probably not.

MarkW said:

I have no problem with the 40 hours if I can get a good result and good performance. My original question was more "how".

Click to expand...

You're right to think about coupling to the cowl inlet. It tends to be an afterthought when it should be primary. The plenum lid itself is just a lid...a sealing method.

As to how to build a lid, I started with shaped foam. The yellow is pour-in-place over a plastic sheet to protect the engine.



The blue is shaped block urethane. Plastic sheet over the top prevents bonding to the foam:



Dark epoxy is high temp stuff.



See the baffle flanges for attachment below? They cracked repeatedly above both front cylinders. Same at the ends of a nice curved flange I ran across above the propshaft. My lid has a long unsupported span across the top of the soft rubber inlets, so the loads were high at those points. Looked nice but wasn't holding up:



So, I ditched the aluminum flanges on the front cylinders, added flanges to the glass, and fastened directly to the cylinder ears with clamp plates for insurance. Also ditched the aluminum flange across the center, again adding a flange to the glass and placing the heavily loaded fasteners in shear. It has been working well since:





Details will depend a lot on which model RV you're building. Don't underestimate the loads. I've measured 94 lbs/sq ft at 180 knots indicated, and the plenum lid is more than a few square feet.

Have fun, hope this helps.

MARK

I built 2 plenums, one for each side. My reason was a quest for better efficiency. Won't go into them all.

You don't need to learn vacuum bagging or any elaborate lay-up techniques, unless you want work of art for all to admire.

I made mine with std Aeropoxy. It is good to a little over 300 deg when cured there and can use it everywhere else. I wouldn't use West because it's GT temp is much lower. I have heat soaked my engine in Texas September heat, no issues. The first, the left side was beautiful when first finished. I made a foam form and simple wet lay-up of glass/carbon on it. But, I then had to cut and trim to get clearance and add an oil cooler pick-up and now it looks as bad as the right one. For the next one, I just taped a cut cardboard soda case tray in place and laid my glass on it. Looks like ****, but it works great. For the sides I cut aluminum pieces to rivet/lay into the glass upper. The big thing with a plenum is you want a good seal with the inlet. If you are not going to do that then a plenum is not worth the effort, IMHO.

I was able to get 2 red tagged angle valve cylinders and made a simulated half engine to do cylinder wraps and engine layups. The wraps are important to keep the air inside the cyl fins all the way around. I made them with high temp epoxy and they are clamped to the cyl bases with band clamps.

My inlets are around half the area of stock and my cylinders are very well cooled. No louvers either. Sorry I don't have a great deal of pictures. I am still working out a less than successful oil cooler experiment. Once I get all my mad scientist tinkering and data done I will post everything, including my top secret jet nozzle.


Right Plenum


Left Plenum



Early pass at cyl wrap, mark 1 if you will.


Simulated engine half




Dang! Dan beat me to it while I was trying to 2 finger type! If you want beautiful fiberglass, follow Dan's examples.

Another option...

Entry #13 on this thread http://www.vansairforce.com/community/showthread.php?t=67276&highlight=plenum+update briefly describes how I did mine.

Be warned tho' it's fair a bit of work to do from scratch and there has been much discussion re the pros and cons of plenums for RVs. Only you can decide if it is worth the effort for your aircraft.

Clive Whittfield
Auckland
New Zealand

Plenum

Hi Mate
Email me at [email protected] and I will send you a pic of my Plenum that was made down at HWO.
My CHT are all in the mid 350 range and I'm only running in my engine currently.
Regards

MarkW said:

I am only talking about replacing the standard baffles and not the cowl. As a first time plane builder you have to look at the standard baffle and say " what the heck". All I have seen show leaks on the bottom side of the top cowl. I may not see improved drag reduction but at least there would not be added drag becuae of leaks.
Less pressure on the top cowl meaning less pressure on the top hinge line.
Less vibration as the seal rubs on the top cowl.
All in all it seem the plenum follows "Best Practices" if it doesn't add too much build time.

Click to expand...

OK now we are getting there.
Vlad, thanks for the link. Pete has many good pictures just none of the inlets finished.
Dan, from other comments you had made I was thinking of using fiberglass flanges around the perimeter for mounting and strength. However I was stuck on the idea of using the cowl for a mold and could not think of a good way to get the flanges molded. Using Pete Howell's mold style works for that.
RV8RIVETER, thanks for the ideas and pictures.
Clive, I had seen your thread in the archives. Very nice work.

I still haven't seen a good inlet idea. I guess when I start fiting the cowl and baffles I wil get a better idea of how that will come together. At this point the inlets are my biggest unknown.
Thanks,

MarkW said:

?.. However I was stuck on the idea of using the cowl for a mold and could not think of a good way to get the flanges molded. Using Pete Howell's mold style works for that...

Click to expand...

Ok, the "...I'm 50/50..." comment led me to believe you were still on the fence .

But if you're set on doing a plenum, you could easily make a splash of the cowl (inside or outside) slightly larger than the expected finished part. You'd do this using mat and polyester resin because its cheap and you're just making a mold. You want it heavy and stiff. For the flanges, you can either build a picture frame of 1 inch MDF bonded to the "top" of the new mold right at the flange locations (female mold) or on the "bottom" (male plug). The MDF gives you your flanges in the finished part. Once you have your mold/plug primed and waxed and perfect, you can crank out the finished part in no time. Using the vacuum bag approach will give you nice sharp corners, but be careful about the cloth type (some will turn a sharper corner than others) and getting the deep corners packed down at the start of the vacuum. Deep corners will tend to bridge if the bag is not loose enough.

MarkW said:

Clive, I had seen your thread in the archives. Very nice work.
I still haven't seen a good inlet idea. I guess when I start fiting the cowl and baffles I wil get a better idea of how that will come together. At this point the inlets are my biggest unknown.

Click to expand...

Clive, are you flying now? Love to hear a report.... static pressure numbers and structure/durability?

Mark, measurement says the stock inlet generates good static pressure. Clive's work incorporates tight inlet sealing with good shapes leading into the plenum. It would be interesting to see if the result is increased pressure.

MarkW said:

... I am yet to see a inlet for a plenum using the stock cowl. This seems like it would be a difficult portion....

Click to expand...

If I was going to do this with a stock cowl, I might seriously consider making two inlet snorkles that were trapped inside/located by the stock cowl inlets. Just make a slightly undersized plug in the shape of the stock inlets, wrap the resulting duct in a little rubber so it could move around some, and forget about trying to seal the stock cowl altogether.

I helped a buddy do a fiberglass plenum on his 10, the top layup was done in the inside of the upper cowl, he used flexible 1/2" insulation foam to space the layup from the cowl, as it would be in actual life.

Packing tape for release over the insulation.

We did 4 layers of normal glass cloth, and one of carbon fiber, then one more of glass. Vinyl ester resin for a bit better heat tolerance.

The factory fiberglass inlet ramp parts were used to make the transition to the cowl inlets, they were NOT bonded to the cowl, but to the plenum lid.

After it all was fitted and trimmed to shape, a temporary "L" bracket or three held the part in place on the baffles, and another layup was made to create the mounting flanges over the side of the baffles.

The last item was to make a door in the plenum so you can check the oil, this was done with a hole saw, about 3", and an aluminum cover that swings out of the way on a pivot bolt.

Sorry, but I do not have any photos of this.

Sides

MarkW said:

After reading you thread I believe I can do the layout with a vacuum bag using a sandwich layer. I kinda wonder about molding side flanges to add strength and mounting support. I am yet to see a inlet for a plenum using the stock cowl. This seems like it would be a difficult portion.

Click to expand...

I made the same sandwich layers on a flat surface. I then cut them to size and mounted them on the existing metal baffles with wax paper between them and the baffle. Trimmed and glued the top portion on. Once dried a good filet and fiberglass strips on the inside made it a good strong joint for positive pressure.
For the inlet I used the metal bottom and sides then packed with clay to shape the top in-place. One layer of cloth over the clay to hold the shape. Once that was dry the flimsy inlet was vac bagged from the outside with the same core material and fiberglass used on the rest.
Once everything was test fitted I went back and routed hard point where the plate nuts would go

Michael, This is exactly what my hangar mate did with his cowl. The plenum top was laid up against the top of the cowl (easy to use mold shape), It was trimed and mated to the stock Van's baffles with fiberglass formed angles. Transitions to mate to the stock inlets were made and bonded to the plenum top.

The stock Van's cowl was used as a female mold to form two "snorkles". The snorkles are mated to the transitions with neoprene boots.

The nice thing here is that when the cowls come off, there is no need to break the inlet to plenum connections. The intake snorkles just hang there, waiting to be sandwiched back between the cowl halves when the cowl halves are put back on.

I have converted the transitions on my -6 to the same design. One flight with no problems but no data collected.

After much consideration, I think that this is the best inlet to plenum arrangement - perfect sealing with no drawbacks. I will try to get some pictures tonight.

So you've got a form-fitting plenum inside a cowling. The plenum is attached to the engine and the cowl's attached to the airframe, and we know the engine moves around a bit.

Doesn't the cowl hit or rub on the plenum? Or did you put some space between the plenum and the cowling when you made it?

I don't understand how this works.

Dave

David Paule said:

Or did you put some space between the plenum and the cowling when you made it?



Dave

Click to expand...

Yes, it does need to be spaced down from the inside of the top cowl.

Dave, Yes it is spaced down about 1.0 inches.

Forming to the cowl only serves as a simple mold that is readily available to form a cover of the correct shape.

The engine and plenum move, the cowl and snorkles remain motionless, and the neoprene sleeves allow the movement between the two while maintaining a seal.

Hopefully the pictures will make it clear.

"Clive, are you flying now? Love to hear a report.... static pressure numbers and structure/durability?

Mark, measurement says the stock inlet generates good static pressure. Clive's work incorporates tight inlet sealing with good shapes leading into the plenum. It would be interesting to see if the result is increased pressure."


Hi Dan

Sadly not flying yet. Getting closer all the time but tight finances and other commitments have been slowing things down somewhat. This year tho...!

I noted your comments in an earlier post about pressures inside the plenum. This is one of the reasons I have gone this way to relieve potential loads on the cowl retainers. Your post gave me pause for thought about how mine is secured. I'm reasonably confident in that most of the screws are in shear, but I'm not sure if the screws across the top will be sufficient. Looking at my photos and based on your experience, any thoughts on the adequacy of this set-up?? Not asking for hand-on-heart declaration, just an informed opinion.

Regards

Clive Whittfield

I hear a lot about this "stress on the cowl fasteners" now and again...

While I'm sure the forces are large, has anyone ever heard of a properly secured cowl coming off in flight?

Back when I worked at NASA, our airplane would use 100 pound lead bars bolted to 1/8 inch thick aluminum plates which were then bolted to the equipment racks as needed for balast. Most of the time, these plates were bolted in from the bottom so the balast was hanging from the bolt heads. Equipment on this airplane was stressed for a 10G landing load, so there's a half a ton hanging there...

...from four little #10 bolts.

So I'm thinking that the 20 or so #8's and full length piano hinge people have on their cowl is plenty of margin.

500+ Hours

Hi Guys,

My plenum has been on and maint free for 4+ years now. Cools fine, but I only a have 160 fire-breathing ponies in my little -9A . No cracks - EZ-Poxy was easy to use(but stinky) and has held up just great.

Not near as pretty as Dan's (much respect, SIR!) and he is right about the inlets - plan for them. I have visions of tearing up the cowl and creating a nice seal, but what I have works and I like to fly too much.

Would I do it again? For the research, learning, and interaction with smart people to discuss how and why -absolutely. For performance and cooling, just over 50% of me would vote to do it again. No great gains, but I chickened out before I got into a repaint and restripe scenario to seal the inlets. My dad always said I could never finish a job

I hope that helps - shoot me questions if I can help.

whittfic said:

..any thoughts on the adequacy of this set-up??

Click to expand...

Looks fine to me. Like Mike said, fastener failure is not likely to be an issue. You might see edges raising up due to pressure and causing a leak, or you might create a stress concentration like my initial dumb design, but we tend to sew the lids on with a lot of fasteners.

As for flanges, the way you and Pete did it is superior.