How to build a plenum

[DanH]

Originally Posted by MarkW

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

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.

[RV8RIVETER]

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.

[whittfic]

Entry #13 on this thread http://www.vansairforce.com/communit...=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

[aussieflyer1]

Hi Mate
Email me at aussieflyer654@gmail.com 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

Originally Posted by MarkW

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.

[MarkW]

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,

[Toobuilder]

Originally Posted by MarkW

?.. 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...

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.

[DanH]

Originally Posted by MarkW

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.

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.

[Toobuilder]

Originally Posted by MarkW

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

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.

[Mike S]

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.

[jdeas]

Originally Posted by MarkW

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.

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