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Tip: Building a plenum in a vacuum

jdeas

Well Known Member
Ok, so the title has a bit of a double meaning here. Being new to fiberglass and having a building partner absolutely adverse to epoxy, I decided to build my own composite plenum. The only problem is I didn't know anyone versed in composite construction or have the proper tools. I only knew the concept of using a plenum seemed sound. Less distortion in the cowl, less cooling drag and a lack of the sealing issues was my goal. I considered aluminum, after all I have done 10,000+ rivets in the last few years but this task needs compound curves. That I did not want to tackle in aluminum!

The first step regardless was to trim the baffles to the proper height. Using the paperclip trick I assembled the baffles and trimmed them down to 1/2? below the cowl.
34hgt5j.jpg

At this point I am still on track with Van's instructions so I could abort and make the traditional baffle seals.
The next step was deciding how to mold the plenum. I thought of all sorts of ways to cover the engine and use foam or some other filler to make the part but in the end (thank you VAF) I saw an example simply using the cowl top!
70kols.jpg

Yes, that is my cowl top, primed, filled, waxed, two layers of epoxy, plastic and a load of sand. It did appear to work. The next day with some careful pulling and swearing I managed to release the plenum cover from the cowl.
xo2xdj.jpg

2z3nwx1.jpg

OK, not so good. The shape was great but the epoxy was not well distributed, heavy and leaving a less than stellar part. Even with the 4 layers of crossed cloth the part was too flexible for me to trust.
Now I really don't want to lose the ability to retreat to a standard baffle! Time to stop working in an information vacuum!
 
A different king of vacuum

I did some more research and decided to do this right I would have to use core material and a vacuum pump. I was able to find a used pump for around $100 on Ebay and the local supplier had some surplus honeycomb material that looked about right but was thicker than what I wanted. I purchased a few scraps anyway and ordered vacuum bagging consumables and a product called Aero-mat from ACP composites.
This was getting expensive so the first order of business was to make a few test stamps.
ih8eu1.jpg

Standard honeycomb, expanded cell honeycomb and Aero-mat. I also put a strip of absorbent matting on one end to remove any excess epoxy from that area. After some destructive testing a few conclusions were made.
  1. Honeycomb is very expensive, light and very strong
  2. Even expanded cell honeycomb has trouble with compound curves.
  3. Aero-Mat is low cost and seems to have more than enough strength
The Aero-mat was not as strong and slightly heavier than honeycomb but it was much less expensive and had no problems with my light compound curves so I decided to do a full on layup.
alq3wm.jpg


This time I was determined to get it right. 4 layers of crossed cloth and the Aero-mat core in a vacuum bag with release film and an absorbent mat.
mjps49.jpg

Not perfect but what a difference! Stiff enough to layout on the baffles and cut to size, light weight and a good even epoxy fill.
Now for the mounting and ramps.

Using the same process on a flat surface I made a sheet of the same composite structure. This sheet was cut into strips and temporally screwed to the metal baffles along with the proper relieve cut to flush the plenum cover with the baffle top edge.
280mukk.jpg

The strips were rough cut, temporarily mounted to the baffles, tacked with epoxy to the top then carefully removed the next day. I was then able to re-enforce the connections with a micro-balloon fillet and 1? tape. I was also able to create hard points for the plate nuts using flox under the same 1? mounting tape
2nis48w.jpg

25hzkg3.jpg

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egy0sn.jpg

With the back and sides sealed I am now working on the ramps.




To this point I have to say, you really have to want to learn epoxy to not simply purchase a composite plenum. I like this approach because I always maintained a retreat path in case my little adventure got out of hand. Using the Aero-mat was another plus. This is a great core material for the light and medium strength parts. At a later date I plan to use it again to replace some of my epoxy mat parts for a cleaner lighter part (remote oil cooler shroud).
Now on to the ramps and front seals:D
 
Lookin good

Couple of comments for you.

Epoxy resin should be post cured to a heat higher than it will see in use. This is typically done in an autoclave while it is still in the mold.

As I recall, vinyl ester resin is better for high temp use in many cases.

Carbon fiber is a quick way to make a layup more rigid.
 
Plenum

JD
Nice work. One suggestion I have is to build a lip on the aft baffle for the plenum to sit on, so that the attach screws are installed vertically. I find it really hard to instal or remove screws horizontally on the back baffle. There is just not much room back there.

Matt
 
Composites

See how easy this composites stuff is? Come to the Dark Side...

I actually think you could have gotten away without using a sandwich, and simply used stiffeners in strategic places. As for the hard points, you can skip those too. Just use a Click-Bond with a ~2" round two BID patch over the top for a stud, or an EZ point for a nut.
 
JD
Nice work. One suggestion I have is to build a lip on the aft baffle for the plenum to sit on, so that the attach screws are installed vertically. I find it really hard to instal or remove screws horizontally on the back baffle. There is just not much room back there.

Matt

With only a 1/2" clearance I wanted to keep everything level or below the existing baffle. I have seen how much a coughing O-360 moves in those mounts and didn't want to hit anything!
 
See how easy this composites stuff is? Come to the Dark Side...

I actually think you could have gotten away without using a sandwich, and simply used stiffeners in strategic places. As for the hard points, you can skip those too. Just use a Click-Bond with a ~2" round two BID patch over the top for a stud, or an EZ point for a nut.

Hard points were just to say I did it. I have used the click bonds on other items and they really can save effort. Stiffeners were my first thought but then your looking at airflow inside the plenum or clearance outside. Once I saw what the Aero-mat could do it was not longer worth the time.
 
Couple of comments for you.

Epoxy resin should be post cured to a heat higher than it will see in use. This is typically done in an autoclave while it is still in the mold.

As I recall, vinyl ester resin is better for high temp use in many cases.

Carbon fiber is a quick way to make a layup more rigid.

I have never used VE, always epoxy. I understand there are some dimensional gothchas with creating VE parts this large. In addition Carbon is much more than Aero-mat.

Now I have to question an assumption, is the cowl VE or Epoxy based?
 
Glass Transition Temp

Cure temperature will determine the ultimate Operational Service Temperature. Most advanced composites are not room temperature cured. Even though epoxy is a thermoset which stays hard at normal temperatures, there is a glass transition temperature (Tg) where the composite will become "rubbery". The actual matrix and the cure temperature will determine the Tg.

Regarding VE vs Epoxy and other matrix system:
Vinyl Ester with an initial cure temp of 200 deg F has an Operational Service temp of 120-320 deg F.

Epoxy with an initial cure temp of 350 deg F has an Operational Service temp of 120-360 deg F.

BMI with an initial cure temp of 375-550 deg F has an Operational Service temp of 400-540 deg F.

Cyanate Ester with an initial cure temp of 250-350 deg F has an Operational Service temp of 200-600 deg F.
 
Cure temps

That is quite a service range. Looks like I need to get the West System docs out and take a look.
 
I used WEST PRO-SET system. It has to be post cured in an oven and the final service temps really go up. I don't have the data at the moment but you can look it up on line. The results using three layers of carbon cloth is super light and very stiff.
 
That is quite a service range. Looks like I need to get the West System docs out and take a look.

I should have put in a bit more detail. The low end of the range on the Vinyl Ester and Epoxy are the room temp Tg's. You get the higher Tg's by curing at the higher temperature.

Most composite matrix systems, other than room temp cure, have somewhat complex cure cycles - there is temp ramp up time, hold time and cool down time. There also maybe requirements for intermediate hold times before reaching the final cure temperature. There is a lot of expensive equipment required to do advanced composites correctly.

I used plain old West Systems for my glass work but I don't have anything that will be subject to temperatures over 120 F.
 
JD, my compliments. Good to see a fella jump in and learn.

I think your flanged glass edge with hardpoints and screws in shear is a good idea. Originally I built this one with 90 degree aluminum flanges all around:

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Note it takes a lot of screws (tight screw spacing) to obtain reasonable sealing. Your scheme will need fewer (but larger) screws, nice for maintenance. In addition the aluminum flanges were structurally too weak in the area on each side of the inlets. This revision eliminated all aluminum flanges near the front. The area across the center got a glass flange and screws in shear:

p0p77.jpg


The RV-8 cowl doesn't offer enough room for any kind of vertical baffle wall above the angle-valve cylinder heads, so I have the front cylinders tied directly to the plenum lid with the two 1/4-20 screws. There's a clamp plate over the glass to make sure it doesn't bow outward between the screws and leak air.

Tg doesn't seem to be a big issue in this application because the plenum lid is loaded as a membrane, i.e. the primary load is tension. The glass fabric is doing most of the work and does so even when the epoxy matrix is a bit rubbery.

BTW, you mentioned using 1/2" clearance to the cowl. That's a little tight and the plenum may knock on the cowl at the front corners during startup. It's worse with a metal prop.
 
BTW, you mentioned using 1/2" clearance to the cowl. That's a little tight and the plenum may knock on the cowl at the front corners during startup. It's worse with a metal prop.

I am worried about that aspect. I have already rough cut the ramp top and sides trying to use the bottom ramp and it's aluminum sides for support. I may simply abandon it and go more to something like what you have done in front.
 
West 105 working temp

Got some info back from West Systems today. Their standard 105 system is good up to 200F but will be a bit softer. Given the airflow through the plenum, I can't see a problem outside of heat soaking after shut down. While that may indeed hit 200F for a short period it would be under a no-load condition.
 
My West Systems epoxy/fiberglass plenum worked great for 325+ hours. The only post cure it ever got was when it was on top of the IO-540 after a summer flight.

I never noticed any softness, warping, deformation, etc..

I'm building a new plenum out of carbon fiber, not because the old one wasn't functional, it just wasn't my best work. Kinda wavy because it was the prototype, and it had a big patch on it where I modified some other stuff... but it worked.

I'll likely use West with the CF too.
 
Obviously there is a ton of work involved to construct a plenum, and I admire the workmanship seen here but.....

Has there been any real world evidence that all this effort will pay some real dividend from the stock cowl/cooling system?
 
Has there been any real world evidence that all this effort will pay some real dividend from the stock cowl/cooling system?

That is a very good question Walt.

A plenum is merely a sealing device, a way to ensure air is not bypassing the engine entirely. You and I have seen lots of leaky flap seals, and some that were an outright joke.

There have indeed been serious efforts to measure flap seal leakage. The best known is probably NASA CR3405. They measured total mass flow for the stock Aztec installation (think typical GA less-than-perfect flap seals) and a sealed plenum:

ayo751.jpg


They found approximately 38% of the inlet air was doing nothing useful, and even the sheetmetal doghouse they put together for the test didn't seal well enough to match Lycoming cooling chart data. Obviously a really well-sealed plenum is better than a leaky plenum, which is better than leaky flap seals.

Ahhh, but we've also seen flap seal installations which were outright artistry, beautiful exhibits of craftsmanship which clearly suffered minimal leakage. Is a plenum better than those? My guess is probably not......but I've not had a chance to instrument one of them.

The practical question in the field is how to judge sealing quality. CR3405 offers a clue; they measured outlet air temperature for the two configurations:

Flow temperature measurements made directly below the engine and at the cooling flow exit showed a significant reduction, and suggested the mixing of heated and unheated cooling air, lending support to the leakage theory.

Air following a leak path doesn't pick up as much heat as air passing between cooling fins. Leaky baffles exhibit low exit temperatures.

Nothing says you can't cool with leaky sealing. You can, and you can see it done with most of the Cherokees and such parked on any ramp. You just need to run a lot more mass through the system to compensate for the leakage and low pressure differential....and the extra mass flow (x momentum loss) makes the airplane slower.
 
Years of service

Obviously there is a ton of work involved to construct a plenum, and I admire the workmanship seen here but.....

Has there been any real world evidence that all this effort will pay some real dividend from the stock cowl/cooling system?

I doubt I will see much if any of improvement in my install over a new well maintained baffle. That said, I rarely see a well maintained baffle on aircraft with years of service. Add to that the cowl deformation I have witnessed on flying RVs, the long term benefits of a plenum may prevail.
 
Dan, I don't see any flex coupling on the air inlet/cowl interface, isn't this going to be a problem?

p0p77.jpg
 
Air following a leak path doesn't pick up as much heat as air passing between cooling fins. Leaky baffles exhibit low exit temperatures.
.

Dan, it seems to me like this would be true if the inlet and exit were also made larger in the leaky baffle case. But is it going to be true if both remain constant and the only change is to the baffles? The engine needs to lose the same amount of heat in either case, even if cht rises in the process. For the cooling air temperature to decrease would seem to require more total mass flow through the cowl.

On my cowl the flap seals genreally are pretty tight. It's very difficult or perhaps impossible however to get a tight seal between the upper cowl area and the inlets. I suspect that this results in a lot of leakage.
 
Plenum in service

Just an update, the plenum has been in operation for about 30 hrs in SoCal. No sign of deformation. It appears that the West Epoxy is up to the temps in this application. Been on and off dozens of times during phase one without issue.
 
JD,
Im glad to hear your plenum is working as you hoped. I like the idea of a plenum to seal everything off. It just looks like it should work better! I did have a concern about being able to see the engine for a pre-flight inspection. I know that crappy little oil door doesn't give much of a view, but I am making my oil door larger. The other thing I was not sure of is how long it takes to take it off once the cowl is off.

Overall, how has your experience been? The construction part of it sounds fun so I may give it a whirl. Thanks for the info and update.
 
16 screws

With the deep side mounts and screws in sheer, I only needed 16 to get a good seal. 4 on each side, 6 in the back and 2 up front. A short ratchet screwdriver is needed to get the screws out between the front of the plenum and the flywheel but the others are in the clear.

Mine comes off in about 2-3 minutes.

open
 
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Duck under and take a look

I did have a concern about being able to see the engine for a pre-flight inspection. I know that crappy little oil door doesn't give much of a view

Baffle or plenum you will not see the top of the engine from the oil door. Look inside the intakes for that. I'm in a hangar so it's easy to roll under and look inside the cowl exit. With a good flashlight you can get a pretty good idea if anything is leaking.
 
JD,

Thanks for posting your results. I am just about done with the mold for my plenum and will be laying it up soon. Nice to see yours is holding up well.
 
Just an update, the plenum has been in operation for about 30 hrs in SoCal. No sign of deformation. It appears that the West Epoxy is up to the temps in this application. Been on and off dozens of times during phase one without issue.

Just curious if you have an update on how the West Sys epoxy is holding up now several years in. I'm in the plenum stage and have half a can of WS105 Id like to use vs buying HTR212 or something else.

Thanks for any info.
 
My West Systems epoxy/fiberglass plenum worked great for 325+ hours. The only post cure it ever got was when it was on top of the IO-540 after a summer flight.

I never noticed any softness, warping, deformation, etc..

I'm building a new plenum out of carbon fiber, not because the old one wasn't functional, it just wasn't my best work. Kinda wavy because it was the prototype, and it had a big patch on it where I modified some other stuff... but it worked.

I'll likely use West with the CF too.

Did you eventually go to a CF plenum with WS105? Thats exactly what I'm embarking on and just wondering if you have some lessons learned with the 105 in this situation. Thanks
 
Pardon the thread drift

But I wanted to tie this question to the plenum issue.

Anyone considered making the 'lid' from flexible, airtight fabric or membrane? The top cowl would likely still take most of the pressure loading as the flexible 'lid' pressed against it, but the goal of sealing the top of the engine would be achieved. Seems like it would be a bit less work.
 
250 hrs

4 years in Socal. including some really hot summer flying. There has been no deformation of the structure. Having done one, I'm sure I could improve if I did it again but there is no reason. The West system epoxy and core material are holding up very well.
 
But I wanted to tie this question to the plenum issue.

Anyone considered making the 'lid' from flexible, airtight fabric or membrane? The top cowl would likely still take most of the pressure loading as the flexible 'lid' pressed against it, but the goal of sealing the top of the engine would be achieved. Seems like it would be a bit less work.
Interesting idea. You would need a flexible material that is heat resistant, and you'd probably still need baffles to attach to the engine. But it might be an interesting option for an alternative to baffle sealing.
 
West Systems will work

I used the same core material as Jim, fiberglass, and West Systems. Test flew in 100+ degree heat. Mine is holding up fine. Jim's is doing fine. I don't think you need some special epoxy to make it work. There are really no structural loads on it at the same time as it is exposed to high heat. Yes, there will be increased air pressure on one side of it in flight, but it is going to be cooled by all the airflow. On the ground when it heat sinks from the cylinders, it doesn't sag or run like water as one might fear. I put a heat sensor in there after a recent flight and it was about 220 and cooled fairly quickly. If I were going to re-do it, I would use carbon fiber instead of fiberglass for the weight savings. We did one for a buddie's airplane that hasn't flown yet and it is very light and stiff.

Ed Holyoke


4 years in Socal. including some really hot summer flying. There has been no deformation of the structure. Having done one, I'm sure I could improve if I did it again but there is no reason. The West system epoxy and core material are holding up very well.
 
If it helps with anyone's confidence in using FG, Vari-Ezes were using FG plenums bolted directly to O-200s back in the '70s. I seriously doubt that any of the high temp epoxies were available to EZ builders back then.
 
High temp resin

Way back when I was researching something for a plenum I finally settled on Resin Services Inc. I don't remember all the specifics but has worked great on quite a few different plenum designs.

30714299336_24e9a05b52_b.jpg



Plenum is on the Carbon Cub in case you are confused on this one...
30862051825_b13c16abae_b.jpg



West Systems has a lot lower deflection temperature but if it is working, I wont argue with success.

A plenum probably has a fairly low temperature in operation when the loads are highest.

After shut-down when the temperature goes high its not such a problem because there is not much load on the part.
 
4 years in Socal. including some really hot summer flying. There has been no deformation of the structure. Having done one, I'm sure I could improve if I did it again but there is no reason. The West system epoxy and core material are holding up very well.

Interesting for a Tg ultimate of 142F. I guess that is why it is good to experiment and get real world data.
 
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Interesting for a Tg ultimate of 142F. I guess that is why it is good to experiment.

Which is why I am pleased you used high temperature resin on your plenum. I can only imagine the under plenum temps after shutdown on a hot day.

The guys at Synergy were amazed at the ease of installation, fit and finish in comparison to the other plenums available. I had no previous experience so no basis of comparison, but we were able to do this first time install along with the baffles in less than a week.
 
That's a very interesting idea!

Interesting idea. You would need a flexible material that is heat resistant, and you'd probably still need baffles to attach to the engine. But it might be an interesting option for an alternative to baffle sealing.

There are plenty of materials available that would offer a flexible sealing membrane that could billow against the inside of the cowl but bring seal leakage to near zero. I'm imagining cutting up a large roasting bag or getting some Kapton from a plastic wholesaler and taping up a see-through plenum with Gorilla tape at the edges to mate it to the aluminum baffles. Possibly cheap enough to tear off and replace at every spark plug change interval?

Who's going to be the first? :)
 
Concept would be to allow the fabric to billow against the cowl top, so the air pressure would be acting directly only on the perimeter. A few tens of square inches instead of hundreds.

Perhaps the same idea with a conformal FG lid (as in earlier posts), but without the need for stiffness. Just a couple of layers of Bi for tension strength & let the cowl hold the shape. Silicone baffle material for the transition?

Shucks, make the whole lid out of Dan H's silicone impregnated FG that he used for his diffusers. Flex, high temp, done. :)
 
The elastic baffle -- It is an interesting concept. Dan's silicone or urethane-impregnated fiberglass would not be elastic enough, it won't stretch out like a bladder. You could make pleats or expansion folds in it to allow it to expand. An UN-reinforced elastomeric membrane, such as urethane or silicone, might work very well. The high-temperature Strechlon 800 vacuum bagging film might work, although it is very thin, care would be required to avoid punctures.

BTW, there are much better room-temp epoxies, often a lot cheaper than West.

Most room-temp epoxies just get their post-cure as their exposure in service occurs, although it is important to have low loads applied as the temps go up and Tg tracks with it. There is sometimes a modest strength increase with the post-cure, not enough to worry about either way.

We really like the Jeffco 1307, now sold by Rhino. It has a Tg of 168F.
Another good one is MGS.

When I say 'better', one of the poor characteristics of West epoxy is that it will cold-inhibit, if exposed to temps in the low 50s F during the curing process, it will not cross-link fully. After such exposure, no amount of higher-temperature exposure will make it cure. It will just stay rubbery. After I scrapped a couple of parts because of this, I said 'never again'.
 
relative motion

There are plenty of materials available that would offer a flexible sealing membrane that could billow against the inside of the cowl but bring seal leakage to near zero

I have to wonder what that relative motion would do to the inside of the cowl and membrane long term. Lots of vibration and dirt FWF to deal with.
I think Dan's silicone intakes have a ridged connection where part come together.
 
Very true. A slick inside cowl surface would help, but keeping it clean in that environment would be a trick. Maybe some periodic application of a silicone lubricant between the two surfaces?

I imaging a breach of the envelope by abrasion would be a "soft failure" - where rising CHT's would be the tipoff that the seal isn't as good as it once was. If the envelope wore through and then ripped to shreds, well that would be bad. Things would get warm fast.

Like I said, "Who will be the first?";)
 
Plenum pressure distribution question ?

Cowl clearance is clearly a concern. On most installs, the oil cooler takes air off one side and cabin heat can be a consumer also. So, how close to the top of the engine case parting flange can the plenum be for reasonable ( functional) pressure distribution above the cylinders ?
 
wet dog startup shakes

Cowl clearance is clearly a concern. On most installs, the oil cooler takes air off one side and cabin heat can be a consumer also. So, how close to the top of the engine case parting flange can the plenum be for reasonable ( functional) pressure distribution above the cylinders ?

I installed the standard baffle kit and trimmed to std length (I think that's 1/2" below cowl) Once done my plenum attached flush with the top of the baffles leaving 1/2" max clearance. As for the inside, the #1 plug required a small bump out to keep the plug wire from being deformed. Plenty of room for airflow above the cylinder. The place to be careful is the transition to the intake. That's where I like Dan's setup (Post #13) better than my fiberglass ramps.
 
I can't see much advantage to a flexible balloon plenum lid as compared to a hard shell lid, except maybe a little less load on the engine attach points, i.e less cracking of the aluminum sheet upright baffle walls. You can get around that by attaching the hard shell plenum directly to the cylinder hard points. Anyway, the real design trick is sealing either style to the cowl inlets.

What might interest me is molded-in-place flap seals. You know, flap seals that actually seal well?
 
I can't see much advantage to a flexible balloon plenum lid as compared to a hard shell lid, except maybe a little less load on the engine attach points, i.e less cracking of the aluminum sheet upright baffle walls. You can get around that by attaching the hard shell plenum directly to the cylinder hard points. Anyway, the real design trick is sealing either style to the cowl inlets.

What might interest me is molded-in-place flap seals. You know, flap seals that actually seal well?

Ease of fabrication? 30 minutes vs a month...? (Spitballin' here but it's just duct tape and an oven bag)
 
Ease of fabrication? 30 minutes vs a month...? (Spitballin' here but it's just duct tape and an oven bag)

Yep; that's the reason I brought it up. Especially if you've got an already flying plane, & want to improve cooling efficiency and you're a bit lazy, like me.

edit: BTW, I understand the concern about rubbing on the upper cowl, but it's worth remembering that non-plenum a/c have that issue by design. With a flexible membrane around the edges, there could well be only a rolling motion of the membrane around the edges as the engine moves around, instead of the scrubbing action that we get with regular non-plenum flexible baffle material.
 
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A flexible plenum might need battens to keep the fabric from flopping down on the cylinders.

Perhaps zippers can be used to open it up for maintenance or inspections.

Dave
 
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