Tip: Building a plenum in a vacuum

[jdeas]

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.

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!

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.


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!

[jdeas]

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.

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.


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.

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.

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





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

[Mike S]

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.

[Beancounter]

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

[krwalsh]

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.

[jdeas]

Originally Posted by Beancounter

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!

[jdeas]

Originally Posted by krwalsh

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.

[jdeas]

Originally Posted by Mike S

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?

[Rick_A]

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.

[jdeas]

That is quite a service range. Looks like I need to get the West System docs out and take a look.