bret

bret

Well Known Member
Going to try my non artistic hand at shaping some foam for the diffuser connection thing to the baffle-plenum contraption, how are you guys making an air tight connection from the fiberglass to sheet aluminum? I would like to keep everything smooth and flowing without much of an overlap or joint?
 
I was watching Crabandy's build using Dan's rubber ducky mold technique, not sure if I want to tackle a new skill, just now getting the hang of FG, It looks like Dan used a Nutplate strip to sandwich the boot. I may be overthinking all this, been reading too much on here about all this cooling drag, baffle, plenum stuff, funny, starting the build 6 years ago I never even had one thought about this part of the build. now I need to stop and think what I should do here.....time to go spray top coat on baffle-plenum stuff and air out house before the wife gets home..........(What dear? what paint smell?).........
 
Connection

Hi Bret

I made my own plenum and reshaped the inlets on the vans cowl to make them round. For a connection from the cowl to the Plenum I used some 3mm neoprene. It is cut so it fits over the cowl intake stub where I use a large jubilee clip to hold it in place.


The neoprene then sits inside to plenum intake to a depth of about 3 inches and is held by nothing more than its shape and air pressure. I used 5 inch diameter intakes on the cowl and T's and P's are spot on.

There is photo on the first place of my blog which capture this and a better one of the plenum about 5 pages back.
 
I plan to do like another in my hangar, which is to use some neoprene or similar tubing material (flexible) to tie them together and allow for engine movement.
 
Last edited:
Having gone down the path of fiberglass inlets with the SJ cowl for use with the neoprene boots, I might go a different way.

I think there might be a good opportunity to make a hybrid approach. That is to make the aft 2/3 of the inlet in glass, then graft on a custom urethane boot bonded to the glass, and bonded to the SJ inlet ring. This given that I already have the ring glassed in. This way it is custom fitted to this plane, and no straight section needed for a clamp. A gentle expansion could begin much sooner. Even better if the inlet ring was shortened or modified.

Just a thought.
 
Last edited:
Very interested in what you guys come up with, I've got the Sam James cowl and plenum and I'm not happy with the neoprene junction either.
 
airguy said:
Very interested in what you guys come up with, I've got the Sam James cowl and plenum and I'm not happy with the neoprene junction either.
Click to expand...

I'm using a James cowl on the RV-10 as well. The first time I did the neoprene junction I was not thrilled with the looks. It worked for four years but they were not pretty. I called Will James and he walked me through the process (which was as provided in the instructions but I was too bull headed to follow them). I just finished doing a new set and they came out perfect. Total time was less than an hour.

The part I kept tripping on was making the butt joint alone the seam of the cylinder as I figured it would not hold. This time I did a pull test after gluing a couple of scrap pieces and was surprised to find the neoprene ripped before the joint failed.

Here is the contact adhesive I used and recommend: http://www.amazon.com/dp/B002DZRFIK/ref=pe_385040_128020140_TE_3p_dp_1

Carl
 
airguy said:
Very interested in what you guys come up with, I've got the Sam James cowl and plenum and I'm not happy with the neoprene junction either.
Click to expand...

Do you think you have an air leak with the neoprene? are your temps good? did the plenum line up with the intake rings? I am ready to do some kind of diffuser but still looking for ideas....
 
bret said:
Do you think you have an air leak with the neoprene? are your temps good? did the plenum line up with the intake rings? I am ready to do some kind of diffuser but still looking for ideas....
Click to expand...

No, No (sort of) and no.

No indication of air leaks, I've been very vigilant about sealing all gaps and holes, and the neoprene joints are not leaking - the recommended James glue seems to be working just fine. My temps (CHT's here, oil is another story) show significantly cooler temps on the right side than the left. The alignment of the original James plenum is the issue in my opinion, the right side is near perfect but the left side is about 1.5" high and 1" inboard, (plenum versus cowl) causing some wrinkles on the neoprene sleeve and spoiling the airflow. That needs to be corrected. I have had a rather negative experience with the James plenum in total. The cowl has been great, the plenum not so much.
 
Last edited:
FWIW I used following parts from McMaster-Carr to seal custom inlet rings to fibreglass radiator ducts (turbo Subaru engine) :

1) Fabric Reinforced Hi Temp Silicone Rubber Sheet (1/16 x 6 x 36 p/n 3635K13)
2) 304SS Smooth Band Clamps (6 in dia 5574K29)

Cut the strip to 3 in wide - wraps around just under twice (6 in rings)
Relatively cheap, seals well & no issues in 200+ hours.

John Moody
RV-8A
 
Last edited:
On the left side the distance between the ring and plenum is a bit longer than on the right. Because of that, the air coming in is able to expand the neoprene outward and then the pocket between the plenum inlet and the neoprene creates turbulence in the airflow. Doubling up the neoprene on the left side fixed the problem and it's all good now.
 
Never thought of that? there is enough pressure to blow up, expand the neoprene that far out of shape? I am thinking of building from the baffle to the rings, half way with glass, go from rectangle to round, about 6-7" diameter and then transition with the neoprene or the sheet mentioned above, to the rings, make it all flowing......any thoughts?
 
There may be 12" of water pressure, but the neoprene is not reinforced and will expand, how much I have not tested. This is why I think that using glass at the engine end and then 2-3-4" of hand laid urethane (DanH material) with one glass layer at a 45 deg angle to the flow, then bonded to the inlet ring will provide:

- Relatively smooth wall contours from the inlet ring (Lenny's point)
- no need for clamps
- stiffer resistance of the elastomer to pressure expansion
- longer distance to absorb motion of the engine (needed with a stiffer material)


A question to be investigated is bond strength to aluminum and to the fiberglass section. Stripping some glass strips from roving and wrapping them around the glass and aluminum ends could greatly stiffen that area and put less stress on peeling off the bond. That might be a good idea.

Custom made inlets with consistent growth of area from inlet to the head face can allow a perfect alignment from the onset.

A custom plenum cover will allow you to use all the room above the engine up to the cowl for airflow, particularly important for getting cool air back to the oil cooler. And not having it sit below the head fins on #4.

Just my thoughts.
 
Last edited:
BillL said:
A question to be investigated is bond strength to aluminum and to the fiberglass section.
Click to expand...

Bill, the urethane rubber is itself quite adhesive, but when cured, bonding it to anything else can be a problem. Smooth-On makes a urethane adhesive for the job (Ure-Bond II), but they don't make strong claims for it.

I reasoned that bonding a fabric reinforced urethane to an inflexible metal ring was less likely to be successful (long term) than bonding it to a flexible rubber ring...a matter of stress concentration.

So, first I mounted the ducts on the engine and trimmed them a little short of the cowl.



Then I machined aluminum reinforcing rings, 1/16" thick with a "L" cross section. They got a few dabs of Ure-Bond on the outside, and then they were slipped into the ducts.



The cowl was added immediately, and the rings were pulled forward so they were against the inner surface of the cowl's glass inlet.



When cured, the aluminum rings were adhered to the urethane duct...not strongly, but well enough to precisely set the position of the duct ends, so I could next cast the rubber rings. The ducts were set nose down in a machined nylon mold. I painted some Ure-Bond on the exposed urethane in the casting area, then poured in liquid urethane. I reasoned that the co-cured Ure-Bond would improve the interface between the cured and uncured rubbers.



Note that the glass reinforced urethane is on the inside of the cast rubber ring; inlet pressure cannot exert a peel stress. The joint is pretty much just shear, something the rubber does well, in particular when the cast rubber keys into the fabric weave pattern on the duct surface.



After cure the aluminum rings were easy to remove. There is no need for them to be fastened to anything, as they cannot escape. In service, they eliminate any possibility of the rubber ring collapsing inward, and they look nice as polished trim in the inlets.



Overall, it's an entirely repeatable procedure. I actually made two sets of ducts, the first being multiple plies of 8-harness, and the second being a single ply of 9 oz plain weave. The second set incorporated a spigot for the prop governor. Here the first four photos were taken while doing the original set, thus no spigot. I still have all the molds, and can make another set when these wear out or fail. Right now (five years, 550 hours) they're doing fine.

The cowl itself got matching flanges cast from micro and flox, but that's another story.
 
Last edited:
DanH said:
Bill, the urethane rubber is itself quite adhesive, but when cured, bonding it to anything else can be a problem. Smooth-On makes a urethane adhesive for the job (Ure-Bond II), but they don't make strong claims for it.

I reasoned that bonding a fabric reinforced urethane to an inflexible metal ring was less likely to be successful (long term) than bonding it to a flexible rubber ring...a matter of stress concentration.

So, first I mounted the ducts on the engine and trimmed them a little short of the cowl.

Then I machined aluminum reinforcing rings, 1/16" thick with a "L" cross section. They got a few dabs of Ure-Bond on the outside, and then they were slipped into the ducts.

The cowl was added immediately, and the rings were pulled forward so they were against the inner surface of the cowl's glass inlet.

When cured, the aluminum rings were adhered to the urethane duct...not strongly, but well enough to precisely set the position of the duct ends, so I could next cast the rubber rings. The ducts were set nose down in a machined nylon mold. I painted some Ure-Bond on the exposed urethane in the casting area, then poured in liquid urethane. I reasoned that the co-cured Ure-Bond would improve the interface between the cured and uncured rubbers.

Note that the glass reinforced urethane is on the inside of the cast rubber ring; inlet pressure cannot exert a peel stress. The joint is pretty much just shear, something the rubber does well, in particular when the cast rubber keys into the fabric weave pattern on the duct surface.

After cure the aluminum rings were easy to remove. There is no need for them to be fastened to anything, as they cannot escape. In service, they eliminate any possibility of the rubber ring collapsing inward, and they look nice as polished trim in the inlets.

Overall, it's an entirely repeatable procedure. I actually made two sets of ducts, the first being multiple plies of 8-harness, and the second being a single ply of 9 oz plain weave. The second set incorporated a spigot for the prop governor. Here the first four photos were taken while doing the original set, thus no spigot. I still have all the molds, and can make another set when these wear out or fail. Right now (five years, 550 hours) they're doing fine.

The cowl itself got matching flanges cast from micro and flox, but that's another story.
Click to expand...

Thanks for the refresher, Dan, inside is definitely a better situation. That was the reason for the "to be tested" comment. Peel forces will definitely be concentrated at the edge of the transition from urethane to glass or aluminum. As you point out, it might exceed the peel strength and fail will cycling. It is NOT a developed proposal.

Your method is proven!
 
Thanks for the pics Dan, I was wondering how the rubber met the rings. How did you come up with this? just amazing!
 
You must log in or register to reply here.