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Soft Spot Plenum Lid

DanH

Legacy Member
Mentor
RV-8 builders know stuffing an angle valve Lycoming into an RV-8 cowl results in very tight engine to cowl clearance above the front cylinder corners, and down the left side. It gets worse with a plenum lid, a classic case of stuffing six pounds of stuff in five pound sack.

My plenum lid has routinely bumped the inside of the upper cowl. If I look close, I can see some fine spiderweb cracks in the cowling clear coat, so if allowed to continue surely it will develop an ugly spot. In recent times it has not been bumping so much, but that was because the mount isolators were sagging. With new isolators, it was time to fix the bumping.

The problem spot, both sides:

Plenum%20800w.jpg


For a first cut, I simply sanded a big flat spot on the lid and glassed it over, in an attempt to lower the height of the lid in the contact area. It wasn't enough to truly cure the problem, so I decided to try an experiment...plenum soft spots.

Plenum%20Soft%20Spot%20left.jpg


Plenum%20Soft%20Spot%20right.jpg


We've had past conversations about soft membrane lids and similar. These are partials. They are made from two plies of 9 oz plain weave fiberglass cloth, with rolled-in Permatex Ultra Black (Loctite 598) silicone sealant to fill the weave. Process is like the cylinder seal wraps; roll in the silicone between two sheets of 4 mil plastic, cut to shape right through the plastic, peel one side, stick into place, allow cure and peel the other plastic sheet.

Here the forward edge and outboard edge of each soft panel is clamped under fastener strips. The inboard and rear edges are simply 2" overlaps of the plenum glass, and rely entirely on the shear strength of the sealant (which for UB/598 is quite good).

The soft inlet ducts are now at 850 hours and 8 years. They are still doing fine. It's a similar fabrication, a single ply of 9 oz and room temp cure urethane rubber. I'll see how the silicone soft spots hold up, and if generally satisfactory, I may replace them with urethane panels. A very future mod might be to make new inlet ducts with a considerable extension rearward, i.e. make the soft inlet and the soft panel as one piece.

More later.
 
Thanks

Dan,
Thanks for pointing out this problem and actually showing a FIX.
I know you have a 0-390.......but this problem exists even in a 0-360 with vans baffeling. Well may not be exactly the same but......
This was one of many issues that urged me to convert to a James plenum and Cowling. Paying attention to the "corners" in the install. Fixed my issue.
So far anyway....:rolleyes:
 
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Curious why you're using fiberglass. In my opinion, it's an obsolete material.
Kevlar would be a better choice in this case. Just an uninvited ombudsman's opinion.
 
The Five Rules for Good Engineering are:

1. Part of the art of being a good engineer is knowing where to steal good ideas.

2. Get it good enough and get on with it.

3. The best way to solve an engineering problem is to guess the right answer.

4. Any **** fool can figure out another way to do it; i.e., the first solution to a problem is often entirely adequate.

5. If you can't fix it, feature it. There's always a way to market the problem as a benefit.

--- written by Mark Drela.

Seems to me that Dan has hit number two pretty well.

Dave
 
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Thanks Dan!

I've been using glass impregnated with black RTV for the RV-8 ducts as you taught us, and also have used it to seal the corners of the flap seal on my 9-A. It's in contact with the upper cowl continuously, and neither the RTV or upper cowl show any abrasion damage after 300 hours.

Now if I could just move on with your cut-and-buff tutorial!

Roger
 
Curious why you're using fiberglass. In my opinion, it's an obsolete material.
Kevlar would be a better choice in this case. Just an uninvited ombudsman's opinion.

It would be a better choice, technical speaking. On the other hand, (1) the fiberglass is hanging in there pretty well in the duct application, and (2) I'm too cheap to buy the special scissors which will actually cut kevlar. ;)
 
Dan, you're the only guy I know with plenum featuring a finish as fine as the outside of your plane... Nice:)
 
From an uneducated, what are the benefits of Kevlar vs fiberglass? and to add to the mix, where does carbon fiber fit (or not) in this specific discussion?

Thanks.
 
From an uneducated, what are the benefits of Kevlar vs fiberglass? and to add to the mix, where does carbon fiber fit (or not) in this specific discussion?

Fatigue resistance. Pretty sure they rank (best to worst) kevlar, glass, carbon. I'm too lazy to look up the numbers. If the membrane would be subject to a lot of flexing, it might be worth consideration. Here? Naaa.
 
It would be a better choice, technical speaking. On the other hand, (1) the fiberglass is hanging in there pretty well in the duct application, and (2) I'm too cheap to buy the special scissors which will actually cut kevlar. ;)

At our shop, we turn a standard $2.00 pair of scissors into Kevlar scissors in about 3 minutes on a grinding wheel. Hopefully Bob Kuykendall will chime in with the finer points, but basically you just grind the cutting edge to be 90 degrees from the faces.

We don't consider fiberglass to be obsolete. It is the right material for small parts where weight is not going to matter, and/or if a lot of fasteners or trimming has to be done (just because Kevlar is sometimes a pain), and it is so inexpensive compared to the alternatives that it keeps earning its way into parts.

For general purpose parts, Kevlar is not great because it has so little compression stiffness. Glass, while not anywhere near as stiff as carbon, still makes nice stiff parts without a lot of thickness.
 
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