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:
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.
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.
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:
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.
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.