Cowl Cracks In Paint

[Pmerems]

Gents,

I want to share with you my pain and disappointment as I embark on my cowl modification/repair.

A year ago when I took my plane to the paint shop I noticed small crack in the primer paint in my lower cowl. I had 50 hours on the RV and never saw this before. The crack was on the outside of the cowl adjacent to the mixture pivot shaft. I have a Silverhawk FI system with Vans standard vertical induction cowl. Additionally I have the standard rotation mixture setup on the Silverhawk FI (CCW for ICO).

Since my plane was in pieces at the paint shop when I noticed the damage I asked them if they could ?blister? the cowl in the area where the damage was. They agree and I told them I would email them instructions and dimensions for the blister the following Monday (I dismantled the RV on Friday at the shop). On Monday morning I called and told them I was emailing the instructions. They told me the blister was completed and that it looked great.

Fast forward 5 weeks later when I re-assembled the RV. The blister looked great but it wasn?t the dimensions I wanted (or believed I needed).

Another 30 hours passes flying my newly painted RV. During my conditional inspection I noticed the paint was cracking where the ?blister? was added. I knew there wasn?t enough clearance that I had hoped for. Add another 30 hours of flying the RV with my wife and now it is time to take the RV back to paint shop so that they can fix/detail several areas they missed the first time. Additionally they agreed to repaint the ?blister? fix.

After loosing sleep worrying about cutting the cowl up I started on the fix yesterday. I decided to make a mold of the finished outer shape. Using modeling clay I sculpted the final shape (with some setback for additional external fiberglas. Then with some PVA and wax I covered the clay and adjacent areas on the cowl and glassed up the mold. 12 hours later I removed the mold and cut out the section of the cowl. Today I will glass up the (from the inside) of the cowl. I will keep you posted on the progress.

I believe the excessive vibration of the engine is caused by poor hot starts. I don?t recall any really bad hot starts however it only takes on to cause this damage. It is my hope that once completed and painted the blister will be unnoticeable and never crack again.

The photos below show the cracks in the cowl (paint) and the inside of the cowl where the aluminum tape shows the impact marks from the bolt on the mixture arm.

[az_gila]

Paul,

Could it be a "O-360 wet dog shake" when you shut the engine down?

[roee]

Paul,

How much clearance did you have between the mixture control and the cowl where it was contacting? How much clearance are you going to give it now?

I'm working on my cowling now, and have already noticed that I have insufficient clearance there. Planning to put a blister in the cowl preemptively. As is, I have about 1/4" clearance. I'm thinking I want to have about 3/4" to be safe. Your data point would be helpful in validating this number.

Thanks,
-Roee

[Pmerems]

Roee,

It is very hard to see the exact clearance when the cowl is on. The blister is approximately 1/4"+ from the original cowl contour. Keep in mind that if you have the Silverhawk FI with CCW mixture rotation for ICO you could have the same problem.

Send me a private message with your phone number if you would like to talk more about.

[roee]

Thanks Paul.

Mine is a Silverhawk with the reverse mixture rotation, so I have the mixture lever clocked facing up. You'd think that that would be better, but the same problem still exists. The end of the mixture arm still dips down into the cowl scoop in the full forward (full rich) position and comes too close to the side. And even the nut at the base of the mixture arm comes a little too close for comfort. So I want to relieve enough clearance for both.

I don't have an exact measurement of my clearance either. Just guesstimating. It's close enough that I can't put a finger between the mixture control and the cowl, which is definitely too close. I think if I could put my thumb through there, it would be fine. Again, just guesstimating. All a matter of how much the engine actually moves around under vibration, probably the worst case being the shudder of startup/shutdown. And since we typically use a 1/2" or so clearance for the cooling baffles to cowl (I used 5/8"), I figured about the same should do for the mixture control, being about the same distance out from the engine's axis of rotation.

We could definitely chat offline and share ideas on this if you like. Sent you a PM with my contact info.

-Roee