Cowl Fit Question

JVolkober · #1 04-17-2010, 10:45 AM

I am fitting the upper cowl on my -9A. I have the cowl set with the spinner plate 1/8 inch higher than the cowl and the cowl center from left to right. I am having difficulty figuring our how to get the cowl to line up square, parallel, to the spinner plate. Horizontally I have a larger space on the left side (facing forward) then the right when measured at the bottom of the upper cowl; 13/32" on the left, 5/16" on the right. I probably can live with this. However, vertically I have a difference from top to bottom of the top cowl. At the top the gap is 3/32" compared to the measurements above made at the bottom of the top cowl. My concern is that the gap will be significantly larger at the bottom of the lower cowl unless I correct how the upper cowl fits. It appears that the engine is pitched up relative to the way the cowl naturally fits to the firewall. Has anyone experienced this and if so, what is the solution to getting the cowl in vertical alignment?

Finley Atherton · #2 04-17-2010, 04:07 PM

Fit the cowls the best you can then you will need to build up the face of the cowl behind the spinner so that you have a constant, parallel gap between the spinner and the cowl. This has been covered in more than one thread in the past. I used micro/epoxy as the fill then sanded it to shape then covered the area with one layer of glass cloth to better secure the micro and prevent any chance of the micro cracking in the future.

Fin
9A

scsmith · #3 04-18-2010, 01:35 AM

Hi John,

What Finley said above is true to a degree, but your measurements sound a bit extreme to me. My -8 cowl fit much closer than that. I probably had about a 1/16" of variation all the way around the spinner.

It really sounds like your engine might be sitting higher in front than it should.
I would suggest triple-checking the assembly of the rubber motor mounts to be sure you have all the rubber and metal parts in right, the thick and thin sides in correctly (different on top and bottom), steel compression tube in right, etc. so that the engine is where it should be.

The other factor is the position of the cowl as it drapes over the firewall. If it is sitting too high in the back, naturally that will close off the gap at the top of the spinner.

Keep us posted what you learn.

Jeff R · #4 04-18-2010, 03:01 PM

I had a similar problem, but it was my fault. I did not properly mount the propeller and hub to the engine. When I discovered the problem, after doing the prop/hub thing the way I was supposed to, I had already cut the rear part of the upper cowl and had to go back and glass in a new aft edge. So, make sure you have the prop and hub properly attached to each other and then tightly bolted to the engine. For me, that made all the difference.

Kyle Boatright · #5 04-18-2010, 07:18 PM

Quote:

Originally Posted by JVolkober

I am fitting the upper cowl on my -9A. I have the cowl set with the spinner plate 1/8 inch higher than the cowl and the cowl center from left to right. I am having difficulty figuring our how to get the cowl to line up square, parallel, to the spinner plate. Horizontally I have a larger space on the left side (facing forward) then the right when measured at the bottom of the upper cowl; 13/32" on the left, 5/16" on the right. I probably can live with this. However, vertically I have a difference from top to bottom of the top cowl. At the top the gap is 3/32" compared to the measurements above made at the bottom of the top cowl. My concern is that the gap will be significantly larger at the bottom of the lower cowl unless I correct how the upper cowl fits. It appears that the engine is pitched up relative to the way the cowl naturally fits to the firewall. Has anyone experienced this and if so, what is the solution to getting the cowl in vertical alignment?

I belive the cowls are designed to have a bigger gap at the bottom of the spinner than the top to make it easier to remove the cowl on tailwheel models. In my case, the difference is ~3/16, and I could still lose a little more room between the bottom cowl and spinner.

jimgreen · #6 04-18-2010, 07:37 PM

My cowl was pretty bad in its raw state. In addition to not being planar with the spinner, the part behind the spinner where the two halves are screwed together had to be rebuilt as the edges of the flanges were 3/16ths apart.

I think there is a lot of variability in these moldings and you just have to be prepared to put a lot of time in to get it right.

pmccoy · #7 04-19-2010, 10:19 AM

I had the same issue with my cowl. You can still see it a bit in this photo. I ended up setting the cowl just a bit high on the engine. Half the people I talked with told me to expect engine sag, and the other half said they have never seen any sag. So moving the cowl up just a tiny bit, helped the gap on mine. B.T.W. I also put a couple of clecos in the front of the cowling for fitting. Locking the front in position made it much easier to figure out where to sand the aft edges to. Once done, its easy to fill the drill holes.

erich weaver · #8 04-19-2010, 10:33 AM

Same problem on my cowl. Instead of building up the front plate on the cowl to make it parallel with the spinner, it is far easier to cut a slit parallel to, but behind the front plate, then wedge it open the appropriate amount to make the front plate parallel with the back of the spinner. Then you just have to make a small fiberglass repair to fill in the wedge-shaped gap you created and strengthen it with a few layups on the inside of the cowl. Not nearly so much filling and sanding.

erich

JVolkober · #9 04-19-2010, 04:43 PM

Thank you all for your input.

My situation now looks pretty much like Peter McCoy's. I worked on it quite a bit yesterday trying to get the best alignment. I complicated things a bit by not getting it aligned properly before I made the cut along the back. Apparently, I did not have the flange behind the spinner centered; it appears to have been off to the right (facing forward), perhaps because I was trying to get the flange to be parallel to the spinner plate, compensating for the engine angle by cutting the back at a slight angle. Once I agreed with myself that I cut the back incorrectly, and consequently will have to add back to the rear flange, I was able to get things lined up better, pretty much as Peter described in his post above.