SonexBuilders.net

I've started working on my cowling this week and it seems I have a bit of an issue with the cowling itself, it looks like it is too short in some areas.
I know the first thing to check is the engine positioning which I have done over and over again now, as far as I'm concerned the prop hub is positioned as per the VW engine installation drawing.
Have a look at the pictures, they show the problem, the cowling is at worst 3/8" short in some areas.



What now? Modify the cowl and if so, how is this done? I know people have done a lot more complex things with fiberglass modifications to cowlings etc. but this is completely new to me.

My understanding is most people end up needing to trim more than they initially expect, especially across the bottom.

When the cowl is loosly positioned it does not reach as far on the prop hub side, making you feel like the cowl is smaller than it really is, but once you start pressuring the sides, top, and bottom into position, the prop hub side will stick out further.

I ended up having pretty close to a 1/8" gap between the cowl and propeller until I installed the rubber engine baffling. I may have been too aggressive trying to guarantee no leaks, but it seems to have stretched the cowl enough to close that gap on the bottom. I ended up using a fiberglass repair kit to build up material behind the lip and sanding it down for propeller clearance.

If your cowl really is too small, I wonder if building it up near the prop hub would be a simpler fix?

I'm working on this a bit more today and it appears as I trim more and more off the bottom side of ther cowl where there is exessive material, the gap on the side seems to be getting smaller and smaller. This is counterintuitive but that's how it is.
It's now less than 1/8" on the side and I still have some trimming to do.
I just realized as well I starter with the left side of the cowl, the instructions say to do the right side first, I can't see any reason that would make a difference?

rizzz wrote:I'm working on this a bit more today and it appears as I trim more and more off the bottom side of ther cowl where there is exessive material, the gap on the side seems to be getting smaller and smaller. This is counterintuitive but that's how it is.
It's now less than 1/8" on the side and I still have some trimming to do.
I just realized as well I starter with the left side of the cowl, the instructions say to do the right side first, I can't see any reason that would make a difference?

Yes, its counterintuitive. The instructions do work and the cowl will fit.
Its a complex 3D object being fitted.
I don't think it really matters which side to start with, they probably just picked one for the pictures and stuck with it.

It is only ok to start on the other side in the Southern Hemisphere.

-----

Bryan Cotton wrote:It is only ok to start on the other side in the Southern Hemisphere.

LOL!
I guess I'm OK then.

Trimming the bottom and clearing more and more space for the gear legs definitely does the trick,
I've managed to completely close the gap on the side, in the end I needed to material away from the side to start closing the top gap (strange). Only a small 1/8" gap on the top center remains, I'll do more work on this tomorrow.

I feel that solving the cowling fit problem was a rite of passage. One should get a merit badge for doing it. I trimmed and trimmed more than I thought I should and was surprised that I got a smooth transition on the bottom. No matter how hard I tried, I couldn't get the cowl to follow the curvature at the canopy and ended up having to cut a long narrow triangular dart out on each side in the curve and then splice the edges together. The lower center joint was fairly easy but I did have to add about a 1/8" wide filler tapered to 0 over about the last 10" to get the halves to meet. I also had a problem with the inside of one lower rear corner of the cowling hitting the lower motor mount. Additionally, the front cowling flange did not end up parallel to the plane of the prop shaft so I had to fill at least 1/4" at the bottom of the cowling front flange.

Since I am a glutton for punishment and felt that taking the cowl halves on and off at least 200 times was not enough suffering, I also did a horizontal split mod--I am thrilled with how this came out and think this is absolutely the best thing for easy FWF inspection and access. My hat goes off to anyone who builds a glass aircraft--after wrestling with the cowl I can't imagine mixing and spreading that much resin.

I would like some recommendations from those who have suffered through this and survived--I need to order the fasteners for the cowl halves. I know many of you use the Southco's and others the Skybolts and some just screws and nut plates. I am concerned about getting the cowl top off with Southco's or Skybolts because they protrude so far and I don't think the top cowl half will flex enough to get them out so I am leaning toward plain nut plates and screws but don't want to have to replace nut plates after removing and reinstalling the cowl ten times. So please tell me what you used, what you like and dislike about your choice and what you would do if you were sent to hell and forced to do it all over (perish the thought).

Thanks for your inputs.

Best

Stuart near Boston
Sonex 1106 hoping to fly in 2015

Hi Stuart, On my current plane I have both Southco's and nut plates and I don' t have problems with either. Both are installed on metal panels. I've taken the nut plate screws in and out about ten times more than the Southco's and they are still serviceable. With Southco's the key is to use the proper length.

David