Fiberglass mod needed on cowl

McFly · #1 08-16-2006, 10:02 AM

Long story but in the end, I don?t have enough cowl/spinner clearance. I thought I was done with this but I have to revisit and I have painted myself into a corner.

I took as much fiberglass off of the spinner as I could to increase clearance but it is not enough. I have to shave the cowl. The cowl face (the area directly behind the spinner) has been built up on the bottom cowl (micro balloons with super light glass on top) but not much on the top cowl. The 11:30 ? 1:30 clock position has no build up at all. I need to take almost the entire cowl face down at least 1/16? and I need to take some additional material off the 11:30 to 3:00 position. During the shaving process I will end up going though the original fiberglass at the 12:00 to 2:30 position.

Before I start shaving I am going to build up the inside of the cowl in the area where the original fiberglass will get reduced. I have thought of two ways to do this but not sure if either is the best approach.

1) Apply layer cotton flox or micro balloon inside cowl in the affected area, shave cowl, add medium weight bi-directional cloth inside cowl on top of cotton flox layer and surrounding area for structural support, fill outside with micro as needed, add extremely light cloth on outside of cowl for final dressing (sorry I don?t know the weight but it looks like you could make clothes out of it).

2) Apply multiple layers of medium cloth inside cowl to affected area, shave cowl, fill outside with micro as needed, add extremely light cloth on outside of cowl for final dressing.

I am partial to method one and prefer using micro balloons. Will this work OK? Other suggestion? Thanks guys. Man-o-man am I depressed.

Mike S · #2 08-16-2006, 10:25 AM

Option 1 will work just fine, be sure you put a layer of cloth over the filler inside the cowl. Flox is stronger, micro easier to work.

When you remove material on the exterior, take off a little bit extra so you can put some cloth over the now exposed filler----------you MUST have cloth on both sides of your filler for needed strength, if using micro. Still a good idea if using flox.

Dont forget breathing protection.

Good luck, Mike

RV7Guy · #3 08-16-2006, 11:15 AM

I'd be curious as to how you got to this point

Good process with a couple of thoughts. Clean the inside thoroughly and then use a Dremel with a stone bit and rough the inside area of the spinner area of the cowl.

I'd use a mix of epoxy with Flox in a consistency that the epoxy is still liquid, almost pourable. Build this area and use alcohol on your fingers to push the mix in and smooth.

Make some reference marks on the cowl ring and use a hand held belt sander to take it down. Block sand to flat.

When you get done add a layer of cloth to the outside to bond everything together.

Keep us posted.

Dan_E_Root · #4 08-16-2006, 11:44 AM

I had the same problem. Flox the inside of the cowl, then sand it down till it clears with a power sander. Then bring it back to glossy smoothness with epoxy and microballoons.

I didn't use b'loons on the inside, but I guess it would be OK when mixed with flox or chopped fiberglass to maintain structural strength.

Keep in mind that the engine will drop some, when you hang the prop and start it up, so don't try to be too precise with your clearance just yet. You might need to fine tune it later anyway. I sprayed a light coat of a dark color on the face, then started the engine with the prop on. If it rubbed even a little with the engine running, it showed through the paint.

RV_7A · #5 08-16-2006, 02:42 PM

Same issue I had... see this page. Easy fix.

-Jeff

http://www.jeffsrv-7a.com/COWLING/06-08-05.htm

Results....

http://www.jeffsrv-7a.com/COWLING/06-08-06.htm

f1rocket · #6 08-16-2006, 03:11 PM

A word of caution, you should NEVER apply microballons between layers of glass. The glass does not adhere to it over the long hall and will separate eventually. I would build the areas up with layers of glass cloth from the inside, then sand the outside with a belt sander until you get the clearance you need. If it becomes too thin, you can always add additional layers to the back to build it back up again.

Micro should only be used as a sandable filler on the outside of layups. This is from the man that wrote the book on glass airplanes! (Rutan, not me.)

gmcjetpilot · #7 08-16-2006, 03:18 PM

Quote:

Originally Posted by f1rocket

A word of caution, you should NEVER apply microballons between layers of glass.

Concure. Micro filler is not very good. If you want a "structural" filler, go with the flox. Not sure why you can't just use additional plies. Pay spcial attention to surface prep of course.

McFly · #8 08-17-2006, 09:08 AM

When posted this yesterday, I did not have the boot cowl clecoed in place (the sheet between the canopy and the cowl , RV-7 tip-up). Looking at it last night the light bulb came on. This lifted the cowl and now the cowl/spinner fit is much improved. I will still need to do some surgery on it but not to the extent that I though originally.

Darwin you asked ?how I got to this point?. Two things happened. One: I had two problems with my prop spacer where I had to send it back. I built the spinner/cowl with the old spacer installed. When I installed the new spacer, the fit was off and I jumped to the conclusion that maybe the original spacer was longer than 2.25?. Yeah I know, unlikely but it already had two other problems so it made some sense at the time. Two: I have a plenum where the baffle material is attached to the mouth of the plenum; there is no baffle material attached to the cowl. This was my first attempt to install the cowl on/off with the baffle cloth installed while the prop was on and it was not easy. It was one of those ?I didn?t think that through very well" moments.

George: The reason for not going with the multiple plies for the build up is that my track record for doing that in a concave area is bad. Despite my best efforts, I get lots of voids between the layers. This is purely a problem with my technique but I am not sure where I am going wrong.

Thanks for the all of the advice; I will not use micro between layers. Could someone tell me why use alcohol on my fingers when putting in the flox, is this to keep it from sticking to fingers?

RV7Guy · #9 08-17-2006, 10:21 PM

Quote:

Originally Posted by McFly

Thanks for the all of the advice; I will not use micro between layers. Could someone tell me why use alcohol on my fingers when putting in the flox, is this to keep it from sticking to fingers?

Thanks for letting us know what happened. Stuff like that takes place no matter how much you try to plan things out.

The reason for the alcohol is to allow you to smooth things to a finished look. Yeah, your fingers won't stick the epoxy while doing this.

Keep us posted.

David-aviator · #10 08-18-2006, 08:26 AM

Quote:

Originally Posted by McFly

Long story but in the end, I don?t have enough cowl/spinner clearance. I thought I was done with this but I have to revisit and I have painted myself into a corner.

I took as much fiberglass off of the spinner as I could to increase clearance but it is not enough. I have to shave the cowl. The cowl face (the area directly behind the spinner) has been built up on the bottom cowl (micro balloons with super light glass on top) but not much on the top cowl. The 11:30 ? 1:30 clock position has no build up at all. I need to take almost the entire cowl face down at least 1/16? and I need to take some additional material off the 11:30 to 3:00 position. During the shaving process I will end up going though the original fiberglass at the 12:00 to 2:30 position.

Before I start shaving I am going to build up the inside of the cowl in the area where the original fiberglass will get reduced. I have thought of two ways to do this but not sure if either is the best approach.

1) Apply layer cotton flox or micro balloon inside cowl in the affected area, shave cowl, add medium weight bi-directional cloth inside cowl on top of cotton flox layer and surrounding area for structural support, fill outside with micro as needed, add extremely light cloth on outside of cowl for final dressing (sorry I don?t know the weight but it looks like you could make clothes out of it).

2) Apply multiple layers of medium cloth inside cowl to affected area, shave cowl, fill outside with micro as needed, add extremely light cloth on outside of cowl for final dressing.

I am partial to method one and prefer using micro balloons. Will this work OK? Other suggestion? Thanks guys. Man-o-man am I depressed.

This is probably a very dumb question to everyone but me, but could not the problem be solved by simply trimming the aft edge of the cowling to provide the required spinner clearance?
I had the same problem with the MT prop and it worked OK. Sure was a lot less work than rebuilding the forward cowling area.

dd