Saturday, April 4, 2015

problem with Cowl vs Spinner

As mentioned several posts ago, I’m really pushing toward 1st start.  To that end, the prop needs to be attached.  Last night I spent about an hour figuring out the installation process.  The Book states to refer to the Sensenich manual, but that manual isn’t really applicable to the RV-12.  I finally figured it out, but discovered a Problem.

The photos below show the spinner backing plate rubbing against the cowl.  According to posts on VAF, there should be about 1/8” clearance, but I may be as much as 1/16” overlapped instead.  You can also see that the spinner plate is ~1/4” too high relative to the upper cowl.  I’ve got an email out to Support@vans and have posted these pics on VAF.

That discovery left me in a bit of a funk last night, but sleeping always helps.  I decided to press forward with the engine start, probably sans cowl (or at least, without the backing plate) for now.   The A&P from my airport does a lot of work with Rotax engines, and he’s agreed to come over to the house and give my project a review from an engine point of view. We might try to start it tomorrow.  Stay tuned.


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May 26th

Time for an update, and it’s mostly all good news.  Ever since I found this mismatch between the cowlings and the prop plate, I’ve been slowly creeping up on a solution.  It was pretty obvious from the start that the cowl (I’m going to use the generic singular to refer to both the upper and lower portions) was too long.  That wasn’t good, as it meant that I’d have to remove the hinges and sand away the excess to re-align things.  I was pleased to note that it wasn’t short, as that’s an entirely different solution—buy  >$1000 of new fiberglass parts and start over!

I have noticed that I really drag my feet when it comes to doing re-work as opposed to new assembly.  This is especially true when I can foresee a problem for which I don’t have a solution.   I could shorten the cowl to fit by sanding and re-installing the hinges, but I couldn’t figure out what to do about the radiator tunnel.  When I finally got the cowl to fit, I had shortened it back by about 1/8”, and that’s roughly the thickness of the lip to which the gasket seal is placed to provide an air tight fit between the cooling duct and radiator.  I am very leery of re-gluing the duct seal back on because then it’s going to be too close to the radiator and will have a much higher spring-back force than ever before (and I think it was just barely acceptable before I trimmed the cowl.)   If I sanded back the face by 1/8”, there would be no face left upon which to glue the gasket seal.  

I finally figured out a solution:  mix up thickened epoxy and add to the back of the lip so that there’s something remaining after I sand away 1/8”.  I added blue food coloring to the epoxy  mixture (it turns pink, but then back to blue when it cures) and glopped it on the back of the lip.  The picture below shows the inside of the cooling duct lip with blue (previous application) and pink (wet) flox/epoxy on the inside lip.  Photo #2 is a bit farther back so you can get oriented as to where the close up is located.

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Close up of cooling duct lip build-up of epoxy



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cooling duct lip w/ epoxy build-up


The next time I get to the shop I will sand down the lip until I get to the blue/pink epoxy (thus having removed 1/8”) and re-attach the duct seal.  When that cures, I should have a finished cowl that fits!  Stay tuned.




Next time I say that “I will sand down the lip until...”  just remind me that it’s not so easy.  It was about 2 hours of sanding with a dremel drum to get down to the blue (pictured below).  I verified that I could get the 1/4” cardboard spacer in between the radiator and the lip of the cooling duct.  Shortly after this picture was taken, I started reattaching the duct seal.


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“new” surface of cooling duct lip





Finally.  It’s done.  After sanding down to the blue flox, I reattached the original rubber seal, let it cure and finally reattached the cowling.  It’s not an easy fit, and there’s a bit of a gap on the L lateral seam.  It’s also pretty ugly with the visible marker on the inside of the cowl and obvious drips of the epoxy.  It was also nearly impossible to get the top cowling pins in place, and they definitely require more “oomph” than they should (IMHO.)  I have to use a pair of pliers to get a firm enough grip to install / remove them.

I’m too impatient to redo the cowling this year, but I may bite the bullet and buy a whole new fiberglass assembly for the front end next winter.  For now, it’s flight worthy.

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Front End, fully assembled.

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