R

RV8R999

Well Known Member
I was actually looking forward to fitting the cowling as it seemed to mark a milestone... but now I realize why others don't like it :)

I'm using the Skybolt complete kit rather than hinges.

I fit the upper cowl to the spinner back plate with 3/16" clearance using shims around the perimeter and trimmed at the firewall to match. It all came out great and I was happy.

So I spend some time cleaning up the lower cowl inner inlet flanges and cut away about 1/2" from the outer inlet flange to about 6" aft to allow the two halves to mate (roughly).

When I trial fit the lower cowl to the spinner backplate using the same 3/16" spacers I notice the whole cowl is too short at the firewall by about 1/16". Then when I put the upper cowl clecoed in place and align the forward faces of both cowls the lower cowl ring is a full 1/2" lower than the spinner backplate - basically these things are not round when matted together are they? I guess I can shorten the upper female flanges to allow the lower cowl face to slide upward to match the spinner, however; this increases the gap at the lower firewall corner as I cannot simply translate the whole lower section upward but rather have to rotate the forward face instead - a conundrum???

I'm surprised the lower cowl is too short and expected to trim a bit off just like the upper. Hmmm

Im not so worried about the interface between the cowl and lower firewall as the skybolt system provides plenty of structure but I'll have to fix the cosmetic 1/16 to 1/8" gap though :(

I'm assuming most of you had to really work the spinner flange area quite a bit to get it round and even? Any tips? I suppose an alternative would be to increase the gap to 3/8" and redo the upper cowl fit to allow some excess to be trimmed from the lower :( but that seems like a really large gap to me. Any thoughts?

Thanks!
 
Here is a picture of the lower cowl and spinner plate. At this fitting the inlet flanges are nearly, but not quite matted (without having trimmed them at all). You can see about 1/2" of flat area extending below the spinner...not good.

http://i45.tinypic.com/24det53.jpg
 
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Independent of the order in which the cowl is fit, I suppose a better question is if 3/16" is too close to the spinner? The fact that my lower cowl is slight shorter than I need it to be isn't due to the upper cowl but the distance I've chosen to the spinner. Maybe I answered my own question :)
 
HI Ken -

A couple of quick thoughts - I think I used 1/4" as a nominal spacing between the cowling ands the back of the spinner - if you get it too close, you're going to have a devil of a time getting the cowling on and off. It might look really, really good, but not be practical on a plane that is going to be flown and maintained a lot.

I have fitted the top cowl first - I think that the instructions (at least for the -8) said to do it that way, nd if they didn't, that is what I gleaned from other builders before I did mine.

Adding a little length to the back of the cowlings is really not very difficult. I messed up fitting my top cowl and created a gap (measure THREE times, not TWICE!), and it was a simple matter to add it back with some fiberglass. If you fit the top cowl and your bottom is still short, just add on a little flange - no one will ever know.

Paul
 
Paul - thanks! I guess adding a little would be ok considering is wont be structural just cosmetic.

Ken
 
Start by fitting the nose of the cowl together while simply overlapping the sides. Get the spinner area round, the inlets identical, and the seam perfectly bisecting the spinner. Only then fit it to the airplane.

 
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Thanks Dan - I see the same thing in your fit that I have in mine. In the picture the top cowl and the wood disk are nearly flush at the top whereas the lower cowl has what appears to be about 1/2 of flat area below the wood disk. Am I mistaken? Is this how most have fitted their cowls. My idea was to match the upper and lower halves to the spinner with with only the thickness of the spinner itself as the overlap???
 
a work of art Dan. Sent you an email. Your "in process" pic was worth a 1000 words...why didnt I think of putting a straight edge bisecting the spinner...DUH!!! Totally made sense when I saw the upper flanges needed about 3/4" trimmed.

Even with the two halves matched perfectly the lower cowl is about 1/4" too short at the firewall. I'll redo the top half and increase the gap to somewhere between 1/4-3/8" to give me something to trim to fit on the lower half.

Ken
 
Mistakes to avoid on the cowl

Ken,
Mistakes that I made in my cowl that didn't become apparent until flying:
1. Not enough clearance between cowl and spinner. Now the paint on the cowl is all chewed up from taking it on and off and rubbing it against the spinner. Like Paul said, 1/4" is a good place to start.
2. Not enough clearance between the cowl fiberglass and the fuselage skins at the firewall. Remember that the edge will be built up somewhat with paint, and that paint can chip if the fiberglass cowl rubs against it in flight or during cowl removals/replacements. My advice is to add a slight gap in there (1/16" maybe?) so your paint doesn't get chipped up (like mine).
3. Leave a fair amount of room between the Filter Air Box and your air intake (see previous thread I started a couple of weeks ago).

Hopefully that helps a bit,
 
Ken, A couple of things I recall from cowl fitting:

1) A heat gun to soften the glass for reforming and fitting the two halves goes a long way in this area.

2) It is pretty easy to shorten the cowl face behind the spinner by building glass thickness behind the face (on the inside of the cowl) and grinding/sanding it back to set the distance between cowl face and spinner. This also helps match the bottom cowl face to the top in a smooth, flat plane. Certainly it could be "built out" just as easily.

Good luck
 
Consider this...

I have two suggestions for consideration in fitting the cowl.
  1. You may not want the spinner bowl on the cowl to be perfectly round. There is a general agreement that the engine will sag to varying degrees over time, but if the spinner bowl is somewhat elongated along the veritcal axis, the engine can sag without the spinner hanging below the edge of the bowl. Mount the cowl a little low relative to the spinner in anticiation of the sag.
  2. For constant-speed props, if you are mounting the spinnner backplate to the crankshaft flange using spacers instead of a prop, be aware that the backplate must be perfectly flat to achieve an accurate fit. My backplate was dished such that the rear flange was about 1/8" forward of where it would be after the center of the plate was cut out for mounting on a constant-speed prop hub. Fortuntely, I discovered the problem before I went too far in the fit-up process.
 
great suggestions from all and many thanks. This forum is absolutely invaluable. I built my BD4 from plans only without the benefit of a forum like this and can tell you it was very, very difficult. So thanks!

I am going with a FP prop but also noticed the spinner backplate is cupped so am using a straight edge instead.


Ken
 
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