-9A cowl fitting

[ge9a]

I have a quick cowl-fitting question and I'm looking for input. Right now, the top of my cowl is on straight and looks pretty good relative to the fuse (the rear of the top cowl fits the fuse pretty well at the firewall, side-to-side, where it is). But when I measure the distance from the trailing edge of the prop to the forward face of the cowl (at the far outer face of the air intake, left and right, on the cowl), the difference is about 25/32?. I don?t really think it is necessarily a looks issues, it is just different. Do I care for any reason?

I can fix this by adjusting the front of the top cowl so the distances between the aft edges of the prop and the front of the cowl on either side are the same, but then the top cowl won?t fit the fuse very well at all at the firewall. Or I can just proceed with the cowl fitting the fuse well, and add foam (or whatever) to the front of the cowl so it is more even behind the prop, if it really needs to be ?even on both sides?. Or just leave it ?uneven?. Either way, I need to figure this out before I cut any more on the cowl to fit the plane.

Do I need to worry about this "mis-match" between the two sides between the aft edge of the prop blades and the leading edge of the cowl intakes?

Any advice would certainly be appreciated.

Thanks.

[docskj]

I simply referenced everything to the spinner back plate. The flat area of the cowl is the same diameter as the spinner backplate. You want the edges to transition nicely to the profile of the cowel. I think the gap between the cowl and back plate is fairly irrelevant as long as you have adequate clearance so there is no rubbing. The gap in my plane is not constant and will definitely not be constant (at least on the vertical aspect) if you anticipate for sag. The only murky issue I remember dealing with was the amount of anticipated sag to account for. Left/right alignment was easy--probably because I did not think about it too much. I never measured distances from prop to cowling but I bet they are not even. Getting the cowl to fit well at the firewall with proper alignment was the main challenge.

Steve Johnson
RV-9A flying 170 hours

[rv9av8tr]

You want to center the top cowl bowl to the spinner back plate. I bondo'd a wood spacer to the front of the bowl so it touched the spinner back plate, establishing a consistant fore-aft postion. With the front centered, start trimming the cowl to the firewall. https://picasaweb.google.com/mikerv9a/FWF

This is a really tedious process, so don't over trim! Like the canopy, you can't add back on what you cut off. Good luck!

[DanH]

Originally Posted by ge9a

But when I measure the distance from the trailing edge of the prop to the forward face of the cowl (at the far outer face of the air intake, left and right, on the cowl), the difference is about 25/32?. I don?t really think it is necessarily a looks issues, it is just different. Do I care for any reason?

Greg, you're asking about the difference between A and B in this sketch?



Remember, the engine is pointed slightly to the right. The propshaft is centered in the cowl face with the rear of the engine to the left. There was no reason to rotate the entire cowl face to the right, just the spinner face. The result is what you measure at A and B.

Best I know they're all like that.

[ge9a]

Yes, Dan, that is exactly what I'm talking about. Thanks for the clarification. So what I'm hearing is that a different dimension between "A" and "B" on either side of the cowl is pretty standard? Mine is 25/32" or so different. If not a big deal, I'll build on!

Thanks for the replies, everyone.

[Rupester]

... but I advise against making any final cowl trims until you have the spinner and backplate in hand. I did not do so and trimmed the entire cowl incorrectly, not realizing that the backplate has a flange that extends rearward > 1/2". I assumed (there's that word again) it was flat on the backside like a Cessna spinner. Thankfully, Sam at Saber saved my bacon by making me a custom prop extension that corrected the difference. The Voice of Experience ....

[ge9a]

Well, it appears the differing dimensions, left and right, behind the trailing edge of the prop, is normal. I wanted to be sure before proceeding. Now I'm building on.

Thanks for the help and the input.