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Cowl to Spinner Gap

mikeyj350

Well Known Member
Hey guys, I know this is a common subject on the forums here but I didn't find anything on this specific problem. I'm working 47-6 Step 11/12. I've fitted the cowl to this point following the plans exactly, with the exception of I'm using SkyBolts instead of hinges around the perimeter of the firewall. Both the upper and lower cowl halves seemed to fit perfectly, and with the 1/8" spacers taped to the front face of the cowl, everything lines up great.

The problem came with the step saying remove the 1/8" spacers taped onto the front. Upon reinstallation of the lower cowl, it is now MUCH closer to the spinner bulkhead than with the 1/8" spacers, and it seems pretty well-set in that position. I can pull and flex the cowl aft and upwards, and with a fair bit of force I can hold it in the desired 1/8" spacing position. As soon as I let go though, it goes back to its "happy" position of only 1/32"-1/16" gap. In contrast, with just the upper cowl skybolted into place, the gap is fine.

I've already trimmed the split line for the lower cowl and have traced that line onto the top cowl, but haven't cut the top cowl yet in case there is some more adjusting I'll need to do.

Am I worried about nothing? Build on and it will fix itself? Do I need to go back and re-trim my firewall line a bit more to get the extra clearance so it sits correctly without the spacers taped into place? Any ideas from folks who may have been here would be great... thanks!
 
I don't think you have anything to worry about, but also don't think it will fix itself. You are just into fiberglass work, or as some call it fiberglass ""ell. Because you have not fitted the top cowl seems you still have the option of trimming the cowl to firewall gap and that may be easiest.

I discovered the same problem after my entire cowl was fit and ended up sanding almost 1/8" off the front of the cowl. Because of the extensive sanding mine required building up the back side of the cowl where it met the spinner. Regardless, the gap fit turned out well. So you have options, work it now or work it latter. Either way its work.

Remember, anything can be done with fiberglass. I just takes time.



Eulice Curington
N104EW Painting
 
I messed around for ages trying to deal with this on my cowls - then I finally decided to just grind a slit all around the bit of the cowl just behind the spinner. Made a 5/16 thick ply ring and gaffer taped to the rear of the spinner backplate and then simply fibreglassed the cowl back into the correct place. Took about 20% of the time spent messing above and produced a much better result.

You really need to have at least 1/4" gap - especially if you intend pulling a few G's . Also bear in mind that the Engine can rotate (in the crankshaft axis) against the torque by as much as 3/8" measured on the fuel servo
 
There is more than one of us that has fallen into this trap. If there is any tension or torque on the cowl is being trial fitted, you pay for it as soon as you make that back cut. The cowl is long enough that you don't notice until its too late.

Like Eulice mentioned, you'll have to sand down the front. But before you start sanding add a few layers of glass on the inside. You may end up sanding down to the new glass layers depending on how much material you need to take off.

I'm not sure if I posted any of my photos on my build site. It did set me back a few days to remedy the issue. Now that it's painted, you can't tell the difference anymore.
 
Look at the Hartzell prop manual. It gives you some options on adding washers on the spinner bolts. This will move the spinner out enough for what you need.

Note - the prop says where it is, the washers just space out the spinner back plate.

Carl
 
Look at the Hartzell prop manual. It gives you some options on adding washers on the spinner bolts. This will move the spinner out enough for what you need.

Note - the prop says where it is, the washers just space out the spinner back plate.

Carl


What Carl said sounds like it might be a good solution for you.

I just want to throw out there that I also opted for a .25" gap. I think it looks perfectly fine and I wanted to allow for engine movement and sag over time and perhaps make it a little easier to pull the cowl on and off.
 
Me too

I just want to throw out there that I also opted for a .25" gap.
Good advice and if you have any pressure between the spacers and the spinner plate, it will reduce to less than 1/4" as soon as you remove them.
My first RV-8 I wanted to keep it as small as possible, 1/8th and it looked very good until you need to take the cowl off.
You'll thank us later if you go with a larger gap.
 
In my experience, it's better to have that gap a little too wide than a little too narrow. Mine is 1/4" and it looks just fine ... have never had an "RV Looker" ask about the gap or observe it was wider than normal.
FWIW ... free advice is worth what you pay for it. ;)
 
gap

Mine on the RV-8 is 9-10/64. Looks good and have had no contact between spinner and cowl with maximum positive and negative G' during phase one. Lord mounts seem stable with no change in last 10-15 hours good luck with yours...
 
Look at the Hartzell prop manual. It gives you some options on adding washers on the spinner bolts. This will move the spinner out enough for what you need.

Note - the prop says where it is, the washers just space out the spinner back plate.

Carl

Adding spacer washers would move the spinner closer to the cowl.
 
Ah, Yep. It sure would.

prop.bmp
 
Thanks for all the responses guys.

Yeah I did consider the bulkhead spacing idea, but unfortunately the gap isn't parallel to the spinner either (it's about a perfect 1/8" on top, and down to 1/16" or a little less on the bottom) so even if it could be done it wouldn't get things perfect. Since on the firewall side everything is already perfectly aligned, cut, and drilled, I'm probably going to have to take some material off the front face of the cowl, reinforcing as necessary from behind as suggested by a few.

Now y'all have me worried about the 1/8" gap not being enough though! Anyone out there build to the spec 1/8" gap and have problems to the point where they wish they had made it bigger? Frankly, I'm not really as concerned about show-quality tight gaps or anything like that... if it looks ok, flies the same, and is just as safe I can see the justification for widening it up a bit. That said, I've already built it now with a tight gap, and it will be less work to make it an even 1/8" all the way around vs. taking additional material off.

As an aside, I've also aligned the cowl to be about 1/8" low to account for eventual engine sag. I'm hoping this is a good number to go with, as it too is pretty well set in stone (well, fiberglass...) and would be pretty difficult to alter at this time.
 
Ah, Yep. It sure would.

prop.bmp

The spinner back plate will move forward (increasing the cowl to spinner gap) if you went from 8 washers to 4 washers or no washers between the hub and the back spinner plate (but still using the spinner to bulkhead spacer).

Look at page 3-12 of the Hartzell manual. You must retain the spacer, but which side of the spinner back plate you put the three washers is your decision.

For you, I would put one washer with the spacer, and two washers under the back plate nut. That 1/16" should be all you need gain for your gap spacing.

Carl
 
sag with age.....

I saw closer to 1/4" sag after about 400 h of flying (RV7), but did not bring out the caliper to confirm...So I think you did good and if it drops a little, that looks and flies just fine too! This is clearly one of those topics/areas that one can get "spun around the prop" about, measure in microns, etc.... and in the end, when it is flying, and paint is on the bird, it just won't matter.
 
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