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Cowl Fitment

rockitdoc

Well Known Member
Sponsor
I started trying to get the cowl to fit in accordance with the plans today. Lots of on and off, sanding, on and off, sanding....

But, even with virtually no gap at the aft end of the cowl and firewall with everything centered left and right, the gap between the cowl and the spinner mount at the top (red arrow) is double the gap at the bottom (green arrow). But, the plans say this should be reversed, ie, bigger gap at the bottom so when the engine droops the gap evens out. I can't figure out why the gap is larger at the top or how to reverse the situation.

Any ideas appreciated. Oh, the spinner mount is square and flat.

Cowl Foreward Fit Initially.jpg
 
Been sitting here thinking about it ... can you post a pic of where the cowl meets the fuse? Perhaps side and top?

Also, can't tell from the pic, do you have the appropriate spacers (washers?) under your angle bracket?
 
Been sitting here thinking about it ... can you post a pic of where the cowl meets the fuse? Perhaps side and top?

Also, can't tell from the pic, do you have the appropriate spacers (washers?) under your angle bracket?

You bet.

Side View

Cowl Aft to Fuse.jpg

Top

Cowl Top View Aft to Fuse.jpg

Washer/Spacer

Cowl with Washer Spacer.jpg

Plans indicates 1/32" washer. Too thin, perhaps?

Cowl Plans Page with Spacer.jpg
 
Cowl

Scott, sorry but you 14 builders are spoiled. :D
That's just a little Vans fabrication error on the nose bowl. The fit looks beautiful.
Mark the outside where you want it. Remove the cowl. Apply some flox or layers of fiberglass inside equal to the amount you need to remove on the outside. That way the finished thickness after sanding will be correct. Grind it down to the mark and move on.
You should have seen mine. That's pretty nice.
 
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Scott, sorry but you 14 builders are spoiled. :D
That's just a little Vans fabrication error on the nose bowl. The fit looks beautiful.
Mark the outside where you want it. Remove the cowl. Apply some flox or layers of fiberglass inside equal to the amount you need to remove on the outside. That way the finished thickness after sanding will be correct. Grind it down to the mark and move on.
You should have seen mine. That's pretty nice.

Thanks Larry. I feel a lot better!
So, just adjust the glass in the front to get the tolerances called for. 3/16" at the bottom with less on top to account for sag after engine drops a bit.
 
Adjust

Thanks Larry. I feel a lot better!
So, just adjust the glass in the front to get the tolerances called for. 3/16" at the bottom with less on top to account for sag after engine drops a bit.

Unless other builders have a better solution. That cowl fit at the firewall is perfect.
You may want to install the bottom and treat them as a unit before modifying the top cowl. That way they match.
 
Rocketdoc: an option to consider

You can trim the lower rear of the cowl to drop the front and open the gap to the spinner. This will likely eliminate a lot of custom micro/flox work and preserve the molded periphery of the spinner area. If the cowl top cannot be lowered a little, you may have to fill and sand as wirejoc suggested.

Just put tape on fuse/boot cowl carefully corresponding to edge of the metal you want to mate nicely with the cowl rear edge. Position the cowl with spinner gapped as you wish (more gap at the bottom you said). With a compass or scribe set the width of the tape, scribe a line on the cowl a constant distance ( tape width ) from the BACK of the tape. Then sand to the line. After you install your fasteners, you can fine tune the cowl to fuse gap you desire with sanding block.
 
You can trim the lower rear of the cowl to drop the front and open the gap to the spinner. This will likely eliminate a lot of custom micro/flox work and preserve the molded periphery of the spinner area. If the cowl top cannot be lowered a little, you may have to fill and sand as wirejoc suggested.

Just put tape on fuse/boot cowl carefully corresponding to edge of the metal you want to mate nicely with the cowl rear edge. Position the cowl with spinner gapped as you wish (more gap at the bottom you said). With a compass or scribe set the width of the tape, scribe a line on the cowl a constant distance ( tape width ) from the BACK of the tape. Then sand to the line. After you install your fasteners, you can fine tune the cowl to fuse gap you desire with sanding block.

Hmm. Since the top of the cowl is fixed with washer and angle, I’m not sure trimming the lower aft cowl does the trick. That would work if front of cowl could drop. But, I may not understand. Not unusual.
 
Sag allowance ?

The washer makes a nice reference and aesthetically pleasing starting point. The wild card here is how much sag to anticipate and accommodate. Some folks say they get none and others inquire about mounts that don’t sag so much. The motor mount shim option is another tool you can use to optimize this cowl/spinner relationship.
 
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Be careful about having the cowling too tight at the fuselage joint. Remember to allow for paint thicknesses. You don’t want to damage it when you take it on and off and the edges hit. Just my experience.
 
Remove the cowl. Apply some flox or layers of fiberglass inside equal to the amount you need to remove on the outside. That way the finished thickness after sanding will be correct. Grind it down to the mark and move on.

The spinner-sized hoop at the front of the cowl is structure, and it's stressed more than most folks realize. Dynamic pressure inflates the cowl like a balloon, and total load (available dynamic pressure x Cp x area) can be hundreds of pounds. Take a look at the cracks near the inlets on some of the older polyester cowls.

So, don't fill from the inside and cut away the original layup. There's a good chance the result will not be as strong as the original.

Instead, continue trimming the rear edge of the cowls, shifting the entire cowl assembly rearward until the most narrow dimension in the spinner gap matches the desired gap. Finished fitting the cowl...all fasteners or hinges in place, all edges gapped for paint, completely done.

Spinner%20Gap.jpg


Now remove the cowl, set it upright on the floor with all the side and nose fasteners in place, and scuff sand the face of the flange with a coarse grit. Mix a batch of epoxy/flox and spoon on a generous quantify. Press it with a board or other flat surface which has been prepped for mold release...slick tape, wax, PVA, whatever works. Leave it in place until after cure, then pop it off, and sand the perimeter to a nice even radius, just like the original edge. If there are some voids in the surface, touch it up with dry micro. When if looks good, make two easy cuts across the flox with a hacksaw blade to separate the halves. Dress the cuts and you're done.

Spinner%20Gap%20Fill.jpg
 
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Cowl fit

Thanks guys. Funny how the brain gets stuck.
Dan's method is the way to go.
The gap at the tip will increase, but that can be filled in as he recommends.
 
cowl nose ring buildup

Dan's method is perfectly acceptable for building up the cowl nose ring.

But, when I did mine, I wasn't that confident in my epoxy/flox buildup techniques, or my ability to get the shape to come out right. The nose ring on mine was out of parallel with the spinner from left to right, and the cowl halves were off top to bottom. The left side of the top cowl was recessed almost 1/4" aft from where it should have been, tapering to "almost right" on the right side. The bottom cowl was off in a similar way, but not as bad as the top. The net result was absolutely nothing on the nose ring was remotely parallel to the spinner back-plate, the gap varied in size all the way around the circle.

So, I ended up epoxying balsa wood to the front of the nose ring, then block sanding the parts back down so there was a constant gap between the spinner back-plate and the cowl. After that I put two layers of glass over the balsa, and sanded everything down to blend it in. The result came out great. (See....all that model airplane building as a kid did come in handy! :) )

build 2-282.jpg
build 2-284.jpg
build 2-290.jpg
build 2-294.jpg
 
Instead, continue trimming the rear edge of the cowls, shifting the entire cowl assembly rearward until the most narrow dimension in the spinner gap matches the desired gap. Finished fitting the cowl...all fasteners or hinges in place, all edges gapped for paint, completely done.

As I indicated above, trimming the aft edge of the cowl on both sides at the bottom would allow for the top cowl to rotate counterclockwise looking at it from the port side, but the gap between the top front of the top cowl would not change unless the angle and washer/spacer installed are removed. By doing this, the front of the cowl at the top moves down in relation to the spinner mount. This would decrease the gap but change the elevation of the top of the cowl in front.

Of course this downward change in elevation could be adjusted with fiberglass/epoxy and flox bodywork. Is this what you had in mind? That would also add a bit of structure.
 
Cowl fit

As I indicated above, trimming the aft edge of the cowl on both sides at the bottom would allow for the top cowl to rotate counterclockwise looking at it from the port side, but the gap between the top front of the top cowl would not change unless the angle and washer/spacer installed are removed. By doing this, the front of the cowl at the top moves down in relation to the spinner mount. This would decrease the gap but change the elevation of the top of the cowl in front.

Of course this downward change in elevation could be adjusted with fiberglass/epoxy and flox bodywork. Is this what you had in mind? That would also add a bit of structure.

No. What Dan is suggesting is to sand the entire aft edge uniformly till the bottom edge of the nose bowl is the correct spacing. This retains the dimension with the washer at the top. When everything is done, the nose bowl to spinner gap will be the even wider at the top. That is repaired by adding material to the nosebowl instead of grinding off. It retains the structural integrity. I was wrong.
 
No. What Dan is suggesting is to sand the entire aft edge uniformly till the bottom edge of the nose bowl is the correct spacing. This retains the dimension with the washer at the top. When everything is done, the nose bowl to spinner gap will be the even wider at the top. That is repaired by adding material to the nosebowl instead of grinding off. It retains the structural integrity. I was wrong.

Ahhha! Thanks Larry! That makes total sense. And, thanks Dan. Now I understand your second drawing.
 
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Is this what you had in mind?

No rotation. Just move it straight aft.

BTW, When using flox or micro, don't miss the opportunity to rough off excess material when the epoxy cures to the cheese stage...still soft, but no longer sticky. It can be sliced with a knife, or grated away with a Sureform, or shaved with a vixen file.

So, I ended up epoxying balsa wood to the front of the nose ring, then block sanding the parts back down so there was a constant gap between the spinner back-plate and the cowl. After that I put two layers of glass over the balsa, and sanded everything down to blend it in.

Nicely done. More than one way to skin a cat!
 
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As I indicated above, trimming the aft edge of the cowl on both sides at the bottom would allow for the top cowl to rotate counterclockwise looking at it from the port side, but the gap between the top front of the top cowl would not change unless the angle and washer/spacer installed are removed. By doing this, the front of the cowl at the top moves down in relation to the spinner mount. This would decrease the gap but change the elevation of the top of the cowl in front.

Of course this downward change in elevation could be adjusted with fiberglass/epoxy and flox bodywork. Is this what you had in mind? That would also add a bit of structure.
As Larry also has mentioned, you will need to readjust the lower cowl to get the proper alignment to the spinner. Then setup the top cowl with the correct alignment with the spinner and then the alignment between the top and bottom cowl.
If you have not cut the sides to a perfect fit yet, the only area that you might need to redo between the top and bottom is the screw holes on the inward part of the inlets which is easy to do.
 
Not to argue with anyone on repair techniques for the cowling nose but I just got off the phone with Vans tech support.

My cowling is already fitted and needs the gap widened between the spinner backing plate and cowl nose. I had planned to add fiberglass/flox to the back side of the nose and remove material from the front side as Larry suggested.

Vans agreed this was a good approach as long as I added the same or more to the back side.
 
No rotation. Just move it straight aft.

Yes. That makes perfect sense, and what I will do.

BTW, When using flox or micro, don't miss the opportunity to rough off excess material when the epoxy cures to the cheese stage...still soft, but no longer sticky. It can be sliced with a knife, or grated away with a Sureform, or shaved with a vixen file.

Thanks for that. I found out this the hard way once by epoxying/floxing in the wrong place, then removed it fairly easily using the procedure you describe.
 
Hate to be High Maintenance, but....

I've almost got the top cowl fitted thanks to all the help here.

Now, I have another issue.

The top cowl fits pretty well all the way round the firewall except on the port side near the top where it makes the curve down the side of the firewall. I can push the cowl down against the Skybolts so the elevations of both cowl and firewall match, but it takes quite a bit of effort, ie, the cowl does not fit nicely by gravity. If I construct it this way there will be upward tension on the Skybolts in this location. I am concerned the stresses will cause trouble down the line especially considering the pressures inside the cowl as DanH has pointed out.

Two solutions I can think of:
1. Change the shape of the firewall to match the cowl :D
2. Add material to the underside of the cowl in a thickness sufficient to match the elevation difference then grind the top down to match the firewall elevation.

I hope there is a third option because neither of these, especially #1, seem questionable.

The starboard side fits pretty well with little effort required
Cowl Top Starboard Fit.jpg

The port side, not so much

Cowl Top Port Side Gap.jpg
 

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  • Cowl Top Port Side Fit.jpg
    Cowl Top Port Side Fit.jpg
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The top cowl fits pretty well all the way round the firewall except on the port side near the top where it makes the curve down the side of the firewall. I can push the cowl down against the Skybolts so the elevations of both cowl and firewall match, but it takes quite a bit of effort, ie, the cowl does not fit nicely by gravity.

Glass/epoxy will creep given warm temperatures and time. Before making any big moves, just put a cinch strap around the cowl and pull it tight. Check for good fit at the seams with the gap pulled down, adjust as required. Cinch it tight again, and put a heat source in the cowl, something to push air temperature up around 200F. Hold it there overnight, then let it cool before removing the strap.
 
Reshape

Glass/epoxy will creep given warm temperatures and time. Before making any big moves, just put a cinch strap around the cowl and pull it tight. Check for good fit at the seams with the gap pulled down, adjust as required. Cinch it tight again, and put a heat source in the cowl, something to push air temperature up around 200F. Hold it there overnight, then let it cool before removing the strap.

I've had good results heating and reshaping like this. Just don't go too hot.
 
Thanks Guys! Progressing Rapidly

I've had good results heating and reshaping like this. Just don't go too hot.

I put a 25w bulb in one of my lead lights and hung it inside cowl port side where cowl fit was marginal. I will let you know what the temp gets to.

Fitment is pretty good thanks to you guys. Once the heat treating is complete I will trim the bottom cowl to fit and start the process of getting the spinner clearance spot on.

Is bodywork recommended between top and bottom on the longitudinal seam so there is no 'step'? Or, do the two halves fit better when Skylocks/hinges are in place? I know there is a recess in the bottom cowl that should accommodate the top.

Cowl Top and Bottom Test Fit with Strap.jpg

Cowl Port Front View Test Fit.jpg

Cowl Heating Lamp.jpg
 
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Is bodywork recommended between top and bottom on the longitudinal seam so there is no 'step'? Or, do the two halves fit better when Skylocks/hinges are in place? I know there is a recess in the bottom cowl that should accommodate the top.

Duh. The bottom is trimmed to butt up against the top. The 'step' only occurs because the top overlaps the bottom until trimmed. I'm catching on slowwwwly
 
Cowl to spinner gap

Don’t forget the Dave Anders approach to capturing lower cowl airflow with a felt seal on the crank extension. This gives some flexibility to the spinner/cowl gap options. Your sense of aesthetics becomes the defining dimension and it could make it a lot easier to get the lower cowl half off .
 
Don’t forget the Dave Anders approach to capturing lower cowl airflow with a felt seal on the crank extension. This gives some flexibility to the spinner/cowl gap options. Your sense of aesthetics becomes the defining dimension and it could make it a lot easier to get the lower cowl half off .

Hmm. Not familiar with this approach. Where can I find the writeup?
 
Lightbulb Cowl Heating

The cowl on the port side upper did not fit quite right. At DanH and Wirejock's suggestions I put a 25w bulb in a trouble light and hung it under the cowl on the port side to heat up the cowl. Temperature of the upper cowl got to 80F, just 3F over ambient. I replaced the 25w with a 100w and left it over night with a blanket on top the cowl to hold temp in. This morning I measured 105F on the top cowl at 74F. Not sure if I should try to go to higher temps.
 
The cowl on the port side upper did not fit quite right. At DanH and Wirejock's suggestions I put a 25w bulb in a trouble light and hung it under the cowl on the port side to heat up the cowl. Temperature of the upper cowl got to 80F, just 3F over ambient. I replaced the 25w with a 100w and left it over night with a blanket on top the cowl to hold temp in. This morning I measured 105F on the top cowl at 74F. Not sure if I should try to go to higher temps.

Use a heat gun and temp gun. Heat it to 200 and let cool.
 
This morning I measured 105F on the top cowl at 74F. Not sure if I should try to go to higher temps.

Yes, higher temperature, held for some period of time. No guarantee, but it should work.

The experience basis is the behavior of cowlings subjected to normal engine compartment heating. I have removed hot cowls and set them on floors and benches, then during reinstallation days later, found they had changed shape slightly...they had cooled while slightly deformed, although not enough to prevent installation. Come back and R&R them later, after a few flights (i.e. heat and cool cycles in place), and they fit just as they did originally.

200F is roughly the air temp in the cowl after shutdown on a hot day. You won't hurt it with the same in the shop.

Heat gun is fine if the plan is to aim it into an inlet as a space heater. Aiming it at a surface tends to result in uneven heating, with little dwell time.
 
In reference to the heat molding/changes.
I agree.
I have an RV-6A 550 hours. I noticed the cooling ramps were delaminating from the the underside of the top cowl and they never had the outboard ends closed.
No big deal. Put the cowl top on the table and went to work sanding, cleaning, and reinstalling the ramps. Sealed the ends and re-glassed the entire inner surface. Real smooth and pretty now. Painted with high gloss.
Looks good enough to eat off of.
NOW it's all pretty and I goes to put it back on the plane.
OH NO it won't fit. Hangs out 3/4" on the right side.
I didn't even think about the cowl being upside down when I did all my repair work.
What to do???? I put that crooked cowl top on knowing/hoping heat would/should be my salvation. 10 hours later the heat did the work I screwed up on.
It fits perfectly, with no bulge.
I am one happy camper/flyer.
My three cent worth of luck and yours may vary.
Yes, I have worked in glass and epoxy for many years, But, thought I had really screwed up this time by not restraining the part before glassing it up.
I won't make the same problem on the bottom cowl repairs that are upcoming this winter.
Art
 
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