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Cowl fitting issues

madmaveric

Well Known Member
I'm in the process of fitting the cowling but have come across an issue, I've searched on here but I can only see issues with spinner clearance and nothing around the back of the cowling.

I cut both cowls to the scribe lines and fitted the bottom cowling with the two holes wither side as per plans and then added the lower hinge. All seemed to be going good and I was happy with the fit.

I then came to fit the top cowling and realised I needed to add the panel cover for this part. I soon realised that the top cowl is too high. On close inspection it seem the right side cowling is higher than the left.

Measuring the step it seems that the right side is higher by around by around 1/8 inch. You can see on the photos that there are no scribe lines visible so I haven't under trimmed the right side by 1/8 inch (and I find it unlikely I over trimmed the side that fits by 1/8 inch which would happen to be just the right amount).

Could this be an error on the trim lines or manufacture.

I have a couple of options now.

1) Bottom cowling, Trim the top, meaning filling and redoing the piano hinge and two mounting holes drilled
2) Bottom cowling, Trim the bottom of the step, only two holes to fill and redo. This may mean I then get the same error of bulging on the bottom but this may be lost in the center slot.
3) Top cowling, Trim the right hand side overlap down by 1/8 inch at the back progressively getting less to the front.

Option 1) would probably take a few days of work and option 3) is the easiest option.

The bottom cowling appears to fit the skin ok so it feels like the top right side is just 1/8 too tall.

I feel like I must be missing something as incorrect scribe lines seems unlikely.

Any tips on how to measure/align the cowling would be useful. I can't use the engine clearance as I can only hang the engine on the top mounts (fighting the nose gear/bottom engine mount where I'm working would be a pain) so the engine isn't a good reference point.

Maybe I'm being overly concerned and just need to trim the upper cowling but I thought I'd run it by you guys first.

Right side: Join line is ~1/8 inch higher than the bottom of the top cover
IMG_20170513_141216_zps0x4ie6vy.jpg

Left side: Join line is ~level with the bottom of the top cover
IMG_20170513_141228_zpsvbt7hjqv.jpg

Right side cowling sits above the hinge.
Untitled_zpsmz0kauwj.png
 
Usually if it doesn't fit, I did something wrong. Except with the fiberglass parts, I do not have much faith in the scribe lines.
 
Usually if it doesn't fit, I did something wrong. Except with the fiberglass parts, I do not have much faith in the scribe lines.

That's my gut feeling normally, Vans produced parts are of Vans quality but I suspect these are outsourced, It seems a lot of issues on here are with parts not made by vans themselves (mounting brackets etc).

From what I read on here the scribe lines are normally very accurate (and embedded in the mould) so shouldn't be an issue.
I didn't want to continue until asking on here, as I didn't want to cut the top cowling down only to find the issue was somewhere else I hadn't looked at (and end up having to buy another cowl).
 
Seems like the high edge of the bottom cowling needs to be trimmed. I initially trimmed about an 1/8" outside the scribe lines and then pretty much ignored them while I trimmed to fit. In the end I ended up right where the scribe lines were.
 
The scribe lines don't require any faith to see their value.... just compare the cowl installation to the extra work that would be required measuring and fitting if the installation is started with no scribe lines at all.

The scribe lines are usually pretty close. Why yours is off slightly, I don't know... perhaps one of the molds has the line position off slightly.
Regardless, they are meant to get you close quickly, and then allow you to see what minor adjustments need to be made with a sanding block during the final fitting.
Yours is a little more than minor, but I wouldn't worry about it. Just make adjustments as necessary. If it were me doing it I would probably trim the top edge of the bottom cowl to make the interface the same on both sides.
 
Thanks for the advice.
I'm still considering if trimming the bottom (meaning filling and re-drilling the hinge holes) is going to be better than trimming the top and having the line a little higher on the right.

The issue with re-doing the bottom cowl would be the possibility causing some weakness in my filling (will be the first structural fibre glassing I have ever done).

I wish I had noticed before I put the hinge on :( .

I will sleep on it tonight (UK here so just about to go out for a few beers and think about it) and come check for more comments in the morning.
 
I would leave everything as is and just sand down where you have shown the red lines at the top of the bottom cowl. You won't have to redo any rivets.
Then just trim the top piece so it comes down to meet the bottom cowl and sit on the rear hinge
You also need to make more of a gap between the fiberglass and metal sides to allow for paint and movement, but you provably just haven't gotten to that yet
Good luck
 
I would leave everything as is and just sand down where you have shown the red lines at the top of the bottom cowl. You won't have to redo any rivets.
Then just trim the top piece so it comes down to meet the bottom cowl and sit on the rear hinge
You also need to make more of a gap between the fiberglass and metal sides to allow for paint and movement, but you provably just haven't gotten to that yet
Good luck

I was mulling this over tonight and came up with the same idea (great minds and all that
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).

I should be able to take a bit off the bottom (need to consult the books to see the edge distance required for the holes to make sure I don't go too far). It will mean the hinge isn't flush to the edge but that shouldn't be a problem and would only be noticeable with the cowling off. Whatever is left can be taken off the top cowl if needed. It should at least make it line up better and not need the rivet holes all re-doing.

The fact you posted the same idea helps with my confidence in the plan, thanks.

Once it is all drilled up (before riveting) I plan on spending some time going over all the joins and adding gaps where needed for paint and clearance when installing and removing the cowling. As much as I'd like it to have no gap I realise the paint wouldn't last long.

Does 1/16 sound about right for the unpainted gap pre paint.

I plan to paint with high build primer and sand back to avoid holes then add 4-6 coats of primer and clear over base 2k paint on top so I guess the build up could high if I'm not careful. multiply this by 2 (for both parts) and I can imagine the gap losing 1/32 easily due to paint on the edges, leaving a finished 1/32 gap (all a bit of guesswork here depending on how heavy/light I end up putting the coats on).
 
Fitting the engine first would be ideal but It won't be able to fit the engine until I have moved out of my build area as I don't think I could get the fuselage up high enough to fit the gear in the shed where I'm building.
Fitting it at the airfield would take longer and be quite costly (the cost of the hanger time needed might be more than the cowls cost around here), but it would mean a better chance of a tighter fit.
Just wish I had a a bigger build area as it would solve a lot these kind of issues (or more money for hanger time :)).
 
Fitting it at the airfield would take longer and be quite costly (the cost of the hanger time needed might be more than the cowls cost around here), but it would mean a better chance of a tighter fit.
Just wish I had a a bigger build area as it would solve a lot these kind of issues (or more money for hanger time :)).

MadMaveric - I too vote to wait until the engine is mounted before doing the final fitting of your cowl.

That said, I can appreciate your conundrum ... I too was working out of a small work area (17 x 17 garage). Between the two workbenches, two completed wings, an air compressor, tool chest, rolling tool cart, a small shelving unit to store parts and the stabilator hanging from the rafters, I found myself constantly walking sideways while assembling the fuselage and needing to be ever so cautious not to poke something into the stabilator hanging overhead. Fortunately, I had a shed to store the tail cone so that enabled the use of the second workbench.

The bottom line ... there is a point that it just makes good sense to move to a larger assembly area (hangar or otherwise) to improve productivity. If I recall correctly, I moved shortly after completing the plumbing in the fuselage and glad I did. Can't tell you how much time was wasted rearranging the shop for a phase of construction only to find myself needing to rearrange things yet again a few days later.

Happy building,
 
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Yup a small workspace does have its issues. I've added to my pain by deciding to paint it myself so the move out has to wait until after I attach the tailcone (so I can paint it in one go).

I had considered seeing if there is a way I can mock up a nose gear/bottom mount (replacing it with a shim/washers) so I could bolt up the top and bottom engine mounts to compress the rubbers correctly and get it in the right position.

I'm not sure if this will give the correct position though. A couple of spacers in place of the landing gear wont give any flex that may be put on the firewall with the real thing ...

I may end up just finishing the two long piano hinges but leaving the firewall attachment until later. That would also mean the cooling duct work would have to wait as well (and probably a some other stuff I haven't got to yet). I'm guessing it would add another week onto the hangar bill but will be worth it in the end to get a nice spinner gap.
 
Having been involved in this process a few times now, I'd suggest you set the cowl aside, and wait until the engine is hung. The prop and spinner is a critical reference point for cowl positioning. Not just the size of the gap - but relative positioning in all directions. The order of the build in the plans in this area is pretty intentional.
 
I've been convinced. I will leave the cowling till later, it will probably add a few hundred pounds to the build cost due to the extra hangar rent but will be worth it.

I had a drive round today to see what is available for hangar space and have prices ranging from ?1000+ per week :eek: to ?200/mth for space in a metal fabricators unit (I wouldn't take that one as I would fear for the aircraft with all that heavy work going on around the it).

At least it is now one less decision to ponder on, it would have been nice for finish something as everything at the moment is being held up by something else but eventually it will get there.

Thanks for the advice.
 
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