Cowl perimeter edge extension - how much is too much?

[eavila1]

Fitting my cowl this weekend. I Purchased the old pepto bismol pink cowl a few years ago from a guy that went horizontal induction rather than vertical. Since I already have my ground adjustable fixed pitch sensenich prop and spinner, and to set the 1/4" gap, I bolted on the starter ring and the spinner which has the saber 2-1/4" spacer bolted/added to the spinner. A beauty.

Then my heart sank as I looked at the aft end cowl to firewall skin gap/shortage. It appears I'm a few inches short...probably a total of 3" or so in order to get a good trim back to the 2" mark per plans

I've researched some of Dan Hortons Scarfed layup extension solution advice. Question is with that much length add on, is it possible to to add this much extension to the cowl, in an area that will have hinges attached? I'd do a long gradual angles scarf with staggered layers and likely carbon and or metal strip included. But I'm open to suggestions. Is this length even feasible. Anyone tackle this issue with their cowl...Looks as if it will be both top and bottom. Not sure why this cowl is so short (doesn't look like it was previously trimmed either) as the spacer is common for fixed pitch.

What do ya'll think?

 

[DanH]

It appears I'm a few inches short...probably a total of 3" or so in order to get a good trim back to the 2" mark per plans.... Question is with that much length add on, is it possible to to add this much extension to the cowl, in an area that will have hinges attached?

Yes, although craftsmanship will be required in order to keep it neat and tidy.

I'd do a long gradual angles scarf with staggered layers and likely carbon and or metal strip included.

This is all about the scarf bond between the old and new, while staying close to profile for minimum filler.

Past discussion has looked at adding a layer of carbon as the outermost plies inside and outside the edge, the goal being to add stiffness and thus reduce bowing between fasteners due to internal air pressure. Note it has nothing to do with the bond strength of an extension. Bond strength won't be any different, carbon or glass.

Forget metal strips.

 

[eavila1]

Yes, although craftsmanship will be required in order to keep it neat and tidy.



This is all about the scarf bond between the old and new, while staying close to profile for minimum filler.

Past discussion has looked at adding a layer of carbon as the outermost plies inside and outside the edge, the goal being to add stiffness and thus reduce bowing between fasteners due to internal air pressure. Note it has nothing to do with the bond strength of an extension. Bond strength won't be any different, carbon or glass.

Forget metal strips.

Ok thank you Dan. Any other tips? But doable correct? I'll plan a good backing strip, long gradual scarf. Would you recommend staggering the plies? Important to scuff the old composite? Any recommendation for glass type or orientation? Add flox as well?

Comment well taken about it being all about the scarf bond. I'll focus on that quality

 

[DanH]

Ok thank you Dan. Any other tips? But doable correct? I'll plan a good backing strip, long gradual scarf. Would you recommend staggering the plies? Important to scuff the old composite? Any recommendation for glass type or orientation? Add flox as well?

Comment well taken about it being all about the scarf bond. I'll focus on that quality

I'll do it with the form (the backing ) on the outside, and do the layups from the inside. Soft aluminum roof flashing might work well, or maybe thin plastic with some mold wax:

Amazon - Plastic Sheet

Cleco sheet strips to the cowl in multiple locations in order to hold tight conformance to contour. It's easy to fill the holes later with a wipe of micro.

Serious 60~80 grit scuff everywhere you expect to bond. No flox, a little dry micro for any subsequent fill.

I'd use ordinary 9oz plain weave (7500). You'll need about a ply for every 0.010" of thickness. Not sure what you're asking re "stagger the plies". Fiber orientation isn't a big deal in this application, so all at 0-90 is fine.

 

[eavila1]

I'll do it with the form (the backing ) on the outside, and do the layups from the inside. Soft aluminum roof flashing might work well, or maybe thin plastic with some mold wax:

Amazon - Plastic Sheet

Cleco sheet strips to the cowl in multiple locations in order to hold tight conformance to contour. It's easy to fill the holes later with a wipe of micro.

Serious 60~80 grit scuff everywhere you expect to bond. No flox, a little dry micro for any subsequent fill.

I'd use ordinary 9oz plain weave (7500). You'll need about a ply for every 0.010" of thickness. Not sure what you're asking re "stagger the plies". Fiber orientation isn't a big deal in this application, so all at 0-90 is fine.

View attachment 71318

Awesome info. Thx Dan. I already have a chunk of UNI and another i think it had marked BDI on it. Can I use either of that? It was ordered some time ago and I probably got it reading previous comments on here. If not, I'll order some 7500. I Already used up the 7781 on the aft canopy fairing that turned out nice.

 

[eavila1]

I'd use ordinary 9oz plain weave (7500). You'll need about a ply for every 0.010" of thickness. Not sure what you're asking re "stagger the plies". Fiber orientation isn't a big deal in this application, so all at 0-90 is fine.

I meant staggered by how the layers are offset laid on your double scarf example. Your single scarf appears to layer on same size strips stacked on top of each other

 

[eavila1]

I'd use ordinary 9oz plain weave (7500). You'll need about a ply for every 0.010" of thickness. Not sure what you're asking re "stagger the plies". Fiber orientation isn't a big deal in this application, so all at 0-90 is fine.

I can see that the same result occurs by the time they are sanded down flush to original thickness. Gotcha. Thx again. I'll report back on how it goes

 

[ryoung]

Fitting my cowl this weekend. I Purchased the old pepto bismol pink cowl a few years ago from a guy that went horizontal induction rather than vertical. Since I already have my ground adjustable fixed pitch sensenich prop and spinner, and to set the 1/4" gap, I bolted on the starter ring and the spinner which has the saber 2-1/4" spacer bolted/added to the spinner. A beauty.

Then my heart sank as I looked at the aft end cowl to firewall skin gap/shortage. It appears I'm a few inches short...probably a total of 3" or so in order to get a good trim back to the 2" mark per plans

I've researched some of Dan Hortons Scarfed layup extension solution advice. Question is with that much length add on, is it possible to to add this much extension to the cowl, in an area that will have hinges attached? I'd do a long gradual angles scarf with staggered layers and likely carbon and or metal strip included. But I'm open to suggestions. Is this length even feasible. Anyone tackle this issue with their cowl...Looks as if it will be both top and bottom. Not sure why this cowl is so short (doesn't look like it was previously trimmed either) as the spacer is common for fixed pitch.

What do ya'll think?

I just did something similar on my cowl when I needed to add about 1 inch to the top cowl but to both horizontal edges, and only the aft portion for the trim line. I used Dan’s single scarf method and it worked well. I used .032 alum on exterior with clecos and tape. The second photo shows the scarf of approx 1/2 inch.

 

[PaulvS]

Fitting my cowl this weekend. I Purchased the old pepto bismol pink cowl a few years ago from a guy that went horizontal induction rather than vertical. Since I already have my ground adjustable fixed pitch sensenich prop and spinner, and to set the 1/4" gap, I bolted on the starter ring and the spinner which has the saber 2-1/4" spacer bolted/added to the spinner. A beauty.

Then my heart sank as I looked at the aft end cowl to firewall skin gap/shortage. It appears I'm a few inches short...probably a total of 3" or so in order to get a good trim back to the 2" mark per plans

I've researched some of Dan Hortons Scarfed layup extension solution advice. Question is with that much length add on, is it possible to to add this much extension to the cowl, in an area that will have hinges attached? I'd do a long gradual angles scarf with staggered layers and likely carbon and or metal strip included. But I'm open to suggestions. Is this length even feasible. Anyone tackle this issue with their cowl...Looks as if it will be both top and bottom. Not sure why this cowl is so short (doesn't look like it was previously trimmed either) as the spacer is common for fixed pitch.

What do ya'll think?

It looks like the cowl in the pic is a little bit short at the sides, maybe an inch or so, but not 3 inches if I read the above correctly. The 2 inch reference line is a reference line to measure from while fitting up and the cowl does not extend to that reference line.

In the pic the ring gear assembly is not installed and that should be on. Also the fwd top skin needs to be fitted before you can figure out the cowl fitting.

I don't think the problem is a big problem!

 

[eavila1]

It looks like the cowl in the pic is a little bit short at the sides, maybe an inch or so, but not 3 inches if I read the above correctly. The 2 inch reference line is a reference line to measure from while fitting up and the cowl does not extend to that reference line.

In the pic the ring gear assembly is not installed and that should be on. Also the fwd top skin needs to be fitted before you can figure out the cowl fitting.

I don't think the problem is a big problem!

You are exactly right. I did put the ring on and fit the top skin and with the 1/4-3/8" gap on spinner I only have about 3/4" lacking on the sides, less than that at 10 and 2 a clock and for much of the very top it's right at my trim line. Thx for the pics Robert that is exactly what i have to do.

 

[Ironflight]

Dan is the master on this of course, and I am a fiberglass hack…..but I did write an article a couple years back when I had to add significantly to our extents cowl - it might give you some ideas on how we did the “mold” work:

Love and Hate - KITPLANES

Mistakes were made. (And fixed.)

www.kitplanes.com

 

[eavila1]

Dan is the master on this of course, and I am a fiberglass hack…..but I did write an article a couple years back when I had to add significantly to our extents cowl - it might give you some ideas on how we did the “mold” work:

Love and Hate - KITPLANES

Mistakes were made. (And fixed.)

www.kitplanes.com

Thank you Paul I'll definitely look at this. You and Dan have convinced me (and out of necessity) join the dark side of improved fiberglass skills
I just printed it....