M20J Cowling rebuild

[jetdriven]

So last weekend, we began to tackle the ratty old, worn out, 34 year-old M20J cowling. This is prep for a repaint this fall. The paint was cracking next to the hump and all the cowl fastener holes were worn out. Additionally, both cowl halves were split right behind the spinner and there were some shoddy repairs made before we owned it.  In other words, time for a total rebuild of the cowling.

 

My first post will be about the carbon fiber stiffener mod and spinner ring. The fastener holes, cowl edges, ram air hole delete, and lower spinner ring will follow as we do those.

 

The upper cowling is made of epoxy impregnated fiberglass, with aluminum strips embedded along the edges where the fasteners go.  The factory also used 3 strips of 1/4" honeycomb about 1" wide under the final layer of fiberglass to act as a stiffener. Where the honeycomb makes the transition at the edge of the hump it is crushed, and fails there first. The fiberglass over it cracks and it begins to flex.  Then the cowl begins to flex, and the paint cracks at that transition next to the hump molding.  It's easy to tell, just set the cowling upside down on your lap and push down on the sides, the gaps open up.

 

 

cracks form here

 

As mentioned earlier, the previous owners attempted to fix this, but the execution was terrible. They used common boat store fiberglass and either contact cement or polyester boat resin to place a large 16" square patch over the entire area. It peeled off in one sheet, and about half the fiberglass cloth was not even wet, it was white, dry, and flexible.  The honeycomb under the hump area was completely crushed down and broken, the only thing holding it together was the very thin skin of 3 layers of fiberglass from the factory.  Terrible.

This is the before picture of the inside of the cowling, after removing the scab patch and during prep for the carbon stiffener.

 

Our solution was the use of 19.7oz/sqyd carbon fiber bidirectional 8HS weave fabric cut to strips and laid up to replace the stiffener. More on fabric selection later, but this is what we had.  For epoxy only one thing will do, and that is MGS. It is a certified epoxy from Germany they build gliders over there with, and Cirrus and Diamond uses it in their airplanes as well. It is a very strong product and has a high T_G, or, the temperature at which the epoxy gets soft and loses strength.  We chose MGS L285 resin with a 50:50 mixture of fast and slow hardener.  Also, per the M20J SMM, Structures section, 5% Antimony Trioxide by weight of resin must be mixed for flame retardant properties. The SMM also lists several epoxys to use for repairs, but none are suitable for laminates. They are more of a PRC-type paste.  I went the modern solution and contacted a few Cirrus repair centers, they all use MGS L285 for cowling repairs.

 

The stiffener was cut down flush 2" from either side of the hump mold line and scarfed back at a 30 degree angle.  You can see this in previous pics.  For the rear two stiffeners, 5 plies of carbon in width from 1.75" to 1.25" were used, to give it a trapezoidal shape, and 2 capstrip plies were used that extends past the scarf line over the top of the factory honeycomb,  2" either side to transfer the load. The front stiffener consists of 6 plies of carbon fabric from 1.75" to 1.25" in width, and three capstrips, one extending 2" either side, the next 2.5" either side, and the final top strip extends 3" either side.  Then the front two stiffeners were covered in three layers of the 19.7 oz fabric, the middle layer cut on a 45 degree bias.  All of this was vacuum bagged with 20" of vacuum.  Due to time constraints, the rear two stiffeners were laid up along with the first 3 layers of the  front stiffener and bagged.  The next day we finished the layup of the front stiffener and the large patch layers, and vacuum bagged that all together to finish the job.

 

Midway through the job, the rear stiffeners are laid up and 3 plies on the front.

 

 

Cutting the carbon proved to be a real issue. The 8HS weave is very loose and excellent for draping over complex curves, but it falls apart with scissors. The solution was a 45mm OLFA razor cutting wheel against a metal straightedge as a guide, cutting epoxy-wet cloth on glass. After a few cuts to refine technique, it comes out very smooth and can be laid up immediately.

 

For vacuum bagging we used a 5CFM a/c vacuum pump and a set of R-12 gauges. That along with a 7$ fitting to connect the vacuum bag fitting worked great.

 

 

It is massively stiff and really was overkill for the work needed. If I were to do it over again, I would use 1.5" wide 19 oz carbon fiber unidirectional tape on the same layup schedule. It would be far easier to cut and layup. We basically made our own strips.  Only 2 plies of the large area patch to tie the front two stiffeners is needed. After stripping of the old shoddy repair, total weight gain was approximately 1.5 pounds.

Final product:

[Steve Dawson]

Nice work Byron and looks like it will last

[dcastor]

Byron, contemplating the same repair on my J. Did you use anything to clean/prep the surface before applying the patches?

Thanks,

Dave

[jetdriven]

Yes, finish the surface to 80 grit using a twist-loc coarse sanding pad disc. The 80-grit sandpaper discs are great for cutting down material, the pads finish the surface nicely. Rough u the factory epoxy finish as much as possible without getting into the fiberglass mat.  Then wash it down twice scrubbing with a clean towel and MEK.

http://www.harborfreight.com/pack-of-5-2-inch-coarse-grade-fiber-grinding-discs-99983.html

[dcastor]

Another question:  Where did you find the Antimony Trioxide?

[jetdriven]

http://shop2.chemassociates.com/PAS-antimonytrioxide.html#2061

[fantom]

VERY impressive, Byron!

[carusoam]

Flies planes and on weekends, he is a chemical engineer!

Very impressive.

Best regards,

-a-

[DonMuncy]

To kinda resurrect an old thread; Byron, after 3 years do you have any additional thoughts about how you would go about the central top cowling crack repair. As to the extent of the repair, the process, the materials and the source of the materials. For instance does anyone sell the epoxy in less than gallon quantities, etc.

[jetdriven]

Aeropoxy is an equivalent structural aircraft epoxy available in quarts. Our repair has held up very well. It's been painted a year and it still won't flex with 50 lbs of force applied to the top cowl.  

[201er]

Byron can lay out one of Marauder's girlfriends on top of his cowl and it ain't going nowhere! Byron, how's all that filler holding up under the paint everywhere? Was the door pin mod worth it?

[jetdriven]

On 2/17/2017 at 1:55 PM, 201er said:

Byron can lay out one of Marauder's girlfriends on top of his cowl and it ain't going nowhere! Byron, how's all that filler holding up under the paint everywhere? Was the door pin mod worth it?

It's all like it was sprayed yesterday. Yes to the door pin, the antenna relocation, the wing smoothing, newer fresh air 

Edited March 2, 2017 by jetdriven

[Brandontwalker]

It's all like it was sprayed yesterday. Yes to the door pin, the antenna relocation, the wing smoothing, newer fresh air 

What is the door pin mod and wing smoothing?


Sent from my iPhone using Tapatalk

[jetdriven]

Mooney sells a kit you install a pin on the upper forward part of the cabin door. It holds the door tightly when in flight. 

Before painting I stripped the front half of the wing and applied a bunch of filler primer and used a sanding spline to flatten the waves from the wing in an effort to improve laminar flow.  In a few areas, like the upper spar cap and the outboard wing skin, we used epoxy and microballooons because the shape was so bad. 

Edited February 18, 2017 by jetdriven

[helitim]

Byron,

You may have seen my post that I am taking my J up to Troutdale for a tank strip and seal next week.  The last step will be paint in September.  Prior to paint, I will be tackling the stiffening process using your technique as soon as she gets back from Oregon.  Do you have any better resolution photos you would be willing to share?  Specifically of the steps during the process.  The photos in the write up are a bit difficult to see clear detail.  I have very little glass experience but a hangar mate who has built several glass planes said he would be glad to help and guide me along.

I ran a search looking for any other writeups you may have posted  regarding how to best repair the worn out fastener holes and cowl edges but did not find anything.  Is that something you posted previously?  

 Thank you again for taking time to post the information as it will be extremely helpful for my project.

Tim

[mooniac15u]

7 hours ago, helitim said:

Byron,

You may have seen my post that I am taking my J up to Troutdale for a tank strip and seal next week.  The last step will be paint in September.  Prior to paint, I will be tackling the stiffening process using your technique as soon as she gets back from Oregon.  Do you have any better resolution photos you would be willing to share?  Specifically of the steps during the process.  The photos in the write up are a bit difficult to see clear detail.  I have very little glass experience but a hangar mate who has built several glass planes said he would be glad to help and guide me along.

I ran a search looking for any other writeups you may have posted  regarding how to best repair the worn out fastener holes and cowl edges but did not find anything.  Is that something you posted previously?  

 Thank you again for taking time to post the information as it will be extremely helpful for my project.

Tim

I think this is what you're looking for.

 

[jetdriven]

18 hours ago, helitim said:

Byron,

You may have seen my post that I am taking my J up to Troutdale for a tank strip and seal next week.  The last step will be paint in September.  Prior to paint, I will be tackling the stiffening process using your technique as soon as she gets back from Oregon.  Do you have any better resolution photos you would be willing to share?  Specifically of the steps during the process.  The photos in the write up are a bit difficult to see clear detail.  I have very little glass experience but a hangar mate who has built several glass planes said he would be glad to help and guide me along.

I ran a search looking for any other writeups you may have posted  regarding how to best repair the worn out fastener holes and cowl edges but did not find anything.  Is that something you posted previously?  

 Thank you again for taking time to post the information as it will be extremely helpful for my project.

Tim

I've got a cowl here in the shop from another member here on MS.  Perhaps I should make a YouTube video 

[DonMuncy]

1 hour ago, jetdriven said:

I've got a cowl here in the shop from another member here on MS.  Perhaps I should make a YouTube video 

That would be nice.

[helitim]

Two thumbs up on the video idea. 

[PeytonM]

My guy is about to start on my 89J model cowling crack repair. Any refinements that you'd suggest since you first posting?

[FBCK]

Mine need works as well, a video would be greatly appreciated.

[bradp]

Byron slightly off topic, but noticed we have the same Frankenstein nav / strobe combo.  Where did you get your glass strobe light cover?  Mine is yellow and cracking. 

Thanks. -B

[N201MKTurbo]

51 minutes ago, bradp said:

Byron slightly off topic, but noticed we have the same Frankenstein nav / strobe combo.  Where did you get your glass strobe light cover?  Mine is yellow and cracking. 

Thanks. -B

https://www.aircraftspruce.com/catalog/elpages/whelenlightheads3.php?clickkey=21471

 

[jetdriven]

1 hour ago, N201MKTurbo said:

https://www.aircraftspruce.com/catalog/elpages/whelenlightheads3.php?clickkey=21471

 

It's a grimes clear tail nav glass cover.  Search ebay or similar 

[jetdriven]

5 hours ago, PeytonM said:

My guy is about to start on my 89J model cowling crack repair. Any refinements that you'd suggest since you first posting?

Nope.  It's pretty optimal.