Cracking Fiberglass Top, Engine Cowling

[M20JFlyer]

This is my first post on Mooney space...so give me a few muligans if procedures are not correct

 

The top cowl of my 1991 J looks terrible.  Plane has 1,500 hours and always hangered. Cracks and fissures, most eminate from center over fuel divider. The problem doesn't present as one hot spot or multiple hot spots but is general structural failure (not discoloration of paint).  

No known airflow problems, and CHT's are always happy on the four probe digital  when in operation, and I am only person flying the plane. Use cowl flaps religious when landing...keep thinking this is a heat degeneration thing but am new to fiberglass cowlings . Have had this J model four years and condition is progressing.

Any one else experienced similar problem?   Am open to sending pictures to anyone with experience in making a fiberglass cowl look legitimate.

Also am I missing some important ingrediant to what may be causing this?

Thanks in advance for any help

 

[Piloto]

I had exactly the same problem. Didn't wait to get worse. I coated the inside witjh a good coating of boat fiberglass resin and fix the problem. You may need to paint touch up the cracks on the outside. The cowling thickness is to thin in some areas so while at it is a good idea to recoat suspected areas.

José

 

[M20JFlyer]

Jose'

Thanks for the feedback with a method to repair cracking of fiberglass cowl top - '91 J model mooney.

Grew up on farm in midwest and have turned wrenches, bent aluminnum,bucked rivets and assist at my annual(s) but know didly squat about using fiberglass resins...Is this a job for an experienced EAA  builder??? Houston is a big town, should be able to find someone who is experienced in crafting a repair. Was the resin specific to high heat? Did you use any reinforcement in addition to additional coats of the resin??

The cowling is still very airworthy in strength..but is butt ugly and will indeed require re-coating the finish plus paint.

Jose' I appreciate the help..anyone else have suggestion please post

[hansel]

If it's any consolation, most J's have this problem. I think there was an STC for a center support rib that was designed to reinforce the top cowling and prevent the spider cracking. Not sure who owns it. Have your paint guy beef up the cowling as mentioned, then repaint it. Should only set you back a few hundred bucks I would guess.

[Cruiser]

go to the boating outlet store and buy a fiberglass repair kit. They come with fiberglass mat. The surface must be completely clean and free of grease/oil. Sand it to roughen the surface then clean with acetone.   Add the catalyst to the resin, mix and apply with a brush. I would start with a small amount so you see how it works. If you use the fiberglass mat, it must be totally and completely saturated with resin. Sometimes it is best to put the fiberglass in the resin and soak it, then move it to the area you are repairing. This is not a structural repair so you can cut small pieces/strips but one piece would be better. Biggest problem is trapped air bubbles.

It is a messy job. Just slop the resin on and spread it around to cover the area you are working on. Check the directions for cure times. In hot, humid air it can dry very fast. Normally takes 24hrs. or so to completely cure.

Don't forget to get the repair signed off by your A&P.

[N201MKTurbo]

You should NEVER!! use polyester fiberglass resin on a Mooney cowl!! It will not bond properly and will cause stress risers at the interfaces because it is stiffer then the original epoxy. The Mooney maintenance manual is very specific about what materials are allowed. Any Mooney cowl with polyester resin is unairworthy! Mooney cowls are made with a high grade fiberglass cloth (not available at a boat shop) and epoxy resin with fire retardant. You are not doing yourself any favors repairing your cowl with the wrong materials. If you must shop at a boar store use West Marine's Epoxy.

[M20JFlyer]

I appreciate the additional input and suggestions for repair of top cowl. An earlier post mentioned the failure of the top cowl is not a rare occasion.

Does any one know if Mooney farms-out the building of the cowls?? ... or are they built at Kerrville. If I learn the cowls are built by a supplier near Kerrville, I live in Houston I may be able to visit, and share what I learn with the MooneySpace members. 

 I hope our beloved Mooney Aircraft Co. or someone familiar with the cowl source would share the information.

I am open to any suggestions.. I flew my F model for over 20 years, the metal never cracked. I have no experience with fiberglass fabricating and putting a glass like finish on my existing cowl may be above my pay grade.

[carusoam]

For the glass like finish part... (sand, gel coat and paint, etc...)

May it make sense to seek out a corvette specialist? 

(These guys are more plentiful and may be in the proper pay grade)

 

-a-

PS - Compared to the M20C cowel.  There were stringers used for stiffening.  My cowel sheet metal was cracked and repaired right on the nose behind the prop.

[KSMooniac]

Mooney no longer builds J cowls, so you'll be SOL as far as I know.  What you have should be repairable by any competent A&P with fiberglass/composite knowledge and training.  It is not that difficult.

[jbreda]

There are several things I see replies to this thread missing:

1) There is virtually no strength in fiberglass resin alone. 

2) You should not just buy fiberglass resin at your boat yard.  You want to use either a Class I flame

retardant resin or use an additive like antimony trioxide per the manufacturer's specifications to convert

a lower grade flame retardant resin (like Class II)  to a Class I flame retardant resin. 

3) Honeycomb or channeling (fiberglassing over foam strips) is a good stiffening method.

4) Use a good grade structural fiberglass cloth which can be purchased at aircraft spruce.

5) Use peel-ply and try to get as much resin out of your hand lay-up as possible.

6) besure to clean, sand, roughen all surfaces to bare fiberglass before applying new fiberglass.

 

[Vref]

Look for a glider repair facility they have plenty of experience in this stuff. Aviation glass has a special woven structure. Always use expoxy. I build my self not so long a go a support in glassfiber for my motorglider wings. You can buy super light weight core cell material if you need to build a rib for additional strenght. Mean concern is bonding with the existing material. This requires a certain angle of material to be removed. Temperature control and some expereince is required though...

IMHO just adding epoxy is not helping to much if you want to increase strength you need to build a rib...you can see this often in modern gliders..The cell foam material I used to build this big rib you can see in this picture is about 5mm thick and it comes in plates pre-punched with 2cm squares this alows to make curves e.g. inside a cowling..

My two cent's from gliding...cracking is standard on older glass gliders it has to do with the gel coat and the constant temperature changes. Polyurethane paint helps avoiding cracks...(this is what they do for re-doing older gliders...). That the cowling cracks is probably a mix of temperature changes and constant flexing when servicing the aircraft...

On my J project I am just redoing the paint with Akzo Nobel Aerodur PU paint hopefully it will not crack..;-))

Mixing epoxy/hardener is critical to keep the right ratio's

-L

[carusoam]

 

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[Lood]

A bit off topic, but while we're at fire retardent or fire proof materials used in airplanes, I don't quite get it. Although these will probably burn slower AND of course cost three times more, everything burns in the end.

A friend of mine smelt something similar to plastic burning in his Ovation last year. He asked ATC to divert to the nearsest airport and while turining from his course, he spotted a small farm strip used by crop sprayers. So, he decided to land there immediately instead of continuing to the diversion airport. When he came to a halt on the farm strip, there was a little smoke coming from behind the panel. He emptied his small Halon extinguisher that is carried on board without much effect and decided this was a good time to get what he could from the airplane and put some distance between him and his beloved Ovation.

I'm not sure how long it took, but IIRC, within 30 minutes everything was gone. I saw the afterwards pictures of the fire and apart from the prop, the two wingtips and the part of the elevator and rudder, everything else burnt into a heap of ash. And I mean just everything.

[jetdriven]

An update on cowling repairs. We are preparing our 201 for paint and the top cowl is pretty flimsy, flexes in flight, and the paint is cracked at the hump in the middle. Some bozo, before we owned it, slapped some green polyeser boat resin and fiberglass cloth in the middle, but all it did was add weight.

 

After doing some serious (I mean 6 or 8 hours) reading, some things are apparent.  Mooney cowlings are built of epoxy resin and fiberglass. For repairs, they call for 4 different types of high-temp epoxy but only Hysol EA934 and perhaps Ren CG1304 are even being made anymore. Both are a heat-tolerant epoxy but they are a paste suitable for, say, gluing a doubler to a composite bulkhead, not for fiberglass layups.  Neither one is readily available. Further, they call for 181 fiberglass cloth which is an obsolete specification. Also, they require antimony trioxide flame retardant mixed in the resin 5% to 10% by weight.  After an hour, I finally found two lab supply stores in the country which carry it.  A pound of the stuff is 37$ shipped.

 

I finally got some current data, newer than 1977 anyways.  The Cirrus service and repair manual calls for German MGS L418 epoxy for cowling repairs. This stuff is popular to build Flugzeigs over there and evidently Cirrus and Diamond build their planes with it. Some of you have seen MGS epoxy for sale at Aircraft Spruce, marketed at Cozy and Long-EZ builders and the like. But not L418. They carry L285 and L335.  L418 has a T_G (the point at which the epoxy softens below its design limit) of 250F.  The MGS L285 is slightly less.  I emailed several Cirrus service centers and of the 4 that replied back, 2 said call Cirrus but don't get excited when they dont have it, one doesnt do structural repairs, and one uses L285 as Cirrus told them to do.  Anyways, Spruce ships L285 but they charge 60$ in hazmat fees to ship one quart of hardener. A gallon of resin and two quarts hardener is around 240$ shipped.

 

Fiberglass 181 is an old mil-spec for 9 OZ fiberglass E-glass unidirectional cloth.  The nearest thing to that is 7781 which is an 8HS (8-strand harness satin) cloth of 9 oz which is structurally the same but conforms better to complex shapes. I am using that to rebuild the ring in the front of the cowl (right behind the spinner) as it is cracked clean through on the top and bottom due to flex, and the glass is nearly a half inch thick there. Further, the front overlapping pieces where the cowl comes together there are completely shot, and the ram air hole needs to be glassed over. Many of the parting line cowl fasteners and the upper boot cowl fasteners are wallowed clean through as well. All those camloc fasteners go into a measured hole and countersunk. After 5000 hours, all the fasteners on the boot cowl have oversize grommets and those are pulling through as well.

 

To solve the upper cowl flex problem is going to take some work. More fiberglass won't fix it, it still flexes too much. My boss tried that on his Piper Arrow and 500 hours later, the paint is cracking again. The cowl inflates in flight and balloons up. I guess I could just put plastic sheet over the engine and fill the whole cowl with epoxy to get it stiff, but you get my point.  So a different approach is necessary. Carbon fiber.

 

Carbon is twice as stiff as fiberglass for a given layup, and you must be careful using it. Besides the hellish cost, the use of carbon in a fiberglass layup can cause part failure. Since it is so stiff, it picks up all the load. So that layup schedule must be strong enough to carry all the load. This is why I'm not using it in the front of the cowl, it is too stiff and could crack a the interface between glass and carbon. It also corrodes aluminum so it cannot make contact with it. So this calls for 4 layers of 9oz carbon fiber, laid up and vacuum bagged on the inside of the upper cowling. It also requires a couple stiffener ribs to be placed on the cowling before laying up the carbon. A bridge so to speak.  Scarfing and laying up new fiberglass on the outside of the cowl where the parting cowl fasteners go, and new .257 holes countersunk to a diameter of .437 should fix that.

 

In other words, its a hell of a lot of work, and expensive, around 700$ for materials.  But new paint and glass will look a damsight better than this noise.

 

as a side note, look at that spinner gap and position. If yours doesnt look like this, the engine is sagging. You can shim it to line up, and get that extra knot for free.

[Marauder]

Byron -- after reading your post, I'm just happy I have an aluminum cowl! That looks like a lot of work!

[jetdriven]

Well, its going to be 40 hours of work. But I spend 2 hours removing and reinstalling an M20E cowl the other day, so let's just say until you get 20 oil changes done, you'll be ahead of me

[KSMooniac]

It is a lot of work, but worth the effort.  How are you laying up the carbon fiber and ribs?  Are you making a local sandwich rib and laying the carbon over it?  or attaching some aluminum rib after the repairs?  If it were me, I would use some foam about 0.5" thick and make some strips with a trapezoidal cross section, and layup fiberglass over the top of those, not carbon fiber, as the stiffness difference of the carbon is likely going to cause cracking elsewhere...

[DonMuncy]

I hope more folks will jump in here and let us know their fixes and results.

[N201MKTurbo]

My cowl was in the same shape as yours. It took about 80 hours of work over three months to get it right. There are cheaper epoxy alternatives. I did all my repairs with multiple layers of 3 oz.  E glass. My cowl Had been repaired with polyester resin and about half the effort was removing the old repairs.

[OR75]

as a side note, look at that spinner gap and position. If yours doesnt look like this, the engine is sagging. You can shim it to line up, and get that extra knot for free.

Did you change those floating nuts that hold the lower cowling to non floating ?

My spinner gap looks great after reinstalling the cowling but the cowling moves up a tad after flying

[DonMuncy]

There are cheaper epoxy alternatives. I did all my repairs with multiple layers of 3 oz.  E glass.

 

What is E glass.

[KSMooniac]

What is E glass.

 

It is a generic term for some non-structural fiberglass.  S-glass is another generic term for fiberglass with better structural properties.

[jetdriven]

Did you change those floating nuts that hold the lower cowling to non floating ?

My spinner gap looks great after reinstalling the cowling but the cowling moves up a tad after flying

They are camlocs which seem to be unique to my plane. They also allow the cowl to move and tear out the holes so I am going back to screws and nutplates, likely non-floating.

[OR75]

They are camlocs which seem to be unique to my plane. They also allow the cowl to move and tear out the holes so I am going back to screws and nutplates, likely non-floating.

 

I also have Camlocs (4002 studs).  The receptacle can be floating or rigid.

I belive e changing the lower cowling only to rigid would make the big difference and prevent the cowling to lift a few degrees 

[KSMooniac]

I have camlocs as well throughout my '77 cowl.  Maxwell changed a lot of the lowers to fixed a few years ago to help with cowl lift, and then added the LASAR strap kit on the lower edge to try to further restrain it...with marginal success.  I should've done what Byron is doing before getting mine painted, but the paint shop said they would rework the cowl and make it nice again.  All they did was glass over the ram air intake (and did a great job) but didn't address the worn camloc holes.