M20J Cowling alignment -- solved.

[PT20J]

My 1994 M20J cowling aligns with the spinner when installed and there is about a 1/16" gap between both the upper and lower cowling and the fuselage skins. However, as soon as I fly it, the air pressure rocks the cowling back closing the gap between the upper cowling and the skin in front of the windshield and misaligning the cowling with the spinner. Some serial numbers have floating receptacles for the 4002 Camlocs that attach the cowling to the fuselage and Paul Lowen purportedly used to solve the problem by replacing a few receptacles with fixed ones. But mine are all fixed. I finally figured out that the problem is that the Camloc 4002-HS grommets have a reduced size shank to hold the retaining ring and this creates some slop. Skybolt makes a "high shear" grommet SK40G1-11S that has a full sized shank with a groove for the retaining ring. I installed five of these (the Skybolt minimum order for the part): two at the bottom of the lower cowling in front of the nose wheel, one on each side at the top of the lower cowling and one in the center of the top cowling and this solved the problem. 

[jetdriven]

You can slow down the problem, but you can’t stop it because the grommets start to wear the holes in the cowling into an egg shape. The way you can stop it is to build a piece of fiberglass inside the cowling that is 1/4” high and contacts a piece of angled aluminum riveted to the inside of the boot cowl. And then it could take the shear load and the grommets can do their job by holding the cowling down. 

[PT20J]

My holes were perfectly round. In fact, I had to ream them slightly to get the SK40G1 grommets to fit as they are slightly larger than the lip that retains the retainer ring on the 4002-HS grommets. But, my airframe only has 1700 hours on it, so I can see how the holes might wear over time. Also, my cowling has a metal strip embedded in the fiberglass where the holes are. I don’t know if all the cowlings have that. The grommet flange is countersunk into the fiberglass gel coat and there was definitely was some out of round wear evident there. Also, I don’t know how much this would help with the floating receptacles.

[EricJ]

56 minutes ago, PT20J said:

Also, my cowling has a metal strip embedded in the fiberglass where the holes are. I don’t know if all the cowlings have that.

I think they all do, as fiberglass isn't strong enough to handle much load that way by itself.

My cowling is a bit ratty in this regard, but it has 6500 hours on the airframe.   I've learned (or I think I have) that the cowl is happier just letting it move around a little than trying to constrain it too much with the fasteners.  

I seem to recall Jose (or somebody, I think it was him) just glued a piece of solid-core wire on the airframe along the top of the cowl where the gap would be, just to keep it from riding up to close that gap.  I've not been tempted to do that, but he said it worked.

[PT20J]

My cowling is pretty stiff -- it has the Mod Works mod to reinforce the top. 

I'm trying to get more clearance between the lower cowling and the muffler. There's about a half inch when it is first installed, but less when it rocked up, and the muffler shroud contacts the inboard right cowl flap hinge bolt head when the engine shakes in the mounts during shutdown, .

I'm going to run with this for a bit and see if there are any issues and then I may replace the remaining 16 grommets with the Skybolt parts. The grommet retaining rings are a pain to remove and the grommets are $6.82 each. Skybolt sells a tool to remove the retainers for about 40 bucks that might be worth it. I removed the others with an angled pick but it's easy to slip and poke yourself. 

[EricJ]

46 minutes ago, PT20J said:

My cowling is pretty stiff -- it has the Mod Works mod to reinforce the top. 

I'm trying to get more clearance between the lower cowling and the muffler. There's about a half inch when it is first installed, but less when it rocked up, and the muffler shroud contacts the inboard right cowl flap hinge bolt head when the engine shakes in the mounts during shutdown, .

I'm going to run with this for a bit and see if there are any issues and then I may replace the remaining 16 grommets with the Skybolt parts. The grommet retaining rings are a pain to remove and the grommets are $6.82 each. Skybolt sells a tool to remove the retainers for about 40 bucks that might be worth it. I removed the others with an angled pick but it's easy to slip and poke yourself. 

The tool is worth it, imho.   There's also a tool for the more plebian camlocks that helps a lot, too.   Maybe I do more cowl fastener maintenance than most... 

[0TreeLemur]

8 hours ago, EricJ said:

plebian

Since Anthony isn't commenting on fantastically underused words, I'll do it.   An interesting way to describe camlocs!

 

[jetdriven]

11 hours ago, EricJ said:

I think they all do, as fiberglass isn't strong enough to handle much load that way by itself.

My cowling is a bit ratty in this regard, but it has 6500 hours on the airframe.   I've learned (or I think I have) that the cowl is happier just letting it move around a little than trying to constrain it too much with the fasteners.  

I seem to recall Jose (or somebody, I think it was him) just glued a piece of solid-core wire on the airframe along the top of the cowl where the gap would be, just to keep it from riding up to close that gap.  I've not been tempted to do that, but he said it worked.

If you wanna stop the cowling for moving, and I mean  stop it from moving a single millimeter, put the stopper block on the inside of the cowling on the upper part and put a piece of angled aluminum on the edge of the boot cowl and when those two pieces butt into each other it can’t move. Now for the bottom, Lasar sold this mod that was a little strap. it was just above the bottom of the cowling, with a nut plate on it, and you put a hole through that in a screw with a smooth shank on it. And then the entire cowl can’t move a single millimeter. Whenever we did all this work, we filed the back edge of the cowling with about a one and a half millimeter gap. And 10 years later, the paint still isn’t chipped and the gap is the same. The harder you clamp down on the cowling with fixed nut plates and short cam locks, the more it wears the holes. The metal in the fiberglass layup is a piece of .020” aluminum which slows it down, but it doesn’t stop it. The camlock’s job is to keep the cowling flush with the skin. The shear loads have to be absorbed by something made to hold shear loads, and a 3/8 in. Square of aluminum, riveted to the airframe, butting  into a piece of fiberglass that’s the same size. It ain’t going nowhere in 1000 hours. 

[PT20J]

The Skybolt grommets are designed for shear loads as opposed to the Camloc 4002-HS which are not. The five I installed stopped the movement. However, if all 21 are changed to the Skybolt grommets, I’ll bet it's better for way more than 1000 hours since I’ve got 1700 hours on the original grommets with no wear in the holes. All my Camloc studs are the proper length to meet the 5 in-lb torque spec. They vary from 4 to 8 due to thickness variation in the fiberglass. I agree that trying to increase the clamping force with short studs won’t help.

I’ve talked to Don Maxwell, Dan Riesland, and Shery Lowen and they all said that the LASAR mod was only to replace floating receptacles with fixed receptacles, so I don’t know about the strap thing.

[blaine beaven]

37 minutes ago, jetdriven said:

If you wanna stop the cowling for moving, and I mean  stop it from moving a single millimeter, put the stopper block on the inside of the cowling on the upper part and put a piece of angled aluminum on the edge of the boot cowl and when those two pieces butt into each other it can’t move. Now for the bottom, Lasar sold this mod that was a little strap. it was just above the bottom of the cowling, with a nut plate on it, and you put a hole through that in a screw with a smooth shank on it. And then the entire cowl can’t move a single millimeter. Whenever we did all this work, we filed the back edge of the cowling with about a one and a half millimeter gap. And 10 years later, the paint still isn’t chipped and the gap is the same. The harder you clamp down on the cowling with fixed nut plates and short cam locks, the more it wears the holes. The metal in the fiberglass layup is a piece of .020” aluminum which slows it down, but it doesn’t stop it. The camlock’s job is to keep the cowling flush with the skin. The shear loads have to be absorbed by something made to hold shear loads, and a 3/8 in. Square of aluminum, riveted to the airframe, butting  into a piece of fiberglass that’s the same size. It ain’t going nowhere in 1000 hours. 

Byron,

I’m getting some work done on my cowls (fixing cracks, new camlocks, paint, etc). Do you have a picture of this strap and angled piece of aluminum set up I can show my mechanic? 
 

thanks, 

[PT20J]

This drawing from the Skybolt catalog shows how the SK40G1 grommets take up the shear load. The top drawing is the 4002-HS grommet Mooney used. The hole has to be larger than the shank to allow the lip that retains the retainer ring to pass through the hole. The SK40G1 has a larger diameter shank and a groove to capture the retainer ring. The hole can be reamed for a tight fit and, since there is no movement, there should be no wear of the hole.