4-place lower engine cowl - one piece?

[John Bickham]

There seems to be a couple of approaches used for fastening the lower section of the nose bowl to the lower section of the engine cowl. Some have used Bob's recommendation of solid rivets and made the lower section one big piece. A significant group has used nutplates on the nose bowl. That allows for removing in sections. The one-piece method for the 4-place seems like a pretty bulky and akward chunk to handle.

I was curious if those with hours of flying and the maintenance required would have some recommendations?

[Battson]

I have a one-piece lower cowl where the nose bowl is riveted on, and also (second time around) built the lower cowl metal from one piece of aluminium instead of having a rivet joint down the middle.
I don't have any issues with it from a maintenance perspective. It does take two people to easily remove the lower cowl without scratching the paint.

For comparison, my top cowl has the nose bowl screwed on, affixed with nut-plates. I have never once removed the top half of the nose bowl from the top metal.

[whee]

We are building ours like Battson’s. It’s been a bit of a trial for us to get it on and looking good, we actually didn’t like our first attempt and are trying again. I like how it looks, clean with no rivet lines.

[John Bickham]

Thanks for the responses. I just finished drilling out the bottom section complete. I understand "bit of a trial to get it on and looking good"!!! I'm on the fence about trying again. It is close but not perfect. I should have slowed down and had a second person here.

[Battson]

I always had problems getting the front of the metal to match up with the nose bowl perfectly. Little scallops or bumps were hard to avoid.
There are some things which can't be made perfect without breaching the Pareto Principle.

[John Bickham]

I have very slowly come to grips with "perfect airplanes never fly" over the life of this project. It might be fair to say I have fabricated 1.5 Bearhawks up to this point !

[svyolo]

Bob's Bearhawk Book calls for An470ad3 rivets to rivet the lower sheet metal to the nose bowl. I was thinking it might be easier to use AN426 flush rivets, back riveted. I will use washers or a strip of metal under the shop heads. Would back riveting damage the fiberglass? I think it would be easier than using a bucking bar on a protruding head rivet.

[Nev]

I used rivets but without a a backing plate or washer. So far they've worked very well. I did add a dollop of epoxy over the rivet tail after setting them.

[TimTall]

I did the same. Flush rivets for upper and lower cowl to the nose bowl. I did use an aluminum backer strip epoxied to the nose bowl for a little extra strength. I spaced them close. Probably about an inch apart. Shares the load and keeps the aluminum from puckering as it curves around the nose bowl.

[AKKen07]

I actually just did this, and used nut plates with flush machine screws, No. 6. I really like how it turned out and it’s a pretty easy installation.

[svyolo]

I thought about nutplates because you could just squeeze the rivets. The cowling has been a lot of work and I don't want to screw it up with normal rivets. I think I am going to go with the back riveted flush rivets.

[Bcone1381]

Bob's Bearhawk Book calls for An470ad3 rivets to rivet the lower sheet metal to the nose bowl. I was thinking it might be easier to use AN426 flush rivets, back riveted. I will use washers or a strip of metal under the shop heads. Would back riveting damage the fiberglass? I think it would be easier than using a bucking bar on a protruding head rivet.

Some thoughts that lack experience.

The soft rivets (for example AN426a3 not the -ad3) don't take much force to form the shop head. They have 16KPSI shear strength as opposed to 26KPSI of our standard rivet. If you wanted a soft rivet then moving up to a AN426-4A might be worth considering to gain back some strength. These are available at Spruce.

Also, the CR9162 pull rivet is a self plugging rivet.

Thirdly, the pull rivets used on aircraft Sling Aircraft are are not self plugging. A slight dab of Dry Micro on the rivet head seals the rivet. After its sanded it looks like a standard rivet.

[svyolo]

I thought about those as well. I had good luck back riveting the lower cowl sheet metal, and did a dry run on the metal to lower nosebowl.