Boot cowl fastener philosophy: Rivet vs nutplate

Zzz

Zzz

Resigned and bitter
I recall that this was somewhat of a debate, but in starting the boot cowl on my quickbuild kit, I see the potential to use rivets for a permanent fastener, or nutplates to allow disassembly.

I tend toward flexible solutions, so even though the nutplates are more work, should I use them for EVERY fastener in the boot cowl? Do any of the joints just work better as rivets alone?

I'm sort of paralyzed by planning and understanding of this phase and I'm terribly afraid to drill any unnecessary holes in my nice firewall flange.I also HATE drilling out rivets. :cool:

I imagine that most of these parts will be painted on my bird and that will necessitate removal of the panel for the painting process. I really don't like spraying assemblies as paint gums up screws and coverage is almost always not as good.

Current progress:

Stainless tunnel clamped in place. I'm ready to poke some holes in the bottom flange of the firewall, but unsure if I should use rivets alone or nutplates for that joint. I stuffed a 1/4" thick piece of wood between the firewall flange and the fuselage tubing to block it for drilling.

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The tunnel fit mostly perfect out of the box from B.A. I only had to trim the forward flange edge to prevent it overlapping the firewall flange when it's turned upward.



I'm obvious a newbie at this and sheet metal work in general. Any tips or pointers are welcome.
 
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I will be riveting the tunnel to the firewall flange. I see no reason to remove it when the plane is done.
I make the side boot cowls removable though. They attach at the bottom with nutplates along the sides of the tunnel. The tunnel is held at the back with screws, along with the belly pan aft of it.
I also rivet the top boot cowl to the firewall, and the aft part is riveted to an aluminum angle which the top of the instrument panel gets screwed to. Again, I don't see the need to ever remove the top boot cowl unless something got damaged. At that point drilling out the few rivets along there would probably be the least of my worries. With the side's removable and the panel easy to remove, I can get to anything behind the panel quite easily. Just don't rivet anything until you absolutely have too, as during the building process you will likely want, or need, to remove the part to do something else.
I don't have time right now, but I can take some pics for you of how I've done it and post them.
 
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Steve, will you be able to buck the rivets at the bottom of the firewall/front of tunnel junction, or will you use some sort of blind rivet? I used #8 screws along the front, and found barely enough room for nutplates and a thread or two of the screw sticking through.
 
Removal of the firewall at this point is pretty simple. It's just cleco'd on. I would likely drill the bottom flange in place then cleco. Later when everything else has been drilled and cleco'd, come back, remove firewall and the tunnel, drill the tunnel for nutplates and rivet them on, then move back into place and fasten with short #8's like Jared suggested. There is about 5/16" between the top of the bottom firewall flange and the bottom tube of station A.

However, I also see the point about the practicality of disassembling this structure in the future, there just isn't much point; there's nothing to access that isn't accessed more easily by removing the floorboards. It just seems weird to have one end of the tunnel riveted and one end screwed.
 
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I don't like removing those floorboards once you've got a few things completed and attached to them / overlaps. Things like the flap handle, carpet, and fuel selector get in the way, so I go through the belly every time.

I used nutplates and screws in every single location, and there's not one place where I have regretted it. I have opened every single panel and part at some point, either during a re-design or for a repair / maintenance.

There ends up being lines and wires running all over the show, and I like having all the design flexibility that ample access allows me.

If you go this way, just be sure to use flush screw heads, countersunk or dimpled as necessary. You never know where you might need to overlap some other panel for a change in design or to deal with a pesky issue. And be careful where you put the screws on the firewall flange, as this somewhat dictates where your cowl doors can and cannot go.
 
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These are 3-3/8" counter sunk.
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The Pacer uses both counter sunk and flush nut plates to secure the bottom cowl and center top cowl .
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Sent from my iPad using Tapatalk
 
Zane, I like to be able to remove the tunnel. So it is installed on my planes with nutplates. I think there are a couple sheet metal screws also. It all depends on what you want removable for inspections or accessing things. Many builders do not think in the future about accessing areas for maintenance. My boot cowl is a combination of screws/nuts and nutplates. I don't figure I'll be removing the entire boot cowl. If I needed to I could get a hand from someone else to get a wrench on nuts in some spots.

Spacing - it is not a bad idea to walk around and look at certified planes on the ramp. I would say in general 4-6" is plenty close spacing in most cases. Mark
 
jaredyates said:
Steve, will you be able to buck the rivets at the bottom of the firewall/front of tunnel junction, or will you use some sort of blind rivet? I used #8 screws along the front, and found barely enough room for nutplates and a thread or two of the screw sticking through.
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Will use SS blind rivets. The floor comes off quite easily with the flap handle up and fuel selector is not in that area. I did however screw the tunnel in place on the Patrol I did so I still might change my mind yet :)
 
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I've come across one good reason for tunnel removal, which is to have a solid place to set the fuselage when there is need to remove the landing gear.
 
Hey all, I'm doing some drilling tonight. I've sunk a few holes in the lower flange of my firewall and cleco'd it to the SS tunnel. I'm just drilling the center holes with a #30 for now of course. On the lower flange of the firewall, I plan to use #8 screws (#6 everywhere else), for which I currently have no nutplates (I only have #6) so I went shopping. I'm embarrassed to say I didn't realize (or had just never thought about it) that nutplates come in 2 basic varieties: MS21047 / K1000 (flat) and MS21049 / K1100 (countersunk.)

I assume the countersunk versions are meant to be used with a dimpled sheet? That begs the next question: On your boot cowl fastening, what did you do for screws? Countersunk or trusshead/buttonhead? Seems like anything in the airstream should be countersunk (even though there wasn't a single flush screw or rivet on my old 170 cowling :) ) but it would be nice I guess to have the glareshield smooth.

.032 and .025 must be dimpled, right? Too thin to countersink with a bit. That dimpled offset has to go somewhere... into the countersunk nutplate?

/n00b

GY2xoBs.jpg
 
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You can use either nut plate if needs be, just be careful not to drastically weaken the nut plate when reaming material off it.

Look at the clearance for the nutplate and screw above that bottom flange of the firewall. There can be a clearance issue there - larger size nut plates may not be a good choice, I used the smaller size on the bottom flange, and most everywhere else EXCEPT supporting the main cowl to the firewall on the main flange. Of course the screws which hold the main cowl in place, can be different or the same as the ones holding the boot cowl to the firewall, depending on your philosophy. But be sure to leave enough room in the right spots, if you are using different screws. That flange gets covered in nut plates really fast.

I used countersunk everywhere, and I used the same screws (and nutplates) to secure the boot cowl and the main cowl. So the screws go through up to three pieces of metal, which all have to have matching countersinks / dimples. I think I used #8 in most places, and #10 top and bottom edges of the lower main cowl. I used #6 in the boot cowl. I hope I am not misremembering the numbers by one size all-round.

For me, the bottom edge screws are critical for keeping the cowl in position as I remove it for maintenance, to keep from massively scratching paint off. So they need to come out last. But my install is complicated with cowl flaps and a flexible air intake connection up front.
 
Battson said:
You can use either nut plate if needs be, just be careful not to drastically weaken the nut plate when reaming material off it.
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Do you mean by dishing out the mouth of a non-dimpled nutplate to make it into one? Or drilling through the wing holes when mounting?

If it's the former, why not just spend a few bucks to buy purpose-built dimpled nutplates?

Battson said:
Look at the clearance for the nutplate and screw above that bottom flange of the firewall. There can be a clearance issue there - larger size nut plates may not be a good choice, I used the smaller size
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Yes, I'm aware of that tight fit below the station A tube. I'll fit what I can. There are "L" series nutplates that have a lower profile, of which i am easily able to fit the 10-32 version in there, so I imagine the 8-32 should fit. By "smaller size" what exactly do you mean? -06?
 
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Zane, I have very rarely used the special nutplates. If I am using a dimpled skin into a normal nutplate, I will take the countersink cutter and run it a bit into the hole where the screw goes. It works fine. .025 sheet must be dimpled. .032 can be countersunk but it leaves minimal material. That changes if there is more than one sheet underneath. Like in the areas around your gas tank bay cover.

Caution about using many different sizes of screws - there is no problem using them except when you start pulling screws out it makes maintenance confusing sometimes if you (or the person helping you) doesn't realize there is a different size screw required for that one special hole. Keeping the same size screws does have advantages. Mark
 
Mark Goldberg said:
Caution about using many different sizes of screws - there is no problem using them except when you start pulling screws out it makes maintenance confusing sometimes if you (or the person helping you) doesn't realize there is a different size screw required for that one special hole. Keeping the same size screws does have advantages. Mark
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That makes a lot of sense. What size have you guys used for engine cowling? My old 170 was 8-32 (actually shitty sheet metal screws, but about that size) but the CubCrafters aircraft I fly uses 6-32 all along the cowling.

6-32 seems adequate for boot cowling.
 
The nutplate saga... Thanks for allowing me to think out loud.
I figure if all the boot cowl and engine cowl fasteners are to be countersunk...then I need to stock up. I have zero countersunk nutplates from my big hardware order.

K1100 is the nutplate with countersunk shape for the countersunk screw (AN509) but it has flat rivet holes in the wings. I learned you can just dimple these with the squeezer for a perfect fit behind dimpled sheet.

K1100-6 (6-32) for the bootcowl.
K1100-8 (8-32) for the engine cowl and tunnel and stuff that might routinely be removed for maintenance.

Two more dimple dies to buy now... #6 and #8 screws.
 
I just finished up fitting all the bottom boot cowl panels, and took most of the panels back off to de-burr and fit all the nut plates. I thought I was going to countersink the firewall flange (dimple the skin), which would require I countersink it too much (using the nut plate as a guide). But I have a piece of .050 5052, so aI drilled it and tried dimpling it with my pneumtic squeezer. I thought generally you don't dimple anything thicker than .040, but the squeezer put a nice, crisp dimple on the panel.

Is there any structural reason not to dimple the firewall flange, which is .050, if my squeezer will do it? I was surprised, as my squeezer did a poor job dimpling 2 .025 pieces together. I ended up doing them separately the first, and then dimpled them together.
 
John, the .050 (5052H32) is a much softer aluminum than the .025 wing skins (2024T3). You did good testing it on scrap to see how it behaved. Those boot cowl panels are just panels that cover the fuselage. Non structural. Not like wing skins which are structural. You should be fine. Mark
 
I picked up a couple of sheets of .050 5052 last week. My aluminum welding is coming along and I wanted to try to make a small header tank. Before I countersunk the flange I figured I would give it a shot. It dimpled really well.
 
Hey guys there is a lot of good info here and I am about to need it myself but I wanted to ask if anyone has considered the weight penalty of using a lot of nut plates? I know they are absolutely necessary but not in all places. I think it might be a consideration worth mentioning.
 
I was about to start installing nut plates on the firewall flange for the boot cowl, but then thought about it for a while. It seems it might be a better idea to hold the that sheet metal together with clecos until the engine cowling is also fabricated so I can match drill it all at the same time.

Is that a better approach, at least for the firewall flange? Door formers and tunnel to sheet metal should be no problem to do now.
 
svyolo said:
I was about to start installing nut plates on the firewall flange for the boot cowl, but then thought about it for a while. It seems it might be a better idea to hold the that sheet metal together with clecos until the engine cowling is also fabricated so I can match drill it all at the same time.
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What did you end up doing ? Has anyone used #6 sized nutplates to attach the main cowling to the firewall ?
 
The reason I ask is that in my very OCD inventory spreadsheet it shows that I’ve got several hundred #6 nutplates in the workshop, and exactly 22 of the larger #8 nutplates. I’m sure I had a reason to order the 22 nutplates at the time, but now I have no idea where they were destined for.
 
I ended up using more #8's than I thought, and stuff like the firewall and boot cowl, and a few other places, some corner and 1 lug #6's are also pretty handy. I try to keep some on hand so I don't have to wait on an order to finish something. An order for me is 3-5 days away.

And I am not on the bottom side of the equator.
 
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