Bonded door skins and nut-plates

[Bcone1381]

I am grateful for Whee. It is rare for someone to say "I did X and it did not work out. I will try Y hoping to improve ."

I just threaded my door and window round tubing for 6-32 machine screws.
-I cut the threads carefully with a Tap Alignment Jig which I fabricated to keep the threads tight and the tap square to the pilot hole.
-Experimentation showed that running the tap all the way in left the threads a bit loose. But 2.25 turns of the tap resulted in a nice reasonably tight thread/screw fit.
-I hope it holds. If it fails, I can upsize the hole to 1/8" rivets.
-I used a high quality screw fabricated from 125,000 psi steel....no ss, no galvanic cold welding. I'm hoping the screw and the tight threads will mate well with each other. Final assembly with Blue thread locker compound will also help I think.

[Bcone1381]

Here are two images of a Patrol Door that was built IAW the Patrol Book. An outer frame of .025 aluminum is attached to the round tubing skelton with machine screws. An inner frame of .020 riveted to the outer frame to sandwich the plexiglass. Rivets are then match drilled through the outer frame, plexiglass, and inner frame and secured together with pop rivets & washers.

I'd like feedback. Is it prudent to secure the plexiglass window insert to the outer frame from the inside of the cabin with 3M VHB tape?
This eliminate hole drilling through plexiglass, to eliminate cracking potential.

VHB hold stregnth varies with specific type, from 14-20 lb/sq inch.

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[whee]

I think this would be great plan.

[svyolo]

I was planning on fabric covering the cargo doors, or at least the aft door. Then I started thinking it might be easier to get a good seal with the aluminum lip. Is that the reason for the aluminum cargo door skin?

[Nev]

I did some tests using VHB tape last year and was very satisfied with its strength. My plan B is exactly what you’ve described - make a frame and bond to the inside. I also think this is a very safe way to do it as if it fails, the window is on the inside already.

[svyolo]

I am skinning the cargo doors also. I am definitely riveting them, but thought about bonding them also to keep them from making noise. Do the .025 skins tend to vibrate?

[zkelley2]

The bearhawk is such a loud airplane you'll never hear anything but the exhaust.

[svyolo]

I am using a 1/2" X 3 X .047 spring cut down to length for the forward cargo door latch. It seems about right. I would have rather used a 7/16", but couldn't feed/thread it past the welded on washer. Century Spring part number is C-738. My local hardware store has a huge selection.

[Nev]

I ended up cutting the washer off and welding it back on with the spring in place. PITA but I wanted a strong spring.

[svyolo]

Nev;
Not to hijack your thread, but to add to it for future searching for builders.

Skinning the cargo doors. The door frames are a little convex. When you try to drape a piece of metal over it, you will end up with a little extra material moving out to the edge. If it is bad enough you might have to cut the metal and take out a pie shaped piece. In the case of the cargo doors, here is what I did, and it worked pretty well.

I laid out the rivets, and drill them, on the steel frame first. That ensured I got the middle (high point) of the round tubing. I used a hole duplicator to transfer them to the aluminum skin. The first two holes in the skin I drilled were along what I consider the "spine" of the convex shape. I then used some clamps to move the sheet around to sort of average out the excess metal. I would kind of "cut in half" the holes I drill in the aluminum skin until all the holes were drilled. If you do it wrong, you can force a bunch of the excess sheet into one or two areas, and have an ugly outcome. I spread mine out pretty evenly, and I think it came out OK.

It would be an easy mistake to make. Dacron can be shrunk. Aluminum sheet, not so much.

[Nev]

I did it a bit differently. I bent the aluminium skin in several places to form a horizontal “M”. The bends are not over the door bracing, so I didn’t find it intuitive. You can see in the photos that the rivets don’t follow the bends. However it worked out nearly perfect. The front cargo door is the same, and the bends line up with the rear door.

Incidentally, I tried to pry the corner of my bonded skin off, and it gave, several inches in fact. So I’ll be riveting it with countersunk rivets. So to all those that pointed out these potential weak points, you were correct, and thanks.

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[svyolo]

The rear door wasn't as bad. The front was a weird, offset cone shaped skin. I didn't put a bend in either, but I thought about it in the rear door. Another option for the front door is put all the extra material in the window, and cut the front edge of the window. Very small repair, and you could probably get the skin to lay down perfect around the rest of the door. In the end, both of mine came out OK.

But it would be very easy to get 70 percent done, and realize you have big bulge of extra material to deal with.

[Nev]

For the front Cargo door, I only attached the perimeter. I found if I tried to bend along the rivet lines, it would deform the skin. My rear door made all contact points.

If you’re looking to put very small bend lines in the door skin, look at where the creases lie on the outer frame and try to line them up (not the rivet lines). It was difficult for me to show this on the front cargo door due to colour and lighting, but you can see it on my rear door. Not the easiest thing to do unfortunately.

[svyolo]

The crease lines do look good. I might try to put them on mine, but I don't want to misalign the holes I already drilled. My front door does have some extra material right where the window cutout will be. I will be cutting it out in a few minutes, I think that will make it lay down better.

[svyolo]

I also keep thinking about how spectacular the visibility would be to the right rear if I just covered the cargo doors in plexi.

[James]

What gauge plexiglass or lexan are people considering for covering the doors / windows?

[robcaldwell]

I covered my gull wing door in 1/8" Lexan, which seems about right. Was told by a few that 1/16" was too thin and billowed in flight. However, I did use 1/16" on my sky light as it is supported more closely.

[James]

Thanks Rob - 1/8" seems pretty heavy - I wonder how that compares to the weight of aluminium... but then you can't see through aluminium! :-/

[robcaldwell]

I may experiment with 1/16" on my doors as I am already seeing cracks in my lexan. I used acrylic drill bits and took all precautions....

[jaredyates]

1/16 has been good for us on everything except the upper front doors. When we replace those, I will go up a size.