I have a friend who screwed up on his flap attachment.....

[bob865]

Good Morning. I've weighed in on this topic, but want to get the collective brain trust's opinion on it as well. I have a friend who deviated from the plans when installing the flap hinge. The directions are not great about build order or how to mount the hinge for the flap. In the directions, riveting the bottom skins comes before mounting the flaps. It even comes before assembling the flaps. He decided not to dimple the skins because he knew he would have to lay the hinge in place to align and drill later and wanted the hinge to lay flat. The problem came when he did do the alignment, now with the bottom skin riveted in place, he couldn't get the dimple die in place to dimple the skin and the support underneath. The two pieces were too stiff to get the dies in place, so he decided he could countersink the rivets. So the stack-up is the wing skin (.032) the flap support (.040) and then the piano hinge itself (somewhere in the .040 range or thicker). The wing skin took the countersink and it is all riveted together is 426AD3 rivets. Per the plans, this would have been dimpled and the hinge would have taken the countersink to seat the dimple. The main span of rivets (37 total) is in the condition described above. The remainder (9 total) are worse. They are countersunk in .020" sheet.

That's the backstory. I shared a few opinions/options with him.

 

[bob865]

Continued: Had to split into two posts.

1) He could go with what he did and monitor
We used AI To try to estimate the impact on the rivet stresses and get this estimation.

Joint Property	Config 1: Machine Countersunk (Top Sheet)	Config 2: Dimpled Top/Middle, Countersunk Base	Performance Difference
Top Sheet Condition	Knife-edged (Undesirable for design)	Full material thickness retained (Cold-worked)	Elimination of stress risers
Est. Shear Strength	~175 lbs	~225 lbs	+~28% strength
Est. Tension Strength	~80 lbs	~140 lbs	+~75% strength

Considering 37 rivets in this condition, the flap would have an ultimate estimated strength in tension of 2,960lbs and a shear strength of 6,475lbs.

Trying to estimate what forces would be applied, I get this:

At 45 degree Deflection and 120kts. (both exceeding the Vfe of 87kts and max angle of 40 degrees)

• Tension (Vertical): ~approx 533.2lbs total or 8.89 lbs per linear inch along the hinge
  Shear (Rearward): ~approx 198.1lbs total or 3.30 lbs per linear inch

Ultimately, even with the derating, this gives the flap 5.5X the required in strength in tension and 32X in shear so operating within limits (87kts and 40 degrees) would be an even bigger safety factor.

I also suggested he may want to take the .020 countersunk which I didn't consider the in above estimations because so much material was removed and drill them out to #30 and replace them with 470AD3 rivets. The edge distance is about .025" below the desired edge distance for a 1/8" rivet, but derating for edge distance would still result in a net increase in strength over the current state.

 

[bob865]

Continued: Turns out I had to split it into 3

2) Replace all of the rivets with 1/8" dome heads.
He could drill out all of the existing rivets and drill up to the next size

Stack-Up Configuration	Joint Design Condition	Est. Ultimate Shear	Est. Ultimate Tension	Primary Failure Mode & Notes
Stack-up 1 .020" / .040" / .040"	Cond 1: Dimple / Dimple / CSK	195 lbs	115 lbs	Sheet bearing failure (Highly reliable).
	Cond 2: Machine CSK Top Sheet	105 lbs	35 lbs	Critical Defect. Fastener pulls straight through top sheet.
	Cond 3: Corrected (470AD4 / Short ED)	265 lbs	190 lbs	Sheet margin tear-out due to reduced edge distance.
Stack-up 2 .032" / .040" / .040"	Cond 1: Dimple / Dimple / CSK	225 lbs	140 lbs	Optimal flush joint configuration. High fatigue life.
	Cond 2: Machine CSK Top Sheet	175 lbs	80 lbs	Knife-edge condition. Susceptible to vibration fatigue.
	Cond 3: Corrected (470AD4 / Short ED)	340 lbs	240 lbs	Maximum strength. Failure is sheet margin tear-out.

This option gives significantly increased strength even beyond the original design needs with the cost of a row of universal head rivets along the otherwise smooth bottom.

 

[Arie]

Just for info. The order should be:
1) Wing Skin—Dimpled
2) Flap Support—Countersink
3) Hinge—-nothing, just the #40 holes
Countersinking of the soft aluminum hinge remove to much material from the hinge for a proper riveted joint.
Regards
Arie

 

[bjdecker]

Kudos on doing the math - I did something a little different; For material loaded in shear, I prefer to keep as much of the material thickness as possible - dimpling vs countersinking. So, I dimpled the Wing skin, the flap support and the hinge. I then dressed the holes with a 100° countersink/single hole debur tool, and then assembled in that order. Yes - dimpling .040" requires the oomph of a pneumatic squeezer, Yes - it will stretch the material and cause a twist or bend, Yes - it's strong AF and you'll sleep better at night.

As for your friend - He could add a length of .025" x 1/2" material - match drilled, dimpled, and then riveted to the outside (bottom I suppose) wing skin. So the stack would be "shim/patch" + wing skin + flap support + hinge. Rivet with AN426-AD3-4 or 4.5.

That solves the knife edge & pull through of the wing skin. .025" is thin and not likely to affect aesthetics or flow field too much in this area - there are a number of trips and stalls already present along this transition - what's one more.