Machine counter sink or dimple?

[DesertBearhawk]

Guys, my question is about the area where the top skin, bottom skin, and main spar flange line up. Did you dimple those rivet holes or machine countersink? Its .082 thick so machining is possible.

Thoughts?

[mswain]

I countersunk the spar flange and dimpled the skins. Wayne Massey did a nice write up on his kitlog (with some pictures) about the various flavors.....in the end they all looked the same and were nice and tight.

I countersunk the spar so in the fuel tank area my nutplates would sit flat (my nutplates had a countersink for the screw but the rivets were flat.) I used a #6 dimple die for the screw holes were the fuel tank cover goes.

[Bearhawkdoc]

For AD3 rivets, .032 is the minimum allowable skin thickness, and for AD4 rivets, .040 is considered the minimum thickness for machine countersinking (although some will countersink .032 for an AD4 rivet). The total thickness of the "sandwich" is irrelevant as the strength of the connection is based on full head contact. When the .025 skin is countersunk, the hole enlarges and the portion immediately adjacent to the hole becomes a "knife edge" and will not hold to the designed strength. The ideal 3 piece sandwich, therefore, would be to machine countersink the spar flange and dimple the skins.

[DBeaulieu]

On a quickbuild wing, the portions of the wing skin which are already riveted, is too close to the fuel bay area to allow for sole countersinking of the spar flange. Simply cannot peel back the wing skin far enough (without drilling out rivets) to gain access.

[Battson]

I believe they were all dimpled on the kitset?

Having extensively reviewed AC43, I couldn't see any conclusive guidance on the sandwich situation (but I may have missed it), although personally I tend to agree with what Bearhawkdoc has said above ^

[stinger]

Dave, forget about the combination. When we refer to the .040 machine count sinking, we are talking about each piece of material/individual parts , Bearhawkdoc is correct. Stinger

[DBeaulieu]

Not to hijack this thread, but it is related...

On a quick build wing (like I have) the wing skin is already riveted along the bottom spar cap. This prevents being able to individually dimple material layers (wing skin and flange) to install nutplates and material lay along the leading edge of the fuel bay. Seems like countersinking thru the wingskin and partially into the flange to accept a #6 screw dimple on the fuel tank cover might work but I have concerns with "knife edging" the wing skin. I understand that this sandwiched between the cover and the flange, but my concern is for cracking of 2023 Al due to the countersink into the thin wing skin material. Additionally, nut plates need to be fastened thru two layers of material (using the 1097 AD3 rivets) - which seems okay for that part.

A solution I see (this may seem like blasphemy to some) is to simply use a non countersunk pan head/dome head fastener for the fuel tank cover. Still do the nutplates, and I would do floating nutplates, using the 1097 AD3 rivets.

Again, my main concern is cracking of the wing skin material due to countersink depth.

Eager for some feedback ideas

Dave

[jaredyates]

I was comfortable with the knife edge in this case.

[Nev]

A solution I see (this may seem like blasphemy to some) is to simply use a non countersunk pan head/dome head fastener for the fuel tank cover.

I can't see any issue with doing that. I re-did my engine cowls with domed head screws. Quite happy with the result both in terms of strength/finish, and looks.

[marcusofcotton]

I think this is a good Bob question. There is generally greater sheer strength in counersink/dimple applications and he may be counting on that for the stress skin wing design.