Dimpling rivets bad for pressurised aircraft? 1

[ddav]

I've heard dimpling aircraft skin is bad practice in pressurised aircraft.
(A) Can someone confirm this?
(B) Direct me to a proper reference?

 

[rb1957]

a) yes, likely crack initiation site (unless you work with a very low pressurisation (hoop) stress. normally you dimple if you have a very thin outer skin, ie so thin that if you put in the CSK (even the shallow 1097 CSK) that you'd get knife edged rivets. normally thin skins mean high skin stresses (or low pressure differentials, like 4psi). most manufacturers go to great lengths to design an efficient fuselage skin, thin to keep the weight down, and thickening along the rivet lines (bonded skins, machined skins, chem milled skins) to provide a suitable thickness for the CSK.

b) i'd consult bruhn or niu

 

[ddav]

rb1957 thanks for the reply. Specific scenario is a proposed antenna installation. a/c skin is 0.032" and internal doubler is 0.040". Was planning to use 0.128 dia 20426 csk with both skin and doubler dimpled. But Guys on the floor don't have the necessary tooling so I might just have to go with an external doubler.

 

[rb1957]

that would be a much better choice, use LZ (NAS1097AD) -4 rivets (full head CSK (BB-4) is not a good choice, given the limited thickness.

a choice might be intrenal doubler with knife-edge rivets ... it's not good practice but that doesn't mean you can't ... maybe look into the fatigue aspect (i figure you're a GA a/c or Pt23).

 

[ddav]

A/C is Pt 23.

SRM for aircraft says to only use LZ (NAS1097AD) -4 rivets in 0.040" or more skin if in a prssurised area.

Am I able to go knife edge as you've suggested if I can show OEM has done it and also fatigue aspect is fine?

Appreciate your thoughts.

 

[rb1957]

0.04" skins ... LZ4 is "borderline knife-edge" (about 75% thk, rule of thumb is 2/3*thk) ... i suspect that this is the typical rivet for yur a/c, so you have a lot of justification using it "standard of type". if the plane is unpressurised then the fatigue stresses will be very low, again supporting the deep CSK.

observation ... 0.04" skins are pretty heavy for an unpressurised plane.