Cowl hinge offset on RV-12is - edge clearance

[BobN54]

Page 37is/U-09 calls for the MS20257-3 cowl side hinge to be offset 1/16" up from the bottom cowl edge. Doing that leaves 7/16" of leaf overlapping the bottom cowl fiberglass. That allows for 1/4" rivet centerline to fiberglass edge (~ 2.5*Rd clearance) and 3/16" rivet centerline to leaf edge (2*Rd clearance) for the aluminum. That feels uncomfortably tight for that long row of rivets in the fiberglass, and seems like it's just asking for a crack. I've been following N122B's builders log photos, and it looks like Barry has more fiberglass edge clearance. Am I missing something? Substituting -4 hinge would give 1/8" more overlap to work with. Moving the hinge down would also improve it, but then the segments may show at the split.

Edge clearance check: Rivet diam. (Rd) = 3/32; Min. rivet centerline to fiberglass edge = 2.5*Rd = 15/64; Min. rivet centerline to aluminum leaf edge = 2*Rd = 3/16. The sum of those is 27/64, which is just 1/64" under the 7/16" overlap.

 

[Tony Kirk]

My suggestion is to stick to the plans, it worked for me.

 

[BobN54]

If you built an RV-12, you probably didn't encounter this because its plans call for the hinge to be positioned with the notch aligned to the cowl edge, 1/16" lower than the plans show for the RV-12is. I'm suspicious Van's raised the hinge on the RV-12is so the segments won't show, and forgot to consider what that did to the edge clearance. One other point is all of the other cowl hinges are the wider MS20257-4, and the edge clearance is greater. Only the side hinges are made from MS20257-3.

RV-12 section 38:

 

[rvbuilder2002]

If you built an RV-12, you probably didn't encounter this because its plans call for the hinge to be positioned with the notch aligned to the cowl edge, 1/16" lower than the plans show for the RV-12is. I'm suspicious Van's raised the hinge on the RV-12is so the segments won't show, and forgot to consider what that did to the edge clearance. One other point is all of the other cowl hinges are the wider MS20257-4, and the edge clearance is greater. Only the side hinges are made from MS20257-3.

RV-12 section 38:

View attachment 100153

It was not forgotten. I admit it doesn’t leave much room for error, but it works fine as specified.
BTW, the intent of the prototype shop was that the hinge be positioned the same on the 12ULS, (the red factory demonstrator was built that way) but the detail was missed during final plans review.
Additionally, I am pretty sure all of the cowl attachment hinges on the RV-12 is 257-3.

 

[BobN54]

We resolved our concern about the tight edge clearance by using -4 on the bottom half and the -3 on the top. The rivet line for the bottom cowl was set at 5/16" below the edge of the fiberglass. That provides a 3.3*Rd fiberglass edge clearance; within the ideal range. The -4 provides plenty of leaf to easily make the 1/16" offset. There's no concern with the top half. One 3' piece of -4 is plenty because half of it is used per side. We were fortunate to have enough extra on hand.

Not to beat the horse to death, but -4 hinge is specified in the RV12is Parts Index for four parts used in the aft cowl. The plans say you can separate the hinge material and use either -4 or -3 hinge on the fiberglass side when doing the aft cowl hinges. If you choose to use -3 there, then there might be enough -4 leftover to do the bottom side cowl. Hope this helps.

RV12is Parts Index:

 

[rvbuilder2002]

We resolved our concern about the tight edge clearance by using -4 on the bottom half and the -3 on the top. The rivet line for the bottom cowl was set at 5/16" below the edge of the fiberglass. That provides a 3.3*Rd fiberglass edge clearance; within the ideal range. The -4 provides plenty of leaf to easily make the 1/16" offset. There's no concern with the top half. One 3' piece of -4 is plenty because half of it is used per side. We were fortunate to have enough extra on hand.

Not to beat the horse to death, but -4 hinge is specified in the RV12is Parts Index for four parts used in the aft cowl.

I stand corrected. Forgot that the wider hinge flange was needed for the rivet line offset on the firewall flanges.
It is hard to keep it all straight with the plans details of ten different RV models scrambled together within my brain…