June 23, 1995
Machine countersinking appears to be another one of those things to be avoided. Reports of cracked skins alone has caused me to change my plans of machine countersinking my inboard wing skins. Yet again, I think about the material and technique here, and this stuff has been done for a long time.
What's going on here?
I knew I could get some real facts and data to go along with this discussion.
MIL-HDBK-5F - Metallic Materials and Elements for Aerospace Vehicle Structures
This is a design guidebook of material strengths and fastener strengths that is approved by the Army, Navy, Air Force and the FAA.
To answer the question above, basically, we homebuilders are not following approved practices in this area, and we are seeing the effects.
For those of us substituting 1/8 rivets at the floorpan/firewall joint, the equivalent shear strength of a 1/8 dimpled rivet in 0.040 material is 388 lbs., a 79% increase over 3/32 dimpled rivets.
These figures are valid for 2024-T3 material, and probably would not be valid for other grades of Aluminium. Here is some more data of interest.
Following is the equivalent data for YIELD strength (in this case, defined as a permanent set of 0.005 inch)
.. Gil Alexander, RV6A, #20701 ... fitting cockpit stuff
PS The gentleman who requested material, rivet and bolt useage and strength information should get a copy of this document .... it is about 5 inches (double-sided) thick though! It is the first time I have seen it, and it is really a wealth of knowledge on materials. It is copyright free, and I presume can be obtained through the Government Printing Office.
Gil Alexander firstname.lastname@example.org
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