(Harry Andreas) wrote:

I read that whole convoluted thread with amusement earlier this
week when I returned from travel. So much figurative arm waving...

As a long time mechanical engineer, let me point out a few things
I saw when reading the whole distended session:
1] someone (MIke?) was absolutely correct when he said that bolts
should never be loaded in shear across the threads. There are
special bolts with unthreaded shanks for shear loading.
2]someone said bolts are roll threaded to increase strength, that
is incorrect. the reason roll threading is used is that it does not
create as bad a stress point as cut threads. Cutting threads cuts
across grain flow and roll threading pushes the grain around the
thread. No increases in strength, but less of a decrease.
3] It is perfectly reasonable that 4 bolts going straight up into the
airframe take the entire loads of a pod. Pod mounting points are
primarily loaded in bending with only a little shear. This is overcome
with tensile strength, not shear strength.
4] any good designer can transfer pod flight loads into the airframe
anyway, without putting the entire load through fasteners
5] cadmium is plated onto fasteners to prevent galvanic corrosion
with aluminum in the airframe
6] pre-loading the bolts puts the structure in compression.
Subsequent flight loads unload the compression before the
structure goes in tension. All this depends on the load paths.
7] I have some experience with "little hooks" and different alloys
and different heat treatments. Size doesn't necessarily matter.

ciao

I was hoping a mechanical engineer type would speak up.
Now getting back to the "nuts & bolts" (pun intended) part of the
issue at hand here, please explain why or why not you think that
an F-4 could pull enough G so as to rip the ECM pod off the belly?

Thanks!

-Mike Marron
CFII, A&P, UFI (fixed-wing, weightshift, land & sea)