Testing On The Cheap

On Jul 6, 5:52*pm, "Peter Dohm" wrote:

(Lotsa stuff clipped)

Dear Peter (and the Group)

I guess I'm just a sissy but I would never use .035 for an engine
mount. In fact, I'd be leery of .049.

This isn't about calculations on a sheet of paper, it's about
incidental damage that can occur to the tubing over heaven knows how
many years of service. I'm talking about dropped wrenches and the
like -- falls down, DINGS the lower mount on its way to the bottom of
the nacelle or even clear to the ground, if you're lucky ( ...but you
seldom are, right? :-) So we fish it out and since there's no
apparent damage we assume everything is okay. But gravity sucks and
sure as the potential student always barfs on the new upholstery
(never on the old), Murphy's Law is ALWAYS waiting in the wings. Ding
round tubing and you may as well cut it out and start over; its specs
no longer apply and in nine cases out of ten the repair will cost more
than the replacement.

3/4 x .035 gottem internal dia, of ,680. Closest match is going to be
5/8ths but using it -- or even making the repair instead of doing a
replacement -- is going to depend on where the tube departed from
spec. Adjacent to a weld? Or a gradual bend across the entire length
of the member... because about the ONLY location where the repair
(using an internal sleeve of .5/8) is practical is right in the
middle of the member. Any where else, the REPLACEMENT is going to be
more practical... except for the engineering.

If you replace the failed member with the SAME SIZE tubing, you know
it's going to happen again. Indeed, the static test has done it's
job; it is telling you to use a sturdier member. But not just there,
EVERYWHERE. Because you have to assume there will be occasions when
gravity is going to be coming at you from different directions. So
that while those upper members may have done fine in the static test,
when they are being subjected to TENSION, there is a high probability
they will fail when the attitude of the aircraft makes them bear the
load in COMPRESSION... or visa versa. Indeed, the static test has
already told you so.

Are you familiar with the 'Jesus Factor'? That is the uncalculable
amount you ADD to every structure -- when ever you can -- so as to
cover the realities of serial production, Monday morning hang-overs,
Friday afternoon hurry-ups and every other thing you can thing of that
might cause the as-fabricated version to depart from the as-calculated
version. In a welded steel tube structure we're generally safe with a
Jesus Factor of 1.5 for the fuselage, 3.0 for a control surface and
5.0 for the landing gear AND ENGINE MOUNT.

Mention the Jesus Factor today and they're liable to start looking for
where you parked your bicycle. Perhaps a better tag would be the ****
Happens Factor. Call it what you will, there are those who upon
learning their engine mount is capable of withstanding 30g's when the
specs only call for six, commence to run in circles, waving their
hands and emitting tiny shrieks. And perhaps with good reason, if
we're talking about a home-built, where the builder is also the test
pilot as well as Line Captain. If the calculations call for .049 then
you can safely assume it won't be something thinner... although
thicker might appear now & then. But whatever you call it and no
matter where it is applied, from Farmington, Long Island to a garage
in San Diego, you can bet your bolly hooly it DOES exist -- and does
so for a purpose.

-R.S.Hoover