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

"Dan" wrote in message
...

If memory serves the B-707 wig tips had something on the order of 14
feet of flex designed in. I wonder how much flex a B-777 has.


http://www.youtube.com/watch?v=pe9PVaFGl3o

rattlesnake wrote:
hmmm.....

my idea was to take out about 1" of the failed tube and fishmouth(spelling?)
both open ends. Inserted tube (with about twice wall thickness) is visible
and can be welded in. How's about this?



I wouldn't even consider it, snake.

This mount BROKE.

It's either too light, or has a stress riser at the break - or both.
(I'm guessing both from the pic)

Consider yourself lucky (you dodged a big bad bullet) and build it again.

Better this time.




Richard

PS: Read Bob's Jesus Factor post again.
And take it to heart...

cavelamb wrote:
rattlesnake wrote:
hmmm.....

my idea was to take out about 1" of the failed tube and
fishmouth(spelling?) both open ends. Inserted tube (with about twice
wall thickness) is visible and can be welded in. How's about this?



I wouldn't even consider it, snake.

This mount BROKE.

It's either too light, or has a stress riser at the break - or both.
(I'm guessing both from the pic)

Consider yourself lucky (you dodged a big bad bullet) and build it again.

Better this time.




Richard

PS: Read Bob's Jesus Factor post again.
And take it to heart...


It would be helpful to know if there are any more samples of this
airplane design flying, and in particular, are there any more examples
of this engine mount in existence?
If there ARE, then that would focus the debate onto material defect or
construction defect or test rig defect.
Richard reminds us that you didn't see a permanent distortion of the
mount - which is the desired failure mode (supposing that any failure is
desirable) but a clean break - presumably two clean breaks. That should
not happen, ever on a test.

Brian W

"Brian Whatcott" schrieb im Newsbeitrag
news ....
It would be helpful to know if there are any more samples of this airplane
design flying,

yes there are and no engine mounts broke (in normal flight) until now (as
far as I know)

and in particular, are there any more examples of this engine mount in
existence?

yes, several

If there ARE, then that would focus the debate onto material defect or
construction defect or test rig defect.

I suppose a testrig issue. Load concentrations build up which do -not-
happen in real flight scenarios (my various previous posts about static load
testing)

Richard reminds us that you didn't see a permanent distortion of the
mount - which is the desired failure mode (supposing that any failure is
desirable) but a clean break - presumably two clean breaks. That should
not happen, ever on a test.

interesting aspect.

Note: because of legal coniderations I will not mention company names,
people's names or types or designations of aircraft here.

--
P.S.
what me -really-surprised is the fact that for about a minute the mount held
the engine more or less in place even with this two severe cracks! 4130 is
really good stuff...

rattlesnake wrote:
...are there any more examples of this engine mount in
existence?

yes, several

If there ARE, then that would focus the debate onto material defect or
construction defect or test rig defect.

I suppose a testrig issue. Load concentrations build up which do -not-
happen in real flight scenarios (my various previous posts about static load
testing)...
P.S.
what me -really-surprised is the fact that for about a minute the mount held
the engine more or less in place even with this two severe cracks! 4130 is
really good stuff...



- -
4130 low alloy chrome molybdenum steel is preferred for engine mounts,
for its strength as annealed (not just 30 ton but 45 ton ) and extended
tenacity in yield.

Some ways I can think of, to break this excellent alloy:

1) chrome-plate it carelessly (or cadmium, nickel, some others too)
or
2) quench it rapidly from bright yellow, to get that brittle 90 ton
strength.
or
3) Decide that it's OK to MIG weld it on a cold day, and there's no need
to normalize or anneal afterwards.
or
4) Buy an engine mount, then leave it standing in the garage for 15
years while the project goes together - without an internal coat of
linseed or waxy film like Boeshield somethingorother. Oh, let the rain
get in it a time or two
or
5) decide that those tubes look really really thin and weedy, and add a
bar or two to "beef it up".
or
6) Decide you will mount a nice powerful 110 HP Subaru up front, with
engine controller, fuel injection, water cooling with the original
radiator etc.

THEN,
load it to the worst case design load, but allow untriangulated tubes of
the mount to load up in bending.

(If there are other similarly equipped examples flying, then you can be
pretty sure they HAVE passed a load test.)

Hope this is food for thought anyway though I am aware that what I am
doing here, is just one step better than making pronouncements on
exactly why the AirFrance liner went down recently....

Good Luck
Brian W

thanks, Brian
your comments are much appreciated

"Brian Whatcott" schrieb im Newsbeitrag
...
rattlesnake wrote:
...are there any more examples of this engine mount in existence?

yes, several

If there ARE, then that would focus the debate onto material defect or
construction defect or test rig defect.

I suppose a testrig issue. Load concentrations build up which do -not-
happen in real flight scenarios (my various previous posts about static
load testing)...
P.S.
what me -really-surprised is the fact that for about a minute the mount
held the engine more or less in place even with this two severe cracks!
4130 is really good stuff...

- -
4130 low alloy chrome molybdenum steel is preferred for engine mounts, for
its strength as annealed (not just 30 ton but 45 ton ) and extended
tenacity in yield.

Some ways I can think of, to break this excellent alloy:

1) chrome-plate it carelessly (or cadmium, nickel, some others too)
or
2) quench it rapidly from bright yellow, to get that brittle 90 ton
strength.
or
3) Decide that it's OK to MIG weld it on a cold day, and there's no need
to normalize or anneal afterwards.
or
4) Buy an engine mount, then leave it standing in the garage for 15 years
while the project goes together - without an internal coat of linseed or
waxy film like Boeshield somethingorother. Oh, let the rain get in it a
time or two
or
5) decide that those tubes look really really thin and weedy, and add a
bar or two to "beef it up".
or
6) Decide you will mount a nice powerful 110 HP Subaru up front, with
engine controller, fuel injection, water cooling with the original
radiator etc.

THEN,
load it to the worst case design load, but allow untriangulated tubes of
the mount to load up in bending.

(If there are other similarly equipped examples flying, then you can be
pretty sure they HAVE passed a load test.)

Hope this is food for thought anyway though I am aware that what I am
doing here, is just one step better than making pronouncements on
exactly why the AirFrance liner went down recently....

Good Luck
Brian W

On Sat, 4 Jul 2009 08:47:51 +0200, "rattlesnake"
wrote:


"Jerry Wass" schrieb im Newsbeitrag
...
....
And the moment legs, moment feet, etc....

EOD


for what it is worth I think rattlesnake was correct in his comments.
he was talking about point loads between the test supports and the
structure being a problem but not really demonstrating anything.

Stealth Pilot

On Thu, 02 Jul 2009 18:36:18 -0500, cavelamb
wrote:

Brian Whatcott wrote:

A striking comment in a wonderful book on engineering design disasters
mentioned casually that the proof load on a particular aluminum airliner
wing would bend it to the vertical at the tips [if buckling did not
occur long before....]
Perhaps you were too hasty to scrub the project?? I have looked out the
window in bumpy conditions to see wingtips flapping a yard or two....

Brian W


I've seen them do that in a 747.

....and here I was expecting you to post "I've seen them do that in the
texas taildragger"

Stealth :-) Pilot

A striking comment in a wonderful book on engineering design disasters
mentioned casually that the proof load on a particular aluminum airliner
wing would bend it to the vertical at the tips [if buckling did not
occur long before....]


When I went through 727 fixit school, they showed us a video of the static
test of that bird. THe wingtips came within about a yard of each other
before the whole thing let go, but when it did it looked like an aluminum
blizzard.

Jim