Testing On The Cheap

performed static testing on Saturday. Went well except engine mount broke
(§&%!?$ one of the few parts I didn't fabricate by myself) So far about
testing.....

www.ph21.de/guest/Pict0035.jpg



"gorgon" schrieb im Newsbeitrag
...
On Jul 3, 10:49 am, "rattlesnake" wrote:


Destruct static testing done in a non-silly and relevant manner. Kind
of cool to see that they predicted the failure mode and location. The
wing was designed within a couple of percent of the goal. No weight
sacrificed here.


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

rattlesnake schreef:
(§&%!?$

You are strong at foreign languages, mein lieber Herr! Bravo!
But then, cursing is always learnt first, isnt'it...?

if you just scrapped a $1000 item wouldn't you, mon ami?


"jan olieslagers" schrieb im Newsbeitrag
...
rattlesnake schreef:
(§&%!?$

You are strong at foreign languages, mein lieber Herr! Bravo!
But then, cursing is always learnt first, isnt'it...?

Ouch,
glad THAT didn't happen in the air.


Seeing an engine depart must be on about the same excitement level as
seeing aeroelastic vibration in the wing tips.
Makes me think of a reason why I sometimes see a wire cable tether round
a couple of engine fixings back to a bulkhead. A loose engine has a
better CofG that a missing engine, no doubt!

Brian W


rattlesnake wrote:
performed static testing on Saturday. Went well except engine mount broke
(§&%!?$ one of the few parts I didn't fabricate by myself) So far about
testing.....

www.ph21.de/guest/Pict0035.jpg



"gorgon" schrieb im Newsbeitrag
...
On Jul 3, 10:49 am, "rattlesnake" wrote:


Destruct static testing done in a non-silly and relevant manner. Kind
of cool to see that they predicted the failure mode and location. The
wing was designed within a couple of percent of the goal. No weight
sacrificed here.


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

"Brian Whatcott" schrieb im Newsbeitrag
...
Ouch,
glad THAT didn't happen in the air.

wouldn't happen in the air. Was caused by stupid static load testing.....

rattlesnake wrote:
"Brian Whatcott" schrieb im Newsbeitrag
...
Ouch,
glad THAT didn't happen in the air.

wouldn't happen in the air. Was caused by stupid static load testing.....


I'm thinking it would be stupid, or at least careless, if the load
were reacted through just one or two mount fixings.
Is that how the test was rigged?
(I noticed there seemed to be a steel strip
perhaps 3/16 X 1 inch welded into the cluster which seemed to
provide a rather abrupt section change.) Is that the way the plans
were drawn?

Brian W

"Brian Whatcott" schrieb im Newsbeitrag
...
....
I'm thinking it would be stupid, or at least careless, if the load
were reacted through just one or two mount fixings.
Is that how the test was rigged?

roughly the setup was like this:
- plane upright
- tail and elevator loaded by about 430 lbs of bricks
- ailerons and flaps loaded by about 290 lbs of bricks
- both wings supported by 1.5 ton car jacks
- engine pushed down by about 1.100 lbs at position of the four attachment
points

I got a bad feeling before the test, but I have only two choices in my
country:
1: do this silly static test
2: reject it and never receive the permit to fly

I think the lower attach points can be repaired. Probably solid 3/4" rond
bars will be welded into the remaining tubes. However I lost some of my
confidence in this flying machine because I don't know what (invisible)
secondary damage may have occured.

"rattlesnake" wrote in message
...

"Brian Whatcott" schrieb im Newsbeitrag
...
...
I'm thinking it would be stupid, or at least careless, if the load
were reacted through just one or two mount fixings.
Is that how the test was rigged?

roughly the setup was like this:
- plane upright
- tail and elevator loaded by about 430 lbs of bricks
- ailerons and flaps loaded by about 290 lbs of bricks
- both wings supported by 1.5 ton car jacks
- engine pushed down by about 1.100 lbs at position of the four attachment
points

I got a bad feeling before the test, but I have only two choices in my
country:
1: do this silly static test
2: reject it and never receive the permit to fly

I think the lower attach points can be repaired. Probably solid 3/4" rond
bars will be welded into the remaining tubes. However I lost some of my
confidence in this flying machine because I don't know what (invisible)
secondary damage may have occured.


While it is really tempting to brag about my father's prescience, in his
decision to leave your country many decades ago, this really does not sound
like it should be an outrageous or damaging test it done correctly.

Assuming that the "engine" was simulated with a rigid fixture that mounted
is essentially the same manner as the engine and about 4 time the weight of
the engine was suspended from the CG point of the engine, or alternatively
that the engine was mounted and that about 3 more time the weight of the
engine was suspended below the CG point of the engine, it would appear that
you simply need a new engine mount which is free of defect. In that case,
the test was well designed and appears to have accomplished its purpose.

OTOH, if this was a "dynafocal" mount, and also if the test procedure failed
to maintain the relative orientation of the four engine mounting points,
then the test procedure would appear to be at fault. It that case, you will
probably ned to perform the test again with a new engine mount.

Peter (Damm)

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

rattlesnake wrote:
....
roughly the setup was like this:
- plane upright
- tail and elevator loaded by about 430 lbs of bricks
- ailerons and flaps loaded by about 290 lbs of bricks
- both wings supported by 1.5 ton car jacks
- engine pushed down by about 1.100 lbs at position of the four attachment
points

I got a bad feeling before the test, but I have only two choices in my
country:
1: do this silly static test
2: reject it and never receive the permit to fly

I think the lower attach points can be repaired. Probably solid 3/4" rond
bars will be welded into the remaining tubes. However I lost some of my
confidence in this flying machine because I don't know what (invisible)
secondary damage may have occurred.


Let me ask you this: how do feel about a tube that was loaded with 275
lb (if the loading was equal and local) that failed?
Pulling numbers out of the air, lets say the failing tube was 3/4 inch
diameter and the material was 30 ton steel - how thick would it be?
There's something evidently very wrong with my numbers, or your loading
conditions: the tube wall thickness would have been
(using 30 ton sq in = 60000 psi steel)
275 lb force = 60000 X pi X 0.75 X wall thickness
So wall thickness = 275 / (60000 X pi X 0.75 ) = about 2 thousandth
inch??
Certainly not! Perhaps it was light alloy tube rated at 20000 psi?
That leads to a wall thickness of 6 thousandth inch? Certainly not!

So maybe they loaded the engine itself though its centroid.
The tube did not fail in crushing, it looked like it failed in shear??
That's the weakest modulus - but not THAT weak - so I am missing
something about the geometry: a long long engine mount over a narrow
area bulkhead?? I just don't get it!

It should not be possible to weld up an engine mount WEAK enough to fail
at the load you mentioned.....
But inserting a solid rod into a thin tube is an unfavorable fix - the
stress concentration is inviting another failure just past the end of
the rod insert.....

Perhaps you might let someone look at the engine mount drawing. There's
something strange about it. At the very least, there was no post weld
heat treat ?? Critical structure should ALWAYS yield (if its metal)
not crack destructively.

Good luck

Brian W
p.s. If the designer was an absolute genius, and the materials all
produced exactly on specification, at the load test, if the engine mount
was just 2% stronger, then one or several other parts would have yielded
(but NEVER cracked) before the engine mount YIELDED.
You can take it for granted that nobody is that good!