Testing On The Cheap

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)

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!

Peter, Brian,

thanks for the bright thoughts. Think I know what caused the problem. Per
the plans my engine requires about 1.5" long spacers between the mount's
attach points and the engine itself. This creates an extra bending moment
which may have triggered the failure.

The mount is made of 3/4" 4130 tube with 0.035" wall thickness.

My current idea of repair is to weld a short round bar into the remaining
tube and so rebuilding the attach points. Maybe I'll shorten the spacers a
bit and bring some ballast further forward instead (battery).

rattlesnake wrote:
Peter, Brian,

thanks for the bright thoughts. Think I know what caused the problem. Per
the plans my engine requires about 1.5" long spacers between the mount's
attach points and the engine itself. This creates an extra bending moment
which may have triggered the failure.

The mount is made of 3/4" 4130 tube with 0.035" wall thickness.

My current idea of repair is to weld a short round bar into the remaining
tube and so rebuilding the attach points. Maybe I'll shorten the spacers a
bit and bring some ballast further forward instead (battery).


See if you can find some tube that will fit inside or outside the failed
tube. perhaps 50 thou wall. fish mouth the ends. Rossette and end weld.
Take a look at CAM18. It gives specific instructions on repairing tubes.

Brian W

hmmm....

found a tube which fits inside the faild tube, but how can I weld it in
then - just asking


"Brian Whatcott" schrieb im Newsbeitrag
...
rattlesnake wrote:
Peter, Brian,

thanks for the bright thoughts. Think I know what caused the problem. Per
the plans my engine requires about 1.5" long spacers between the mount's
attach points and the engine itself. This creates an extra bending moment
which may have triggered the failure.

The mount is made of 3/4" 4130 tube with 0.035" wall thickness.

My current idea of repair is to weld a short round bar into the remaining
tube and so rebuilding the attach points. Maybe I'll shorten the spacers
a bit and bring some ballast further forward instead (battery).


See if you can find some tube that will fit inside or outside the failed
tube. perhaps 50 thou wall. fish mouth the ends. Rossette and end weld.
Take a look at CAM18. It gives specific instructions on repairing tubes.

Brian W

oh, think I got it now with rossettes and fishmouths :-))


"Brian Whatcott" schrieb im Newsbeitrag
...
rattlesnake wrote:
Peter, Brian,

thanks for the bright thoughts. Think I know what caused the problem. Per
the plans my engine requires about 1.5" long spacers between the mount's
attach points and the engine itself. This creates an extra bending moment
which may have triggered the failure.

The mount is made of 3/4" 4130 tube with 0.035" wall thickness.

My current idea of repair is to weld a short round bar into the remaining
tube and so rebuilding the attach points. Maybe I'll shorten the spacers
a bit and bring some ballast further forward instead (battery).


See if you can find some tube that will fit inside or outside the failed
tube. perhaps 50 thou wall. fish mouth the ends. Rossette and end weld.
Take a look at CAM18. It gives specific instructions on repairing tubes.

Brian W

rattlesnake wrote:
hmmm....

found a tube which fits inside the failed tube, but how can I weld it in
then - just asking


....
The ends of the insert are tapered. Holes are drilled in the failed tube
to allow rosette welds of the interior tube, which have much the same
effect as through bolts, but rather lighter and stronger, if well done.

Brian W

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?

"Brian Whatcott" schrieb im Newsbeitrag
...
rattlesnake wrote:
hmmm....

found a tube which fits inside the failed tube, but how can I weld it in
then - just asking


...
The ends of the insert are tapered. Holes are drilled in the failed tube
to allow rosette welds of the interior tube, which have much the same
effect as through bolts, but rather lighter and stronger, if well done.

Brian W