Structural failure due to harmonic vibration

Big John wrote:

Jay

After the two crashes they found the problem and fixed it but then
they didn't sell good to airlines due to crash history.

Navy bought a large number (with MAD gear) after fix, that are now
configured to the P-3C Orion (ASW) configuration and many P-3's were
sold around the world in ASW confguration. Navy is still using them
while waiting for a new bird. There is a request out to industry for a
new bird???

That was a different airplane, and a different problem. The one Jay is
referring to were the orignal Electras, similar to what Amelia Earhart
flew. The Northwest flight was an L-14 Super Electra, a larger, more
powerful version of Amelia's L-10E.

http://en.wikipedia.org/wiki/Northwe...lines_Flight_2

What you are describing are the L-188 accidents in 1959/60. The were a
result of what was called the "whirl mode" failure.

The aircraft would perhaps have a hard landing, which would weaken or
break one of the struts in an engine mount. The engine would then be
able to move around slightly on the remaining mounts. When the aircraft
subsequently encountered moderate turbulence, the mounts would not be
able to restrain the engine from acting like a gyroscope, resulting in
the hub of the prop rotating in a circle. In one of the accidents, it
looked like the engine had moved up to 35 degrees out of line.

Unfortunately, when the crew reduced engine power because of the onset of
whirl mode, the frequency of the whirl mode would reduce, and eventually
would match that of the wing. The sympathetic vibration would grow in
magnitude until the wing structure failed.

Big John wrote in
:

The T-28A had a bad problem. At normal cruise RPM the vibrations of
the big wide paddle bladed prop would cause the crank to fail at a
square cut corner on it.

They restricted constant operation in a wide band of RPM ( and
undercut the square corner (where prop shaft met the crank throw) and
bird still lost props and became a glider.

Navy in their T-28B put a new engine and prop on it and no problems.
Some times it pays to go sloppy seconds )


He he.
Kind of surprising that the engine still had that weakness at that stage
of it's career. Variants had been around for over a decade and those hadn't
exactly been treated with kid gloves.
A friend of mine learned to fly in one of those.. I have to say I was
mightely impressed at the time. IIRC it was during a brief period in time
when the T-34 was being replaced with the initially troublesome T-34C, so
they were just going straight into the T-28b.
I remember seeing the books for the damned thing all over his apartment.
Unreal.


Bertie

Big John wrote in
:

Jay

After the two crashes they found the problem and fixed it but then
they didn't sell good to airlines due to crash history.

Navy bought a large number (with MAD gear) after fix, that are now
configured to the P-3C Orion (ASW) configuration and many P-3's were
sold around the world in ASW confguration. Navy is still using them
while waiting for a new bird. There is a request out to industry for a
new bird???


Hey, why get a new airplane when you can just sit in a lawn chair with your
Wii and hunt virtual subs?

The Orion name was another they re-used, of course.
Why they did this is beyond me, it's not like they were ever going to run
out of constellations. Pity they never made a Cygnus, always been my
favorite.



Bertie

Wikipedia says, "Later, an investigation revealed that the tail structure had
failed on the new design from what is known as "natural resonance, or period of
vibration." Sounds like the natural frequency was too low....

Ron Wanttaja

Is this the same kind of flutter that affects the Bonanza V-tail?

Bertie the Bunyip wrote:

Why they did this is beyond me, it's not like they were ever going to run
out of constellations.

Lockheed ran out of Constellations in 1958. :-))))))


John Galban=====N4BQ (PA28-180)

--
Message posted via AviationKB.com
http://www.aviationkb.com/Uwe/Forums...ation/200711/1

Bertie

I did some service testing at Edwards on the T-28A. Things found
there. Bird had a bad 2P vibration in cockpit. Was caused by each of
the big wide blades of prop blowing big chunk of air back on the
original canopy which was much taller that the one ending up on bird.
Felt like sitting inside a base drum.

A C-47 or T-29 (forget which one) 3 blade'd prop was cut down to the
correct size and the three thinner blades cured the problem but Aero
Products had completed their order for the big fat props and they were
in storage in a warehouse so it was felt too expensive to can them and
put new props on bird.

Also the engine was a 7 cylinder 800 HP Wright and not the smoothest
running engine ever built. Almost 115 HP per cylinder. Chug,
chug,chug.

I ferried one of the first group (5) of birds delivered to the USAF
from LA to Enid, OK. Willie refused to accept the birds like they were
scheduled to due to the prop problem. Give to the multi engine school
)

We didn't have much trouble with the T-6 and students. When we finally
got the T-28A at Willie (after they machined the crank) there were
several wings bent which would have broken off and crashed if plastic.
I like metal wings.

Enough reminiscing )

Big John

*****************************************8

On Thu, 8 Nov 2007 19:35:58 +0000 (UTC), Bertie the Bunyip
wrote:

Big John wrote in
:

The T-28A had a bad problem. At normal cruise RPM the vibrations of
the big wide paddle bladed prop would cause the crank to fail at a
square cut corner on it.

They restricted constant operation in a wide band of RPM ( and
undercut the square corner (where prop shaft met the crank throw) and
bird still lost props and became a glider.

Navy in their T-28B put a new engine and prop on it and no problems.
Some times it pays to go sloppy seconds )


He he.
Kind of surprising that the engine still had that weakness at that stage
of it's career. Variants had been around for over a decade and those hadn't
exactly been treated with kid gloves.
A friend of mine learned to fly in one of those.. I have to say I was
mightely impressed at the time. IIRC it was during a brief period in time
when the T-34 was being replaced with the initially troublesome T-34C, so
they were just going straight into the T-28b.
I remember seeing the books for the damned thing all over his apartment.
Unreal.


Bertie

Jay Honeck wrote:
I've just finished reading the tale of the first round trip coast-to-
coast flight (which was accomplished by mid-air refueling,
occasionally from milk cans) from Spokane, WA to the east coast and
back, way back in 1929. They flew a distance of 10,000 miles (they
circled over airports whilst refueling) without landing. A
fascinating story, printed in a terrific magazine, "Aviation History".

One of the pilots, Nick Mamer, went on to a career with Northwest Air
Lines. The author of the article states that he was killed in 1938
flying a Lockheed 14 Super Electra over Montana when the plane crashed
after suffering structural failure due to harmonic vibration. All
passengers and crew were killed.

This is something I've never heard much about. I understand harmonic
vibration can destroy any structure, given enough time and amplitude
-- but wouldn't a pilot be aware of such an event occurring in
flight?

If the Q is high enough, the amplitude of the vibration might ramp very
quickly to failure. Think control surface flutter. There isn't always
time to react.

Matt

"JGalban via AviationKB.com" u32749@uwe wrote in
news:7aec9d9cbf3fa@uwe:

Bertie the Bunyip wrote:

Why they did this is beyond me, it's not like they were ever going to
run out of constellations.

Lockheed ran out of Constellations in 1958. :-))))))


Groan!



Bertie

Big John wrote in
:

Bertie

I did some service testing at Edwards on the T-28A. Things found
there. Bird had a bad 2P vibration in cockpit. Was caused by each of
the big wide blades of prop blowing big chunk of air back on the
original canopy which was much taller that the one ending up on bird.
Felt like sitting inside a base drum.


OK, you're going to have to tell me what 2P means...

A C-47 or T-29 (forget which one) 3 blade'd prop was cut down to the
correct size and the three thinner blades cured the problem but Aero
Products had completed their order for the big fat props and they were
in storage in a warehouse so it was felt too expensive to can them and
put new props on bird.

Also the engine was a 7 cylinder 800 HP Wright and not the smoothest
running engine ever built. Almost 115 HP per cylinder. Chug,
chug,chug.

Oh, OK, I thought they had 1830s or R2000's In fact, didn;t they
eventually go to R2800's with them? an awful lot bigger than a T-6 for a
"mere" 800hp.

I ferried one of the first group (5) of birds delivered to the USAF
from LA to Enid, OK. Willie refused to accept the birds like they were
scheduled to due to the prop problem. Give to the multi engine school
)

We didn't have much trouble with the T-6 and students. When we finally
got the T-28A at Willie (after they machined the crank) there were
several wings bent which would have broken off and crashed if plastic.
I like metal wings.

I like wood ones!


Bertie

Bertie the Bunyip wrote:

Big John wrote:

Bertie

I did some service testing at Edwards on the T-28A. Things found
there. Bird had a bad 2P vibration in cockpit. Was caused by each of
the big wide blades of prop blowing big chunk of air back on the
original canopy which was much taller that the one ending up on bird.
Felt like sitting inside a base drum.

OK, you're going to have to tell me what 2P means...

These are terms typically used in diagnosing engine vibration problems.

1P vibrations are those which are at the same frequency of the engine RPM.
2P vibrations are those which are at twice the frequency of the engine RPM.
1/2P vibrations are those which are at 1/2 the RPM of the engine.

If you have 1P vibrations, you look at things like prop balance or that you
have all the right size pistons in the engine.

With 2P vibrations, you look at things like the effect of two bladed props,
or anything that might be attached to the engine with 1:2 gearing.