Green Arc Red Zone

"Morgans" wrote in message
...

"Dudley Henriques" wrote

The Lockheed Electra sure could have used a prop AD for this one!

So there was a RPM restriction on the Electra?

What do you recall? Didn't a constant speed prop pretty much bypass it?
--
Jim in NC

I only have a vague recall on this. It came up during a safety meeting at
the Naval Test Pilots School many years ago.
If I remember right, it involved a harmonic mode (whirl I believe) not
transmitting correctly through the outboard engine mounts on the airplane.
They specified certain conditions that caused this to happen but I don't
remember what they were. I seem to recall that after the issue was addressed
by Lockheed and the mounts were redesigned, there was a further wing issue
as well that had to be dealt with.
I think Macklin will probably be the guy who can best deal with exactly what
was going on.
Dudley Henriques

Thanks, but I'm not really an expert on the Electra. But I
also read that they harmonic was such that a whirl-mode
began [as you also said] and this caused several complete
airframe failures.

The NTSB probably has a file.


"Dudley Henriques" wrote in message
...
|
| "Morgans" wrote in message
| ...
|
| "Dudley Henriques" wrote
|
| The Lockheed Electra sure could have used a prop AD for
this one!
|
| So there was a RPM restriction on the Electra?
|
| What do you recall? Didn't a constant speed prop pretty
much bypass it?
| --
| Jim in NC
|
| I only have a vague recall on this. It came up during a
safety meeting at
| the Naval Test Pilots School many years ago.
| If I remember right, it involved a harmonic mode (whirl I
believe) not
| transmitting correctly through the outboard engine mounts
on the airplane.
| They specified certain conditions that caused this to
happen but I don't
| remember what they were. I seem to recall that after the
issue was addressed
| by Lockheed and the mounts were redesigned, there was a
further wing issue
| as well that had to be dealt with.
| I think Macklin will probably be the guy who can best deal
with exactly what
| was going on.
| Dudley Henriques
|
|

"Dudley Henriques" wrote

I only have a vague recall on this. It came up during a safety meeting at
the Naval Test Pilots School many years ago.
If I remember right, it involved a harmonic mode (whirl I believe) not
transmitting correctly through the outboard engine mounts on the airplane.
They specified certain conditions that caused this to happen but I don't
remember what they were. I seem to recall that after the issue was
addressed by Lockheed and the mounts were redesigned, there was a further
wing issue as well that had to be dealt with.


Yeah, I remember that coming up once before, now that the whirl mode is
mentioned. I don't recall the details, either.
--
Jim in NC

Google found this
http://www.cs.clemson.edu/~steve/Spiro/electra1.html


"Dudley Henriques" wrote in message
...
|
| "Morgans" wrote in message
| ...
|
| "Dudley Henriques" wrote
|
| The Lockheed Electra sure could have used a prop AD for
this one!
|
| So there was a RPM restriction on the Electra?
|
| What do you recall? Didn't a constant speed prop pretty
much bypass it?
| --
| Jim in NC
|
| I only have a vague recall on this. It came up during a
safety meeting at
| the Naval Test Pilots School many years ago.
| If I remember right, it involved a harmonic mode (whirl I
believe) not
| transmitting correctly through the outboard engine mounts
on the airplane.
| They specified certain conditions that caused this to
happen but I don't
| remember what they were. I seem to recall that after the
issue was addressed
| by Lockheed and the mounts were redesigned, there was a
further wing issue
| as well that had to be dealt with.
| I think Macklin will probably be the guy who can best deal
with exactly what
| was going on.
| Dudley Henriques
|
|

Google found this
http://www.cs.clemson.edu/~steve/Spiro/electra1.html

And here is part 2.
http://www.cs.clemson.edu/~steve/Spiro/electra2.html
--
Jim in NC

I saw part two is linked at the bottom of the page, so
didn't bother to put both there.
Didn't know the link was bad when I posted. Thanks.


What did you think of the article? It seems to be a an
aviation history students
paper.


"Morgans" wrote in message
...
| Google found this
| http://www.cs.clemson.edu/~steve/Spiro/electra1.html
|
| And here is part 2.
| http://www.cs.clemson.edu/~steve/Spiro/electra2.html
| --
| Jim in NC
|
|

"Jim Macklin" wrote in message
...
I saw part two is linked at the bottom of the page, so
didn't bother to put both there.
Didn't know the link was bad when I posted. Thanks.


What did you think of the article? It seems to be a an
aviation history students
paper.

Perhaps more like a journalist student who is interested in aviation. I
noticed the poetic license, and the big buzz words, like "tube of death."

Some technical items not quite right, but if this is a journalist, it is
better accident reporting, than most! g
--
Jim in NC

"Jim Macklin" wrote in message
...
Google found this
http://www.cs.clemson.edu/~steve/Spiro/electra1.html

The second page of the above story is located at:
http://www.cs.clemson.edu/~steve/Spiro/electra2.html
(The link embedded at the end of the first page is obsolete)
Peter



"Dudley Henriques" wrote in message
...
|
| "Morgans" wrote in message
| ...
|
| "Dudley Henriques" wrote
|
| The Lockheed Electra sure could have used a prop AD for
this one!
|
| So there was a RPM restriction on the Electra?
|
| What do you recall? Didn't a constant speed prop pretty
much bypass it?
| --
| Jim in NC
|
| I only have a vague recall on this. It came up during a
safety meeting at
| the Naval Test Pilots School many years ago.
| If I remember right, it involved a harmonic mode (whirl I
believe) not
| transmitting correctly through the outboard engine mounts
on the airplane.
| They specified certain conditions that caused this to
happen but I don't
| remember what they were. I seem to recall that after the
issue was addressed
| by Lockheed and the mounts were redesigned, there was a
further wing issue
| as well that had to be dealt with.
| I think Macklin will probably be the guy who can best deal
with exactly what
| was going on.
| Dudley Henriques
|
|

Assuming a certain amount of artistic license, the reference article is
probably correct. I worked on another project with Prof Robt Scanlan
of Princeton Univ who was in charge of that Electra analysis.

As I recall he said that when an Electra was newly manufactured, the
whirl mode resonant frequency of the engine mount was about 5 Hz & the
flutter frequency of the wing was 3 Hz. After a period of service, the
engine mounts would crack (not that unusual in large aircraft) and
eventually the engine mount frequency would decay to the point that the
two frequencies would become so close that the engine motion would
couple into the wing flutter. Eventually the dynamic system became
divergent resulting in wing structural failure.

You must recognize however that the Electra whirl mode problem was and
is unrelated to the RPM restrictions with certain engine propeller
combinations. These restrictions address torsional resonance modes of
the propeller engine crankshaft combination - not whirl modes.

I understand the frequencies of concern are in the 220 Hz range (a
couple of half-steps below middle C on a piano) which is the sixth
harmonic of crankshaft rotation in a 4 cylinder engine. The resonant
mode shape is the back of the crank going to-and-fro while the prop
tips go fro-and-to assuming a theoretical observer riding on the
spinner (read that again carefully!). Changing the stiffness of the
crank (i, e, the solid core) changes the natural frequency of the
system enough to get it out of the operating harmonics range. Adding a
damper mass on the back of the crank is another way to reduce the
torsional vibration buildup and the resulting prop and crank stresses.


I could hear the 6th harmonic in my 172M at low cruise too even though
it was not placarded against it. There was a certain ringing harshness
in the noise, and any musical ear could recognize that component in the
cabin noise once it is pointed out. As a precaution I stayed away from
that RPM.

Frankly from my limited experience in vibration, I am surprised that
narrow a restriction is sufficient to prevent problems in the fleet.