Green Arc Red Zone

Should read: "Although there is no red marking..."

Although there is red marking in the green arc, the POH for the 1979
Piper Turbo Arrow IV/PA28RT-201T, the LIMITATIONS SECTION lists the
following placards:

AVOID CONTINUOUS GROUND OPERATION 1700-2100 RPM IN CROSS/TAIL WIND OVER
10 KTS.

AVOID CONTINUOUS OPERATION 2000-2200 RPM ABOVE 32" MANIFOLD PRESSURE

I believe this applies to the two-bladed prop only and not the
three-bladed prop, but I can find nothing in the manual to support this
theory. I do not find the placards on the panel of the aircraft I fly
which has the three-bladed prop.

These two lines in the LIMITATIONS SECTION are the only place in the
POH which mention this condition.

john smith wrote:
Jack Allison's discussion of the Hartzell AD for his Arrow has gotten me
to thinking.
Wow...attribution and getting Mr. S to think. My work here is done :-)

What aircraft do you fly/have you flown/do you have first hand knowledge
of which have a red zone somewhere in the green arc of the tachometer?
Ours is a yellow arc that goes from...rats, I don't have it written down
anywhere. IIRC, the upper end of the yellow is 2300, maybe 2350. I'll
have to check tomorrow, make a note, and report back.

At any rate, it's really not much of a deal for our plane since,
basically, we use three prop settings:

1) Full forward for takeoff
2) 2500 rpm at 1000 AGL (and 25 inches MP)
3) 2400 rpm at cruise (and 21 inches MP...or, if you want to burn more
100ll for only a couple kt gain, go with 24 inches MP).


--
Jack Allison
PP-ASEL-Instrument Airplane

"To become a Jedi knight, you must master a single force. To become
a private pilot you must strive to master four of them"
- Rod Machado

(Remove the obvious from address to reply via e-mail)

Jack Allison wrote:

john smith wrote:

Jack Allison's discussion of the Hartzell AD for his Arrow has gotten
me to thinking.

Wow...attribution and getting Mr. S to think. My work here is done :-)

Should I respond to that?

Naw! I don't feed trolls. :-))

I don't think anyone mentioned the early Cherokee 180s. They have (or
should have) a red arc on the tach from 2150 to 2350 and a placard
stating that continuous operation should be avoided in that range.
The reason was harmonic vibrations developed between the Sensenich 76"
prop and the hollow cranked O-360-A3A. There were some prop tip
losses in the late 60s that prompted the restriction.

The restriction was removed in the '68 and later models, with the
introduction of the solid cranked O-360-A4A engine.

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


--
JGalban
Posted at www.flight.org

john smith wrote:
Jack Allison's discussion of the Hartzell AD for his Arrow has gotten
me to thinking.

Wow...attribution and getting Mr. S to think. My work here is done :-)

Should I respond to that?

Too late, you just did! :-)

Naw! I don't feed trolls. :-))

Oh sure, break out the 'T' word. :-)


--
Jack Allison
PP-ASEL-Instrument Airplane

"To become a Jedi knight, you must master a single force. To become
a private pilot you must strive to master four of them"
- Rod Machado

(Remove the obvious from address to reply via e-mail)

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.