running over-square

wrote in
oups.com:

On Sep 17, 5:36 pm, Roy Smith wrote:
wrote:
The old no-more-than-square thing was a rule of thumb for pilots
who flew engines that had little or no operating instructions,

Keep in mind that there's nothing magic about "square" operation.
Square means "the manifold pressure is that same as the prop speed".
That's hogwash; the numbers only work out the same because of an
accident of what units we use.

There's nothing that says we have to measure prop speed in RPM; we
could just as easily measure it in radians per second or Mega-degrees
per fortnight. There's also nothing that says we need to measure
manifold pressure in inches of mercury. It could be in mm/Hg, torr,
atmospheres, PSI, Pascals, etc.

No, there's nothing magic about it. Just that the old guys
often avoided oversquare operation unless they could find
manufacturer's data recommending it.

Some of these old practices get
carried forward into newer engines where they make no sense. Old
engines often had to run on low-octane fuels that suffered detonation
at low RPM and high MP, and the accident of RPM vs. MP was a handy way
to avoid it. Detonation was a sure way to end up on foot miles from
anywhere hospitable, and since fuel was cheap and the boss was paying
for it anyway, it was safer to use more and get home.

It's not an old practice. Many prewar engines were run over square and
100 octane fuels were commonly available immediatly after the war, not
to mention 115/145.
The only time I ever ran an engine like that was in training, and that
was only for ease of operation as a quick rule of thumb. never during
actual revenue operation.


IMO it's a practice that crept in over the years out of ignorance of the
way engines operate and ignorance of the loads and forces at play when a
piston is whizzing up and down.


bertie

Newps wrote in news:
:



wrote:


I have the cylinder from an IO-520 here that had been
detonating. The head is blown clean off the cylinder; the aluminum
fractured at the top of the cylinder threads. Things would get very
noisy, shaky, smoky and scary if that happened. Cylinder pressures go
out of sight during detonation, as do CHTs.


That sounds like preignition, not detonation. Detonation is rarly fatal
to an engine.


No, he's right about that,definitely. High BMEP's will cause the fuel to
burn so that it gives insanely high peak pressures and that will trash an
engine pretty quickly. That's the definition of detonation and it's the
quickest way to blow a hole through the top of a piston.


Bertie

"Thomas Borchert" wrote

And the B-36. Berlin and Dresden never would have been bombed without
running oversquare...

Berlin and Dresden were bombed with B-36's?
--
Jim in NC

Thomas Borchert wrote in
:

Blueskies,

That was one of the 'tricks' Lindbergh taught the P-38 pilots to
increase their range in the Pacific...


And the B-36. Berlin and Dresden never would have been bombed without
running oversquare...


A, the B36 was only a pipedream in WW2, and running oversquare in a
supercharged airplane is not relevant to the discussion anyway. For one
thing, the engines were geared, for another, the MP pressures for takeoff
for even the lowest boosted airplanes were in the order of 32 inches, with
some running well over 40 inches.
IOW you're comparing apples with oranges.

Bertie

"Clark" wrote

Right after the Germans bombed Pearl Harbor.

chuckle Yeah, that's the ticket! The German's bombing Pearl Harbor;
that's the ticket!

What I would give, to see a real, full sized B-36 flying overhead, and doing
a few takeoffs and landings and fly-bys, just one time!

Think it will ever happen again?
--
Jim in NC

"Morgans" wrote:

What I would give, to see a real, full sized B-36 flying overhead, and doing
a few takeoffs and landings and fly-bys, just one time!

When I was a 6-year old Air Force brat on Williams AFB ca. 1953, some B-36s
went overhead at low altitude and were so loud I peed my pants. The sound was
so overwhelming I didn't even realize what had happened until I went back in
the house.

What an airplane!

--
Dan
T-182T at BFM

"Dan Luke" wrote

When I was a 6-year old Air Force brat on Williams AFB ca. 1953, some
B-36s went overhead at low altitude and were so loud I peed my pants. The
sound was so overwhelming I didn't even realize what had happened until I
went back in the house.

What an airplane!

Yeah!

I've heard that they were so loud that the sound pressure could rupture your
spleen! ;-))
--
Jim in NC

On Sep 18, 1:40 pm, Newps wrote:
wrote:
I have the cylinder from an IO-520 here that had been
detonating. The head is blown clean off the cylinder; the aluminum
fractured at the top of the cylinder threads. Things would get very
noisy, shaky, smoky and scary if that happened. Cylinder pressures go
out of sight during detonation, as do CHTs.

That sounds like preignition, not detonation. Detonation is rarly fatal
to an engine.

http://en.wikipedia.org/wiki/Engine_knocking
http://www.streetrodstuff.com/Articl...ne/Detonation/
http://www.sacskyranch.com/deton.htm
http://www.americanaviationinc.com/effectsNavajo.html

Dan

Clark,

Right after the Germans bombed Pearl Harbor.


Uhm, not really. Not that Hitler wouldn't have liked to. ;-)

I meant the 17s...

--
Thomas Borchert (EDDH)

wrote:
On Sep 18, 1:40 pm, Newps wrote:

wrote:

I have the cylinder from an IO-520 here that had been
detonating. The head is blown clean off the cylinder; the aluminum
fractured at the top of the cylinder threads. Things would get very
noisy, shaky, smoky and scary if that happened. Cylinder pressures go
out of sight during detonation, as do CHTs.

That sounds like preignition, not detonation. Detonation is rarly fatal
to an engine.


http://en.wikipedia.org/wiki/Engine_knocking
http://www.streetrodstuff.com/Articl...ne/Detonation/
http://www.sacskyranch.com/deton.htm
http://www.americanaviationinc.com/effectsNavajo.html

Dan


Back to the original post somewhat... I knew about the over squared and
my Lycoming O-360 manual has a chart that shows conditions acceptable to
over square for the same % hp. I tried it last night and set the two
conditions for 65% and leaned. I noticed that the CHT were lower in the
over squared condition rather than the "under squared?". I have a C/S
prop with a redline condition between 2000 and 2250 RPM. Really limits
my choices. Do you folks see the same?

--

Regards, Ross
C-172F 180HP
KSWI