running over-square

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.

On Sep 17, 7:36 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.

Where did the term "square" come from? I mean, we are not squaring a
number, and there is nothing square as in a four-sided thingy.

The same is true in an automobile. If you go up a hill, stay in high
gear and floor it as opposed to shifting down and NOT having to floor
it. It is not intuitive. For maximum fuel economy and least engine
wear go for the least revolutions per MILE.

It's a bit different in a plane, because you are dealing with a
constant speed prop instead of transmission with gears, but the
principles are the same.

There is one other caveat, and it can be a big one. That is to run the
engine where it is smoothest. You can feel it.Vibration shakes thiings
apart, and the less the vibration the better off your aircraft is
going to be. So keep that one in mind too.

Doug wrote:
The same is true in an automobile. If you go up a hill, stay in high
gear and floor it as opposed to shifting down and NOT having to floor
it. It is not intuitive. For maximum fuel economy and least engine
wear go for the least revolutions per MILE.

The trouble is that is isn't true in either an airplane or an
automobile. Jay is correct that the tach records less time, but you
can't be sure that you are using either less fuel or incurring less
engine wear. If you run too slow with cylinder pressures to high, you
will run hotter, often much hotter in a water cooled engine that depends
on the water pump speed to circulate coolant. And running way below the
torque peak is not efficient fuel-wise at some point either.


It's a bit different in a plane, because you are dealing with a
constant speed prop instead of transmission with gears, but the
principles are the same.

To a degree. There is an optimum point and running too slow with
cylinder pressures too high is not the most efficient at some point.


There is one other caveat, and it can be a big one. That is to run the
engine where it is smoothest. You can feel it.Vibration shakes thiings
apart, and the less the vibration the better off your aircraft is
going to be. So keep that one in mind too.

That is generally too, but there are exceptions here as well. Some
frequencies of vibration are much more damaging to certain instruments
and components that are others. What feels good to a human may be
killing a sensitive instrument.

Matt

Andrew Sarangan wrote:
On Sep 17, 7:36 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.

Where did the term "square" come from? I mean, we are not squaring a
number, and there is nothing square as in a four-sided thingy.

Well, if you have a 4" square you might say it is 4" x 4" or 4" square.

The analogy is that 24" and 24 hundred RPM are the same number (yes,
different units, but the same raw number) and thus the 24 square
nomenclature. Seems to make some sense to me.

Matt

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.
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.

Dan

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.

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

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

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