running over-square

Dan Luke wrote:
The settings I mentioned, and some that are even more over-square, are listed
as permissable in the POH.

Here's what I found in a Lycoming flyer for a higher-power version of the
TIO-540:

"A power setting of 2200 RPM and 31" Hg manifold pressure is recommended for
all cruise flight."


Your quotation refers to a turbocharged engine which by definition would always
be operated oversquare in cruise anyway. I doubt you will ever see 31" of
manifold pressure in a normally aspirated engine. They just can't do it.

That being said, I have been a long time fan of using the lowest RPM I can get
away with for cruise flight mostly because of my experience with bigger twins.
The slower they turn, the quieter they are, and the less tired everybody in
earshot becomes.

I look for whatever combination of low RPM and manifold pressure can give me
65-75% power. I still want the speed but don't need the extra noise trying to
get an extra 7 knots out of the beast. All things in moderation...



--
Mortimer Schnerd, RN
mschnerdatcarolina.rr.com

"Dan Luke" wrote in message ...
This weekend the Cessna Pilots Ass'n put on one of their 2-day Systems & Procedures classes at Sporty's in Batavia
Ohio. This one was for 182S, 182T and T182S/T owners. About 20 owners attended, and it was well worth the trip and
cost.

Anyhow, one of the best tips I picked up from John Frank was to cruise my engine more over-square than I usually do,
i.e., instead of 26"/2400 RPM or 25"/2300 RPM, run it 27"/2200 RPM or 26"/2100 RPM.

I tried this on the way home and found I got the same performance, maybe a hair better, by trying to get the same %
horsepower with more MAP and less RPM. It was quieter and the fuel flow was down a smidgen, too. According to John,
most of the advantage comes from less internal mechanical horsepower loss at the lower RPM settings.

This is probably not news to a lot of you folks, but it was to me, and it's the way I'm going to run from now on.

--
Dan
T-182T at BFM


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

"Mortimer Schnerd, RN" wrote:

"A power setting of 2200 RPM and 31" Hg manifold pressure is recommended
for
all cruise flight."


Your quotation refers to a turbocharged engine which by definition would
always be operated oversquare in cruise anyway. I doubt you will ever see
31" of manifold pressure in a normally aspirated engine. They just can't do
it.

My engine is a turbocharged engine.

http://tinyurl.com/2jskz7


That being said, I have been a long time fan of using the lowest RPM I can
get away with for cruise flight mostly because of my experience with bigger
twins. The slower they turn, the quieter they are, and the less tired
everybody in earshot becomes.

Yep.

--
Dan
T-182T at BFM

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.

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

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

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.

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...

--
Thomas Borchert (EDDH)