running over-square

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

"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.


WOTLOPSOP.

"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.


Is John with the CPA or Lycoming? Just wondering what Lycoming thinks of the
recommendations.

On Sep 17, 10:13 am, "Maxwell" wrote:
"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.

Is John with the CPA or Lycoming? Just wondering what Lycoming thinks of the
recommendations.- Hide quoted text -

- Show quoted text -

Go by what the POH says. If it says you can do it, then do
it. The old no-more-than-square thing was a rule of thumb for pilots
who flew engines that had little or no operating instructions,
especially higher-compression engines that would detonate easily. The
POH will have a cruise chart, along with a lot of other advice, that
is often ignored or overlooked. Lycoming will have worked with Cessna
to establish those limits.
Dan

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.

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.