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

"Maxwell" wrote:


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

CPA

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."


--
Dan
T-182T at BFM

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

"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

Dan,

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.


Just like the book (POH) says...

You might want to read the engine management columns by John Deakin over at
avweb.com.

--
Thomas Borchert (EDDH)

"Thomas Borchert" wrote:

Just like the book (POH) says...

Yep; it's all in the POH.

You might want to read the engine management columns by John Deakin over at
avweb.com.

I'm sure I've read Deakin's thoughts on this and not paid much attention. I
was always concentrating on the LOP/ROP stuff.

--
Dan
T-182T at BFM

"Dan Luke" wrote:
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.

In general, I recommend running at the highest MP and lowest RPM
combination in the engine manufacturer's power setting table for the %HP
you want to achieve. This will give you the lowest noise, lowest
vibration, and best efficiency.

On top of that, the tach runs slower too, so if you're paying by tach time,
you save money. If you're a commercial operator watching the hours tick
down to an obligatory engine overhaul or inspection, it's good for you too.

"Roy Smith" wrote:

On top of that, the tach runs slower too, so if you're paying by tach time,
you save money. If you're a commercial operator watching the hours tick
down to an obligatory engine overhaul or inspection, it's good for you too.

I hadn't thought about that angle. Even if you're a part 91 owner, it saves
you money.


--
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.

--
Dan
T-182T at BFM


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

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)