Running props over square?

kiloalpha

kiloalpha

Well-Known Member
Out company for years and years have told us to run at 2300 rpms and 23" on the MP. Usually trues us out at 170-172kts and burns about 30-32 gph.

Now we are being told to run at 2200 rpms and 24" of MP. We are told this has no negative effect on the engine as is better for the props since they work more efficiently now. Is this true? Are we going pop cylinders and crack cases now? or is this actually going to work?

We have Lycoming IO-540s that make 290hp. Company research shows a fuel savings of 10% with a 1-2% drop in airspeed.
 
Check your POH. The old rule of squaring up prop speed and MP is a great way to SWAG power settings, but it's usually not the most efficient way to run the engines. Most POHs will show economy power settings with lower prop speeds than MPs.
 
The squared ideal has been around forever... and is a good way to ball park figures... but it really depends on the airplane... most turbo'd applications are actually identical engines, but run in a different level of pressure. cranks valve train etc is all the same, and they are nowhere near squared.

i would not worry about damage, esp as 24"2200 is not exactly radical power settings. POH will tell you though.
 
On our TSIO-520's we run 30"/2450 for climb and high cruise, and 26"/2300 for normal cruise. Pretty good reliabililty so far.
 
kiloalpha said:
Out company for years and years have told us to run at 2300 rpms and 23" on the MP. Usually trues us out at 170-172kts and burns about 30-32 gph.

Now we are being told to run at 2200 rpms and 24" of MP. We are told this has no negative effect on the engine as is better for the props since they work more efficiently now. Is this true? Are we going pop cylinders and crack cases now? or is this actually going to work?

We have Lycoming IO-540s that make 290hp. Company research shows a fuel savings of 10% with a 1-2% drop in airspeed.
Click to expand...
MP is just a measure of the level of vacuum in the intake manifold. the higher the number, the lower the vacuum, meaning the engine is not working as hard to get air. unless we are talking about turbo, the engine will only suck as much air as it can use.
 
For a little perspective of how long this debate has been going on, Lindberg flew lean of peak AND over-square. When he went into the Pacific in WWII he demonstrated the techniques in the P-38 and almost doubled the range. Later (1943) this increased range was used to fly a low level intercept of Japanese Admiral Yamamoto's airplane. One way the intercept was over 400 miles, beyond the range of other US fighters.

Later, the airlines used lean of peak and oversquare to increase the life of their big radials and reduce fuel burn.
 
tprops4me said:
So then that old rule of "keep the prop on top" is just a myth.
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see my post above.

most people just understand what MP is really reading. at low throttle settings in a non-turbo a/c you have have more pressure in the manifold than when at a high throttle setting.
 
kiloalpha said:
Out company for years and years have told us to run at 2300 rpms and 23" on the MP. Usually trues us out at 170-172kts and burns about 30-32 gph.

Now we are being told to run at 2200 rpms and 24" of MP. We are told this has no negative effect on the engine as is better for the props since they work more efficiently now. Is this true? Are we going pop cylinders and crack cases now? or is this actually going to work?

We have Lycoming IO-540s that make 290hp. Company research shows a fuel savings of 10% with a 1-2% drop in airspeed.
Click to expand...

All sounds good to me.

Read this book
 
This discussion always cracks me up a little. The numbers are purely arbitrary based on which units of measurement are chosen. It's just a coincidence that pressure (inches of water column) has the same numbers as the first two digits of the rotation speed (revolutions per minute). I have a hard time imagining that the same rule of thumb would have come into being if we were talking about 43 mm Hg and 38 Hertz. Or 57 mbar and 241 radians per second.

They're just numbers.
 
SteveC said:
I have a hard time imagining that the same rule of thumb would have come into being if we were talking about 43 mm Hg and 38 Hertz. Or 57 mbar and 241 radians per second.

They're just numbers.
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Keggers. If we're talking hypothetically about what to call the arbitrary units of measurement we're going to use, I want one of them to be called keggers. Set the MP to 44 ft pounds of diesel dust and set the prop to 720 keggers. Unless its a three blade prop, in which case you'd set it to 1122 quarter keggers. :rawk::rawk::D

Kegger silliness aside, I agree completely with your statement. The units are arbitrary. I think the reason the no oversquare myth continues to exist is because its really easy to teach and its pretty much always safe in terms of being harmful to the engine.
 
Most planes I have flown the POH's show many over squaring settings. Prop on top is a myth as others are saying. It reminds me of the lean of peak scare everyone has. Same type of deal.
 
SteveC said:
This discussion always cracks me up a little. The numbers are purely arbitrary based on which units of measurement are chosen. It's just a coincidence that pressure (inches of water column) has the same numbers as the first two digits of the rotation speed (revolutions per minute). I have a hard time imagining that the same rule of thumb would have come into being if we were talking about 43 mm Hg and 38 Hertz. Or 57 mbar and 241 radians per second.

They're just numbers.
Click to expand...

How about the WWII German and Brit fighters in 'atmospheres'?
 
BuickCFI said:
see my post above.

most people just understand what MP is really reading. at low throttle settings in a non-turbo a/c you have have more pressure in the manifold than when at a high throttle setting.
Click to expand...

untrue statement, at low power settings you have less pressure and at full throttle you have ambient pressure (whatever the pressure is at the altitude you are at)
 
spbrian said:
untrue statement, at low power settings you have less pressure and at full throttle you have ambient pressure (whatever the pressure is at the altitude you are at)
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more vacuum pressure, i didn't put that in there. the engine is working harder to get air so the vacuum goes up. at full throttle it is ambient because there is no vacuum pressure
 
jynxyjoe said:
All sounds good to me.

Read this book
Click to expand...

Inverted said:
Most planes I have flown the POH's show many over squaring settings. Prop on top is a myth as others are saying. It reminds me of the lean of peak scare everyone has. Same type of deal.
Click to expand...

Thanks everyone for your input. I'll look into that book.

It's in the POH as okay to go over so I'll believe anything printed in that. As far as lean of peak, I do that on occasion, but I've seen people with GamiInjectors on their BE35 lean it way out and blow out rings all the time. I always try to lean to the point of engine roughness then back rich about "a smidge." We don't have EGT (I don't understand why) but we have a CHT and I try to keep it around 150-180 degrees.
 
BuickCFI said:
more vacuum pressure, i didn't put that in there. the engine is working harder to get air so the vacuum goes up. at full throttle it is ambient because there is no vacuum pressure
Click to expand...

You need to stick to either referring to it as pressure or vacuum. Personally, I find it easier just to refer to everything in terms of pressure and pressure differential when talking about engine load. What we are referring to as vacuum is really a difference between atmospheric pressure and the lower pressure in the manifold. In this way, it also makes it easier to understand what is really going on with a turbocharger/supercharger. Both are merely devices that either work to keep manifold pressure equal to what it would approximately be at sea level or to provide pressure higher than ambient. Generally, most aircraft turbochargers fall into the former category.

But there is no "vacuum pressure."
 
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