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FDX8891

Well-Known Member
Alright, I know this question has been asked before, but I can't seem to find a concrete or consistent answer (maybe because there isn't one):

Why is it taught by some that a Higher Manifold Pressure than RPM is "bad" for an engine? I've heard several instructors preach it, but it doesn't make any sense to me; there are plenty of published power settings in airplanes that are over-square.

Any input or links to resources would be greatly appreciated.

Thanks!
 
As far as I can determine, merely higher MP than RPM has never been a problem even with the old radials. DC3's, for instance have MP's in the mid to high 30's and RPM at 2500 for take off and even lower RPM on approach to landing. Some warbirds will pull MP's in the 40's with 26-2700 RPM. From what I gather, the oversquare thing is more of a problem with moderate to high MP and really low RPM, like in the 12-1500 range

On the other hand, I could be totally wrong and running 2499 RPM with 25" MP will cause you to instantly explode.
 
Mike H said:
On the other hand, I could be totally wrong and running 2499 RPM with 25" MP will cause you to instantly explode.
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Seems like the more likely scenario IMO.

Anyways, sarcasm aside, the next time an instructor tells you that ask them to site their source. I suspect they won't find a technical writing that agrees with this old wives tail. It can be bad, with extreme differences, but you can even find out what those differences are for the engine you operate. There is a chart right in every engine operating manual I've ever read that tells you what difference between RPM and MP is acceptable for continuous operations.

It is called the Sea Level and Altitude Pressure chart. Remember, it is located in the aircraft engine manual, not the aircrafts POH.
 
They are two different units of measure. 23" in no way equates to 2300 RPM. Your engine has no clue that both of those values start with a 2 and a 3.

Buuuuut, there is a such a thing as excessive manifold pressure which would lead to detonation.
 
Pick your poison.
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Look at your engine manufacture's operating procedures.... Next... Are there any propeller limitations (other than redline)? Generally, you want to avoid HIGH MAP with LOW RPM... Most square Lycomings have a limitation not to exceed 23" below 2300 RPM. Nothing wrong with running full throttle (within MAP limitations) at 2500 (naturally aspirated). Lycoming actually recommends it.
 
I was always told that high MP with low RPM will stress an engine with a lower HP rating, due to the engine having to work harder to compensate for the flatter prop pitch. The comparison I got was that it was like trying to drive a car at 10MPH on 4th gear. Too much torque and it will wear the engine out faster.

I just teach the proper method to increase/decrease MP/RPM and tell my students to follow the POH settings for all phases of flight. If they want to screw around, they can do so in their airplane, and under their insurance.
 
Somebody likely taught this as a rule of thumb in another time, and it stuck around for who knows why.

Read the POH. Fly that. Can't go wrong doing what the engineers told you to do.
 
Flying High Performance Singles and Twins said:
As MP rises, so does BMEP. And when BMEP goes up, so does the pressures and temperatures in the cylinder-and this will push the engine closer to its detonation limits. But when the manufacturer set up the RPM/MP envelope, detonation was definitely being considered, and you can safely operate anywhere inside the envelope, including oversquare settings.
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Pretty good explanation as to why there are limits of flying oversquare.

BMEP = Brake Mean Effective Pressure

Definition: BMEP is the theoretically determined constant pressure that could be exerted in the combustion chamber during each power stroke to produce the actual Brake Horse Power. (definition from the book quoted above)


Douglas recommended this book to me and I also highly recommend the book for CFIs or anyone else that wants to learn more about piston engines. You don't even need to be flying high performance aircraft for the book to be a great tool.
 
The "over square" rule of thumb (and that's all it is -- a ROT, which means it doesn't technically apply to all engines under all circumstances) comes from a time when big pistons ruled. With big pistons swinging big props, the inertia of the prop was as important a factor in the health/life of the engine as the inertia of all the pistons moving inside the engine.

Read up on "reciprocating loads" from Randy Sohn to get some background understanding of what drove the MP = RPM rule of thumb.

http://www.douglasdc3.com/sohn/3.htm

The argument can be made that these same forces are at work inside smaller pistons swinging smaller props, but as has been stated, manufacturer's operating specifications always trump ROTs.
 
Firebird2XC said:
Somebody likely taught this as a rule of thumb in another time, and it stuck around for who knows why.

Read the POH. Fly that. Can't go wrong doing what the engineers told you to do.
Click to expand...

I am an engineer... We don't write rules like that, we test and make tables & charts. We only put rules like that in the book when the lawyers tell us to.
 
So the flight school doesn't have to worry about students when they go on solo's. It is much easier to give somebody a rule of thumb than it is to go into the POH and go "ok, here is when you can and cant i before e except after c divide by 7 carry the 3 etc. etc."

Are you getting all this hyperbole? :cool:
 
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