On May 1, 7:18 am, Dylan Smith wrote:
On 2007-04-30, Brian wrote:

It makes sense to me to charge by Hobbs on constant speed props
because charging by Tach encourages running High MFP and Low RPM. Low
RPM is ok but need to keep the MFP in line with it as well.

That's an old wives' tale, I'm afraid. The best regime to operate an
engine (most efficient and least maintenance) is the lowest RPM and
highest MP for the desired power setting. There are some caveats (some
aircraft have a range of RPMs which you should not continuously operate
the engine), but generally speaking, using the lowest RPM possible for
the desired power setting results in less noise, less vibration and less
wear on the engine.

The thing about operating engines (specifically normally aspirated flat
engines like most of us use) 'above square' is hogwash, I'm afraid.


Agreed, But how do you make a simple rule of thumb for this for pilots
with less then 100 hours or like my latest student that is starting
his primary instruction an an airplane with a constant speed prop.

The problem with your statement is that with new, or low time pilots,
as is often typical in clubs, that are going to take it literally and
is you will end up with a few pilots running around WOT and 1500 RPM.
I doubt that is what you ment and the cyinder pressures have got be
quite high in this configuration, especially if they don't lean
properly either.

The "above square rule" is a great simplifcation of how you "can"
operate constant speed props. It certainly is not a hard and fast rule
but you have to understand what you doing operate otherwise safetly.
For a pilot that barely knows how to operate the throttle and mixture
properly they probably ought to just stick with the "over square rule"

Brian
CFIIG/ASEL