On 14 Oct 2005 08:00:10 -0700, "cwby-flyer" wrote:

Thomas Borchert wrote:

No, but your table doesn't show MAP/RPM settings. So the MAP/RPM settings at
best economy would have to be higher for the same power output. As an aside,
50 degrees ROP is not a good point to run your engine at.

I definitely agree that 50 ROP is not a good place to run your engine,
much better to keep it around 100 - 125 ROP.

As for your assertation that MAP/RPM setting, I must repectfully
disagree (and if I'm mistaken, please let me know - I'm always looking
to learn more). Your power combinations are determined by your MAP &
RPM at a given density altitude and expressed as a percentage of HP.
When you lean the mixture, the RPM/MAP combination will move (assuming
you're not at sea level on a standard day) and then to place the engine
back on your desired % power, you re-adjust the throttle and Prop
levers so that the MAP/RPM match what is in the POH for your density
altitude.

Mike

Mike,

If I understand you correctly, I don't think I agree with you g.
Air-fuel ratio as set by the mixture control also significantly effects
fuel consumption.

POH tables take into account not only altitude (and the ones I'm familiar
with is PRESSURE altitude, not density altitude) but also whether one is
leaned to best economy or best power mixture.

For example, for the same MP/RPM settings on my Lycoming IO-360, bhp will
be less at best economy vs best power.

On the other hand, IF I keep bhp unchanged by adjusting the MP, then fuel
consumption will decrease at best economy vs best power.

Example: IO360A; 65% power (130 bhp); 2400 rpm
@ best power (125°F ROP): 64 lbs/hr
@ best econ (at Peak EGT): 54.5 lbs/hr

Here's another example for a Continental IO550-G
10,000' pressure altitude; 2400 RPM 65% power
@ best economy (50°F LOP) 21.0" MP 12.3 gal/hr
@ best power (50°F ROP) 19.8" MP 14.0 gal/hr

The airspeeds at the same power settings will be the same.

The settings in the manuals for best power and best economy are defined in
terms of degrees rich or lean of peak EGT.

One difference in addition to fuel burn is that, since you require a lower
MP at best power setting, you can achieve that power at a higher altitude
than with a best economy setting.


Ron (EPM) (N5843Q, Mooney M20E) (CP, ASEL, ASES, IA)