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From: W Atkinson <Walter@advancedpilot.com>
Subject: Re: [LML] Re: airspeed vs. power

Scott:

**In general, @6500 MSL, @WOT the first 100 RPM reduction gave me about=20=

a 7.5% increase in FF, a 1% reduction in TAS and, interestingly, a 90F=20=

drop in EGT.=A0**

Certainly you meant a drop in FF, not an increase?   The drop in EGT=20
has little to nothing to do with any mixture change.  As the rpm drops,=20=

the pulses of hot gas decrease in number.  This increases the=20
refractory time that the probe sees and it reports a lower average=20
temperature of time.  The pulses of hot gas can be the same=20
temperature, while the reading drops AT THE SAME MIXTURE.

**The next 100 RPM drop (to 2300) had the=A0same FF as 25x2500, with a =
3%=20
reduction in TAS (from the 25x25 speed) and, unfortunately, I did not=20
record the EGT.=A0 Looks like the fuel flow went rich (I did not touch=20=

the mixture)?**

I doubt the mixture changed much at all.  The EGT reading changed.  See=20=

above.
=A0
**Back at 25x2500, I dropped the MAP 1" and the FF dropped about 8.5%,=20=

TAS about .5% and there was a 50F rise in EGT.=A0 The next MAP reduction=20=

of 1" (to 23") had a FF drop of 15% (from 25x2500), TAS drop of 3% and=20=

a 40F increase in EGT.=A0 Looks like the FF went lean?**

I doubt it.  As the MP drops, the EGT changes even if the mixture stays=20=

the same for two reasons. 1) decreased temp of combustion, and 2) the=20
movement of thetaPP away from TDC.  The latter has the greatest effect.
=A0
**If I were to repeat this experiment, I would re-lean each power=20
change for say, best power at 100F ROP.**

That would be an educational exercise.  I think you will find that the=20=

mixture isn't changing very much at all with rpm changes and very=20
little with MP changes ONCE you are out of the enrichment feature range=20=

for MP.

**At the throttle reduction from WOT, maybe I should back the throttle=20=

down until the MAP just begins to drop, then slowly put the MAP back at=20=

the WOT reading without the throttle being firewalled (thus,=20
eliminating auto rich), then lean for best power.=A0 Maybe I should use=20=

this as the WOTx2500 base line.=A0 Does this make sense?=A0 Any=20
suggestions?**

You can do that, or you could re-lean between setting changes.
=A0
Walter=