Paul,

 

Ahhhh, See the attached engine performance chart.  HP does have a relationship to the product of RPM and MAP, but note that relationship must change when running LOP vs ROP.  Why?  Because of the variable gearing achieved thru a constant speed prop (by definition).  Because of the untouched prop control, rpm remains "constant" and the pitch bite is reduced (AOA reduced) producing less lift (forward thrust delivered).  Thus, airspeed is reduced.  On a NA engine using ram induction air, the MAP may be slightly reduced when the LOP operation airspeed is also reduced from the loss of thrust HP - even though the throttle position is unchanged.  

 

Maybe someone using a fixed pitch prop can comment on the loss of RPM when going from ROP to LOP operation.

 

I think the flaw in that argument is the GPH per HP for a specific regime (ROP versus LOP) as your specific fuel consumption will change depending on that setting.  Your HP should be directly proportional to RPM and MP at any mixture. I think I'm correct in that last statement regardless of whether the engine is ROP or LOP.  When I ran the TSIO-520NB with GAMI the procedure would be to bump up the MP to keep the desired HP (65%, 70% or whatever you choose).   You have to bring the MP back up as the fuel flow drops off.    So, I would pick your desired power setting for LOP ops (say 65% for talking purposes) and get the MP and RPM for that power setting.   When you go LOP work towards bringing the MP back up to maintain your target power.  That is what I recall for the GAMIs so I would be interested if that might be what was being taught more recently.  As I said in an earlier posting, the only problem with high altitude turbo ops LOP was the need to bump up the MP to keep the wastegate from bootstrapping.  Others may need higher MP for pressurization but I never experienced that in the twin.  Hope that all makes sense.  

Paul Miller

Calgary (Legacy) 

On 2009-10-08, at 6:13 PM, Isaac Heizer wrote:

My ES-P has a TSIO-550E with GAMI injectors, and i've recently been through advancedpilot's web LOP course.  But I'm still unclear on at least one point.

My TSIO-550E will happily run at 17.5 GPH at a MP of any of 30, 31, 32 (among others).  All cylinders remain below 380, and TIT is reasonable.

My understanding is multiplying 14.7 x GPH yields HP, in this case 73.5% power

Is it 73% power no matter how I set the MP, so long as temperatures remain in control?  If that's true, why would I select one MP over another (realizing I can't run wide open throttle)?

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