LOP Speed Loss

[Agflyboy]

Hi all, I was discussing with a POA member the other day what speed loss we could expect in out Turbo Comanche's when running lop. It depends on how far lean on the lean side of peak you run of course. I snapped a couple pictures of my engine monitor playing around rop and lop at a couple different power settings. Iirc the lower power setting the engine was 100 degrees rop and 20 lop. At the slightly higher power settings the engine was 120 degrees rop and 25 lop. All lop & rop were set off the tit readings since the turbo was engaged.

I don't hold myself out as an expert on LOP operations or are the pictures an example of a scientific test, just a couple in flight quick comparisons.

 

[John Collins]

One thing to consider, slower speed means lower power setting. LOP power settings are linearly related to fuel flow. Lower fuel flow, lower power. If you maintain the same power setting, there should be no loss of speed. ROP power settings are based on the air flow into the engine whereas LOP power settings are based on fuel flow. A given ROP power setting for MP and RPM at a specified altitude and temperature will always be greater HP than the LOP power setting if these parameters are left unchanged. IOW, if ROP is 65%, LOP may be 60% if only mixture is changed. To obtain the same power LOP, you have to increase the MP and or RPM so that you can obtain the 65% fuel flow while still remaining LOP. With a turbocharged engine, you can add MP (within the limits of the engine) to restore the fuel flow required by a given power setting LOP. In this example, if you add MP to permit a higher fuel flow (you may have to adjust both MP and mixture) that is required for LOP 65% power, you won't have any speed loss when comparing operation at the same power ROP of 65%.

 

[Agflyboy]

John makes a good point on the power output not being the same and one could add back power in MP and or RPM lop to make up for the reduced power output. Although looking at the (3rd,4th) pictures with the higher power settings I "lost" 7 knots, but burned 4.3 gph less and had 20 degree cooler temp on my cht. Not a bad trade imho.

Kevin

 

[gismo]

John makes a good point on the power output not being the same and one could add back power in MP and or RPM lop to make up for the reduced power output. Although looking at the (3rd,4th) pictures with the higher power settings I "lost" 7 knots, but burned 4.3 gph less and had 20 degree cooler temp on my cht. Not a bad trade imho.

Kevin

That's not a "fair" trade. If you simply reduced power enough to drop the same 7 Kt while leaving the mixture ROP you're fuel consumption would likely drop about half of that 4.3 gph. That said the remaining 2+gph saving due solely to LOP operation is nothing to sneeze at.