Lancair IVP Pilots:
We have been recently reviewing LOP cruise numbers for the benefit of sharing with outher LIVP drivers. Here's some numbers that have come out of the discussion:
LOP Cruise: 28" MAP 2300 RPM 14.0 GPH
LOP Cruise: 31" MAP 2400 RPM 15.1 GPH
LOP Cruise: 34" MAP 2500 RPM 18.0 GPH
One individual offered: "Climb - full power - full rich" (I assume this is 38.5" MAP, 2700 RPM, 43 GPH ROP, 350 HP), then "Level off at FL22, FL23 - full power - full rich" then "After this LOP at FL22, FL23 at 28 inHG, 16.1 gph" which sounded interesting with outstanding speed numbers.
I have some questions for the group of more (than me) informed pilots.
1. Pressurization: When climbing at full power (38.5" MAP) or even 36" or 34" or (the original recommended 31" MAP), upon leveling off at the flight levels, if you reduce your MAP, you will lower your cabin pressurization, perhaps somewhat significantly. If I were at 38.5" MAP (or even 34" MAP) during full power climb to the flight levels, I would have 5.0+ psi cabin differential. If I quickly reduced MAP to 31", and particularly to 28" MAP, the cabin altitude would quickly climb to >14,000' and the Duke's regulator would take several minutes to equilibrate. And even then, it might not be able to restore 5.0 psi differential (at 28" MAP). So, all this talk of lower MAP
2. LOP Fuel Flow equals HP Output: We have previously recognized that LOP becomes a simple conversion of GPH to HP output. In a Nov 2010 LML post, we learned "Typically, the 8.5:1 compression ration engines use 14.9 hp/gph while the 7.5:1 CR engines use 13.7 hp/gph." In my TSIO-550E, the ratio seems to be 14.6 to 14.75 HP/gph. Once LOP, the MAP is not important to the calculation (only to control detonation). More MAP produces more compressive heat (both induction temperature, and cabin inlet temperature) which alters performance and where one is on the LOP side of the curve, but not the HP output, as every bit of fuel vapor is being consumed by an excess of available oxygen.
3. Airspeed: My recent exploration of these cruise numbers(above) revealed the following indicated airspeed at 8200 MSL, OAT 80*F:
LOP Cruise: 28" MAP 2300 RPM 14.0 GPH....173 KIAS (206 HP, 59%)
LOP Cruise: 31" MAP 2400 RPM 15.1 GPH....177 KIAS (222 HP, 63%)
LOP Cruise: 34" MAP 2500 RPM 18.0 GPH....187 KIAS (265 HP, 76%)
Obviously, the more fuel you provide LOP, the faster you go. An example of this is 31" MAP, 2400 RPM, 15.6 GPH (229 HP, 65%) gave me 183 KIAS (3.3% more fuel, 3.3% faster). Note that these fuel flow (power settings above) are VERY lean of peak (LOP), some 150-180*F LOP, and the engine is not that happy. An extra tenth or two of a gallon lower (mixture), or a change in OAT as you transition across a front, will sometimes make an engine cylinder cough...upsetting the delicate spouse sitting next to you.
4. Adjust GPH, not MAP: If when in climb or upon reaching cruise, if I move quickly to LOP (with the big mixture pull), I can control HP output at a fixed prop speed and MAP by adjusting fuel flow. If I climb at 34" MAP 2500 RPM 20 GPH (294 HP, 84%) and then cruise at 34" 2500 RPM 18.0 GPH (265 HP, 76%), I do not experience any cabin pressurization issues. If I roll back the RPM to 34" MAP 2400 RPM and push back up the mixture to 18.0 GPH, I am less lean (the engine is happier), I have less internal friction (from the lower RPM), and my HP output remains the same, and cabin pressure is static.
So I don't understand the allure of big reductions(or any reduction) in MAP after establishing a satisfactory climb configuation...please explain the benefits.
5. Economy Mode vs Fast Mode: And regarding the economy mode of LOP at different power settings, we see that 14.0 GPH yields 173 KIAS and 18.0 GPH yields 187 KIAS. (I will let your TAS be whatever based on altitude you chose, the same for both fuel flows.) If I have 110 gals in my tanks, it seems that (in principle) that 14.0 GPH (7.9 hrs at 173 kts) gets me 1360nm downrange. If I use 18 GPH (6.1 hrs at 187 kts), I can go 1143 nm (just using simple calculations); your mileage my vary (YMMV). 19% further, 30% longer flight time, no potty break). While this economy mode vs get there fast mode is important, it seems like the missions and the fuel breaks usually come every 4.5 hours. With 110 gals, 19 gph seems a good blend of all criteria, typically getting me 1100nm on a single tank (noting climb to flight levels and TAS>>IAS).
This is a combination of shared experience and inquiry. Perhaps some piltots would like to contribute.
I have taken the Engine course, do the BMP (big mixture pull) shortly after departure (during climb), and typically keep my engine at 34" MAP 2500 RPM 19 GPH throughout the entire flight. Opinions welcomed.
Jeff L
LIVP in New Jersey