2200 or 2400 cruise

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perossichi
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Re: 2200 or 2400 cruise

Post by perossichi »

Chris is absolutely right about the advanced pilot course. I learned so much from it.

Around 8000 ft, you get the best cruise performance from the DA40. At that altitude, the famous red box is very narrow or non existent, so it doesn’t stress the engine to run low rpms even rich of peak.

I always cruise 30-50 degrees lean of peak and wide open throttle. If I want to go fast, I’ll set 2500 rpm, 9.5 gph. If I want endurance, I’ll set 2100-2200 rpm, about 8 gph.
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Re: 2200 or 2400 cruise

Post by astaib »

Thanks all for your comments and advises.
I will make me note and think about how I will make my settings.

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Re: 2200 or 2400 cruise

Post by Roxi5m9 »

perossichi wrote: Thu Dec 12, 2019 9:28 am Chris is absolutely right about the advanced pilot course. I learned so much from it.

Around 8000 ft, you get the best cruise performance from the DA40. At that altitude, the famous red box is very narrow or non existent, so it doesn’t stress the engine to run low rpms even rich of peak.

I always cruise 30-50 degrees lean of peak and wide open throttle. If I want to go fast, I’ll set 2500 rpm, 9.5 gph. If I want endurance, I’ll set 2100-2200 rpm, about 8 gph.
Chris, when you are running LOP that far, is your motor very smooth? Do you have GAMI injectors? I have a 400 SMOH motor, fine wire plugs, mags tweaked to within a degree, and after a GAMI test with the G1000 spreadsheet was told by GAMI my cylinders all peak too closely for their injectors to help me. Yet, there is a vibration to the motor that I don’t like when lean of peak but she runs smoothly at peak, maybe 10 LOP. Any experience or thoughts you have would be great. Would love to run 50 LOP. Maybe I’m too sensitive to the vibrations?
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Rick
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Re: 2200 or 2400 cruise

Post by Rick »

If I am going somewhere, I'm always WOT and 10-20 LOP, and it's very smooth. At 8000' or 9000', 2450 RPM, my CHTs are around 350, 8.2 to 8.4 gph, and I plan for 140 kts (but usually see slightly better). In my DA40, I think 50 degrees LOP would be too much of a power loss.

For you guys who have taken Deakin's class, what is his recommendation as to how far LOP we should run? I thought at 8000' and above, the "red box" went away and you could (theoretically) run AT peak EGT all day without any problem, so even "slightly" LOP would be just fine. By 50 LOP you are losing a LOT of power, aren't you?
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Re: 2200 or 2400 cruise

Post by Roxi5m9 »

Rick wrote: Sun Dec 29, 2019 11:15 pm If I am going somewhere, I'm always WOT and 10-20 LOP, and it's very smooth. At 8000' or 9000', 2450 RPM, my CHTs are around 350, 8.2 to 8.4 gph, and I plan for 140 kts (but usually see slightly better). In my DA40, I think 50 degrees LOP would be too much of a power loss.

For you guys who have taken Deakin's class, what is his recommendation as to how far LOP we should run? I thought at 8000' and above, the "red box" went away and you could (theoretically) run AT peak EGT all day without any problem, so even "slightly" LOP would be just fine. By 50 LOP you are losing a LOT of power, aren't you?
That is true Rick that the box pretty much goes away between 7-8000. You can really run as lean as you want as long as the motor keeps running and is smooth. I had a long conversation with a guy at GAMI who said Lycoming puts in the POH to run at either 100 rich (power) or at Peak (economy) because they would love to say run LOP but they wont factory install GAMIs and they cant guarantee smooth operation LOP with standard injectors, but they can Guarantee smooth operation at peak. For what all that is worth.
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Chris B
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Re: 2200 or 2400 cruise

Post by Chris B »

Roxi5m9 wrote: Sun Dec 29, 2019 8:59 pm Chris, when you are running LOP that far, is your motor very smooth? Do you have GAMI injectors? I have a 400 SMOH motor, fine wire plugs, mags tweaked to within a degree, and after a GAMI test with the G1000 spreadsheet was told by GAMI my cylinders all peak too closely for their injectors to help me. Yet, there is a vibration to the motor that I don’t like when lean of peak but she runs smoothly at peak, maybe 10 LOP. Any experience or thoughts you have would be great. Would love to run 50 LOP. Maybe I’m too sensitive to the vibrations?
Yes, I have GAMI injectors, and spent a lot of time tuning them. John-Paul is *extremely* patient! I recently replaced the engine and now need to re-tune the injectors. :(

The fundamental problem is that each cylinder is functionally an independent 'engine' that is connected to the others by the crankshaft, but shares one mixture control. Changing fuel injector nozzle size helps each cylinder respond similarly to the mixture control, but each cylinder has a unique response curve. Compared to most fuel injected Continental engines, ours use a pressure compensating RSA fuel controller that requires less routine in-flight mixture tweaking with altitude changes, but adds extra response curve complexity.

IME it is only possible to get 'perfect' tuning at one combination of RPM, MP/FF, and altitude. I wasted a lot of time before figuring this out. Every other combination is a compromise, and even the target combination is subject to probability noise (where the larger spark of fine wire plugs helps).

Mine was tuned to <0.1 GPH spread at 2450 RPM & WOT at 3500' density altitude, and extremely smooth LOP at that operating point. By 12,500 DA the spread opened to ~0.6 GPH, and LOP was not smooth. This is due to a complex combination of RPM, intake plumbing & injector nozzle behavior at various air & fuel flows, plus RSA fuel controller pressure compensation impact. IME tweaking RPM is one in-flight "knob" for balancing cylinders that is sometimes helpful. But results presumably vary by engine, so this might not work for you.

My recollection from the APS course is that generally best engine efficiency is ~20 degrees LOP at higher MP, decreasing to ~10 degrees LOP at lower MP. If that is too much fuel flow (e.g., sightseeing), reduce MP. Further LOP typically gets increasingly difficult to run smoothly. OTOH, WOT is also most efficient, so if the engine is smooth at 50+ LOP, that may be comparable efficiency to 20 LOP with a partially closed throttle. :scratch:

IMO the "less than 0.5 GPH GAMI spread" rule of thumb originated with GAMI's primary target market: six cylinder Continental IO-520/550 engines running ~2x the fuel flow of our engines. Four cylinder engines are inherently rougher than six cylinder engines with comparable tuning, and 0.5 GPH is obviously a much larger percentage of our fuel flow. Consider shooting for ~0.2 GPH spread at your target RPM / MP / altitude sweet spot if you are willing to invest the time.

But if the GAMI spread is small (<0.2 GPH) at your operating point, the propeller is well balanced (very important), the ignition system is in good shape, and you are 10-20 LOP, then that is probably as smooth as it is going to get when LOP.

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Re: 2200 or 2400 cruise

Post by Roxi5m9 »

Wow. Awesome response, Chris! Thanks for taking so much time to share all that. It sounds exactly like what I experience so often with my plane regarding the complexities of how well I can get the motor to run LOP at different altitudes/different days/different power/RPM settings. It is frustrating it is so complex, but good to hear its not just a problem with the way I’m operating my plane or the health of the mechanics of my motor. Everything you wrote did bring some clarity, as it seems my motor most consistently runs LOP between 4500 and 5500 msl at 2300 RPMs. But often I cruise higher than that on long cross countries. I often settle for running at peak, which always gets me the smoothest running engine, and cooler CHTS that anything rich of that, a respectable fuel flow and TAS.
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Chris B
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Re: 2200 or 2400 cruise

Post by Chris B »

Roxi5m9 wrote: Fri Jan 03, 2020 3:24 pmThanks for taking so much time to share all that.
No worries, Nick. Glad you found it helpful.
But often I cruise higher than that on long cross countries. I often settle for running at peak, which always gets me the smoothest running engine, and cooler CHTS than anything rich of that, a respectable fuel flow and TAS.
Sounds familiar! :thumbsup:

As you note in an earlier post, above ~8000' any mixture setting is "safe" for normally aspirated engines. Running at ~peak EGT is probably slightly less efficient than 10-20 LOP, but much more likely to be smooth, particularly as the GAMI spread changes.

One subtle advantage of the supercharger is that with tuned injectors it should enable smooth LOP operation at a target RPM/MP sweet spot over a wider altitude range. Though the "safe" altitude for aggressive mixture settings will also increase dramatically.

Chris
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