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RV-10 LOP vs. ROP Flight Testing

rv6ejguy

Well Known Member
I posted a video a few weeks ago exploring this a bit.

Last week we went up again after a customer posted an observation that fuel flows would be close to the same at the same power setting ROP. His theory was that the lower fuel burns were primarily the result of the lower power LOP.

We set out to examine that theory.

The procedure was to go LOP at WOT, observing % power on the Garmin, then go ROP to the same % power by throttling the engine slightly. Did the tests at 10,500 and 12,500 feet as before.

You can see the results here: https://youtu.be/lWxDt2Sd9uA

On another note of interest, we compared the barometric pressure from the SDS ECU with what the Garmin was seeing for MAP when WOT. This installation uses the Show Planes induction system and we were seeing about 1 more inch MAP than what the barometric pressure was at altitude. You can see this in the video as well.

This engine has the SDS EM-5 EFI/EI on it with our 80mm billet throttle body. At anything over about 2/3rds throttle running 2450 rpm, there is no change in MAP, so this offers no measurable restriction to the induction air.

Thanks again to Les Kearney for getting me out of the shop for a while. We had a beautiful glass smooth day again for running these tests.

Unfortunately the wideband O2 sensor wasn't working again. We hoped to gather some data on EGT vs. AFR on this flight. Maybe next time.
 
Oh man, Les, you're setting the bar high! I'm still breaking the engine in but woof, I'm seeing 20 gph at 155 TAS at 3500! I know that will drop once I pull the power back and get up higher after break in. I'm excited to see your numbers, that's great motivation!

Ross, great job on the SDS stuff! I'm really enjoying getting mine all tuned up.
 
Good information. Definitely substantiates LOP operation. Also good to know about the Showplanes cowl intake performance.
 
I figured the % power would be an issue. How these computers calculate (estimate) % power needs to be know to understand if you can really use % power as a plotting variable in this type of flight test. Most likely they use fuel flow in their calculation of % power. At least that is what I deduced based on my flight testing.

Also, what spark advance did you use when you when you went LOP? I am guessing the results would be different if you don't have the ability to advance the spark?

This is cool stuff.
 
I believe Les is running about 28-30 degrees BTDC when LOP. He's still tweaking and experimenting with some settings.

Next time I think we'll either set the same FF and look at TAS or same TAS and look at FF, ROP vs. LOP.
 
Oh man, Les, you're setting the bar high! I'm still breaking the engine in but woof, I'm seeing 20 gph at 155 TAS at 3500! I know that will drop once I pull the power back and get up higher after break in. I'm excited to see your numbers, that's great motivation!

Ross, great job on the SDS stuff! I'm really enjoying getting mine all tuned up.

I know waiting for it to break in can be a bit tedious but is will be worth it. Les' RV-10 is awesome.
 
FF vs TAS

I believe Les is running about 28-30 degrees BTDC when LOP. He's still tweaking and experimenting with some settings.

Next time I think we'll either set the same FF and look at TAS or same TAS and look at FF, ROP vs. LOP.

Hello Ross,
I have used your SDS system for nearly 400 hours. I have documented more than half of those hours both ROP and LOP. Changing the ignition advance will change the displayed delta from peak making clear comparisons difficult.

I came to the conclusion the only really important data was TAS as a function of fuel flow (FF) either ROP or LOP. The advantage to this approach is I have a chart with TAS as a function of Ignition Advance, Density Altitude (DA) and FF. I programmed Curve 2 for a specific Advance (Baseline +5), DA (9000’) and FF. I get to altitude, flip to Curve 2 and fly on. If for some reason, I am not using 9000DA I can adjust FF with the rheostat.

I read with great interest any RV14/14A data provided by posters on VAF. A true comparison is very difficult because I seldom know the quality of the calibration; however, it appears EI saves ~0.8 to 1gph and an angle valve engine requires less advance than a parallel valve.

Marvin
 
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Though I've doubted the accuracy of EFIS' displayed % power years before we did these flights, I wanted to do some flight testing to see for myself.

After these flights, I agree with you and said as much in an earlier thread a few years back which just considered FF vs. TAS. This is really what we are interested in at the end of the day.

How we get there with the best compromise in AFRs vs. ignition timing is the interesting part and you've probably done more documented testing than anyone on VAF on AV engines.

I hope to do some more flights with Les to learn some more.
 
I think the most interesting is just FF vs TAS for LOP (fuel and ignition map adjusted) against ROP. What ROP point? Assuming everyone leans their engines in cruise, some value ROP that represents where most folks with a carb would start running rough, and richen up a bit. 80 deg ROP?
 
The flight was really an exercise to learn what the FF differences would be at similar power settings ROP vs. LOP. One of my engineer customers posed the question on YT from the previous video.

Actually, many folks haven't embraced the LOP world, and even some of the big engine builders are not behind the practice, believing it shortens engine life.
 
Actually, many folks haven't embraced the LOP world, and even some of the big engine builders are not behind the practice, believing it shortens engine life.

Hoping to not drift this thread into a LOP debate, I just wonder why individuals and magazine article writers think they know more than the engine manufacturers who have decades of test and operational data and can be financial responsible if there are issues with their products?

Is there a performance improvement? Potentially. is there a cost to that performance improvement in engine life or maintenance? Potentially.

This thread is interesting to me because it is touches on a new variable that has a definite ability to change the answer, namely adjusting the timing for LOP operation. It definitely has potential for performance improvement but long term effect on engine life will require a long time of collecting data and concerted effort of studying the effects, which won’t happen until engine companies get involved with electronic ignition and variable timing. Until then it just us experimenters making claims, but pushing on the status quo.
 
Hoping to not drift this thread into a LOP debate, I just wonder why individuals and magazine article writers think they know more than the engine manufacturers who have decades of test and operational data and can be financial responsible if there are issues with their products?...

But there are decades of data available on aircraft piston engines operating LOP. Since WW-II and through the piston engine airliner era LOP was SOP on radials and V types alike. Further, Lycoming has specifically endorsed Peak EGT for decades. By definition, LOP ops is lower temps and lower pressure and stress than Peak or ROP, so it's a bit more than a wild shot in the dark for those of us who do it.

Sure, not all the manufacturers endorse it, but considering it takes some skill and attention to perform properly, can you blame them? The piston airline crews had strict procedures and BMEP charts to operate LOP - with the state of primary pilot training, frankly it's a miracle that GA pilots are allowed to touch the mixture knob at all.
 
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I was referring mainly to 3rd party engine builders.

I think there is tremendous weight of evidence both from the R3350 engines used in airline service in the '50s and '60s and in the last decade or so, on flat engines running LOP. Engines last just fine IF power setting, timing, CHT/EGTs are within limits. A modern engine monitor goes a long way in ensuring this last factor. Many old timers still believe you'll damage the engine running LOP no matter what you do and that could well be the case without proper instrumentation as we have available these days.

With regards to the variable ignition timing with LOP operation, our recommendation is to only advance ignition timing enough to restore the point of peak cylinder pressure to where it would be ROP with typical fixed timing. This puts no more stress on the engine and the EGTs and CHTs are also lower LOP. Cleaner plugs, cleaner oil, cleaner chambers are another benefit along with saving some money on fuel.

In the end, the choice is yours.
 
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