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-   -   LOP and HP (https://vansairforce.net/community/showthread.php?t=179075)

macrafic 01-27-2020 07:29 PM

LOP and HP
 
As I understand it, LOP operation is thought to be acceptable as long as engine HP is below 75%. Please, don?t want to debate this here).

Is that 75% power BEFORE the engine is leaned LOP? Or after.

I can start my LOP process with the engine at X % power and, by the time I am done, the power level is much less than X%. I generally do not run my engine above 75% power so this has not been an issue. On occasion, however?

Cannon 01-27-2020 07:43 PM

I think the number is 65%, generally speaking, and it?s after leaning. Lots of people (myself included) will do the ?Big Pull? and quickly lean the engine down to a fuel flow that is well below 65%.

BobTurner 01-27-2020 07:47 PM

This is my imperfect understanding:
At "high" power settings (when rich of peak), there is a "danger" area near 50 deg F rich of peak, where CHT's can get out of hand, and/or detonation risks exist. Operators who choose to operate LOP at High powers talk about "the big pull" - where they rapidly move the mixture from full rich, thru the danger area, to well lean of peak. The lower the power (rich of peak) you start at, the smaller the danger area, until at sufficiently low powers the danger area isn't dangerous any more at all. Some take 75% (best power mixture) as the break-point: above that, mixtures near 50 rich to peak must be avoided except for momentarily, while below 75% you can take your time. Others take a more conservative view and start at 70%, or even 65%. Clearly, the higher the power actually is, the more careful (all cylinders peaking at the same setting, staying away from the peak get area) you need to be.

N941WR 01-27-2020 08:17 PM

I only run LoP above 6,000 DA and set the power to 65%, according to the Dynon. Then I start leaning.

I end up around 55% power, +/-, depending on how aggressively I lean.

Last year I was playing with extending my range and here are two data points from my test flight. Remember, I have a carb under my O-360 and two P-mags firing auto plugs gaped to .032.

50% 150 kts @ 6.1 gph = 24.6 KMPG LoP
40? 135 kts @ 4.8 gph = 28.1 KMPG LoP

JDeanda 01-27-2020 11:27 PM

Mike Busch's Book
 
"Mike Busch on Engines", recently published has a lot of info on how to lean, among other things regarding Lycoming and Continental engines. I'm about halfway thru the book. He bills the book as being for owners, but there's a ton of detailed info to be had. Maybe more than most owners would be willing to read, but it's sure interesting to this A&P, and I learned a lot. $30 well spent.

rzbill 01-28-2020 04:32 AM

You will not fall out of the air by leaning at a power level 1% over someone's rule of thumb power level.
Engines differ in design and a single rule of thumb does not cover all engines correctly.
A stock (like my M1B) parallel valve 8.5 comp 360 is nearly impossible to detonate. The same cannot be said of angle valves or higher compression.
Use CHT as a guide. If they are low, you have plenty of buffer. You may choose to do the big mixture pull and then find peak from the LOP side. That will avoid the problem area Bob described earlier. I do not do it this way simply because I do not have to. My CHTs are low.
So....I run LOP over the whole power range available to me although operation above 80% is rare just because the manifold pressure is not available to use due to altitude.

Mark Dickens 01-28-2020 05:23 AM

This article by George Braly at GAMI does a good job of explaining the dynamics of running LOP: https://gami.com/articles/bttfpart1.php. I personally only run ROP at takeoff and landing. LOP the rest of the time, of course, I have an injected engine with a 0.2 GAMI spread which makes that easy.

Dave12 01-28-2020 06:15 AM

Read any articles you can find written by Walt Atkinson, George Braly or John Deakins . These guys are the gurus of lop operations. They have done years of research on the subject. There are many good articles on AvWeb. I lean as soon as the engine is running, enriched a little for the run-up, full rich for takeoff. When I go to 25/25 for climb, I’ll start leaning. You have to have the means of closely monitoring each cylinder and a well balanced injection system to get full benefits from lop operations. When I level out at altitude, I’ll do the big pull back to 20-30 lop. The bottom line, watch those cht’s. They will tell the story. I’m running dual pmags and Precision Silverhawk Injection and this combination is very efficient for lop operations. It saves a lot of $.

svyolo 01-28-2020 06:42 AM

Mike Busch also has a bunch of very long youtube videos. Several involve LOP. His claim, that makes sense to me, is that when LOP you can calculate hp by just multiplying the FF by a fixed multiplier. The multiplier changes a little because of compression ratio, which makes sense to me.

For "normal" NA Lycoming (8.5-1) he uses 14.9 X FF (in lbs per hour) equals hp. If running lower or higher compression pistons it would change the same percentage as the C.R., at least theoretically.

There is a very simple formula that calculates HP for an engine, and it varies linearly with C.R.

rv6ejguy 01-28-2020 07:36 AM

Quote:

Originally Posted by svyolo (Post 1403731)

There is a very simple formula that calculates HP for an engine, and it varies linearly with C.R.

Hp doesn't vary linearly with CR so if that's what these guys claim, they are incorrect.


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