LOP Power Settings Epiphanies

[Bob - S50]

Just had two epiphanies about power settings for LOP.  Haven't tried either yet but I will on my next flight.

 

#1

1.  Power output is determined by fuel flow when LOP.

 

2.  Approximate fuel flows for a 200 HP engine are:

 

10 GPH = 75%

9.3 GPH = 70%

8.7 GPH = 65%

8.1 GPH = 60%

 

3.  To set my desired power setting I can level at cruise altitude (I'm assuming something reasonable like 6000+) I can set an initial RPM with WOT then lean to LOP.  I personally like all cylinders to be in the 10 to 30 LOP range.  I can then check the fuel flow.  If it is higher or lower than my desired power fuel flow, I can adjust the RPM to get what I want, then fiddle with the mixture to ensure I'm still LOP.  Adjusting RPM should have minimal impact on mixture.

 

For example, let's say I want 65% power or about 8.7 GPH.  I level off, leave the throttle wide open, set 2400 RPM, and lean to LOP.  I then note that my fuel flow is only 8.3 GPH.  I could increase my RPM to about 2500 and I should see the desired 8.7 GPH.  I would then fiddle with the mixture to see if all the EGT's increase if I richen or decrease if I lean.  If they do, I'm still LOP and all set for 65% power.  Using the Normalize EGT mode on a JPI should make it easier to check.

 

#2

If I have my power set to the desired level but my CHT's are a bit higher than I like; further leaning with an increased RPM that results in the same fuel flow should give me the same power output (but I'll lose a little to friction) but with lower CHT's.  That's because higher RPM puts the peak pressure further past TDC.  Further leaning has the same affect.  Therefore I'm doing two things that put my peak pressure further after TDC.  That should decrease peak pressure and lower my CHT's.

 

That is, if 20 LOP and 2400 RPM is too warm, I might be able to get the same fuel flow and lower CHT's at 30 or 40 LOP with 2500 RPM.

 

Agree?  Disagree?

 

Bob

[pinerunner]

This makes pretty good sense to me based on the 14.9 HP/GPH rule when LOP. I mentioned that on another thread and saw one reply that the rule didn't always hold.  I think if you go too far LOP it starts to falter. After all if you you go far enough LOP the engine actually quits. I've heard that LOP slows the flame front propagation (but I can't back that up personally) so that should move the point of highest pressure forward and lower the CHT's as you say. 

 

At 60% power you can get away with almost anything (they say) so why not do the experiment on your plane and report back to us. Drop it to 50% if you like for even more security. I love it when people come on with real data.

[RobertE]

That makes sense, but one question.  What is the purpose of producing a particular % of HP?  Unless I'm just killing time I tend to go as fast as I can via WOT, then lean as needed based on HP%.  We all know the "red box" regime.

 

Do you happen to always want to fly at a given, low, HP% to lessen the burden on your engine?

[Jeff_S]

This makes pretty good sense to me based on the 14.9 HP/GPH rule when LOP. I mentioned that on another thread and saw one reply that the rule didn't always hold.  

 

Does this metric still hold true when talking about a Continental big-bore such as the IO-550, or is it specific to the IO-360 in the J? As I inch closer to my move into an Ovation I'm boning up on the metrics I need to use as rule-of-thumb for that engine. I have them down pat in the J so just trying to quickly get to the same level of comfort with the bigger engine.

[podair]

works nicely on a 200hp Mooney with fuel flow in litres: double the flow = % HP

ah the metric system... royal cheese :-)

[Bob - S50]

That makes sense, but one question.  What is the purpose of producing a particular % of HP?  Unless I'm just killing time I tend to go as fast as I can via WOT, then lean as needed based on HP%.  We all know the "red box" regime.

 

Do you happen to always want to fly at a given, low, HP% to lessen the burden on your engine?

I like to fly at a compromise of speed and economy.  I find something in the neighborhood of 65% gives that to me.  If it turns out to be 63% or 68% that's fine, but something close to 65% is my goal.  That tends to give me about 155 knots and about 16 NM/Gallon.  We can obviously fly faster, or you can fly slower and get better gas mileage, but as someone once said, "I didn't buy a fast airplane to fly slowly."   I just posted the other numbers in case someone likes to fly faster or more cheaply.

 

Bob

[carusoam]

The red box theory will apply to the IO550 as well.

The same 65% hp is the level to be aware of...

The O provides a blue box (EGT) for ROP climbs... White box for the G1000 crowd... (100- 200dF range?)

%HP available is directly related to altitude being flown...

POH provides adequate, but not ideal, information. The power chart can be saved to the iPad for quick reference...

Go IO550,

-a-