Leaning/power setting

This seems rather basic, but it’s got me confused. After reaching cruise I throttle back to about 2500 rpm. I go to the lean assist to get the “Best Power” setting. After doing this, it seems like I always end up at 65% power indicated. Should the % power go down like that? I would expect it to stay at the 75%. If I end up at 65% like I describe, what is the problem/effect of adding throttle to get back to 75% once you’ve set mixture?

In reply to:


This seems rather basic, but it’s got me confused. After reaching cruise I throttle back to about 2500 rpm. I go to the lean assist to get the “Best Power” setting. After doing this, it seems like I always end up at 65% power indicated. Should the % power go down like that? I would expect it to stay at the 75%. If I end up at 65% like I describe, what is the problem/effect of adding throttle to get back to 75% once you’ve set mixture?


Dear Mcooper;
I am assuming from your post that you are flying a Cirrus, and yet your post is not on the “Members” side of the forum.

If you will but spring for the $50 membership, you will find well over 100,000 posts, many devoted to variants on your question & perhaps other important points which might save more that your Gaas

Dear mcooper,

There’s a section of the POH dedicated to this sort of thing. You can reference altitude, MAP, RPM, OAT and fuel flow to % power. After getting that stuff under control, you might look into John Deakin’s articles on Avweb titled “Where Should I Run My Engine?” (starting here) for a more in-depth explanation of the mixture control.

Best Regards,
Paul

…that said, everything Dennis says (well, not everything, but at least what’s in the post above [;)]) is very true. The Member’s side is a tremendous resource for all topics Cirrus and then some. [:)]

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…which might save more that your Gaas


Dennis,

Let’s at least proof-read the posts where we try to convince non-members to experience all of the intelligence on the Member’s side. [:D]

Yes. I am familiar with the info in the POH. My question relates to the use of the Lean assist. Unlike other planes, this little tool is useful, but my point was that it seems like I always end up at a lower power setting than originally desired when using it. I’m not looking at the which is better best power or best economy question.

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Dennis,
Let’s at least proof-read the posts


For once I believe Dennis intended the word exactly as he spelled it…

In reply to:


For once I believe Dennis intended the word exactly as he spelled it…


In reply to:


…which might save more that your Gaas


Shouldn’t it read “…which might save more than your Gass”?

Walt

In reply to:


For once I believe Dennis intended the word exactly as he spelled it…


OK, I get it. [:)]

In reply to:


In reply to:
…which might save more that your Gaas
Shouldn’t it read “…which might save more than your Gass”?
Walt


Yes it should. . . but I was trying to be subtle.
And yes, I do indulge the double entrandre periodically.

In reply to:


Shouldn’t it read “…which might save more than your Gass”?


Walt,

Thanks for the save. [:)]

In reply to:


And yes, I do indulge the double entrandre periodically.


Dennis,

Isn’t that illegal in most states? [:D]

In reply to:


… but my point was that it seems like I always end up at a lower power setting than originally desired when using it (lean assist)…


Mr (Ms?) mcooper,

How does the % power shown on the Avidyne compare to the book value for the OAT, MAP, RPM, altitude and fuel flow that you’re seeing?

Paul

ok, lemme go through my example (better) again…

At 8000 ft. I set the pwoer at about 2500 rpm. I then proceed to hit the lean assist and lean back until everything peaks and then add approx 75 degrees ROP per the procedure. Once I get it back to this point 75 degrees ROP I am always at about 65% power. However, I would like to have been at 75% power. In order to get back to 75% power I am thinking I need to add power while leaving the mixture where it is, but I am thinking this will make the mixute too rich thus, I would have to start the lean process over again.

Am I making this too complicated? Should I just be using the chart in the POH to set the figures? If so, what good is the lean assist?

In reply to:


ok, lemme go through my example (better) again…
At 8000 ft. I set the pwoer at about 2500 rpm. I then proceed to hit the lean assist and lean back until everything peaks and then add approx 75 degrees ROP per the procedure. Once I get it back to this point 75 degrees ROP I am always at about 65% power. However, I would like to have been at 75% power. In order to get back to 75% power I am thinking I need to add power while leaving the mixture where it is, but I am thinking this will make the mixute too rich thus, I would have to start the lean process over again.
Am I making this too complicated? Should I just be using the chart in the POH to set the figures? If so, what good is the lean assist?


The MAP can be a lot of different places while the RPM is 2500, so this doesn’t really tell the whole story. This table from the POH shows how different %power can be at the same 2500 RPM:

8000 Feet Pressure Altitude
ISA – 30° C ISA ISA + 30° C
RPM MAP PWR KTAS GPH PWR KTAS GPH PWR KTAS GPH
2700 21.7 83% 180 19.6 78% 183 18.9 75% 186 18.4
2600 21.7 79% 178 19.1 75% 181 18.4 71% 184 17.9
2500 21.7 75% 174 18.4 71% 177 17.7 67% 180 17.2
2500 20.7 70% 170 17.7 66% 173 17.0 63% 176 16.4
2500 19.7 66% 166 16.9 62% 169 16.3 59% 172 15.7
2500 18.7 61% 162 16.1 58% 165 15.4 55% 167 14.9
2500 17.7 57% 158 15.2 54% 161 14.6 51% 163 14.0

You might try flying with the table in your lap and using the Avidyne to verify. That way you’ll have all the numbers.

Paul

PS For LOP, the Avidyne %power is wrong

At 8000 ft you can’t get 75% power at 2500 rpm. I don’t know if you have an SR22 or a 20, but in a 20, you have to leave the lever full forward (at 2700 rpm) to get 75% at 8000 ft.

I’ll also join the chorus suggesting you spend the few bucks to join COPA and get access to the enormous archives from the forum.

Clyde

I guess that’s what this all boils down to. The numbers ARE correct for the power setting I end up at…65%. However, I want to be at 75% after using the assist for best power. Obviously setting 75% power and then leaning doesn’t accomplish this. So, do you have to experiment with what power setting to set before leaning so you can end up at 75%? For example, do you have to experiment to know to set the power at 85%/2500rpm (just a guess) and then do the lean assist which hopefully will take the power down to the desired 75% when your computer starts indicating Best Power?

In reply to:


The MAP can be a lot of different places while the RPM is 2500, so this doesn’t really tell the whole story. This table from the POH shows how different %power can be at the same 2500 RPM:
8000 Feet Pressure Altitude
ISA – 30° C ISA ISA + 30° C
RPM MAP PWR KTAS GPH PWR KTAS GPH PWR KTAS GPH
2700 21.7 83% 180 19.6 78% 183 18.9 75% 186 18.4
2600 21.7 79% 178 19.1 75% 181 18.4 71% 184 17.9
2500 21.7 75% 174 18.4 71% 177 17.7 67% 180 17.2
2500 20.7 70% 170 17.7 66% 173 17.0 63% 176 16.4
2500 19.7 66% 166 16.9 62% 169 16.3 59% 172 15.7
2500 18.7 61% 162 16.1 58% 165 15.4 55% 167 14.9
2500 17.7 57% 158 15.2 54% 161 14.6 51% 163 14.0
You might try flying with the table in your lap and using the Avidyne to verify. That way you’ll have all the numbers.
Paul
PS For LOP, the Avidyne %power is wrong


Paul is correct. Perhaps because it is a new plane I have been keeping data as I fly. Today at 8000 feet and EMax Best Power I was getting 83% power (I did not write down the rest of the data). At 10,000 feet, EMax Best power the data was 2660 RPM, 21 In MP, 46 Degrees F OAT, 73% power and 184 knots. look at the performance tables, if you are wanting best performance you are going to get it at higher RPM, all other factors the same. Try it at 2700 RPM next time and I think you will be surprised.

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…do you have to experiment to know to set the power at 85%/2500rpm (just a guess) and then do the lean assist which hopefully will take the power down to the desired 75% when your computer starts indicating Best Power?


Yes, I can see that you would. I typically cruise LOP, so I had a hard time understanding what you were seeing. It’s easier to set the engine LOP, because fuel flow relates directly to %power.

Paul

Ahhh. OK. Now we’re getting somewhere. So it seems like the best use for the lean assist is to get the best LOP mixture since it will optimize for the power setting you’re already at. However, it sounds like there is no real method to using the lean assist to get one of the configurations in the POH charts. Soooo, it sounds like the best way to achieve one of these setting is the old fashioned way of just setting per the chart.