Power setting table and best economy/best power...

When a power setting table shows a "best economy" and "best power" setting
for a given power level (say 75%) using a given MP and RPM this is just a
starting point, right?

Once the MP and RPM has been set and the leaning done, the actual power
settings would be more like 78% for best power, and 73% for best economy,
wouldn't they? (%'s are just guesses, no science involved).

Thanks in advance.

"xerj" wrote in message
...
When a power setting table shows a "best economy" and "best power" setting
for a given power level (say 75%) using a given MP and RPM this is just a
starting point, right?

Once the MP and RPM has been set and the leaning done, the actual power
settings would be more like 78% for best power, and 73% for best economy,
wouldn't they? (%'s are just guesses, no science involved).

Depends on the table. However, generally speaking a power setting table
will indicate how to lean the engine for that power setting, and the actual
power is "exactly" what the table indicates. "Exactly" is in quotes
because, of course, it's unlikely one can get the exact combination of MP,
RPM, leaning, and other factors in operation. But you'll be pretty darn
close.

Assuming the table shows the desired leaning (and most do), you would not
adjust the indicated power setting for any other factor, not even leaning.
If it says you are getting 75% and you've leaned as indicated, then you're
getting 75% (more or less).

If you have a specific table in mind and would like to post the details,
then information based on that table in particular can be provided to you.
Otherwise, the general rule is that the table tells you the correct power
resulting from the power setting. No adjustments required or desired.

Pete

On Thu, 13 Oct 2005 22:47:25 GMT, "xerj" wrote:

When a power setting table shows a "best economy" and "best power" setting
for a given power level (say 75%) using a given MP and RPM this is just a
starting point, right?

Once the MP and RPM has been set and the leaning done, the actual power
settings would be more like 78% for best power, and 73% for best economy,
wouldn't they? (%'s are just guesses, no science involved).

Thanks in advance.


It probably depends on the power table.

For example, the Mooney Ovation2 has separate tables for Best Power and
Best Economy. The former calls to lean 50°F ROP, and the latter 50°F LOP.

Others that I've seen give you a specific leaning parameter in order to
obtain the power in the table.


Ron (EPM) (N5843Q, Mooney M20E) (CP, ASEL, ASES, IA)

Here's a link to a checklist with a power setting table for a Lyc IO-360.

http://www.dauntless-soft.com/PRODUC...son_n222bh.pdf

Down the bottom of the MAP/RPM combos it has this:-

************************************************** ************************************

Fuel Consumption Approximations
75% Best Power 11.5 GPH 65% Best Power 10.5 GPH
75% Best Economy 10.0 GPH 65% Best Economy 9.5 GPH
Best power = 50° C rich of peak EGT Best Economy = Peak EGT (or LOP)

************************************************** ************************************
The 75% "Best Power" and 75% "Best Economy" couldn't both be actually 75%,
could they?

Fuel Consumption Approximations
75% Best Power 11.5 GPH 65% Best Power 10.5 GPH
75% Best Economy 10.0 GPH 65% Best Economy 9.5 GPH
Best power = 50° C rich of peak EGT Best Economy = Peak EGT (or LOP)

************************************************** ************************************
The 75% "Best Power" and 75% "Best Economy" couldn't both be actually 75%,
could they?

Yes, they could. You are using more gas at the 75% best power setting
than at the 75% best economy setting.

It depends what you hold constant in the comparision. If we held fuel
flow constant, then for the same amount of gas, best power gives you a
higher percent of power, and best economy gives you less power (so as to
stretch the gas longer).

But if you hold power constant (instead of fuel flow), then best power
=uses= more gas ("wasting" some to give you the same power), and best
economy uses less gas, making the best (most efficient) use of the gas.

Jose
--
Money: what you need when you run out of brains.
for Email, make the obvious change in the address.

The 75% "Best Power" and 75% "Best Economy" couldn't both be actually
75%, could they?

Yes, they could. You are using more gas at the 75% best power setting
than at the 75% best economy setting.

Sorry, I mean they couldn't both actually be 75% for the same MAP/RPM combo.

The leaner setting would have to be developing less power, wouldn't it?

The 75% "Best Power" and 75% "Best Economy" couldn't both be actually
75%, could they?
Yes, they could. You are using more gas at the 75% best power setting
than at the 75% best economy setting.

Sorry, I mean they couldn't both actually be 75% for the same MAP/RPM combo.

The leaner setting would have to be developing less power, wouldn't it?

The thottle and mixture levers may be in a different position in order
to achieve the same MAP/RPM, as a result of using the fuel more or less
efficiently.

Jose
--
Money: what you need when you run out of brains.
for Email, make the obvious change in the address.

Xerj,

Sorry, I mean they couldn't both actually be 75% for the same MAP/RPM combo.


No, but your table doesn't show MAP/RPM settings. So the MAP/RPM settings at
best economy would have to be higher for the same power output. As an aside,
50 degrees ROP is not a good point to run your engine at.

--
Thomas Borchert (EDDH)

No, but your table doesn't show MAP/RPM settings.

It does in the upper section above the bit I cut and pasted. It has % power
settings columns, and then the various MAP/RPM settings.

So the MAP/RPM settings at best economy would have to be higher for the
same power output.

That's what I was pretty sure of.

Thomas Borchert wrote:

No, but your table doesn't show MAP/RPM settings. So the MAP/RPM settings at
best economy would have to be higher for the same power output. As an aside,
50 degrees ROP is not a good point to run your engine at.

I definitely agree that 50 ROP is not a good place to run your engine,
much better to keep it around 100 - 125 ROP.

As for your assertation that MAP/RPM setting, I must repectfully
disagree (and if I'm mistaken, please let me know - I'm always looking
to learn more). Your power combinations are determined by your MAP &
RPM at a given density altitude and expressed as a percentage of HP.
When you lean the mixture, the RPM/MAP combination will move (assuming
you're not at sea level on a standard day) and then to place the engine
back on your desired % power, you re-adjust the throttle and Prop
levers so that the MAP/RPM match what is in the POH for your density
altitude.

Mike