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

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.

I have a O-360 and if you look at the Lycoming manual you can have
different settings for the same power and save fuel one compaired to the
other. Usually it is the MP over the RPM that gives you the better fuel
burn.


-------------
Regards, Ross
C-172F 180HP
KSWI


Jose wrote:
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

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

On 14 Oct 2005 08:00:10 -0700, "cwby-flyer" wrote:

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

Mike,

If I understand you correctly, I don't think I agree with you g.
Air-fuel ratio as set by the mixture control also significantly effects
fuel consumption.

POH tables take into account not only altitude (and the ones I'm familiar
with is PRESSURE altitude, not density altitude) but also whether one is
leaned to best economy or best power mixture.

For example, for the same MP/RPM settings on my Lycoming IO-360, bhp will
be less at best economy vs best power.

On the other hand, IF I keep bhp unchanged by adjusting the MP, then fuel
consumption will decrease at best economy vs best power.

Example: IO360A; 65% power (130 bhp); 2400 rpm
@ best power (125°F ROP): 64 lbs/hr
@ best econ (at Peak EGT): 54.5 lbs/hr

Here's another example for a Continental IO550-G
10,000' pressure altitude; 2400 RPM 65% power
@ best economy (50°F LOP) 21.0" MP 12.3 gal/hr
@ best power (50°F ROP) 19.8" MP 14.0 gal/hr

The airspeeds at the same power settings will be the same.

The settings in the manuals for best power and best economy are defined in
terms of degrees rich or lean of peak EGT.

One difference in addition to fuel burn is that, since you require a lower
MP at best power setting, you can achieve that power at a higher altitude
than with a best economy setting.


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

For example, for the same MP/RPM settings on my Lycoming IO-360, bhp will
be less at best economy vs best power.

That's what my original understanding was, and why I was confused by the
presentation of the power setting table I linked to.

You can prove it simply by leaving the prop and throttle where they are, and
moving the mixture in between best econ and best power. At best econ, you
will be slower than best power. Slower = less power being produced.

If you actually could just get the same power at a lesser fuel flow without
adjusting MP and RPM, putting aside CHT considerations for the moment,
there'd never be a reason to run at best power. It'd just be burning more
fuel for the same result.

On Fri, 14 Oct 2005 21:45:42 GMT, "xerj" wrote:

For example, for the same MP/RPM settings on my Lycoming IO-360, bhp will
be less at best economy vs best power.

That's what my original understanding was, and why I was confused by the
presentation of the power setting table I linked to.

You can prove it simply by leaving the prop and throttle where they are, and
moving the mixture in between best econ and best power. At best econ, you
will be slower than best power. Slower = less power being produced.

If you actually could just get the same power at a lesser fuel flow without
adjusting MP and RPM, putting aside CHT considerations for the moment,
there'd never be a reason to run at best power. It'd just be burning more
fuel for the same result.



Your understanding is correct. The older a/c power charts only showed best
power MP/RPM settings.

One method:

1. Decide on your power setting -- e.g. 65%
2. Set MP/RPM appropriately per POH with adjustments for altitude,
temperature.
3. Set mixture to best power.
4. Note IAS.
5. Set mixture to best economy.
6. Increase MP to regain lost airspeed.

You will now be at 65% power, but with best economy setting, and burning
less fuel for the same airspeed.

It's hard to apply this method unless air is calm.

You might want to obtain the Operator's Manual for the -360- series of
engines from Lycoming. It has a wealth of charts that are much more
detailed than that in the older Mooney's, and might be interesting reading
for you.



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