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

"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).

Thanks in advance.


Yes, the chart (which you may or may not have for your airplane) which gives
best economy is "maximium range chart" which shows the best power setting
for each altitude.

Mike
MU-2

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

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)

"xerj" wrote in message
...
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.

However, as Jose pointed out, RPM and/or MP will not remain constant as you
adjust the mixture. Just because the controls have not moved, that does not
mean that the power setting hasn't changed.

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.

Well, that's a matter of current debate, as it happens. The primary
argument in favor of a product like the GAMI fuel injectors is that you CAN
get the same power at a significantly lower fuel flow. In that argument,
you ARE just "burning more fuel for the same result" by using the "best
power, rich of peak" mixture settings suggested by engine operating manuals.

I find the argument compelling. The proponents explain that the
rich-of-peak, "best power" mixture settings exist to provide enough excess
fuel to ensure that no cylinder is actually running at peak EGT (since in
most engines, the actual mixture from one cylinder to another varies by a
significant amount), and to provide cooling for all cylinders to compensate
for the high power setting used.

They go on to explain that if the fuel mixture is actually well-matched from
one cylinder to another, one can accomplish the same effect by running the
engine lean-of-peak, ensuring that all of the fuel in the air/fuel mixture
is burned (rather than some of it being used to cool the engine), but still
keeping the EGT low enough in each cylinder to avoid heat-related problems.

Whether all of that is correct, I do not know. I'm not the expert. But it
does makes sense to me, and yet is still consistent with operating the
engine rich-of-peak for engines that aren't designed to ensure evenly
matched fuel/air mixtures for each cylinder.

Pete

However, as Jose pointed out, RPM and/or MP will not remain constant as
you adjust the mixture. Just because the controls have not moved, that
does not mean that the power setting hasn't changed.

It's one of those "I just can't remember, exactly" things and I'm very much
grounded by my wallet at the moment, dammit. Although this could be a really
good excuse to get in and fool around "in the name of science". Better
justification than a $200 Coca Cola.

RPM with a constant speed should stay the same (or at least quickly revert
back to the same) shouldn't it? If so, then it's the MP that would vary as
you pull the mixture back. Off hand, do you remember by how much it varies
in a typical type you fly when you go from best power to best econ?

"xerj" wrote in message
...
RPM with a constant speed should stay the same (or at least quickly revert
back to the same) shouldn't it?

Yes, the governor will ensure that the RPM remains constant (hence "constant
speed" ).

If so, then it's the MP that would vary as you pull the mixture back. Off
hand, do you remember by how much it varies in a typical type you fly when
you go from best power to best econ?

No, not specifically. To be sure, it doesn't vary by much. But a few
percent change in power (what I might expect with changes in mixture changes
alone, at the most) wouldn't require much of a change in MP. It might not
even be detectable with the typical 2 or 3 inch MP gauge found in most small
airplanes.

Once the mixture is "in the ballpark" on my airplane, whatever change occurs
in MP isn't enough to prompt me to readjust the throttle, I can tell you
that much.

Keep in mind the rest of my post as well. The reference you posted doesn't
provide the details, but it's entirely possible that the "best power"
setting and "best economy" settings DO provide essentially the same power
(within a percent or so), but that the "best economy" setting incurs some
additional engine wear and tear, due to higher operating temperaturs (note
that the "best economy" setting is "Peak EGT").

Pete

"Peter Duniho" wrote in message
...
[...]
Once the mixture is "in the ballpark" on my airplane, whatever change
occurs in MP isn't enough to prompt me to readjust the throttle, I can
tell you that much.

Upon re-reading my own post, I'm not convinced the MP would change at all in
this situation. MP is simply a measurement of the air pressure in the
intake manifold. It *ought* to be, as far as I know, strictly a function of
engine RPM and throttle position. I wouldn't expect fuel flow to affect it
at all.

So I think that part was in error. It's not that the change is too small to
notice. It's that it just doesn't exist (not counting some completely
inconsequential effects that alter the pressure due to temperature and
density changes as a result of the fuel).

However, I still don't see anything inconsistent with the table you posted.
It may very well be that the only difference between the "best power" and
"best economy" power settings is fuel flow and engine temperature. I
certainly don't see anything in the table to suggest otherwise.

Pete

On Sat, 15 Oct 2005 00:23:34 -0700, "Peter Duniho"
wrote:

"Peter Duniho" wrote in message
...
[...]
Once the mixture is "in the ballpark" on my airplane, whatever change
occurs in MP isn't enough to prompt me to readjust the throttle, I can
tell you that much.

Upon re-reading my own post, I'm not convinced the MP would change at all in
this situation. MP is simply a measurement of the air pressure in the
intake manifold. It *ought* to be, as far as I know, strictly a function of
engine RPM and throttle position. I wouldn't expect fuel flow to affect it
at all.

So I think that part was in error. It's not that the change is too small to
notice. It's that it just doesn't exist (not counting some completely
inconsequential effects that alter the pressure due to temperature and
density changes as a result of the fuel).

However, I still don't see anything inconsistent with the table you posted.
It may very well be that the only difference between the "best power" and
"best economy" power settings is fuel flow and engine temperature. I
certainly don't see anything in the table to suggest otherwise.

Pete


Peter,

Some comments regarding your assertions.

It may very well be that the only difference between the "best power" and
"best economy" power settings is fuel flow and engine temperature.

In the Lycoming O-360 engine operators manual, there is a chart that
indicates an 8% drop in BHP going from best power to best economy settings.

There seems to be approximately a 5% drop looking at power tables for a
Continental IO550 in Mooney Ovation2 which has separate tables for best
economy vs best power.

---------------------------
(from a different post)

... but that the "best economy" setting incurs some additional engine wear
and tear, due to higher operating temperaturs (note that the "best economy" setting is "Peak EGT").

I think that it is extremely arguable on several grounds.

Lycoming data shows that as a percentage, CHT's drop considerably more than
EGT's rise. Also, even at peak EGT, in a normally aspirated engine, one is
well below the "red line" for exhaust components whereas even under normal
operating conditions, say 425°F CHT, one is still stressing the cylinders.

Furthermore, data from both Continental and GAMI show that at best economy
and especially LOP settings, the cylinder head pressure pulse waveform is
more gradual and, although more sustained, has a lower peak pressure
(GAMI). Continental charts indicate just that the interior cylinder
pressures are lower.

So to claim that there is "higher operating temperature" causing "some
additional engine wear" without noting that, other than in the immediate
exhaust area, the engine operating temperature is actually lower, and the
power pulse pressure waveform is less destructive, seems to me to be
overlooking essential data.

Of course, some engines are unable to run at peak EGT or LOP EGT due to
imbalances in fuel or air flow. If an operator is not operating any leaner
than, let us say, 25°F RICH of peak EGT, he may indeed cause increased wear
and tear on his engine at those settings. I believe the original (1965)
manual for my Mooney recommended that setting for best economy. But I do
not believe that either of the current engine (or airframe) manufacturers
still make that recommendation.




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