Best Power vs Best Economy

[201er]

My POH says the expected cruise speed is the same whether best power or best economy mixture is set which seems to contradict the concept of best power. Is the difference in speed so insignificant or did they go wrong somewhere?

 

Example:

Economy: 6000ft 24.1" 2400RPM 75% 10.5GPH 164kts

Best Power: 6000ft 24.1" 2400RPM 75% 12.3GPH 164kts

According to the POH, you'd be nuts to waste 1.8GPH for no gain (unless needed strictly for cooling). Have you noticed a speed difference in reality though? Likewise I wonder if there is an actual speed difference between 23.3"/2700 and 26.8"/2400 because the latter saves .5GPH?

[danb35]

A given amount of power at a given density altitude will always give you the same airspeed--however, 24/24 at best power is not the same amount of power as 24/24 at "best economy" (which is likely not true best economy, that being found LOP, which is also likely to be cooler than the "best power" setting).  I'd wager that the "best power" setting is actually more than 75%, and would give you more speed.

According to the POH for a '74 F, best economy is 25 ROP and best power is 100 ROP.  I understand some other models recommend peak EGT for best economy; if yours does so, the "best economy" you list will be close to 75% power.

In general, though, I'm a little skeptical of the power charts in the POH--maybe it's because they're quite different from an F to a J, despite having nearly-identical engines.  Maybe it's because they show the engine being able to make over 70% power at 12.5k.

[Cruiser]

I think you mean 2600 RPM not 2400

75% power is 75% power no matter how you configure it. Unless Shadrach will share the secret to "git er on the step" where she gets more efficient...

[201er]

Oh yeah, it's 2600. Doesn't really change the point though. Can't seem to edit to fix that.

And danb, the highest altitude my POH says can produce 70% is 10,000ft at WOT and 2700RPM.

[Shadrach]

My POH is a joke... It has probably 20 hand written corrections through out the power, climb, take off and landing tables. These were done by the dealer, as the mistakes are obvious and all of the numbers were written in the same ink and at what appears to be the same time...

[Shadrach]

Interesting... my POH says I can make 70.2% at 12,500ft - 19"mp at 2700rpm WOTRAO 100ROP.

Tom, as hard as I've tried, "the step" still elludes me...;-)

[Shadrach]

It does change the point 201er, 

All other things being equal except mixture and RPM...an extra 200rpm is 400 extra powerstrokes per minute and all of the fuel and air that is burned up with them, additionally, as one leans from best power towards ~45LOP (lowest specific fuel consumption), more work is being done with less fuel. The same power setting at 100ROP and Peak should produce different FF...

[201er]

Huh? I'm comparing absolutely everything being equal except mixture. I put 2400 by accident instead of 2600 but for both examples. The point I'm making is that GPH is lower but speed is the same in best economy. Best power is supposed to make more power than best economy but where is that power? Is it just wasted as drag? The POH shows a higher fuel consumption and nothing else channging by going from economy to best power. It doesn't say it requires a lower MP or lower RPM or gives more speed. The POH makes it seem like best power just uses more fuel for no benefit. I don't understand this. So I'd like to know in the real world when RPM/MP/Altitude all remain the same and mixture is advanced from a stable economy ROP cruise to a best power ROP cruise, what happens? Does speed increase? Does % power increase and require you to decrease throttle or rpm to maintain the same power setting?

[Magnum]

Best power will produce more power than economy. Between 100 ROP and 25 ROP (the recommendation in the POH) the difference should be around 3-4% (see attached picture).

IMHO you'll see approx. 3-4 KTAS between Best power and best economy.

[Magnum]

Correction: It should be max. 2% difference in power, and mabe max. 2 KTAS difference.

[Shadrach]

201er,

I thought you're 2600 correction was to just one of the settings, not both. Hence I was thinking 2400 @ 75% vs 2600 @ 75%

I cannot speak to your POH... But as I stated earlier, Specific Fuel consumption curve goes down as you lean towards peak, getting pretty shallow as you go past about 10 LOP and bottums at out at 45 LOP... This means that in fact more...or the same...in terms of power, can be done with less fuel, depending on mixture setting.

[danb35]

To your question, if you're cruising level at some MP/RPM combination leaned to peak EGT, and then enrich to 100 ROP, you will be making more power.  If you trim to stay level, your airspeed will increase.  If you're really concerned that you cruise at exactly 75.0% power, yes, you'll need to adjust throttle, prop, and/or mixture to retain that setting.

Or, you can be a bit more practical, and pretty much ignore the concept of setting a specific amount of power.  For example, I usually cruise at 7-9k MSL, WOT (with ram air open), 2500 rpm (for noise and smoothness), leaned to ~ 9 gph LOP.  I might adjust these a bit; if I want to go faster I might run at 2600 and/or enrich the mixture a bit; if things are getting too warm I'll lean a bit more.  I don't really consider (or see any reason to consider) what percentage of power I'm using, though I know that 9 gph is 67%.

[201er]

Well when you're that high, you inherently can't exceed 75% power because the pressure is too low. Now if you were to level off for a short XC flight at 2,500' would be a different story huh?

[Shadrach]

Even at lower alt, I doubt that Dan calculates power... I don't.  I go as fast as I can at a level of cooling that satisfies my  engine temp limitations. If the engine is happy, then I'm happy. If I wanna slow down I remove fuel, if I wanna speed up I add it...so long as I can see the engine is happy, I'm happy...

[fantom]

Again, well stated Ross. After decades of flying just about everything, KISS still rules.

[201er]

So what's behind 75% max power for cruise rule of thumb? Is it just a dumb and simple way to keep pilots from overheating the engines or is there more to it?

Also I'm curious if anyone actually cruises at 2700RPM? Looks like avove 6,000' the only way to achieve 75% is by having the prop up there. Do you take advantage of TAS by going high any leaving prop full, still go high but lower RPM for a lower power setting, or go lower to get 75% at lower RPM? Which is the best compromise of fuel, temp, and getting there?

[Shadrach]

Either my POH is complete BS or there is something to the "Ram-air" function... My POH says that at 7500ft, 32df (standar day) I can make 76.2% at 2500rpm full rich (yuk) 14gph...

 

The 75% is not a hard number. It was picked a a place in which detonation margins widen and the ability to over-heat you engine becomes more difficult...as in even a Monkey randomly twisting the mixture knob would probably not hurt anything... not that it is impossible to shorten cylinder life span at that level, it is...it just requires that you be some what dumber than a monkey... ;-)

[201er]

Anyone good at math? How do you derive power given RPM, MP, DA, and GPH at ROP?

[Magnum]

Get a EDM 800 or 830, it will calculate it for you (including mixture setting, you'll see how much power you have left when LOP).

If you have an Iphone you can use the app IO360. The calculations are based on the Lycoming operators manual, so they might differ a bit to the POH. IMHO values are better than from the POH as no marketing department influenced them.

[tony]

201er,

There is a cute app for the iPhone I've played with called IO360.  I can not attest to the accuracy but I think it will do what you want.  It was free when I downloaded it.  You put in the alt and temp, tell it you want best power or best economy, and it will give you a set of paramaters for MP and RPM.

[Cruiser]

>Posted Jun 23, 2011 11:08 AM by 201er

>Huh? I'm comparing absolutely everything being equal except mixture. I put 2400 by accident instead of 2600 but for >both examples. The point I'm making is that GPH is lower but speed is the same in best economy. Best power is >supposed to make more power than best economy but where is that power? Is it just wasted as drag? The POH shows >a higher fuel consumption and nothing else channging by going from economy to best power. It doesn't say it requires >a lower MP or lower RPM or gives more speed. The POH makes it seem like best power just uses more fuel for no >benefit."

 

You make a compelling case for LOP operations

[jetdriven]

take IO-360 and add 2" of MP when operating 10-50 LOP to get the LOP power setting. Check your FF and it will be right on that way.

 

Like Shadrach I run full throttle, 2500 RPM, and lean to the speed I am looking for.  9 GPH is 75%.  8.5 is 65%.  If it gets near 380, lean more.  If it is getting slow, richen a little.  Above 8-9000 feet, you will be near peak and maybe even raise RPM to 2600 or 2700 because your % of power is making your airplane just get too slow.

[201er]

Are you sure 9GPH is 75%? Now I'm getting confused because last time I heard 10GPH is 75% cause 10x15=150 which is 3/4 of 200HP so 75%. How do you get 9gph?

BTW how many of you guys that run LOP in a Mooney are doing it without GAMIs?

[jetdriven]

its 10 and 8.72 GPH.  I think the fine wire plugs add another .02 BSFC so it would be slight lower.

[jetdriven]

I have noticed the difference between pea and 40 LOP~  is around 7% less power.

 

I also get the feeling Shadrach could get in my plane and it wouldnt know the difference in who is flying it.  Just make sure to climb 300' above final cruise altitude then descend to accelerate quickly so as to get her on the step.  20 MPH over book that way.