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2 Answers

Acheiving best power vs. best economy

Asked by: 8365 views ,
General Aviation

I'm trying to learn how to achieve best power vs. best economy.  

Are cruise performance tables generally set to best power?

Is best power rich of peak while best economy would be at peak (or more lean of peak)?

*Here is how I have seen the two terms defined; please correct me if I'm wrong:

Best power: The power setting that gives the most power for any specific RPM and MP, i.e. - best speed.

Best economy: This power setting that gives the best specific fuel consumption (least amount of fuel), i.e. - best range.

Therefore, if I used best power, would I be traveling at a faster speed and arrive more quickly at a destination?  If not, what is the advantage?  Why not always fly with best economy if you get better fuel efficiency?

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2 Answers

  1. John D. Collins on Feb 22, 2011

    Usually the aircraft manufacturer states the assumptions for the various power charts.  The POH power charts are often part marketing and part information.  It is not uncommon for the fuel flows to be based on a mixture that is leaner than best power.  If you examine a piston engine chart that shows power verses fuel flow, EGT, and CHT, it will show that when richer than peak EGT, the power is relatively flat as a function of mixture.  Even though it is relatively flat, the peak power occurs at a mixture that is 100 to 150 degrees F rich of peak.  On the lean side of peak, power falls as a strong function of fuel flow.  A few points that can be seen on the chart is that CHT is the hotest at about 50 degrees F rich of peak and falls off richer or leaner, with significantly cooler operation on the lean side of peak EGT.  To some degree, CHT is an indication of how much strain the engine is operating under, with a hotter CHT indicating that internal cylinder pressures are higher than at a cooler value.  It is interesting to note that most POH will suggest leaning to a point of an EGT that is 50 degrees rich of peak, which is where the CHT is the hottest.
    The best economy or lowest specific fuel consumption occurs somewhere around 30 to 50 degrees lean of peak EGT.  Since most engines don’t run smoothly when lean of peak, this setting is not used with these engines.  Engines with a balanced induction system or that use fuel injection systems with balance injectors are capable of running at best specific fuel consumption lean of peak settings while still operating reasonably smoothly.  The reason that the engine does not run smoothly when lean of peak EGT has to do with the power curve and the lack of good fuel distribution.  Since power falls off rapidly when operating lean of peak, it is imperative to provide the same fuel to each cylinder, otherwise those cylinders receiving more fuel will generate more power than those that receive less fuel.  This imbalance in power from cylinder to cylinder makes the engine run rough.  This does not occur on the rich side of peak because the power generated by each cylinder is largely independent of the fuel flow.  In fact what determines the power on the rich side of peak is the amount and distribution of air to the cylinders, as there is more than enough fuel for complete combustion.  On the lean side of peak, things reverse and there is more than enough air for complete combustion, and it is the amount of fuel that determines the amount of power.  In fact, if you are operating lean of peak, the only thing that determines power is the fuel flow rate.  Depending on the compression ratio of the engine, there is a simple relationship between power and fuel flow.  For my IO520BA engine that has a compression ration of 8.5 to 1 and has balanced fuel injectors, power in horsepower when operating lean of peak equals fuel flow in gallons per hour times 14.9, end of story.
    There is a similar relationship between air density flow and power on the rich side of peak EGT, it is just that we don’t have a “air density flow” meter on our aircraft, so we have to look up in the POH power table that allows us to determine power based on all the things that affect air density flow.  What are the parameters that affect air density flow?  They certainly include the RPM, the manifold pressure, the altitude, and the temperature.  One advantage to operating lean of peak is the simplicity in determining power.  Again on my engine, lean of peak at 12.5 gallons per hour equates to 65% power.  So if I want to operate at 65% power, it is simple, pull the mixture back until it reads 12.5 GPH on my digital fuel flow indicator.
    If the engine doesn’t run smoothly when lean of peak, then usually peak EGT is the best economy point.  For Lycoming engines, peak EGT is usually authorized at 75% and below whereas with the typical Continental engine, peak EGT is usually authorized at 65% and below.  You should follow whatever the manufacturer says for your engine.
    For engines that have a fixed propeller and there isn’t an EGT available, leaning is relatively simple.  Pull the mixture until the onset of roughness which usually occurs at peak EGT or slightly lean of peak.  Then enrichen until smooth operation is achieved.  For best power, continue to enrichen until the RPM peaks.
    Finally, you asked why would someone ever fly at best power verses at best economy.  It all has to do with who is paying for the gas and how much you value your time.  If someone else is paying for the gas, I use the airplane as a time saving machine and fly it at 75% with a best power mixture (about 16 GPH and 175 Kts).  On the other hand if I am paying, I operate at 12.5 GPH and 65% power (about 165 Kts). 

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  2. Susan on Mar 31, 2012

    That’s a very good explanation. Thanks! My interest is only slightly specific to flying – and the reason for visiting has more to do with making design changes to an APU that’s a single piston gasoline unit. The explanation with respect to the shift from combustion air distribution being significant at rich to fuel distribution being significant at lean is very welcome! (and accords with flight instruction from many years ago) As the APU in question cannot suffer from either problem, being a single piston layout, I will refine the settings for peak minus x degrees – we’ll see how it goes. An auto peaking EGT feedback loop gadget ought not to be too difficult to jigger – a girl can actually use the electrical resistance of the exhaust tube as a sensor…then electronically produce a proportionate variable percent duty cycle air bleed between the carb-a-tooter and the intake valve.

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