How to set power for best range & endurance

[lookahead]

With regards to light piston planes, how do I set the power to fly for maximum endurance and maximum range? Please reply as to the actions needed in the cockpit. I've read up on the theory, the power required curve. But that doesn't tell me what to do while in the cockpit. Thank you.

[rans6andrew]

Maximum range/best economy is not a simple calculation. First you need to know the exact fuel consumption (litres per hour) of the aircraft, at the weight that you are flying at, at a range of AIRSPEEDS (AS). Then you need to know the GROUNDSPEED (Gs) you are achieving. If you know the litres per hour (As) and divide by the miles per hour (Gs) you will get a figure for the litres per mile. Then consider if you fly a few miles per hour faster (As), how does that change the litres per hour and how does it change the groundspeed (Gs)? Run the calculation again. Find the speed that give the minimum litres per mile. Change the miles to km or nautical miles and litres to gallons or US gallons or pounds to suit your needs.

The microlight competition types go out and fly, to calibrate their fuel usage at precisely 40mph, 45mph, 50mph, 55mph etc and measure the exact quantity of fuel used at each speed. This gives them the fuel consumption to speed relationship for their aircraft which they use later in competition.

What you will find is that it pays to fly faster than the normal best economy speed if flying into a headwind and slower if flying with a tailwind.

There are a few fuel computers about that measure the fuel flow with an impeller and take a groundspeed reading (from a GPS) and do the calculation in real time.

If you are after maximum duration this is much simpler. It is simply the AIRSPEED that gives the lowest litres per hour.

Rans6...

[BackPacker]

What sort of engine does your airplane have, what sort of equipment (RPM, MAP, CHT, EGT, GPH, ...), can you do per-cylinder analysis?

What does the POH for your aircraft specify?

[IO540]

Assuming constant engine efficiency versus power, the best MPG is achieved at Vbg (best glide speed).

This is however far too slow for most people... (95kt or so for a TB20).

Also, engine efficiency is not constant. As the power output is reduced (and at Vbg most types will be running at maybe 40% power) the pumping losses become increasingly significant.

So the best MPG speed will end up being somewhere above Vbg.

In practice (NON turbo context) one flies by choosing a power setting which is good for engine life, say 65%, and sets up that power setting at peak EGT or slightly lean of peak, and accepts whatever speed one gets.

[Piltdown Man]

In basic terms, climb to full throttle altitude. ie. roughly the point where to set any reasonable power setting, you have to have the throttle fully open (there is a far more scientific description, but I've forgotten it and can't be bothered to look it up). Then, using the manufacturer's book, set the RPM & mixture. From memory, the IO-470 as fitted to the Cessna 206 could be set up in a multitude of ways. The one that used to float my boat was running over square (where the RPM was less greater than manifold pressure) for example: 23" and 2,100 RPM and leaning "lean of peak". Worked for me for a few hundred hours "Downunder".

But the important thing is, read the manufacturer's book.

PM

[Captain Smithy]

For the purpose of flying basic fixed-pitch 1970s/1980s Yank spamcans without any posh stuff like CHT, EGT or FF gauges, analysers etc., I was taught to lean until the RPM rises slightly (<50RPM) then when it starts to fall again enrich the mixture slightly.

It is unusual that so many people aren't taught the basics of leaning and managing power settings, or seem to be completely ignorant of them. Flying around on full rich all day isn't very efficient and doesn't do the engine much good either.

Smithy

[IO540]

I was taught to lean until the RPM rises slightly (<50RPM) then when it starts to fall again enrich the mixture slightly.

Yes, that sets peak-EGT pretty well.

The efficiency is a flat curve around peak EGT so one doesn't have to be accurate. I've done accurate tests and going LOP at the same IAS i.e. thrust makes no additional MPG improvement. Any such improvement comes from the acceptance of a lower IAS And any lower IAS will yield better MPG - all the way back to somewhere near Vbg.

[RatherBeFlying]

The motorglider folks sometimes use sawtooth flight when they want to go A to B.

Run the engine and climb at efficient speed to a good height.

Turn off engine.

Glide towards destination.

Restart engine when low (preferably over suitable field in case restart fails)

[BackPacker]

Yeah, but your typical motorglider has a glide ratio of, what, 1:25? And they presumably have had training to make use of any thermals they encounter en-route to gain some altitude without restarting the engine.

Your average spamcan will have a glide angle of 1:10 if you're lucky, and the average PPL will not know anything about thermals other than that they're bumpy, which tends to mess up the interior after a while.

Furthermore, you've got to be very careful in managing the engine temperatures/shock cooling when you do that. For an air-cooled engine a full power/engine-off glide cycle is not a great idea. In motorgliders, the Rotax 912(s) is very common, which is water-cooled and doesn't suffer from shock cooling all that much.

[AdamFrisch]

It is unusual that so many people aren't taught the basics of leaning and managing power settings, or seem to be completely ignorant of them. Flying around on full rich all day isn't very efficient and doesn't do the engine much good either.

I agree that there's not enough knowledge about this (myself included not long ago). Especially when operating out of high elevation fields where full rich mixture can actually be dangerous from a safety standpoint.

[Tinstaafl]

Best endurance in a fixed pitch:

Fly LOW, with minimum drag configuration, use the lowest power setting that keeps you airborne (the speed will be at or close to minimum descent speed in a glide), lean for peak EGT or - even better - lean of peak.

If variable pitch prop: use the lowest authorised RPM with whatever MP is needed to maintain the speed.

Best range fixed pitch:

Min. drag configuration, leaned to peak EGT or - better - lean of peak, high enough that full throttle is needed to fly at best range speed (same as best glide speed.)

If variable pitch prop then use lowest authorised RPM and an altitude that requires full throttle to maintain Vg.


Some notes:

Min drag config: Flap, gear & cowl flaps up. Flown in balance with heading & altitude flown accurately.

Lean of peak: To maintain the same power % as a given Rich of Peak setting will require the throttle to be open a bit more say, 2 or 3" MP or a hundred or two of RPM (approximately). If you're already at full throttle then you may require a slightly lower altitude for more MP or RPM to make up the power lost from setting LOP.

If there's a head or tailwind then a power vs speed curve is useful for finding the Best Range speed. Adjust the origin of the best range tangent to the left for tailwind & right for a headwind. Amount to adjust is the same as the head or tailwind component eg 20 kt headwind then adjust the range tangent to the right by 20kts.

[lookahead]

Capt Smithy:
By your method of leaning, will the resultant mixture be slightly ROP EGT since you enrich slightly after the rpm drop?

To get the best SFC, I would have to be slightly LOP EGT. So does that mean I don't enrich after getting the rpm drop?

Tinstaafl:
Thanks for your suggestion on the method for best endurance in a fixed pitch. That is what I've been looking for. Does this "minimum power setting to hold altitude" results in an airspeed that is the same or very close to Carson's formula for best endurance speed (best glide speed / 1.316)?

As for best range power setting in a fixed pitch, I don't quite understand. Do you mean that at my desired cruise level, I reduce throttle and trim for straight and level until I reach the best glide speed. Next I lean mixture to slightly LOP EGT, followed by increasing throttle and further leaning to get the corresponding slightly LOP EGT. Repeat until full throttle is reached. The airspeed will still be maintained at best glide speed. Is that correct?


Also, I'd like to clarify on the terms "best economy" and "best endurance".
Best economy refers to a fuel/air mixture that gives optimal combustion. So to get best economy above 3,000 ft, we lean the mixture to slightly LOP EGT.
Best endurance on the other hand refers to the power setting that gives the lowest fuel flow (gallons/hr), thus allowing maximum time aloft.
Is my understanding correct?

[BackPacker]

Capt Smithy:
By your method of leaning, will the resultant mixture be slightly ROP EGT since you enrich slightly after the rpm drop?

To get the best SFC, I would have to be slightly LOP EGT. So does that mean I don't enrich after getting the rpm drop?

Actually, what you would theoretically want to achieve is to run at exactly max EGT all the time. This gives the best SFC. However, because the induction system isn't 100% perfect, injectors aren't perfectly matched and you typically don't have the instrumentation to find out what's going on in each individual cylinder, peak EGT doesn't happen in all cylinders at the same time. You've got to err on the side of caution. That's where the 25/50 ROP rules comes from, to a large extent. Of course, this can only be done if you have an EGT gauge.

If all you have is an RPM then it's nearly impossible to lean accurately for best economy. On Robins, the advice I've seen is to lean until the engine no longer runs smoothly, then increase the mixture to restore smooth running. On Pipers, I've seen the advice to run with wide open throttle, then reduce the mixture until a certain RPM is reached (the table is on the pilots sunvisor, of all places). But as IO said, the fuel flow near the flat top of the engine performance curve means that finding the exact setting doesn't matter all that much. Heck, even running with an engine leaned for "best power" instead of full rich all day long will give you maybe a 25% fuel savings vs. 30% fuel savings when leaned for "best economy".

Also, I'd like to clarify on the terms "best economy" and "best endurance".
Best economy refers to a fuel/air mixture that gives optimal combustion. So to get best economy above 3,000 ft, we lean the mixture to slightly LOP EGT.
Best endurance on the other hand refers to the power setting that gives the lowest fuel flow (gallons/hr), thus allowing maximum time aloft.
Is my understanding correct?

The other thing you are confusing yourself with, is the difference between airspeed and power.

"Best endurance" and "Best range" all refer to a certain airspeed. In the first case, it refers to the absolute lowest drag airspeed (bottom of the power curve) and in the second case it refers to the best trade-off between total drag and speed (the tangent). The first thing you need to do for max performance is to know these speeds and fly them.

Second step is then to set and optimize the engine power to maintain this speed. That's where the terms "best economy" and "best power" come in. Best economy refers to the mixture setting that uses the least amount of fuel to deliver a certain amount of power. Best power referes to the mixture setting that allows you to get the most power out of the engine in the circumstances.

So in both the "best endurance" and "best range" scenarios you will want to set the power mixture to "best economy". In fact, you will want to fly at the "best economy" mixture settings in all flight conditions, with the exception of situations where maximum power is required or may be required shortly.

That's the theory at least. Practically speaking, there is an influence by the engine efficiency on the power curve, and IO540 mentioned that earlier. It's got to do with pumping losses, alternators producing electricity and so forth. You can think of this as the "parasite drag" of the engine. Because of this, by increasing the power output of the engine, engine efficiency increases. After all, more total energy is generated and since the "parasite drag" (pumping losses etc) stays more or less the same, more energy is converted into thrust.

Because of this, it may be more efficient to fly slightly faster than the airframes "best range" speed. Because at this slightly higher speed the engine works more efficiently. But the trouble is that you need to match the exact power/drag curve of the airframe with the exact fuel/power curve of the engine/propellor combination, then factor in the effect of the increased propellor slipstream onto the airframe and so forth. Generally the information to do this is not available and dependent on too many factors anyway (dirty airframe, avionics added etc.).

Your aircraft manufacturer has done testing on the airframe/engine combo, and the result of all this testing, plus all the theory behind it, is in the "performance" section of the POH. In there, with a bit of luck, you will find the best endurance and best range speed and tactics. In the real world, that information is the best you have. The theory behind it is nice to know but since you've got to match two compound (and generally unknown/unpublished) curves to arrive at the best range tactic for your specific circumstances, you can hardly call that a practical method to be used in the cockpit.

[Captain Smithy]

Capt Smithy:
By your method of leaning, will the resultant mixture be slightly ROP EGT since you enrich slightly after the rpm drop?

To get the best SFC, I would have to be slightly LOP EGT. So does that mean I don't enrich after getting the rpm drop?

More or less, yes it would be ROP. It's what I use all of the time.

Without the benefit of an EGT gauge, to set LOP I believe you would lean until maximum RPM is achieved (fixed-pitch) and then back it off a bit further very gently (not so much to cause a >50RPM reduction). Perhaps someone could confirm this?

The thing with running LOP is that you need to be careful that you don't cook the engine. A lot of people are too ham-fisted with the engine controls and this is when problems appear...

Also the mixture needs to be re-adjusted for any change in throttle setting. E.g. don't forget to go to full rich if you go back into a full-power climb...

Smithy

[IO540]

Best power is about 125F ROP. The fuel flow per HP is about 10% higher than the best economy setting which is about 25F LOP.

Full rich is about 30% worse than best economy. That's the price the schools pay for simple teaching

Leaning till there is a noticeable drop of speed (thrust) and then enriching just a little is probably a fairly accurate way of setting best economy - once you get it consistent.

[hatzflyer]

"Fly low" ..Why? what with a tailwind? why not higher where the wind will be stronger and push you along.

[robin]

Yeah, but your typical motorglider has a glide ratio of, what, 1:25? And they presumably have had training to make use of any thermals they encounter en-route to gain some altitude without restarting the engine.

but you can still use cloud streets and save fuel even in a light aircraft. You don't have to thermal, just follow the energy that's there for free.

[hatzflyer]

Ahh.. but don't you have to check the POH about that and ask the instructor?

[robin]

Dammit, I'd forgotten that....and the new EASA difference training...

[hatzflyer]

Mickey taking aside, when someone comes on here asking for help, why do people say ask your instructor, or refer to the POH. don't they think the person asking would do that?
When will they realise that not all aircraft have a POH?
Surely if their answer is "ask your instructor " they are implying that they are flying under instruction themselves and so not in a position to give advice.