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.:E

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... :suspect:

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. :ok:

[hatzflyer]

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

[robin]

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

[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.:ugh::ugh::ugh:

[BillieBob]

What has tailwind got to do with endurance? Maximum endurance is achieved at the lowest practical altitude because for best endurance you must fly at a specific IAS for a given weight. Since, to maintain the same IAS with increasing altitude, TAS increases and power = drag x TAS you will use more power (and fuel) at a higher altitude to maintain the same IAS thus reducing endurance.

[hatzflyer]

BB, of course you are absolutely right.
The thread is entitled max endurance and max range... I missed that.
My post was aimed at max range.
Max range and max endurance are not going to happen on the same flight.:ok:

[IO540]

Max endurance is a useless power setting. It is the power setting at which you stay airborne for longest - regardless of going anywhere. It's what you would use in a holding scenario only, and even then engine temperature might be a problem due to low cooling airflow.

The relationship between max range and wind is nontrivial and few if any pilots play it mathematically correctly because engine efficiency is not linear over its power output range. With tailwind, one wants to spend more time in it to get the best result, and with headwind one wants to spend less time in it to get the best result. But "we" all fly well above the best-MPG airspeeds anyway, so are not flying optimally.

Most private flight flight planning disregards tailwind, and takes account of headwind only, plus a margin on top.

However, playing this stuff to the limit is hazardous unless one has accurate fuel metering linked to the GPS. Most decent modern IFR tourers do have this but most of the older stuff doesn't.

If one has GPS-linked fuel metering then one can plan some "tight" scenarios which involve strong tailwind (potentially anywhere flying east of here) with pre-planned diversions near the end of the route, and if the computed FOB (fuel on board) falls below a specific figure, one would "drop in" into one of these instead. I have done that many times. Of course, if the fuel metering failed, then I would land ASAP. The metering accuracy is checked against the airport pump, on every fill-up.

In the VFR GA context, most flying is done at low levels, below CAS bases, so one cannot play with different winds at different levels, because flying below say 1500ft increases a mid-air risk dramatically, and often one cannot go above 2500ft.

In the IFR / airways context, unless one has a turbo one is flying with a wide open throttle the whole time, peak-EGT or LOP, and the speed you get is the speed you get... :) You always want more but there isn't any more AIR up there for the engine to suck. If you get a tailwind, great.

So the scope for planning is much less than most would think.

They crucial thing is setting the engine operating point for best economy, and knowing one's fuel flow accurately so one can plan long flights without cutting it fine.

Fuel metering is not expensive. A Shadin Microflo is about $1000, the 201B transducer is $600, then you have some pipes which any hose shop can knock up (£100), and some hacking to install it.

[Tinstaafl]

lookahead,

For best range you want the engine to operate at full throttle to reduce inefficiencies caused by the throttle plate at part throttle settings. The problem is that at lower altitudes the engine will produce too much power for the airframe to fly at its best range speed so you have to throttle back to achieve the power needed to maintain any speed less than maximum level flight speed at a given altitude.

As altitude is gained a normally aspirated engine suffers a reduced power output. More & more throttle will be needed for it to produce a certain amount of power. At some altitude full throttle will be reached to get that power. Full throttle would give 100% power (and a certain speed at sea level, but only 75% and a lower speed several thousand feet higher, 65% and even lower speed another couple of thousand feet up. Eventually some altitude will be reached where full throttle would only give enough power to maintain the speed for best range.

[RatherBeFlying]

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.Some self-launch or sustainer gliders have glide ratios better than 40:1.

Cloud streets are wonderful but please keep your eyes open for gliders blasting along 500' below cloudbase -- well, 80+ kt. is blasting along for us;)

[Pilot DAR]

Though I do hear recommendations for lean of peak operation, in the Rockwell 114 I was flying today, lean of peak operaton is actually specifically prohibited in the flight manual. I have to believe Rockwell and Lycoming had their reasons...

[Tinstaafl]

Their reason was simple: It was cheaper (for them) for you to pay for extra fuel operating rich of peak, than for them to fit an all cylinder engine monitor and balanced injectors.


Interestingly, a PA31 I fly *is* approved for 50 deg F LOP - and and it uses Lyc. TIO540s, similar to the Rockwell. That was with the factory's crappy single point EGT & CHT probes. We've since had a JPI EDM fitted and it's much better to ensure no operations in the EGT, CHT & TIT danger zones

[Big Pistons Forever]

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.

Just to make sure we are on the same page

Endurance = maximum time in the air for the fuel available

Range = maximum distance travelled for the fuel available.

Both are dependant on the conditions at the time. Part of the problem is flight schools have turned what should be a very practical problem in to a largly theoretical execise in POH graph hair splitting. What is unfortunately often missing is applying these concepts in a way that is relavent in getting the aircraft from A to B

Endurance: For the average PPL flying at true endurance speed is for all practical purposes an emeregency measure. Due to a massive flight planning failure on your part (usually involving bad weather, most often fog, or a runway that is unavailable without enough fuel to go to another airport). In other words you find yourself in the air with no place to go. Your only hope is to remain in the air untill the weather clears or the runway becomes available.
So you establish a very gentle orbit with about 3 degree's of bank and very slowly reduce power untill you can just maintain level flight (note RPM). Now lean the engine untill it is just starting to run rough and readjust throttle to above RPM. You are now flying for max endurance.

Range: Again for the average PPL flying for range is an emergency manoever because the best range speed for the average trainer/tourer is going to be so slow it is for of little practical value. To find the airspeed go to the POH and look up the range chart. For the, C152 for example, the best range airspeed is at 45 % power and 77 knots.....not a very commonly used power setting in real life :hmm: BUT this number in practice means nothing because it only applies with a tailwind or on a no wind day. If you have a strong headwind best range may actually be at a higher power setting. (for illustrative purposes and using the C 152, I will give an extreme example assume a 52 kt headwind and a 100 trip. At best range the flight will take 4 hrs (77kt TAS - 52 kt headwind = GS of 25 kt divided by 100 nm and use 15.6 gals [3.9 GPH from cruise performance chart]. Now if we fly at max cruise we get 101 kts TAS at 6.1 GPH, but total time is just over 2 hrs (101 kt TAS - 52 kt headwind = GS of 49 kts divided by 100 nm). Total fuel burn is on 12.4 gals.

So what does all this mean.....simple calculating the airspeed for "Flight for best Range" doesn't exist in the real world. Instead the question that matters is what is the best power setting for the conditions affecting my flight today?
A handy rule of thumb (for light aircraft ) is use 65 % for flight with winds aloft of less than 20 kt and 55% for a tailwind of more than 20kts and 75 % power for headwinds of more than 20 kts. This will in my opinion give a prcticable balance between fuel efficency and time enroute.

What really matters IMO is knowing how much fuel you have on board and what the actual fuel consumption is (It is almost certainly going to be more than the POH numbers). Since virtually all light aircraft fuel guages are basically useless the only time you know how much fuel you have on board is at teh start of the flight when you have physically measured the fuel. The problem then becomes what is the actual fuel consumption you are experiencing in flight ? The ideal solution is to have an electronic fuel flow guage or if you always fly the same aircraft carefully note how much fuel was loaded with every fill up and over time you will get a good feel for actual fuel consumption. If you are flying your typical beater rental plan on 10% higher fuel consumption and 10% lower TAS than the book numbers. When in cruise lean untill the engine starts to stumble and then enrichen untill the engine smooths out ( It is important that the misture control is moved slowly to get an accurate lean indication). Always plan to land with 1 hour reserve fuel and go to a closer alternate if you start encroaching on your reserve (and no practical alternate always means the flight is cancelled) . Do that and you will never have to use flight for range or endurance.:ok:

No POH.....No problem. Go out on an average weight on smooth air day and find the airspeed for minimum power to maintain level flight (ie endurance).
This speed will be a good proxy for best rate of climb speed. Add 10% and you will get best still air absolute range speed. For fuel consumption take the horsepower of the engine and divide by 10. This will give you full power full rich fuel consumption. 65% cruise with leaned mixture will be half of this value. While not perfect , these numbers will be close enough for all practical purposes. If I again use the C152 as an example , 108 hp = 10.8 GPH at fuel power and 5.4 GPH at 65 % ....and the POH says 5.0 to 5.2 GPH depending on altitude...:cool:

[Checkboard]

Take the test, learn a bit about how the theory works, and how to calculate the speeds for any light piston aircraft:
:ok:

MAXRNG (http://www.db.erau.edu/research/cruise/)

[IO540]

Take the test, learn a bit about how the theory works, and how to calculate the speeds for any light piston aircraft:
http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/thumbs.gif

MAXRNG (http://www.db.erau.edu/research/cruise/)

I didn't do so well on that test :)

Firstly, some of the questions are somewhat ambiguous e.g.

Assuming a situation without complicating wind, weather, or terrain factors, the airplane is capable of greatest range when flown at what altitude?
a. the lowest practical altitude
b. the highest practical altitude
c. an altitude roughly midway between these extremes
d. any altitude, since it has little effect on available range.

the last option, d, where it says "little effect" that includes a b and c too. How little is "little"?

It's an excellent site and all advanced pilots should read it.

However I think it was written before the days of more efficient engine operation became fashionable.

For example the concept that range does not depend on altitude needs some qualification. It is true if all climbs and descents are done at best (and constant) engine efficiency i.e. peak EGT or LOP. I found this on my long flights, where I got a computed FOB very early on (once in cruise) and to my suprise it hardly changed as I then climbed FL100-FL160 to get above some weather, etc. But if I had to go to 100F ROP to get the plane to climb to say FL190 (which one has to do when pushing the ceiling of any piston plane especially a non-turbo one) range suffered noticeably. Also, one climbs ~ 150F ROP and even if using the constant-EGT method (which is the most efficient way to climb) this is ~ 30% less efficient than peak-EGT or LOP flight, and this excess fuel burnt during the climb is not recovered during a subsequent descent which will obviously be peak or LOP.

A less obvious effect on engine efficiency is wide open throttle operation, which reduced pumping losses. Nobody seems to know how much, however, and due to the self-imposed 65% power limit one cannot fly WOT below about FL080.

The bottom line is that between about FL080 and whatever altitude one can fly at while peak/LOP, the range does remain very constant.

[Crankshaft]

Borrowing this thread for a related question.

A normal aspirated engine at 150 to 240 HP: How much (if any) would the Peak EGT differ between say 1500 ft and 5000 ft?

Unfortunately I don't have the possibility to go up and find out my self that often anymore. :(

[IO540]

I will try to do a test today, but I think "not very much".

[BEagle]

IO540 has conducted considerable practical research into obtaining the most efficient settings for long range cruise in his aircraft type - and therein lies the secret. Practical research using accurate navigation and engine instrumentation.

Most 'club' aircraft don't have such luxuries. A colleague of mine owned a Rockwell 112. His preferred activity was touring, so he had also worked out the optimum balance of prop rpm, MAP, cooling gills, fuel pressure with the benefit of CHT, EGT and fuel flow indications for his preferred cruising regime. Once level at cruise altitude with MAP and rpm set at 23/2300, mixture and cooling gills were both adjusted until the appropriate values were seen - roughly 11 gall per hr, EGT less than 1425 and CHT 400-425, then monitored carefully and left set for the rest of the flight until descent for landing. Not many 'club' pilots would bother with such careful engine handling.

Unless you're a long distance tourer, getting the last few miles of range out of your aircraft is rarely necessary and excessive leaning by ham-fisted pilots can cause a lot of damage.

When flying HM's Bulldogs, the 'book' way of setting the aircraft up for a diversion at around FL40 was to set 17"/2200 and Best Economy Mixture (from the table in the flight reference cards). This yielded about 86KIAS and 14.7 nm per imp gallon in still air. However, no-one ever did this in practice. Additionally, from about the early 1990s, students were never taught to set less than 2400 in the cruise, so introducing them to such a different way of operating for an in-flight diversion was imprudent. Instead we told them just to set 19"/2400 and 2.5 psi fuel pressure. This gave a more comfortable 100KIAS at FL40 - the difference being that only 13.7 nm per imp gallon was achieved. But for a 50 mile diversion, this hardly mattered as it would typically lead to a difference of about 0.25 gallon fuel burn.

I once flew down in a 10-ship formation from RAF Abingdon to RAF St Mawgan at about 2000 ft. Everyone set the usual 2400 rpm, best power mixture and MAP to give 100 KIAS. However, I decided to experiment and pulled the prop back as far as I could and pushed up the MAP to just under the limit; I also leaned off the mixture as far as practical, keeping a close eye on the CHT. Engine response was much poorer, not very nice when flying even loose formation. But when the aircraft was refuelled at St Mawgan, it was found that I'd burned 3 gallons less than my colleagues.

On another occasion, I was flying a fixed-pitch PA28 with an FI colleague who was as mean as mouse$hit. Shortly after taking off from Shoreham he whipped the mixture back and the engine began to run quite roughly. After a minute or so of this, I asked whether this was his 'student error' or what. I was quite astonished when he said that he thought that 'a little roughness' didn't matter if it saved fuel.....:eek: The aircraft wasn't fitted with a serviceable CHT or EGT gauge, so he had no idea whether his engine handling was hurting the engine.

Many pilots don't really know how 'lean' is advisable and, without CHT, EGT and fuel flow information, for most '£100 hamburger' flights, leaving the mixture fully rich is probably the safest option, particularly since they're usually paying a 'wet' rate for the hire, so the amount of fuel they use is not going to affect their hire cost.

For a struggling club, burning a little more fuel is a lot cheaper than having to replace a cooked cylinder or four!

[IO540]

Got some data...

1500ft 1530F OAT +13C Q1013
5500ft 1510F OAT +7C Q1013

So, given the ~ 12F drop in the OAT, there isn't much of a drop in the EGT, despite the big drop in the outside air pressure and density.

Everyone set the usual 2400 rpm, best power mixture and MAP to give 100 KIAS. However, I decided to experiment and pulled the prop back as far as I could and pushed up the MAP to just under the limit; I also leaned off the mixture as far as practical, keeping a close eye on the CHT. Engine response was much poorer, not very nice when flying even loose formation. But when the aircraft was refuelled at St Mawgan, it was found that I'd burned 3 gallons less than my colleagues.Yes, low RPM really does save fuel. But I find the engine does not sound comfortable, unless running at a low power setting. So I use it only above FL100 (full throttle). Flying at low levels, one would need to be at 20" or below MP.

For a struggling club, burning a little more fuel is a lot cheaper than having to replace a cooked cylinder or four!OTOH if they spend a couple of k installing an EDM700, they would seriously benefit. The trick is to get everybody flying to get their heads around this, and work past the old prejudices which have been going around this scene for decades. I know of one syndicate which has a well instrumented plane but there are still members who refuse to do it the "modern way". You can also get recriminations if somebody does do it, and then an issue (valve or a cylinder leaking) is found. The issue was prob99 caused by somebody else's gross mismagement but nobody can prove it.

[Fuji Abound]

1500ft 1530F OAT +13C Q1013
5500ft 1510F OAT +7C Q1013

So, given the ~ 12F drop in the OAT, there isn't much of a drop in the EGT, despite the big drop in the outside air pressure and density.



Something doesnt look right with those temps - I assume they are meant to be actuals including the difference.

[IO540]

Yes, actual EGTs.

[Fuji Abound]

No I was meaning the 12 degree drop in OAT.

[IO540]

Ha!

The OAT dropped from 13C to 7C (OAT is measured in C; EGTs and CHTs on American engines are measured in F) which is 6C and in my simple mind is about 12F :)

The peak EGT value dropped by 20F which is obviously only partly accounted for by the air getting colder.

The MP was held at 23" for both altitudes, BTW, as was the fuel flow (11.0GPH).

[mm_flynn]

Here's my thoughts for SEP flying and fuel:

If you want to save money why not just fly as fast as possible all the time, the extra fuel is less than the cost of airframe and powerplant maintenence.

If you enjoy flying and want it to take longer, go slower.

If you are planning a trip where this range stuff really matters, don't go that way, plan it another way with shorter legs.

If it HAS to be that far, eg over the North Atlantic, pay someone else to fly it.????
On my machine variable maintenance and engine reserve are maybe £30/hr and fuel burn some where between 12 and 20 US GPH depending on how you fly it - so the difference between best and worst fuel burn is nearly twice my total variable maintenance.
Most of my cost is fixed (annual, insurance, parking, charts, data subsriptions, general depreciation etc) or fuel - so fuel is pretty high on my list of things to manage.

Also, with careful fuel planning I often find adding 20 minutes to a medium leng trip by optimising range saves enough fuel to get the last PAX or bag in and still be under my target weight and achieve IFR reserves.

[IO540]

and shorter legs is a very very bad way to fly unless one actually wants to stay at the stops.

Stopping for fuel is generally a hassle, and doing it internationally is worse, with customs/PPR/PNR / flight plan filing issues. The gain in doing a trip nonstop is very substantial

[Tinstaafl]

The more take-offs, climbs & circuit manoeuvering there is, the less efficient the flight. Like IO540 and others have written, good fuel management can mean the difference between having to stop or carry a smaller payload, or be able to make the trip with fewer stops &/or greater payload.

In a Panther Navajo I manage, operating at 75% ROP gives 190 kts @ ~A100 & 44 USG/hr. AT 60-65% ROP gives 180 kts &36- 38 USG /hr. That can mean the difference between having to stop for fuel or not.

We had a JPI EDM installed in it. Even though this aircraft was already approved to run LOP, having the EDM meant that we could be certain that no cylinder, EGT or turbine limit was reached. Now it's flown 60-65% LOP and gives ~170-175kts & 28-29 USG/hr. There's also a benefit in being able to confidently lean during the climb without cooking something.

The flight times differences are negligible but the fuel savings/payload gain improves significantly, especially LOP

[Big Pistons Forever]

.

Unless you're a long distance tourer, getting the last few miles of range out of your aircraft is rarely necessary and excessive leaning by ham-fisted pilots can cause a lot of damage.

Many pilots don't really know how 'lean' is advisable and, without CHT, EGT and fuel flow information, for most '£100 hamburger' flights, leaving the mixture fully rich is probably the safest option, particularly since they're usually paying a 'wet' rate for the hire, so the amount of fuel they use is not going to affect their hire cost.

For a struggling club, burning a little more fuel is a lot cheaper than having to replace a cooked cylinder or four!

For the average club light aircraft with a non turbocharged, carburated, 4 cyl engine it is virtually impossible to hurt it by overleaning at cruise power settings. This is because the engine will start to run rough before any cylinder becomes dangerously lean. Therefor if it is running smoothly you are in a safe mixture range. Where it is possible to damage an engine is if it is leaned a bit and then full power is applied and the speed (and therefore cooling airflow) is reduced, This is precisely what would happen if after cruising for awhile the pilot decides to climb and forgets to go to full rich. Unfortunately the frequent club practice of always running full rich for local flights means most students only ever lean the engine on the Navex's and develop the bad habit of just pushing the throttle in without checking the mixture first (because it is always just left at full rich). I get my students to always lean in level flight unless they are in the aerodrome traffic area or in the practice area. This makes them confident and efficent at leaning the engine and forces them to develop the habit of always checking the mixture before changing power. This applies for reducing power as well as depending on the situation going to full rich at start of a long descent can contribute to cylinder cracking from shock cooling.