How to set power for best range & endurance
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.
Join Date: Oct 2008
Location: suffolk
Posts: 399
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
Join Date: Jun 2003
Location: EuroGA.org
Posts: 13,787
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
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.
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.
Last edited by Tinstaafl; 22nd Mar 2010 at 00:39.
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.
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.
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
Moderator
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...
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
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
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.
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
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.
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...
Last edited by Big Pistons Forever; 20th Mar 2010 at 17:44.
Join Date: Jun 2003
Location: EuroGA.org
Posts: 13,787
Likes: 0
Received 0 Likes
on
0 Posts
Take the test, learn a bit about how the theory works, and how to calculate the speeds for any light piston aircraft:
MAXRNG
MAXRNG
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.
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.
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.
Join Date: Nov 2007
Location: Europe
Posts: 46
Likes: 0
Received 0 Likes
on
0 Posts
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.
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 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.....
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!
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.....
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!
Last edited by BEagle; 7th Jun 2010 at 11:15.
Join Date: Jun 2003
Location: EuroGA.org
Posts: 13,787
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
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.
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.
For a struggling club, burning a little more fuel is a lot cheaper than having to replace a cooked cylinder or four!
Join Date: May 2001
Location: UK
Posts: 4,631
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
Join Date: Jun 2003
Location: EuroGA.org
Posts: 13,787
Likes: 0
Received 0 Likes
on
0 Posts
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).
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).
Join Date: Aug 2003
Location: Surrey
Posts: 1,217
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
Join Date: Jun 2003
Location: EuroGA.org
Posts: 13,787
Likes: 0
Received 0 Likes
on
0 Posts
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
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