Determining the maximum endurance speed
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I observe that the original question was asked in the context of endurance, not range. As I recall, endurance is not at all affected by winds. Range is. Endurance is simply what is the greatest length of time the aircraft can remain airborne, going somewhere is not a part of that, so winds are not either.
Though leaning and fuel flow will factor into the endurance time, it is the airspeed which must be determined first, as it is a greater factor. You can lean at any airspeed, and leaning will increase your endurance, but not as much as the wrong airspeed will reduce it!
I'm sure that the protracted discussion about winds and leaning is important, and informative, but is it relevent to the original question?
But then, on the other hand, I've certainly been guilty of thread drift too!
Pilot DAR
Though leaning and fuel flow will factor into the endurance time, it is the airspeed which must be determined first, as it is a greater factor. You can lean at any airspeed, and leaning will increase your endurance, but not as much as the wrong airspeed will reduce it!
I'm sure that the protracted discussion about winds and leaning is important, and informative, but is it relevent to the original question?
But then, on the other hand, I've certainly been guilty of thread drift too!
Pilot DAR
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I'm sure that the protracted discussion about winds and leaning is important, and informative, but is it relevent to the original question? But then, on the other hand, I've certainly been guilty of thread drift too!
But since maximum endurance is obtained at the lowest fuel flow to sustain level flight, I'm truly at a loss to understand how you could argue that a discussion about mixture management to achieve lowest BSFC amounts to thread drift.
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I observe that the original question was asked in the context of endurance, not range. As I recall, endurance is not at all affected by winds. Range is. Endurance is simply what is the greatest length of time the aircraft can remain airborne, going somewhere is not a part of that, so winds are not either.
I think the answer is
1 - An airspeed very close to Vx (max excess power so we have the maximum possible reduction in power available - however Vx is determined at full power and the thrust vs IAS curve may be slightly different at lower power settings)
2 - Best possible efficiency of the engine/propeller
a - Leaned to 25 degrees or so LOP (Best BSFC)
b - Turning the prop as slow as allowable (Reduced pumping losses and I believe slightly improved propeller efficiency)
3 - MP set to the minimum value consistent with level flight
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I observe that the original question was asked in the context of endurance, not range. As I recall, endurance is not at all affected by winds. Range is. Endurance is simply what is the greatest length of time the aircraft can remain airborne, going somewhere is not a part of that, so winds are not either.
Hence I tried to kick the subject into something more practical as unless you are flying for some record or doing police surveillance work its pretty pointless
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unless you are flying for some record or doing police surveillance work its pretty pointless
I'm sure there are also situations you could find yourself in where it's useful to know how to acheive maximum endurance. All I can think of at the moment are stuck above solid IMC (without IR/instruments) which is expected to clear, and lost at night - neither particularly likely, or situations I intend to put myself in, but that doesn't mean I don't want to know the figures just in case.
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Hollo echos my thoughts.
On a nice evening, I take the door off (to be at one with the world) and tour around at a speed more a kin to endurance than range or race. I have had a (fortunately few) occasions, where staying up and orbiting was preferrable to landing right away. Waiting for the clearing of an obstructed runway, and float plane landing area come to mind. On some occasions, where I have a very large tailwind at high altitude, flight at endurance speed is the most efficient, as it becomes about the same speed as flight for best range.
I also participate in public service searches, where flight for endurance is the most appropriate. I have had to interrupt my search to go for fuel, and that's a little embarrassing!
A few additional comments:
Some engine installations are better than others in cooling, but I would expect all engine installations to cool adequately indefinately, during flight at endurance speed. This characteristic would be covered during the climb cooling test at the time of certification for the aircraft. The climb speed would be best rate, which would be very close to the speed for endurance, and the climb at full power and leaned (if appropriate). Presuming design compliance has been shown in this phase of flight, flying at the same speed in level flight with the power pulled well back should have no difficulty cooling.
Turning the prop slowly has merit, but not "over square" unless the engine/propeller design specifically approves this type of operation. Over square operation will rapidly intrduce the risk of detonation, increasing with increased power. Further, it will increase propeller blade strain. We don't think of that very often, but I have been reminded by observations and resulting limitations on a test propeller imposed by the propeller manufacturer, for an aircraft we are flight testing right now (engine change STC). The engine involved (a well know brand in the 200 HP range) is limited with other propellers, so as to have "avoid operation" speed ranges. It looks like this engine/propeller into airframe installation in this case may be similarly limited. Time and testing will tell. We are presently considering the climb cooling performance of this installation.
The mixture setting of an engine will have nearly nothing to do with the speed at which the plane will fly. As the original question was asked about speed, as opposed to fuel flow, I suggest that mixture settings would not be an element of the answer. Mixture is a distinct subject, which will forever be the basis for many discussions. Fiddle with mixture as you will. In a carburetted engine (any older 172) you will never achieve the ideal mixture into all cylinders. There will probably be a power setting at which you come close, but this is not published, varies considerably engine to engine, and can only be determined with careful experimentation with a scanner. For my 150, the preferred RPM for equal EGT for the four cylinders is 2450, If I run more or less power, I'll be leaning only to the leanest cylinder (which changes), and thus wasting gas. Thus, If I fly at the speed for best endurance, two of the cylinders are running un-necessarily rich, and wasting gas. I'm just betting that the amount conserved by the overall reduced power, is better than the amount wasted by the uneven fuel flow. Truth is, I really don't care that much! Fuel injected are a vast improvement over this situation, though still not perfect. Fuel injected dilikes Mogas, and my engine and I like Mogas! In the past, we have converted injected engines to carburettors so we can run Mogas, but I thread drift......
Pilot DAR
On a nice evening, I take the door off (to be at one with the world) and tour around at a speed more a kin to endurance than range or race. I have had a (fortunately few) occasions, where staying up and orbiting was preferrable to landing right away. Waiting for the clearing of an obstructed runway, and float plane landing area come to mind. On some occasions, where I have a very large tailwind at high altitude, flight at endurance speed is the most efficient, as it becomes about the same speed as flight for best range.
I also participate in public service searches, where flight for endurance is the most appropriate. I have had to interrupt my search to go for fuel, and that's a little embarrassing!
A few additional comments:
Some engine installations are better than others in cooling, but I would expect all engine installations to cool adequately indefinately, during flight at endurance speed. This characteristic would be covered during the climb cooling test at the time of certification for the aircraft. The climb speed would be best rate, which would be very close to the speed for endurance, and the climb at full power and leaned (if appropriate). Presuming design compliance has been shown in this phase of flight, flying at the same speed in level flight with the power pulled well back should have no difficulty cooling.
Turning the prop slowly has merit, but not "over square" unless the engine/propeller design specifically approves this type of operation. Over square operation will rapidly intrduce the risk of detonation, increasing with increased power. Further, it will increase propeller blade strain. We don't think of that very often, but I have been reminded by observations and resulting limitations on a test propeller imposed by the propeller manufacturer, for an aircraft we are flight testing right now (engine change STC). The engine involved (a well know brand in the 200 HP range) is limited with other propellers, so as to have "avoid operation" speed ranges. It looks like this engine/propeller into airframe installation in this case may be similarly limited. Time and testing will tell. We are presently considering the climb cooling performance of this installation.
The mixture setting of an engine will have nearly nothing to do with the speed at which the plane will fly. As the original question was asked about speed, as opposed to fuel flow, I suggest that mixture settings would not be an element of the answer. Mixture is a distinct subject, which will forever be the basis for many discussions. Fiddle with mixture as you will. In a carburetted engine (any older 172) you will never achieve the ideal mixture into all cylinders. There will probably be a power setting at which you come close, but this is not published, varies considerably engine to engine, and can only be determined with careful experimentation with a scanner. For my 150, the preferred RPM for equal EGT for the four cylinders is 2450, If I run more or less power, I'll be leaning only to the leanest cylinder (which changes), and thus wasting gas. Thus, If I fly at the speed for best endurance, two of the cylinders are running un-necessarily rich, and wasting gas. I'm just betting that the amount conserved by the overall reduced power, is better than the amount wasted by the uneven fuel flow. Truth is, I really don't care that much! Fuel injected are a vast improvement over this situation, though still not perfect. Fuel injected dilikes Mogas, and my engine and I like Mogas! In the past, we have converted injected engines to carburettors so we can run Mogas, but I thread drift......
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Originally Posted by IO540
You will have a helluva job flying around at anywhere near Vx without the engine getting too hot, due to poor airflow.
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I have never understood why it is ok to run oversquare at full power for the whole climb (maximum power, maximum heat, low cooling airflow). but if in cruise flight you choose 25 in and 2100 rpm this is a problem.
You are of course correct the mixture has nothing to do with speed, However, the mixture setting you choose will have a considerable impact on the amount of time you can stay in the air (or the cost /hr), which is the whole point of flying at this speed.
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I'm sure there are also situations you could find yourself in where it's useful to know how to acheive maximum endurance. All I can think of at the moment are stuck above solid IMC (without IR/instruments) which is expected to clear, and lost at night - neither particularly likely, or situations I intend to put myself in, but that doesn't mean I don't want to know the figures just in case.
I dont disagree with why you may want max endurance so am not trying to discredit it. Normally pilots will need to stay up because they have had stronger headwinds than planned to get to their destination. Maximum endurance will not help you there.
Someone stuck above cloud in a pure VFR aircraft would be ill advised to sit there waiting for the clouds to hopefully disappear.
If my flying involved going nowhere and staying up for as long as possible for as little money as possible I would probably buy a motorglider and only use the engine when I needed to.
Practically saving fuel and thus increasing range has more to do with trip management than engine fuel burn management that was all I was trying to say and that involves so many other vital factors other than how you lean the engine.
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You are of course correct the mixture has nothing to do with speed
For that to be true, there would need to be no significant variation in power across the attainable mixture range ... which just isn't the case, especially for those engines capable of operating significantly LOP.
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mm_flynn, to use the question you posed me - are you sure?
For that to be true, there would need to be no significant variation in power across the attainable mixture range ... which just isn't the case, especially for those engines capable of operating significantly LOP.
For that to be true, there would need to be no significant variation in power across the attainable mixture range ... which just isn't the case, especially for those engines capable of operating significantly LOP.
The reason I (and probably you and IO) keep banging on about it, is that mixture is hugely important in getting the most HP out of a KG of fuel. In this context I need to beg to differ with Pace - in piston aircraft mixture can move specific fuel consumption 30% or more. This is by far the most significant factor unless winds are exceptionally strong (when as in the case posted by Pace he would have been much better off down low - with regard to range), or you are going to really drone along for hours near Vbr (which is very slow)
mm flynn
Could not agree more. Part of the problem is IMO full use of the mixture control is rarely tought during ab initio training (or often even in advanced training)
The mixture control on most training aircraft spends 99% of its time set at either full rich or ICO. And frankly at early stages of training this is a "good enough practice" which allows the instructor to concentrate on teaching more fundamental flying skills/knowledge. However by the time a pilot has obtained his/her PPL it is time to develop a more sophisticated understanding of engine management which includes not only use of the mixture control but also
-engine warm up procedures
-proper runups
-managing CHT's (the No 1 requirement for long engine life IMO)
-selection of cruise power settings
-understanding what the engine guages are really telling you
-recording usefull engine behavior notes for maintainance trouble shooting
Unfortunately there is no "one perfect way" to operate an airplane piston engine, but there sure is a lot of opinions
All POH's provide some guidance but most are pretty useless when it comes to the finer points of engine operation. However there is a wealth of good info on the web starting with the engine makers sites. This forum and Avweb also have a lot of good info. Talking to engineers is also very valuable as most have had to deal with the results of poor pilot practices. A bit of research will give pilots lots to think about and allow folks to move from a "cookbook" approach to a real understanding of what is going on under the cowling.
Could not agree more. Part of the problem is IMO full use of the mixture control is rarely tought during ab initio training (or often even in advanced training)
The mixture control on most training aircraft spends 99% of its time set at either full rich or ICO. And frankly at early stages of training this is a "good enough practice" which allows the instructor to concentrate on teaching more fundamental flying skills/knowledge. However by the time a pilot has obtained his/her PPL it is time to develop a more sophisticated understanding of engine management which includes not only use of the mixture control but also-engine warm up procedures
-proper runups
-managing CHT's (the No 1 requirement for long engine life IMO)
-selection of cruise power settings
-understanding what the engine guages are really telling you
-recording usefull engine behavior notes for maintainance trouble shooting
Unfortunately there is no "one perfect way" to operate an airplane piston engine, but there sure is a lot of opinions
All POH's provide some guidance but most are pretty useless when it comes to the finer points of engine operation. However there is a wealth of good info on the web starting with the engine makers sites. This forum and Avweb also have a lot of good info. Talking to engineers is also very valuable as most have had to deal with the results of poor pilot practices. A bit of research will give pilots lots to think about and allow folks to move from a "cookbook" approach to a real understanding of what is going on under the cowling.
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Hi there,
this is an interesting conversation for a change, but I note that the original question has gone completely unanswered:
(my emphasis)
Ok, so we all know that best endurance speed is Vmd (minimum drag) and this has already been pointed out.
So now the question can be rephrased as: "How does one find Vmd, experimentally, for a particular airframe?" I believe that question had been answered some time ago over at Tech Log or Flight Testing. Those are undoubtedly the best places to ask.
Lastly, I note that the question only asked about the value for best endurance speed so, for all its practical interest, by and large the power settings (mixture and prop pitch) are not relevant as far as this question goes. Best endurance will always be obtained at the same EAS. The amount of endurance you will get out of the aircraft will of course vary significantly depending on the power settings being used but that is not a consideration here, do we agree?
this is an interesting conversation for a change, but I note that the original question has gone completely unanswered:
How to determine the best endurance speed using an experimental method?
Ok, so we all know that best endurance speed is Vmd (minimum drag) and this has already been pointed out.
So now the question can be rephrased as: "How does one find Vmd, experimentally, for a particular airframe?" I believe that question had been answered some time ago over at Tech Log or Flight Testing. Those are undoubtedly the best places to ask.
Lastly, I note that the question only asked about the value for best endurance speed so, for all its practical interest, by and large the power settings (mixture and prop pitch) are not relevant as far as this question goes. Best endurance will always be obtained at the same EAS. The amount of endurance you will get out of the aircraft will of course vary significantly depending on the power settings being used but that is not a consideration here, do we agree?
but I note that the original question has gone completely unanswered
Ok, so we all know that best endurance speed is Vmd (minimum drag)
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bookworm,
Doh, of course you are right, as usual 
Sorry, I meant Vmp (minimum power) everywhere in my previous.
Yes you did, it was so far at the beginning of the thread that I completely missed. I agree that it's a simple experiment with appropriate equipment--I was again thinking about finding Vmd.
Right, now better go and get some more coffee. Apologies for the misinformation
No, it's not. Best endurance comes at minimum power required
Sorry, I meant Vmp (minimum power) everywhere in my previous.
I thought I answered it in post #4
Right, now better go and get some more coffee. Apologies for the misinformation
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Is this of value?
13.4 Aircraft Endurance
I looked into this (not the above link; Internet hadn't been invented) many years ago when I was considering an affordable means of keeping flying; such as build your own aeroplane. I’d been recently inspired by the, then, new BD5. There was a much simpler formula that related the coefficient of lift/coefficient of drag ratios at different speeds for a particular wing section but I’m damned if I know which box (of many) I placed the papers in. Cl squared over Cd cubed Max rings a bell. I never got as far as worrying about power and fuel burn curves.
13.4 Aircraft Endurance
I looked into this (not the above link; Internet hadn't been invented) many years ago when I was considering an affordable means of keeping flying; such as build your own aeroplane. I’d been recently inspired by the, then, new BD5. There was a much simpler formula that related the coefficient of lift/coefficient of drag ratios at different speeds for a particular wing section but I’m damned if I know which box (of many) I placed the papers in. Cl squared over Cd cubed Max rings a bell. I never got as far as worrying about power and fuel burn curves.