Max Endurance Speed / Min Fuel Flow
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Lower speeds meaning less parasites on the wing but more induced drag. That's why even in a low TSFC scenario with no counterwinds still meaning lower angle of attack. And Keith, for the high bypass turbofan engines, more RPM always doesn't mean more TSFC.
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Well, here`s my five acres:
Max Endurance is the maximum time you can get in the air. It is not for going anywhere - it is just to keep you airborne.
For example - if you want to hold, for a time or for a long time, you want the fuel to last for as long as possible so you burn just enough to stay airborne. Thats all that is required. It is effecient for minimum fuel over the longest (time) endurance.
Max Range of course, is completely different. Here you actually want to go a distance, at speed, as far as possible. Therefore you fly at the optimum speed where the aircraft flies at its most effecient speed to give you the maximum distance (range) given the weight and altitiude and temperature and least possible drag for the fuel required.
Max Endurance is the maximum time you can get in the air. It is not for going anywhere - it is just to keep you airborne.
For example - if you want to hold, for a time or for a long time, you want the fuel to last for as long as possible so you burn just enough to stay airborne. Thats all that is required. It is effecient for minimum fuel over the longest (time) endurance.
Max Range of course, is completely different. Here you actually want to go a distance, at speed, as far as possible. Therefore you fly at the optimum speed where the aircraft flies at its most effecient speed to give you the maximum distance (range) given the weight and altitiude and temperature and least possible drag for the fuel required.
Last edited by Natstrackalpha; 7th Oct 2013 at 01:56.
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That sounds odd, I thought with ram compression and everything a jet would perform better at higher speed due to ram compression effects though at the speed optimum for that, the wing's L/D ratio would have already started falling off.
Well, yeh, (see my note) this is indeed true of cruising for range - but like I said you just want to keep the aircraft airborne - this does not mean max thrust at max angle at the back of the drag curve, that would be silly and you would use loads of fuel, very quickly and cause a lot of noise and overheating and a whole load of stuff not good for engines.
Nor, is the aircraft hanging by the thread of its teeth by the same token.
It is not a fast speed but fast enough to warrant good effeciency from the engines, albeit the entire profile is only effecient for letting the engines run and cool but nothing else, except to keep you effecintly airborne by burning the least amount of fuel over time, hence keeping you airborne over a longer time.
Invariably, but not always, you would probably be holding at the end of your trip. Therefore, you would be lighter than at the start of the trip, so this will work in your favour too. It is not effecient in distance . . . but it is effecient in burning the least amount of fuel possible, over time. Therefore maximising your time in the air. Just swan around slowly, around the hold at the max endurance speed, which is quite slow, until they have bulldozed the dead elephant off the landing runway.
Last edited by Natstrackalpha; 7th Oct 2013 at 02:17.
And Keith, for the high bypass turbofan engines, more RPM always doesn't mean
more TSFC.
At idle RPM most of the energy from the fuel is being used just to keep the engine running. Very little thrust is produced, so the TSFC is very high.
As RPM increases the fuel flow is increasing, but a greater proportion of the energy in the fuel is being used to produce thrust. So the TSFC gradually decreases as RPM increase. But this benefit continues only up to a certain RPM.
At very high RPM internal inefficiencies increase, thereby reducing the proportion of the fuel energy that is used to produce thrust.
The overall effect is that over the RPM range from idle to max RPM the TSFC starts high, gradually decreases until the optimum RPM is reached , then TSFC gradually increases again as RPM continues to increase above the optimum value.
But all of this is only part of the story, because other factors such as airspeed, air temperature and altitude all affect SFC. As I said in an earlier post, basic texts make a number of simplfying assumptions. In most cases these simplifying assumptions are not explicitly stated in the texts. So we should not be at all surprised when we find that real-life aircraft do not comply with the (over simplfied) conventional wisdom.
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Whenever appraoching the subsonic speeds (>0.8 mach), at a great flight level(might be FL360 or more) you will face great amounts of parasite drags which is going to increase your Vmd, but still not SFC at horrible levels. But when at lower, for eg inital approach, there is going to be more TSFC because of drag both caused by intense air and less lift of slower speeds. In this instance, greater N1 might be required, which is going to cause also more fuel flow. But this is a aircraft specific subject. Of course, in a DG-1000 glider, we need no thrust either. All on lift and wing design.
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Would the optimum using those excellent graphs be: Mach 0.50
graph 1, and, M0.71 in graph number 2 . . ?
graph 1, and, M0.71 in graph number 2 . . ?
Last edited by Natstrackalpha; 9th Oct 2013 at 18:56.
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Natstrackalpha,
Supposing you are referring to my post -
RE graph 1: The Boeing source does not provide data below M=0.5. The lowest TSFC will probably be static (M=0).
RE graph 2: Yes
Supposing you are referring to my post -
RE graph 1: The Boeing source does not provide data below M=0.5. The lowest TSFC will probably be static (M=0).
RE graph 2: Yes
