Question out of interest.
Noticed during climb fuel flow does gradually decrease (A320) - i do climb with constant climb power on manual throttle, controlling speed with pitch basically simulating an open climb in vnav.
Now obviously fuel flow would decrease at altitude due to reduced drag.
I’m wondering though unless i’m missing something obvious, FF shouldn’t decrease on climb power if constant thrust is applied. is that just the way IF does simulate fuel burn at altitude or does that actually happen in real life
Would be interested to know
This is an interesting question and not something I’ve thought about before. I wonder if engine temperature has something to do with it, since at altitude the temp can be in the negative degrees centigrade.
There are many different factors that effect fuel flow especially during climb, compared to cruise you usually use up to 2 to 3 times more than cruise per hour. This has to do with energy management, ISA conditions and for example wind direction and strength. Usually your fuel flow would be higher during climb no matter what thrust setting your using
Hope this sheds some light on it
Well yes that’s sort of my point - regardless of any other factors like temperature etc - i keep my climb power constant - let’s say at 86% - so my point is why would FF decrease if thrust settings don’t change - might very well be just the way IF calculate things though
So, the question is efficiency: How much fuel does it take to produce a given amount of thrust and why would it change with altitude?
If I understand correctly, if temperature didn’t decrease with altitude, the dropping air density would make thrust more fuel hungry.
But the drop in temperature increases the thermodynamic efficiency of how much thrust you get from fuel (there’s a steeper energy gradient).
And the gain in benefit from temperature dropping, beats out the loss in efficiency from density dropping. (someone correct me if I’m wrong)
So, for a given thrust setting, fuel consumption should decrease with altitude, mostly due to lower temp.
Another efficiency issue: there is optimal efficiency by hitting the “rpm sweet spot.” And I’m assuming that is related to the fixed pitch angle of the turbine blades causing an optimal turbine spin rate in relation to the incoming airflow speed. The turbines are mini wings subject to the same angle of attack efficiencies that the aircraft’s wing is confronted with.
Not really. Your fuel flow and thrust would both decrease. A given RPM setting would take less fuel and give you less thrust. But this is hypothetical and not consistent with reality.
Temperature does play in though because until you hit the tropopause temperature is not nearly constant. Lower inlet temperatures will increase efficiency and decrease fuel flow for a given thrust amount. Thrust specific fuel consumption and temperature are inversely related in this way. So since temperature generally decreases with altitude this is also working to decrease fuel consumption as you climb.
The stated problem is to hold thrust constant, and ask what would happen to fuel consumption and why.
A fixed rpm setting doesn’t imply a fixed amount of thrust, which is why thrust itself was held constant.
Yes, that was the key point.
Yes, thanks, I think I said that:
Perhaps it is the “rpm sweet spot” that fixes why thrust is allowed to vary, in terms of fuel efficiency, but of course thrust must decrease anyway to accompany drag reduction to prevent over speeding?
What I am saying is that constant thrust would not be applied as @ch.glaser describes. He is holding the throttle in a constant spot, that does not mean constant thrust is applied. Thrust available will change with both altitude and speed. In a jet thrust available won’t change a heck of a lot with speed since a couple of effects largely cancel out. (Though of note a reciprocating engine does have thrust change a lot with speed.) But with altitude it decreases. It is more complicated than this, but as a broad oversimplification you can just think that a jet accelerates air, that’s its whole job, if there is less air to accelerate the engine will not create as much thrust.
Also, as we get into the weeds here it is worth uncoupling power and thrust. Jets are thrust producers. Power is thrust x velocity, so while power is not produced by the engine directly (as it is for a recip) we can see that as thrust available decreases so must power available.
I’m also pulling most of this out of my aircraft performance class, and honestly I have no idea how well infinite flight simulates any of this, so your mileage may vary in the simulator.
“while the percentage RPM setting represents a fixed proportion of the engine’s maximum capacity, the actual revolutions per minute can vary based on environmental and operational factors to maintain consistent performance and efficiency” (I haven’t thought it through…)
I know that, and it’s an important clarification, but I was interested in:
Thrust is force, and power is energy per unit time. So, depending on the amount of air mass represented in force expressed as a change in momentum, the power consumed changes to sustain a given thrust force.
That’s why I wanted to look at it from the perspective of holding thrust constant: to consider what causes the change in power demand to achieve that given force.
Temperature for example will vary the maximum RPM that an engine can operate at. Again this circles back to my point that holding throttle at a constant point really does not correlate to a constant amount of thrust.
Is that purely an engine fatigue/stress issue, or is there more to it? (I’m really curious about how the turbine blade profile, turbine blade AoA sweet spot if you will, affects the relationship between air density, actual turbine revolution speed, optimal fuel mixture, and speed of air entering the turbines).
Well AOA will change with speed, the turbine section though will have relatively constant speed so it is easier to optimize. It’s really the fan at the front, and the compressor that will have more variable AOA. I’d hazard to guess most commercial engines are optimized for efficiency in cruise. Though some engines for different applications may be optimized differently. A military focused engine targeting something closer to peak power at lower speeds strikes me as a possibility.
I guess the turbine profile will affect this a bit, and so will density, but this is really more of a mechanical limitation in terms of max RPM. What the given RPM is varies more based on how much fuel you are burning and how much energy that is putting into the turbine section.
The turbine may not even be at the same RPM as the rest of the engine, or even itself. There are split turbine engines out there where part of the turbine powers the compressor then the other part powers whatever needs spinning outside the engine (prop, fan, wheels). The PT6 is a very common example of this split shaft design, though it is used for turboprops, not turbofans, I am not aware of a turbofan that does this. Some newer engines also have a reduction gearbox which allows the fan to run at a lower RPM than the compressor and turbine, which is a huge efficiency advantage as the fan is more efficient at lower RPMs and the compressor/turbine section is more efficient at higher RPMs. This though is all a bit of a side tangent lol
Optimal fuel mixture for maximum power is going to be more an issue of what temperatures your combustor will be able to tolerate. As close to the Stoichiometric ratio as possible will be the most efficient, anywhere below that you are leaving fuel or air on the table and sacrificing efficiency or peak power respectively for lower temperatures. The Stoichiometric ratio will be constant so as air density decreases you will need to add less and less fuel to maintain it.
Speed of air entering the turbine section should be relatively constant, as should exhaust velocity. This is why propulsive efficiency increases with increasing speed because in theory you have perfect propulsive efficiency when exhaust velocity and free stream velocity are equal, though this can never be achieved.
I’m going to need to work this through in the morning because I’m too tired to think through this, I’m not sure if your just wording it differently to how I have been shown but for example power consumed doesn’t really make sense to me in this context. Perhaps a more awake mind will make better sense of it.
Again not sure if it’s just a wording thing but the power demand doesn’t affect force. Power required and thrust available are completely independent, though changed based on some of the same variables.
Honestly this has probably become a discussion on aircraft performance a little out of the scope of the original question. @ch.glaser the fuel flow decreases because of decreased air density. There are a lot of factors at play, and you can take whole classes on this stuff (I just did, it’s as tedious as it sounds 😭) but that is the major variable at play here. There are smaller contributors but I don’t know how much infinite flight is simulating the finer points of how a turbine engine generates thrust and power and how much of both you have available in given conditions.
There are several YouTube videos to explain more in-depth about this exact topic but this is the reason most jets fly as high as possible for their given weight and why step climbs are so important on long flights
Power required does not refer to the fuel required, at least not directly. Power required, measured in horsepower, equals thrust required (which is just the drag curve) times airspeed.
Thrust available is relatively constant in a jet; in a reciprocating aircraft, it decreases roughly linearly. This is because power available remains roughly constant for a reciprocating engine. This is due to inherent differences in how they propel the aircraft, and this is why we refer to jets as thrust producers, and recips as power producers. We say they are “producing” the one which stays (roughly) constant with speed. Yes, both do “produce” both and I’m not sure a physicist would love the term produce here, but that’s the aviation vernacular to differentiate the two. Regardless, power is thrust force x velocity, this is why power increases with velocity in a jet because thrust is remaining (roughly) constant as speed increases. This is also why reciprocating aircraft have a power rating, in hp, and jets have a thrust rating, generally in pounds.
Little disclaimer of sorts
Of note that this is all referring to a pure jet, or a pure reciprocating engine, pure recips are common but pure jets are nonexistent nowadays, so a turbofan for example will have some slightly blurred characteristics, but these are helpful concepts.
Now that we have gotten thrust and power available out of the way let’s tackle required. This stays the same for both a jet and a recip as this has to do with drag. Drag is a force, pushing back on you, opposed by thrust, so the thrust required curve and the drag curve are exactly the same. Assuming we are playing in the same units of course. The power required curve is drag times velocity. Refer to the same P=F•V relationship referenced earlier.
So while thrust and power are related (through the P=F•V equation) they are not the same. Power is work over time, so there is a displacement involved. So while work will be in J, and does therefore refer to energy, power available or required do not refer to the amount of fuel energy available or required, they refer to the amount of work an engine can do over a certain displacement. How efficiently that engine does it (fuel required) is a different matter.
Relating this all back to my original point, as we climb thrust available will decrease, as will the fuel flow for any given throttle setting as the air gets thinner and thinner.
I think we need to go back to the basic purpose of a jet engine:
to provide thrust as one of the 4 fundamental forces acting on the aircraft: weight, drag, lift and thrust.
The balance of those forces determines how the aircraft moves through the 3 dimensions of space. The pilot’s job is to manage the relationship of those forces (for the duration of the flight).
The two key questions then become:
1)how much thrust is necessary in relation to drag, and in the case of a climb, how much thrust in necessary for work done against gravity. But in total, how much thrust?
2)how much power is consumed in providing that necessary level of thrust?
Thrust is achieved in the engine by changing the momentum of a mass of air using the energy from fuel burn.
The change in momentum always exactly equals the amount of thrust.
But power depends on the mix of mass vs the velocity of that mass, in that air that undergoes the change of momentum.
In other words, if in one second the momentum changes by: momentum = m x v, you can juggle around mass and velocity however you want, as long as the momentum change in that second stays the same (to get you a given number for thrust).
But it takes more power as you make v bigger and m smaller (for the same momentum change): because the kinetic energy squares the velocity term (more kinetic energy in that second is more fuel burn, so the burn rate is higher).
As for:
The question I have about this: suppose you climb at constant vertical speed and constant IAS (so TAS continually increases due to air density dropping, making IAS higher in relation to TAS).
Constant IAS means that as you climb the engine experiences constant dynamic air pressure.
So, the question is: does the fuel burn really decrease due to lower air density?
(it’s true that static pressure decreases, but dynamic pressure, the amount of air supply, stays the same)?
But is it true or not, that flying right at the service ceiling may not be the most fuel efficient because the angle of attack necessary to sustain that upper limit, might add a bit too much drag for optimal performance?
edit: It hit me later this question was ill-conceived. The straight line drawn up diagonally through the middle of the optimal step-climb, is the ideal altitude profile to fly, creating the best balance of engine and aerodynamic efficiency tradeoffs for the continual lightening due to fuel burn.