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F-16 Fuel Flow Drift


Glide
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3 hours ago, glide said:

You can fly those tests easily in DCS.  Start at mach .3, deflect to 85 pla, watch fuel flow until mach .75. 

 

I was watching the Grim Reapers turn tests on YT today.  I'm going to try some similar tests with the mach increase and without the mach increase. 

How do you figure out where 85 deg PLA is in DCS?

"Subsonic is below Mach 1, supersonic is up to Mach 5. Above Mach 5 is hypersonic. And reentry from space, well, that's like Mach a lot."

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2 hours ago, Machalot said:

How do you figure out where 85 deg PLA is in DCS?

They said 85 pla is full non-afterburner power.  So, right before the AB detent is my understanding.

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3 minutes ago, SpaceMonkey037 said:

Can you link that paper?

 

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"Subsonic is below Mach 1, supersonic is up to Mach 5. Above Mach 5 is hypersonic. And reentry from space, well, that's like Mach a lot."

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Not really sure what the point of this thread is since it seems like your questions were addressed in multiple replies already, but I chuckled at this bit:

 

19 hours ago, glide said:

I think this is why folks say they don't feel the sense of speed in the game.  I think it's not the sense of speed they miss, but the sense that there's a rocket attached to your chair and it wants to accelerate.  I think if fuel flow was "stable", we would feel the jet's desire to accelerate more as we bleed off energy in turns and such.  Right now it feels like a fast airliner.

 

Unless your computer desk is set up atop an actual rocket sled, I'm afraid you're stuck with the rest of us judging performance in DCS through numbers and not by "feel".

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The throttle simply gives commands to the main engine controller about the desired thrust. MIL power (the AB detent) giving you the most thrust available without AB, and idle giving you the least. The main engine controller controls fuel flow to the engine, nozzle position and the inlet guide vanes. The main engine controller controls these three things so to always keep the engine within envelope and to give you the desired thrust. At high alt for example there isn't as much air, so the engine can't burn as much fuel and so the fuel flow will be less. As you accelerate, deaccelerate etc, the parameters change and the main engine controller must compensate for this by changing for example the fuel flow. That is the reason for why, when you're accelerating, you will see changes in the fuel flow rate.

To make this more logical: MIL power (the AB detent) tells the MEC (main engine controller) that you want max available thrust without AB, as stated above. The MEC will then supply the engine with the highest amount of fuel possible without drowning the engine. The amount of fuel possible to supply to the engine is limited by the amount of oxygen available. A lot of oxygen means that you can supply the engine with a lot of fuel without drowning the engine as the fuel will be able to burn. Little available oxygens means that the engine won't be able to burn as much fuel and the MEC will compensate for this by giving the engine less fuel. When you're low on speed, and then go throttle MIL, you command the MEC to give max thrust. It will then supply the engine with the maximum fuel without drowning the engine. Gradually when your speed increases, available air also increases and the MEC will supply the engine with more fuel to take advantage of this, meaning that you always get the maximum available thrust. At some point your speed will cap out where the engine is either receiving the maximum possible fuel flow, or the MEC will reach a balance where there isn't enough air to increase the fuel flow. Either way you will reach unaccelerated flight.

TL DR: Fuel flow rate varies based on available air and throttle setting. It is normal behaviour that fuel flow will increase when accelerating straight and level. 
I might have slight inaccuracies on places where the DEC (digital engine control) actually controls instead of the MEC, other than that everything should be valid.


Edited by SpaceMonkey037
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Posted (edited)
12 hours ago, SpaceMonkey037 said:

It is normal behaviour that fuel flow will increase when accelerating straight and level.

Yes, we established that the increase is due to the N1 fan speed increasing.  What's incorrect is that the added fuel is not creating acceleration.  It's a straight line increase to airspeed, not a curved increase. 

 

But what about deceleration?  The jets lose fuel flow as the airspeed decreases.  The mechanical nature of the thrust levers would not allow the fuel flow to drop past full military thrust as the airspeed drops.  Full military thrust on the ground test was just below 10000 pph.  Therefore, in a tight turn, my fuel flow should never drop below 10000 pph because my thrust levers are at PLA 85 right before the AB detent.  This is why the jets turn to slushy mush in a turning fight.  As you slow down, you lose the N1 fan speed and the extra push it gives you, but N2 should stay constant as long as the levers don't move.


Edited by glide
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38 minutes ago, glide said:

Yes, we established that the increase is due to the N1 fan speed increasing.  What's incorrect is that the added fuel is not creating acceleration.  It's a straight line increase to airspeed, not a curved increase. 

I am unsure about exactly how the fuel flow increases, all I know is why.

 

38 minutes ago, glide said:

But what about deceleration?  The jets lose fuel flow as the airspeed decreases.  The mechanical nature of the thrust levers would not allow the fuel flow to drop past full military thrust as the airspeed drops.  Full military thrust on the ground test was just below 10000 pph.  Therefore, in a tight turn, my fuel flow should never drop below 10000 pph because my thrust levers are at PLA 85 right before the AB detent.  This is why the jets turn to slushy mush in a turning fight.  As you slow down, you lose the N1 fan speed and the extra push it gives you, but N2 should stay constant as long as the levers don't move.

 

As mentioned in my previous post, fuel flow is regulated by the amount of air available. At ground level you have the most dense air, meaning you will be able to supply the engine with a lot of fuel without drowning it. At higher altitudes the air is less dense and so the engine can't handle as much fuel, speed will increase the available air, but if you're going very slow it might not help that much. Because of this it is definitely possible do dip beneath 10000pph.

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3 hours ago, glide said:

Yes, we established that the increase is due to the N1 fan speed increasing.  What's incorrect is that the added fuel is not creating acceleration.  It's a straight line increase to airspeed, not a curved increase. 

 

But what about deceleration?  The jets lose fuel flow as the airspeed decreases.  The mechanical nature of the thrust levers would not allow the fuel flow to drop past full military thrust as the airspeed drops.  Full military thrust on the ground test was just below 10000 pph.  Therefore, in a tight turn, my fuel flow should never drop below 10000 pph because my thrust levers are at PLA 85 right before the AB detent.  This is why the jets turn to slushy mush in a turning fight.  As you slow down, you lose the N1 fan speed and the extra push it gives you, but N2 should stay constant as long as the levers don't move.

 

I feel like you're not reading any of my posts. 

"Subsonic is below Mach 1, supersonic is up to Mach 5. Above Mach 5 is hypersonic. And reentry from space, well, that's like Mach a lot."

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1 hour ago, Machalot said:

I feel like you're not reading any of my posts. 

Sorry, I had target fixation on making the DCS Viper better.  Nevermind.  It's perfect.

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1 hour ago, glide said:

Sorry, I had target fixation on making the DCS Viper better.  Nevermind.  It's perfect.

That was not what he meant what so ever. Imo, my explanation for why why the fuel flow increases as airspeed increase with throttle in MIL power was pretty good. I added a TL DR for the people skimming through this topic just to give a very brief explanation of what I was referring to in my post. You quoted the TL DR part, which wasn't perfect, i admit that, but it was only meant as a brief summary of what I was talking about so it shouldn't be expected that is is perfect either. In the post I stated the exact reason for the fuel flow changes, yet in your next post you seem to have taken the TL DR a bit too seriously and used it to argue for why the F-16 is incorrectly modelled, completely ignoring the statements made in the meat of my post. For that reason it wouldn't come as a surprise that you might have very quickly skimmed through our posts. 
Of course we want the best for our sim, but the issue you're describing must be assumed to be correct behaviour based on the statements which have been made by, what seems like, people with sufficient knowledge on the matter. You don't seem to have any sources stating anything about how a jet engine should operate in different environments, let alone the actual F-110-GE129 engine.

 If the issue isn't that you're not reading, but rather that you're not understanding, please ask about the unclear things so that we can aid you in understanding. I think everyone who's posted on this topic have an interest in heling people learn and making the sim better, and no doubt that it has it's flaws, but pointing out mistakes without sufficient backing won't get you far.
I hope you are able to understand this and that we can continue the discussion sensibly. Safe flights!
 

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Is there any difference between engine thrust and engine power? Is the PLA to output a linear relationship?

e.g. IDLE, A Watts. MIL, B Watts. PLA at mid setting engine output A+B/2 Watts or replace Watts for Newtons?

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17 minutes ago, Frederf said:

Is there any difference between engine thrust and engine power? Is the PLA to output a linear relationship?

e.g. IDLE, A Watts. MIL, B Watts. PLA at mid setting engine output A+B/2 Watts or replace Watts for Newtons?

For a jet engine, the conventional definition is

 

Power = Thrust x Airspeed

"Subsonic is below Mach 1, supersonic is up to Mach 5. Above Mach 5 is hypersonic. And reentry from space, well, that's like Mach a lot."

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1 hour ago, Frederf said:

Is there any difference between engine thrust and engine power? Is the PLA to output a linear relationship?

e.g. IDLE, A Watts. MIL, B Watts. PLA at mid setting engine output A+B/2 Watts or replace Watts for Newtons?

I'm no engineer, but think of the PLA as a percentage. MIL gives you 100% thrust without AB, aka maximum thrust. IDLE gives you 0 % thrust of what the engine can deliver, you're obviously getting some thrust, but it's the least possible with the engine still running. Mid setting demands 50% thrust. etc. etc.
The power lever angle tells the MEC what percentage of thrust output you want. In PRI the MEC will then give that command to the DEC (digital engine controller) which will then use sensors and fancy math stuff to tell the MEC how much fuel to give etc. The DEC also controls some things like AB fuel flow, nozzle angle etc.. 
Just, the power lever angle gives you a percentage of the maximum thrust the engine can deliver, that simple. Safe flights!

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Just now, Skysurfer said:

 

That's assuming constant PLA (MAX AB or MIL).

I already quite clearly stated how fuel flow increases/decreases with airspeed in my original explanation. Don't take it personal, but you're not adding anything to the conversation except confusion by not including all the assumptions made in your statement to come your original conclusion. Just keep it in mind 🙂 Thanks.

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4 hours ago, SpaceMonkey037 said:

I'm no engineer, but think of the PLA as a percentage. MIL gives you 100% thrust without AB, aka maximum thrust. IDLE gives you 0 % thrust of what the engine can deliver, you're obviously getting some thrust, but it's the least possible with the engine still running. Mid setting demands 50% thrust. etc. etc.
The power lever angle tells the MEC what percentage of thrust output you want. In PRI the MEC will then give that command to the DEC (digital engine controller) which will then use sensors and fancy math stuff to tell the MEC how much fuel to give etc. The DEC also controls some things like AB fuel flow, nozzle angle etc.. 
Just, the power lever angle gives you a percentage of the maximum thrust the engine can deliver, that simple. Safe flights!

OK, you just said "engine power" last time and not "engine thrust". It would be like if 50% gas pedal in a car gave 50% of available torque or 50% of available horsepower. If power is just thrust x speed then those fractions are the same.

 

I also didn't know if the response was linear. Maybe half way between IDLE and MIL isn't 50% available thrust. Nothing is to stop an engineer building in a curve to it.

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On 3/24/2021 at 4:59 AM, glide said:

Yes, we established that the increase is due to the N1 fan speed increasing.

I think you have the causality backward.  The fan is spun by the low pressure turbine, which is driven by exhaust from the core.  The only way the low pressure turbine spins faster is if the core is producing more power, which is caused in this case by increased air mass flow rate.  The air mass flow rate is primarily driven by airspeed, not by N1.  N1 goes up as a downstream effect of increased airspeed.

 

On 3/24/2021 at 4:59 AM, glide said:

What's incorrect is that the added fuel is not creating acceleration.  It's a straight line increase to airspeed, not a curved increase. 

This is an assertion which you have provided no data or physics to support.  On the contrary, the expected trend of acceleration is to initially rise after advancing the throttle, but then to decrease and eventually vanish as drag rises to match thrust.  The airspeed should increase and gradually level off at Vmax.  Stepping through the process:

  • In steady level flight, drag equals thrust. 
  • When you push the throttle, thrust exceeds drag and the jet accelerates. 
  • As airspeed increases, both drag and thrust increase, but drag goes up much faster than thrust.  For example, in test condition A from the NASA paper, as Mach goes from 0.344 to 0.758, gross thrust goes from 16,793 lb to 24,189 lb, which is a factor of 1.44 increase. 
  • Meanwhile, dynamic pressure increases by a factor of nearly 5, driving drag up proportionally.  There's some reduction in the induced drag coefficient because less angle of attack is needed at higher speed, but overall drag should still be higher.

 

Here's a qualitative picture of what's happening with thrust (red) and drag (blue).  The vertical gap between the red and blue curves is net positive axial force (thrust minus drag), which gives acceleration.  The gap shrinks as Mach increases, and acceleration tapers off.  Once they converge, acceleration is zero and the aircraft reaches its maximum airspeed.

 

image.png


Edited by Machalot
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"Subsonic is below Mach 1, supersonic is up to Mach 5. Above Mach 5 is hypersonic. And reentry from space, well, that's like Mach a lot."

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8 hours ago, Frederf said:

OK, you just said "engine power" last time and not "engine thrust". It would be like if 50% gas pedal in a car gave 50% of available torque or 50% of available horsepower. If power is just thrust x speed then those fractions are the same.

Sorry for the confusion, yeah I used power and thrust to indicate the same thing. Might be wrong of me to assume both mean the same thing, will try to stick to thrust from now on.

8 hours ago, Frederf said:

I also didn't know if the response was linear. Maybe half way between IDLE and MIL isn't 50% available thrust. Nothing is to stop an engineer building in a curve to it.

I don't see why it wouldn't be linear, either way if it's linear or not isn't really mandatory as long as it works it works.  

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