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Mirage 2000 Fuel flow


EvilKipper
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Hi, question about the mirage. Normally at a given altitude, fuel flow will increase as mach number increases. If you think about it this makes perfect sense, the engine has to burn more fuel in order to keep the optimal air fuel mixture as more air is entering the engine. In the F-15C for example this effect is quite large. I notice that this never happens in the mirage, is it just not modeled yet, or does the real plane actually behave this way? I can imagine several ways this might happen, e.g. the intake ramps might restrict airflow, or additional air might be forced to bypass the hot section.

 

By the way, love the mirage, it's my favorite plane in DCS by far, thanks for all the hard work.


Edited by EvilKipper
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Hi, question about the mirage. Normally at a given altitude, fuel flow will increase as mach number increases. If you think about it this makes perfect sense, the engine has to burn more fuel in order to keep the optimal air fuel mixture as more air is entering the engine. In the F-15C for example this effect is quite large. I notice that this never happens in the mirage, is it just not modeled yet, or does the real plane actually behave this way? I can imagine several ways this might happen, e.g. the intake ramps might restrict airflow, or additional air might be forced to bypass the hot section.

 

By the way, love the mirage, it's my favorite plane in DCS by far, thanks for all the hard work.

Yes you're right, fuel flow increases as you go faster as mass airflow through the engine increases. However the higher up you go the less dense the air. This means the engine finds it easier to rotate and so uses less fuel to rotate it as fast because overall the mass airflow has decreased as the air is less dense.

 

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I'd say the ram air's pressure increasing with speed reduces the energy needed for the engine to compress it afterwards, leading to less fuel consumption. Of course I'd also say that it depends on the engine designs and their best operating conditions for better efficiency.

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At 40k fuel flow is about 132 on full burner, regardless of speed

 

That just seems odd, even if the engine control system is able to restrict airflow, or reduce fuel flow without risking a flame out. I'd expect some change in fuel flow at a constant altitude at m 0.8 vs m 2.2. I'd be quite surprised to learn that the actual plane exhibits no change at all in fuel flow between these vastly different speeds. If it's just not implemented yet, fine, it's early access, but if that's so here is my vote that it gets implemented properly in the final product!

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It should change, last time i checked currently consumption is somewhere in the middle between high and low speed so the dynamics are probably okay.

 

But there should be change, I agree.

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Do you actually have a source for the data on fuel consumption at different speeds and altitudes? I found the below site, but I'm not sure how reliable it is, and it doesn't have data for different speeds at the same altitude.

 

http://www.mirage-jet.com/COMPAR_1/compar_1.htm

 

Good spot, didnt find that.

 

The data seems believable to me. I dont have hard data but a dry SFC and comparing it to similar engines it seems believable.

 

One should be able to reverse calculate air to fuel ratio and from that should be able to build believable speed vs fuel flow relation.

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Mass airflow increases as speed increases, the intake is designed to slow the air down to Mach 0.4-0.5. the engine cannot accept supersonic air but the intake spike creates Shockwaves to slow the air as it travels down the intake however the compressor still has to compress the air to gain maximum energy conversion. The engine is designed to Increase static pressure and decrease dynamic but I don't know the ins and outs of the mirage design. I know other aircraft in depth IRL but not this airframe.

 

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My gut feeling is that we are not going to find anything very surprising about the mirage's engine. It's designed to be simple, reliable, and easy to maintain first, fuel efficiency and performance seem to have been lesser goals. It's quite amazing to me that Dassalt was able to build such a great fighter with such an unimpressive engine. I suspect it works basically the same as every other supersonic jet. I guess maybe due to its initially low compression ratio of 9.8:1, vs 30:1 on comparable engines, maybe it gains more efficiency from ram compression at high speeds than other engines, but I'd still expect more fuel flow at a higher Mach number (at a constant altitude).

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If you don't alter the thrust lever position you don't alter the fuel flow. No matter what hight you fly.

 

Even in the old jets like MiG-15 you don't control fuel flow with the throttle directly. That's the responsibility of the fuel flow regulator (or FADEC computer in modern airplanes) which adjusts the fuel flow constantly according to the conditions outside, to keep the correct engine operating parameters. Which means changing the fuel flow according to outside pressure, speed and RPM.

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Depending on the engine, the engine controler could target a specific N1, N2, EPR, often also taking EGT limits into account, or even all of the above plus outside conditions, using some more complex algorithms.


Edited by some1

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I think I'm with hekktor on this one. You are settings fuel flow with the thrust lever. Period. Outside environmentals will play a part on what speed you will achieve with that power setting but you are commanding a specific and continuous fuel flow/thrust position. Now different environmentals will have an effect on fuel flow but with all things remaining constant it shouldn't change

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some1 is correct. You don't control fuel flow with the throttle directly. There is a fuel control unit (FCU) which takes into account several information about the engine, such as N1 (low pressure compressor or fan), N2 (gas generator), turbine discharge pressure, EPR, turbine inlet/outlet temperature or interstage turbine temperature, it varies with engine, not all engines have ITT for example.

 

Best example I can give you is the TF-34 of the A-10, at low altitudes, the TF-34 is hydromechanically prevented from reaching its maximum core RPM because the trim amplifier must trim the fuel flow in order to prevent the engine from exceeding it's maximum turbine temperature (865°C), so you aren't controlling the fuel flow with throttle position only, there is the max power trim. You would have direct control of the fuel flow if you were using the override mode of the engine, in this case FF is controlled directly by the throttle position, you can even exceed maximum turbine temperature doing that. Because you simply don't have the ITT amplifier working with the engine anymore.

 

I'm not sure about the Mirage engine, not sure how it works exactly.


Edited by Vitormouraa
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Actually RPM of the engine is controlled by means of fuel flow. More fuel=higher RPM, less fuel=lower the RPM. The discussion is how you are controlling the fuel flow. Older engines had an old fuel control unit which was controlled by the throttle directly.

 

Nowadays we have FADEC which has full authority of literally everything, inlet guide vanes, stator vanes, fuel flow, bleed air etc, the throttle is basically telling the computer how much thrust it wants, then the FADEC does what it needs to do to accomplish that request.

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The M53-P2 is RENPAR, REgulation Numerique Pleine Autorite Redondante (FADEC). http://www.icas.org/ICAS_ARCHIVE/ICAS1986/ICAS-86-2.8.1.pdf

 

Push the lever forward at low speed you send a big error signal to the controller that desired engine speed is higher than current. The controller schedules fuel based on a desired acceleration schedule. It's clear from the Figure 2 block diagram that some aircraft parameters feed into the controller. It stands to reason some of these parameters are changing as EAS changes with corresponding changes in metering. The controller is certainly smarter than to just schedule a fuel rate associated with that lever angle. The power lever is an RPM-command system which the controller accelerates by fuel metering according to the RPM error delta.

 

What fuel rate is associated with the commanded RPM is an accident of the system and the controller is constantly chasing that RPM with whatever fuel rate it needs without the lever touched.

 

Of course this is probably quite predictable as it has to be compared to a simulation channel to know if there is a fault. The simulation is supplied with the same aircraft parameter data as the main channel. What would be great is to have an exact list of what parameters those are.

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I am quite a noob at how turbines works.

 

However, i have read on a forum that the minimum RPM ( before engine fails to keep working ) of the Mirage III's engine was increasing along with speed and height, and that the max ceiling of the Mirage III was XX-XXXX ft because at this height, that minimum RPM was equal to the max RPM of the engine.

 

And what i have found out experimenting with M-2000 modules, but also Mig 21 and FC3 ones, is that the iddle RPM is almost, if not the same at 5.000ft at mach 0.5 and at 40.000ft and mach 2 ( arround 57% with the m-2000 )

 

I know there were years of developement between Atar-9 serie and M-53, but they are still quite close, so we should see some change in iddle RPM, shouldn't we ?


Edited by Vilab
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Same for the L-39

 

attachment.php?attachmentid=170035&stc=1&d=1506720779

 

Note: the L-39 idle rev's vary in DCS but IIRC are a little higher than the above schedule

 

For the L-39

 

a) 106.8% at the takeoff (maximum) rating.

b) 103.2% at the maximum continuous rating.

c) 99.6% at the 0.85 maximum continuous rating.

 

HP rotor speeds are set by the throttle position (and over-speed limiter) .

 

Intermediate throttle positions between idle and max continuous take altitude into account -

... the fuel control unit maintains fuel flow set by the altitude control depending on the pitot pressure at the engine inlet and on the fuel control unit throttle lever setting (HP rotor variable speed envelope).

L-39_AI-25TL_Engine_Idle.jpg.024fc99b6bd9bae0cf1da9db799c62d7.jpg


Edited by Ramsay
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I am quite a noob at how turbines works.

 

However, i have read on a forum that the minimum RPM ( before engine fails to keep working ) of the Mirage III's engine was increasing along with speed and height, and that the max ceiling of the Mirage III was XX-XXXX ft because at this height, that minimum RPM was equal to the max RPM of the engine.

 

And what i have found out experimenting with M-2000 modules, but also Mig 21 and FC3 ones, is that the iddle RPM is almost, if not the same at 5.000ft at mach 0.5 and at 40.000ft and mach 2 ( arround 57% with the m-2000 )

 

I know there were years of developement between Atar-9 serie and M-53, but they are still quite close, so we should see some change in iddle RPM, shouldn't we ?

 

Idle RPM will vary with inlet air density, with temperature and perhaps pressure. This will also vary with the inlet design, so an A-10 will be different from the Mirage which is going to be different from the SR-71 (yeah I know that's kinda obvious).

 

So at sea level, the engine runs more efficiently simply because the compressor needs to do less work compared to an altitude of 45,000ft, especially when the OATC (outside air temperature) is lower, so the air density will be a bit higher, thus the inlet density increases and the engine compressor needs to do less work (lower idle RPM) to keep the necessary air mass flow thru the engine, as you get altitude, the pressure is lower, therefore the oxygen molecules are more separated, so the engine has to do more work to keep the same air mass flow, and it does that with an increase in fuel flow which in turn increases the RPM.

 

Here's a chart showing the TF34-GE-100A minimum idle RPM. That's the N2, (high-pressure compressor - core)

 

 

sbRg42K.png

 

 

With an increase in temperature, air density decreases, therefore the engine has to increase RPM by means of fuel flow. The same thing will happen with altitude.


Edited by Vitormouraa
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