Just for clarity, in my example above I listed 5 5 5 5 to show how to apply the drag equally to all RPM, meaning enter one value into each CN1 column of a specific mach row.
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FS2004 .air Table 1507
- Thread starter Supercruise
- Start date
Just for clarity, in my example above I listed 5 5 5 5 to show how to apply the drag equally to all RPM, meaning enter one value into each CN1 column of a specific mach row.
Roy Holmes
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This is not something invented by MS. It is the basic equation for thrust.
You may find it useful to go here:
http://www.grc.nasa.gov/WWW/k-12/airplane/bgp.html
Then download their EngineSim applet.
If you run the applet and check the net thrust at zero speed and at some non-zero speed you will find that the net thrust does equal gross thrust minus ram drag.
You can also examine TSFC variations with throttle movement, fuel flow and a bunch of other parameters.
I use the applet to compare gross thrust variations with altitude with what FS computes. That is how I can state that there is a 25% approx reduced thrust result from FS at 40,000 ft. I plot the difference between the applet results and the basic FS results and adjust the turbine table values to recover the difference. I have an interactive spreadsheet where an adjustment to any table value is addressed by the other table values giving thrust outputs using the FSX equations and interpolation methods.
In stabilized flight with full throttle net thrust equals aerodynamic drag. With stock FSX turbine table values the resulting maximum airspeed or Mach is always less at high altitude than the Manual for the aircraft states. This is because FSX reduces thrust at altitude in proportion to ambient air pressure ratio. It does not consider the effect of fuel control systems that adjust HP fuel flow consistent with limits on engine temperatures. That is why it is necessary to adjust the tables.
However, 1507 is based on sound theory and calculates ram drag accurately for subsonic flight, so I never adjust it. There are fighters that can go supersonic using MIL, non-afterburning thrust, (EE Lightning, F-104, F-22 for example), but the majority are just subsonic at MIL power. The dominant factor in supersonic FS modeling is the afterburner table and it copes with potential errors in 1507 at supersonic speeds.
The reason for low taxi thrust is usually that the idle rpm is set too low
Roy
Roy, you must give me more credit than that...
I am aware of the equation, but it is not literally a reduction in thrust output. It is the sum of positive thrust coming out the back, and negative thrust from drag at the front. If you don't add the drag, and all you do is scale the thrust, that would be okay for simulation purposes IF the fuel flow remained the same. In other words, the engine MUST become less efficient.
I think you may be making the same mistake Microsoft made by reading that equation literally. What it means is, the engine is producing gross thrust, but there is no physical way possible to transmit that thrust to the airframe without also transmitting the counteracting drag forces. It does not mean the engine is producing less gross thrust pounds at the same altitude/temp. It means the amount of positive thrust transmitted through the mounts is counteracted. Fuel flow would remain unchanged at that gross thrust setting.
In reality, thrust actually goes up with Ram Drag, because in order for ram drag to go up, mass airflow must go up first, but lets keep it simple.
FN net thrust is not the same as NPF net propulsive force, which accounts for everything: cowl drag, inlet spillage, interference drag, fan blade spillage, pressure resistance, pressure recovery, Rn, and a few other things not included in the Ram drag equation you cited.... NPF is the real thrust number that determines speed, not FN, although if you have included everything they would be the same. This is over complicating things for our purposes.
Real world example from inflight test data:
In stabilized flight at FL250 M0.70, airframe produces an SFC of 0.45 with 8,130lbs D drag, 11,437lbs FN net thrust, and 5,188lbs fuel flow.
In stabilized flight at FL360 M0.70, airframe produces an SFC of 0.573 with 7,976lbs D drag, 8,225lbs FN net thrust, and 4,718lbs fuel flow.
In stabilized flight at FL250 M0.80, airframe produces an SFC of 0.605 with 9,909lbs D drag, 10,950lbs FN net thrust, and 6,626lbs fuel flow.
If you notice, FN is more than D. D = NPF, not D = FN ...but now we are getting too complicated for FS purposes. For FS purposes, the goal is to replicate performance. The only way to do that is controlled fluctuation in SFC. If you replicate the SFC of the engine, everything else falls in line.
Also, you must have misspoke -- "with full throttle," most jets will exceed Vmo...
As I said I agree with you, but I disagree that it's the density/pressure calculation on MSFS's part that is the culprit. You haven't taken into account SFC, which means that your test was flawed from the start. If you were to set your SFC to match the real engine at 40,000 feet, I guarantee that if your Cd0, Cdi, geometry, CL, and inflight AoA are correct, the thrust required to maintain speed will be dead on. The sim works as it should.
All MS did was scale thrust and fuel flow by airspeed which is absolutely wrong and the cause of your 25% reduction. If you were to set your thrust pounds to match the real aircraft without increasing the SFC, you would be too fast because you are not incurring the ram drag penalty.
So we (FDE designers) adjust down to make up the difference, then blame the sim.
This is the mistake I pointed out earlier that we all make.
I don't have this problem. And please, never ever use stock tables if you want realism... those were thrown together by programmers.
Also, Table 1506 IS the HP fuel control system. If your system does some fancy increasing at altitude then your tables should reflect that.
You reminded me of another bug... real ram drag is based on true airspeed not indicated. 1507 is based on Indicated. But overall it seems you are missing my points. I never said it wasn't based on sound theory. What I said is it doesn't add drag or reduce efficiency as it should. It MUST do one or the other, otherwise it is not correct. As it is now, it is just a silly scalar of thrust to CN1 by indicated airspeed.
My first recommendation to anyone tweaking their engine performance is to zero out table 1507 completely and start with idle descents and power off glides to verify CD. Then add thrust by thrust pounds and check with SFC settings. You will immediately know if you are there or not. CN1/CN2 tweaking is last.
If you don't have real-world data then of course there's no point, it's all a guess.
LMAO. Once again, PLEASE give me more credit!!! I'm not THAT slow. Of course my RPMs and fuel flow are set correctly.
That being said, this statement is absolutely wrong. If you set the correct RPM and fuel flow with a cruise SFC, you will be short on taxi thrust pounds, requiring more throttle. If that cruise SFC is lower than max altitude SFC then you will also be overpowered at max altitudes, which results in under powered due to typical FDE compensation.
If you search the various MSFS forums (including this one) you will find thousands of complaints about not having sufficient taxi thrust at the correct RPM and fuel flow. It is a well known and documented complaint. There are several sim.dll hacks out there that people use to fix (band-aid) it. What's the number they claim the thrust is off by?? 25%. In a real jet, SFC changes with RPM, altitude, mach, airspeed, temp, and hundreds of other variables. That is the culprit of your problems.
Trust me on this, I have spent almost a decade figuring it out. I have all the RPM/FN/CL/CD/CDi/EPR/temp/altitude/Fuel Flow/SFC data on my real world airline engine. I have dissected FS and the airfile... and have conducted every kind of experiment you can imagine to confirm or deny my own theories. I could never figure it out until one day it hit me....
The real world engine SFC is 0.29 in taxi, while it is in the 0.50s at cruise and in the 0.60s in climb. Of course my thrust is off in taxi, climb, descent, and high altitudes if I have been setting my SFC to normal cruise numbers. *light bulb*
So I set my SFC to 0.29 in the cfg file and viola... perfection in taxi. If I set my SFC to any inflight number corresponding to the real thing it simulates the engine perfectly. So then, my objective changed to figuring out how to manipulate SFC inflight.
One final question,
Is your gripe that thrust pounds are reduced by 25% or that it takes 25% more thrust pounds to maintain a given speed than the real aircraft?
Hope all this helps. If you have questions feel free to ask.
Roy Holmes
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There are really too many points you make to answer.
My main issue is about thrust, not SFC, nor fuel flow.
I fully understand how FSX calculates thrust and SFC is not one of the inputs.
The cfg file entry for TSFC just gives the correct fuel flow variable output. There is no way to adjust fuel flow in the air file.
I also understand aerodynamic theory as it applies to FSX and the real world.
In addition to over 10 years working with FS air files, I flew jet fighters for 20 years so I do have some practical experience on the subject of thrust.
One small point, the equations used with 1507 use true airspeed in ft/second based on Mach number.
Let us just agree to differ.
Roy
My main issue is about thrust, not SFC, nor fuel flow.
I fully understand how FSX calculates thrust and SFC is not one of the inputs.
The cfg file entry for TSFC just gives the correct fuel flow variable output. There is no way to adjust fuel flow in the air file.
I also understand aerodynamic theory as it applies to FSX and the real world.
In addition to over 10 years working with FS air files, I flew jet fighters for 20 years so I do have some practical experience on the subject of thrust.
One small point, the equations used with 1507 use true airspeed in ft/second based on Mach number.
Let us just agree to differ.
Roy
Right, and what I have been trying to tell you is the cause of your issue is the sims inability to manipulate SFC inflight. This would be possible if Table 1507 actually worked properly.
Hop in the sim, and maintain indicated airspeed while increasing true airspeed, then maintain true while increasing indicated. The sim will scale thrust according to indicated airspeed only. I think you get too focused on what the equations say they do as opposed to what the sim is actually doing.
I totally missed that you said this but that's exactly my point. In MSFS, TSFC is a set number in a CFG file....
on a real jet, SFC = mFF/FN
So if table 1507 worked properly and reduced usable thrust without affecting fuel flow that would directly change SFC, and thus fix your problem.
As it works now, it is impossible to get the correct fuel flow output throughout the full performance envelope. This can only be achieved at the SFC point set in the cfg file.