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Boeing 747 NASA Corrected Net Thrust - Where am I going wrong?

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228
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us-kentucky
Hello all,

With all of the stability and control derivatives of my 747 decided, it is now time to get thrust figured out. I am about 95% confident in my values of N1 and N2, and I have begun writing my own fuel flow system so I am not concerned with that.

I am using the method of setting table 1507 ram drag to all zeroes, and using 1506 for net thrust. Thankfully, NASA gives me corrected net thrust as a function of EPR and mach number, seen in the two below pages:

liRqS90.png


XGTg2TI.png


A later page gives CN1 vs. EPR, so I know exactly how to correlate CN1 to thrust by reading from the two tables, and I have formed a graph for Mach 0 and Mach 0.8 in table 1506 (others will come later).
It looks like this:

Gr5nluW.png


However, this graph results in far too much thrust at mach 0.8. I have been referencing this thread...


... And I noticed that jx's JT9D data gives 14600 lbs of net thrust at Mach 0.8 and CN1 of 102.4, whereas the NASA chart is showing around 41000 lbs.

What am I missing here?
 

Heretic

Resource contributor
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germany
I have a really hard time trying to understand the applicability of this chart. All I can take from it is thrust vs EPR at mach zero. The rest does not make any sense whatsoever.

Therefor, I'd just take what jx_ posted as a reference and model parts of the table from there, then set up a flight test condition in FS and start adusting lift and drag because those play a major role in performance as well.

I also have a LibreOffice Calc spreadsheet for modeling table 1506 (and others) with a simple (incomplete) performance test section. Holla if interested.
 
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us-kentucky
I think I found the answer I was looking for during some experimentation, and it comes down to some wierdness in how FS handles thrust.

Just for the heck of it, I set the Mach 0.8 line to 1.0 at several consecutive points and then used AJPC to read how much thrust that was... I don't remember exactly how much, but it was around 78000 lbs of thrust per engine! I am not sure what FS uses to calculate that...

Using this method, I figured out what the "1.0 thrust" for each mach line was, and then set the net thrust from the charts I have against that (I found a different, more accurate set directly from PW which has done me wonders, even includes fuel flow and EGT graphs!)

After doing this and double checking my lift and drag, my thrust numbers are now within about 1% in most places, though certainly not all.

One question I do have, in testing my thrust numbers I've come across an oddity: At 720000 lbs, FL270 and 340 knots, my Northwest performance manual lists 1.22 EPR as the predicted cruise thrust. Converting that EPR to CN1 and then to N1 under ISA conditions gives me about 87-88% N1. However, at the same weight and roughly the same mach at FL320, the manuals give an EPR of about 1.38, which is about 89.8% N1. The Mach number at FL270 is 0.826 and at FL320 is 0.84.

In FS, my airplane requires around 90.8% N1 at FL270 and 90.1% at FL320. I have checked and double checked my thrust graphs and I am really not sure what would cause such a dramatic difference from the sim to my real world. Maybe my Mach 0.9 graph is off?

My theory is that it has to do with CL0 with mach, which I have tuned as closely as possible to give the correct lift coefficient for a given angle of attack. It doesn't seem to affect induced drag at all, and so in order to add in the extra drag I had to add a small amount to CD0 with mach, but that has affected many other numbers as well.

Anyway, enough rambling for now. I appreciate the advice Bjoern!
 
Last edited:
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2,077
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us-ohio
One thing I know... air density at altitude does not match real world. I don't remember the details, but I do know it means that upper altitudes are not in alignment with the real world with regards to density... which messes with thrust results.
 

Roy Holmes

Resource contributor
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I'm fairly sure the atmosphere is correctly modeled up to the tropopause and that instead of the pause it keeps the same lapse rates so is inaccurate above 36500 ish feet. Would not make much difference at the heights 747 fly.
Roy
 

Heretic

Resource contributor
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6,830
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germany
One question I do have, in testing my thrust numbers I've come across an oddity: At 720000 lbs, FL270 and 340 knots, my Northwest performance manual lists 1.22 EPR as the predicted cruise thrust. Converting that EPR to CN1 and then to N1 under ISA conditions gives me about 87-88% N1. However, at the same weight and roughly the same mach at FL320, the manuals give an EPR of about 1.38, which is about 89.8% N1. The Mach number at FL270 is 0.826 and at FL320 is 0.84.

In FS, my airplane requires around 90.8% N1 at FL270 and 90.1% at FL320. I have checked and double checked my thrust graphs and I am really not sure what would cause such a dramatic difference from the sim to my real world. Maybe my Mach 0.9 graph is off?

1) Check that both charts refer to the same engine version. For JT8D's for example, rated thrust EPR varies between the lower thrust -9 and the higher thrust -17, so cruise EPR will differ.
2) Charts are mostly theoretical and what you will read there will not necessarily reflect figures seen on a bog standard airline flight since engine wear, aerodynamic imperfections on the airframe, etc. all influence performance.
3) You can only ever accurately tune cruise performance for a single setpoint as MSFS' flight modeling is too simplified and linear to get accurate figures for any point in your envelope. I've noticed this while tuning the 727 and Lear 35. So pick a cruise condition that may be the most common one and tune for that.
4) As you've noticed, drag also plays a role, but may throw off behavior in other flight conditions.

But generally, I'm happy that you've figured it out. :)
 

jx_

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555
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unitedstates
@Heretic Performance charts are 0% theoretical!!! The charts are demonstrated values only! The whole point of the generalized system is "ALL TEMPERATURE, ALL ALTITUDE". Please don't suggest people try to fudge it to fit one flight regime! It just leads to more badly made airplanes. The sim models the subsonic aerodynamics perfectly with 2 exceptions...

1) Fuel Flow should be re-written from gross thrust instead of net thrust
2) induced drag should include the additional lift resulting from mach effects. It does not.

If you observe an airplane in sim that has numbers that do not match the real airplane in the same conditions, it has a poorly made airfile. Aircraft.cfg scalars should all be 1.0 and no tricks should be used. If the correct coefficients are used and the 2 points I outlined above are allowed for, all scenarios will be accurate under all conditions within +/- 1.5% (rounding errors) from the real airplane.

- of course this excludes Prop engine performance.
- if we don't fudge numbers, we can trouble shoot problems easier. For example, once Chris has the real thrust/CN1 locked in, he will know for sure (100% certainty) if his drag is off or not.


@WarpD atmosphere density is correct as Roy pointed out.




@chris there is no "weirdness in how FS handles thrust. " If you see "weirdness" ask and I will try to explain.



First, you are mixing up terms. We must be careful of our baselines.

#1 My chart is for the -7RA, yours is for the -3 (probably not a huge deal but they will be different somehow)
#2 look at the reference symbol on the charts. My thrust value is FN/δ_total. The chart you posted is FN/δ (will not make a difference at sea level, but makes a big difference elsewhere).
#3 your chart is for installed net thrust, JT9D engines---my chart is for installed net thrust, JT9D engine
#4 my charted thrust is in daN, yours in in lbF; 14600 daN = 32822 lbF

details are important here ;) Be careful to check your data notations. If the data doesn't notate it then don't trust it.



so....to answer your question

Your chart is INSTALLED net thrust, JT9D ENGINES. These two bold words mean two things. The engine demonstrated (not theoretical) coefficients installed on the airplane; and more relevant to your question - all engines. How many engines are there on the B747?


- you need to divide into 4 engines
- you need to convert daN to lbF to compare my chart to yours.


according to your table....

At M0.00 1.6EPR = 58K lbF
58 / 4 engines = 14500 lbF per engine Static Sea Level

At M0.80 1.6EPR = 42.5K lbF
42.5 / 4 engines = 10625 lbF per engine at M0.80 Sea Level


In 1506, your thrust should fall from 14500 to 10625 from static to M0.80 at 1.6 EPR.


We need to enter the per engine values into table 1506. And again, once you have entered the real engine data, if your thrust settings vary from the real aircraft, it's the rest of your aero that's wrong NOT the sim!! You will need to work out the correct lift and drag numbers to get the rest of the picture.


Enter all ten of those lines from your chart above into the airfile after dividing them into four engines, zero 1507, and your thrust is done perfectly. Be sure to calibrate your CN1/thrust relationship. Be sure to set aircraft.cfg scalars to 0 and verify all engine section settings. Verify the thrust set in the aircraft.cfg is set to the value of thrust at 100%CN1!!! Most engines are rated for a specific RPM value that is not 100%CN1.



P.S. Remember the math don't lie, but you can easily break the equation by guessing.
 
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