Mach Drag question!

[Guest LFMNboy]

Hello!I try to correctly adjust the 747-400 climb and descent performance.At 10,000 and 30,000 ft, the perfs are pretty good but I noticed that I need more drag around 20,000 ft.Is TBL 430 the right (and easy) way to correct it?And someone could help me understand how to adjust that particular table?Thanks a lot,LFMNboy:)

[WarpD 824]

I'm not an expert on this, but I don't think table 430 is the one you want to fiddle with. Table 430 simply controls the amount of mach drag created by specific mach speeds. Any change in this table would change performance at all altitudes.It would help a great deal to know exactly what climb/descent performance you're trying to adjust.

[Guest LFMNboy]

Dear Ed,Thanks for your support!Here are the perfs I am trying to adjust:300,000KGS /290KIAS /CLB THRUST /ISA CONDITIONSALTITUDE(FT) CLIMB RATE(FPM) FS DATA (What I get)10,000 3600 370020,000 2500 2700 (+ 8%)30,000 1500 1550You notice that around 20,000FT, the simulated aircraft performs better than expected.So, what parameters do I have to change in order to get that particular moment of the climb right?Cheers,LFMNboy

[WarpD 824]

Table 1506 is probably the one to be looking at. Based on what you've posted, your engines are putting out more thrust than they should at lower altitude(s).While this might seem odd, consider that 290 KIAS represents different mach values at each of those altitudes.

[GusRodrigues 1]

Consider also to check the 401 (dCl/dAoA vs Mach) values.Adjust 150x if you know what to change.if the reading of N1/N2 are wrong, adjust the 1502 (CN1 and CN2). Refine throttle in tables 1503 and 1504. You can adjust your Spool and Turbine CFF (corrected Fuel flow! not pph!) in table 1505.Adjust the turbine thrust factor vs CN1 in table 1506.Adjust the turbine air flow factor vs CN1 in table 1507.You will find a lot of documentation and tools at Mr. Herv

[WarpD 824]

Whoa... I don't think I'd go diving into all of those values for the problem you're having. Based on what you posted, it appears to have more thrust than it should, thus the higher than expected climb rates. Reducing the thrust in 1506 would be the FIRST stop for that.

[Guest LFMNboy]

Hi Gustavo,As Ed mentionned, it sounds a little to difficult for my little knowledge!But thank you for your help, it is very interesting to see how things interact between each others!Regards,LFMNboy

[Guest LFMNboy]

Hi Ed,Once again, thank you for your help and your very clear explanations!I will take a closer look at TBL1506 and see what happens...I'll let you know if I get into troubles:)Best Regards,LFMNboy

[GusRodrigues 1]

is really difficult, but is not the beast that appears to be.you may have much more trust than you needed, this can be a signal that your fm have to much parasite drag, also. this force some editors to send the 1506 thrust curve to the infinity and beyond.or, you have too much lift with mach increase, and there isn't a problem with the 1506.... and there goes on.IMO, the best way is try to not screw with those trust curve, unless you know what you should do, since they affect your PPH, and this can make your plane burn much more fuel than it should, or can burn much less fuel... also, this can affect your glide angle @ idle...

[Guest jcomm]

Assuming you're using correct Oswald Ef. Factor, and didn't mess with cm_q and cm_adot, you should probably start by trying some tweaking over Tb1504 - Turbine HiMach CN2 vs Tr & IAP Ratio.You can add a row for an IAP Ratio aproximate to PressSeaLevel(ISA)/PressFL200(ISA) and try your adjustments for different CN2 values.Beware that CN2 is N2 / Theta^(0.5)where Theta is (288.15 - 1.98*FL/10)/288.15Most of the time, simply "think" CN2 and don't bother finding the corresponding N2.Changes to either 1506 and/or 1507 will be trickier because they'll affect performance independently of the FL...Just my 2cents...

[Guest jcomm]

...around 2.1747, using 1976 Stand. Atmosphere...On the attached picture it means you should create an entry for 2.1747, adapt it based on the ones you already have in Tbl1504, and then it's trial & error....

[Guest jcomm]

>Beware that CN2 is N2 / Theta^(0.5)>where Theta is (288.15 - 1.98*FL/10)/288.15>Wrong...Duh!!!!... That's what happens when your writing while feeling very sleepy...CN(1 or 2) = N(1 or 2) / (theta * (1 + M^2 / 5))^ 0.5theta = (288.15 - 1.98 * FL/10) / 288.15 (assuming ISA...)That's it :-)

[Guest LFMNboy]

Thanks Gustavo!Actually, I have pretty much corrected the problem!So I am happy:)Cheers,LFMNboy