# For FDE editors: How does FS9 calculate N1?

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How does FS9 calculate N1? This value seems very arbitrary! In FDE, everything is done in terms of CN1. I can set CN1 to remain constant from sea level to nearly any altitude with a good IAP ratio.For example, on this freeware panel I'm using, I'll set climb thrust to something like, say, 108% N1. The value is gradually decreasing from sea level and up. By FL350, for example, the N1 value (and N2 for that matter) would have dropped to something around 103% or so, whilst CN1 (via AFSD reading) remained at 108% throughout the climb.My question now is, is there any way to take control of how N1 is calculated the keep it constant? At high altitude, when firewalling throttles, the N1 never reaches the max values whilst CN1 most certainly does.Let me know if I need to be more specific here. This is a real annoyance. Does FS9 simply pull this N1 value out of its rear end? What does it answer to?

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You do understand that N1 and CN1 are not one and the same? N1 should reflect altitude change, CN1 should not. Sounds like it's working as expected to me.

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I know this, but I'm trying to determine the formula FS uses to calculate its N1 decrease per speed and altitude. It falls off so much that seems unnatural.CN1 is a lot easier to use because it's an absolute indication of power in FDE that the aircraft is creating.

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Check with Ron at the airdecode forum: http://www.avhistory.org/scripts/MegaBBS/f...w.asp?forumid=5JohanA LITTLE LESS CONVERSATION, AND A LITTLE MORE ACTION PLEASE

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In MSFS, Nx = CNx * sqrt((TAT+273)/288)where x = 1 or 2, i.e. N1 or N2TAT = Total Air Temp in deg CRegardsBob ScottATP IMEL Gulfstream II-III-IV-V L-300Santiago de Chile

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Hi Bob.I did some testing of your equation and it wasn't working out for me.My example:CN1 at FL330 was reading 94.8%.According to AFSD, the N1 reading is 89.86%.In order for CN1 = 89.86 the following had to be true: CN1 x (273/288.15)= N1. Theta wasn't even a factor.At the time, the TAT was -50.2

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Ok, I think I found the issue. sqrt is not square root of Theta, but Theta_totTheta_tot = Theta(1+.2m

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Hello,I have just answered that on AvHistory.I am glad some of you are doing your homework ;-)It is all spelt out on Herve's site under Turbojet notes.I even provide ISA tables.Ian

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Correct. Looks like you were using OAT in your computation instead of TAT the first time around.TAT = OAT * (1 + 0.2M^2)However, FS computes TAT for you, so the formula I provided you based on TAT uses the already-computed value of TAT as derived by FS from OAT and Mach number.CheersBob ScottATP IMEL Gulfstream II-III-IV-V L-300Santiago de Chile

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Ahh, see, this is why I'm a communications major and not a math major. Thanks!

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