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N1 Settings

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Hey gang,I'm probably opening up another can of worms...but here goes:I've been flying the DC-10 a bit lately, and one of the things that always strikes me is how high most of the N1 settings are for the various stages of flight.For example, for takeoff thrust, the N1 settings go as high as 118% in the really hot and high situations, but even a more "normal" situation can require N1 of 107%. In cruise, if you're in a mid-weight situation, the N1 can range from 95%-104% (This is all taken from performance charts and manuals -- thank you, Matt Zagoren!).Getting around to my main point: I have always been under the impression that N1 numbers were a percentage based on the design speed of that particular stage of the engine (id est, 96% N1 = the first stage of the engine is spinning at 96% of its "designed" full setting.)Now, I'm not an aerospace engineer, and I don't work for General Electric, but I would think that if N1 settings were based of a design rating of 100%, anything over that would not be encouraged for use due to increased wear and tear and the like. I have read before that it's considered "bad practice" to simply push the throttle levers to the firewall on takeoff. Then again, I may be under the misconception that N1 drives the equations for engine performance. Doesn't Boeing often rely upon EPR?Whew! Well, I'll be curious to see what you all have to say. Thanks in advance!Matt

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Not sure of the big iron but in the Falcon 20 the N1 settings are a lot lower than that. N1 is the fan in the turbo fan engine and it's speed translates to direct thrust and is used to set power in some turbofan engines. Some Boeing (as well as other turbine engines) use EPR (engine pressure ratio) which is a ratio of the pressure of air entering into the engine and the pressure exiting the engine to set power. This also translates to direct thrust. On the Falcon 20 takeoff N1 rarely exceeds 97%. Our max N1 is set at 101%, but not all manufacturers rate the fan speeds the same. The speed limitation is placed on the fan by the manufacturer and can be any speed they deem is the max for that rotating component, be it 100% or 120%. It may be that 100% will produce the max rated thrust on a standard (59F,29.92) day but on a very hot high altitude day it may take more speed such as 115% to make that same thrust. Or it could be that 100% is the max physical speed allowed for the fan to turn reguardless of the thrust produced. It all boils down to how the engine was designed and tested and certified as to how high an N1 it will take. Hope this helps explain it a little.Ken

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Matt,Glad you find the info useful. Regarding the N1 and N2 speeds, I thought I'd post a quick little tidbit from the Type Certificate Data Sheet (TCDS) for the CF6-6:CF6-6D, Max Permissible Rotor SpeedsN1 Rotor 3810 rpm (111%)N2 Rotor 9925 rpm (101%)CF6-50C2, Max Permissible Rotor SpeedsN1 Rotor 4102 rpm (119.5%)N2 Rotor 10761 rpm (109.5%)In my unqualified opinion, it seems that the percentages are based on a prototype engine that ran at a N1 of 3432 rpm and a N2 of 9826 rpm. Using this engine as a baseline, it appears that the rpm of both rotors was increased as a way to increase thrust for both the CF6-6D and -50C2. As an example, for the -50C2 if you take the N1 rating of this baseline engine (3432) times 1.195 (119.5% N1) you get a N1 rpm of 4101 - very close to the 4102 listed in the TCDS. I'm not sure if my theory is correct but it sounds good anyway. :)For engine and aircraft TCDSs click the following link:http://www1.airweb.faa.gov/Regulatory_and_...me?OpenFrameSetThanks,Matt Zagoren

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Matt,Well, your theory sounds good to me! I guess if they want to increase thrust by increasing RPM then that's their call. If it were up to me, though, I would have moved the 100% N1 relative to new settings; however, the whole "commonality" of the CF6 family might be compromised by doing that.Oh well...GE must know what they're doing.Thanks Matt!Matt(!)

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