I was wondering what the main differences were between engine types in the 757/767 (aside from the obviously astethic differences)? How much do the performances vary /engine?Thanks a ton!

One of the big differences is in how the different manufacturers measure engine parameters during flight. GE use N1 as their primary Engine Parameter in flight. RR & PW use EPR instead of N1 as their primary Engine Parameter. RR engines also have an N3 measurement. It's a lot more than just a simple difference in produced thrust.

awesome--thanks a ton.I have two other questions then based off of that.Also, what is the main difference between using N1 and EPR? Will there ever be plans to implement an upgrade (I'd definitely pay for it) that would allow us to change between GE/PW/RR.

Also, one last question: between the GE/PW--what is the major difference in apperance, they seem very similar to me.

N1 "only" measures the rotation speed of the turbine while EPR actually measures the produced thrust as it compares the pressure of the air before it enters the turbine and the pressure of the air after leaving the engine. (EPR = engine pressure ratio)Regards,Mark

awesome thanks

So fyiGE and PW are 'TWO STAGE' engines, meaning they have 2 compressors.the RR engine is a 3 stage engine.The difference in engine is basically indeed just produced thrust which varies. but the 'operation' of the engine might be different => different parameters to monitor.Cheers

Rolls Royce!Paul Meyer - Morris, C09http://www.flightsimnetwork.com/dcforum/Us...7b45404593a.jpg

wow thanks a ton for the details--really appreciate it.I've learned a ton!Is there a website that has a lot of technical information about the 757/767 (i know is 757.org.uk/767.org.uk--those are great!) are there any others? Thanks a ton!

Hey, I have a couple of questions...1)Whose bright idea was it to express engine power in terms of a unitless ratio that is inherently difficult to understand if u were not the one who designed the engine.For example, It's pretty clear how much power the engine is producing at 98.5%N1 or 28.7%N1. However, is 1.231EPR closer to ground idle or Max takeoff thrust?2) How does a third stage compressor (N3) work in a jet engine? Is this the stage which drives the N2, which will in turn drive the N1? Is the N3 the turbine that first moves when the ignition selector is placed to "GND"?thanks for any help

Interesting questions, Not sure of the history of the EPR unit-PW and RR both use them so I would blame Frank Whittle since he doesn't read this forum. As far as it being "useless" I would not go that far. It IS a measure of power by comparing the input pressure of air to the output pressure of air. That produces the ratio and is the figure displayed or the indication on the guage. Percentages like those used in GE's are the percentages of the various various internal speeds of the engine and don't really provide a measurement of power, only operating parameters for a given moment. You could argue it is a percentage of the maximum thrust but that would only be true on a "standard day". On hot and humid days the "mileage you get may vary"...(that's how come we get those assumed temperature settings and such for performance calculations to save the engines...).Another area to look at is HOW is the information displayed. This is key in a glass cockpit environment. In the early days of "glass" they experimented with just providing the digital read-out like the 757/767 standby engine indicators. The design was quickly scrapped since like you said, numbers didn't provide the "picture". So they went back to those color coded arcs/needles with a digital display of the value. This shows the pilot "where" the engine is at on the scale.I believe the RR is a true 3-spool engine, with three compressor sections, three turbine sections driving them, compressors don't "drive" the spools, it is the turbine blades aft of the burners that drive the shaft that spins the compressor drawing in and compressing the air for the combustion chamber which then turns the compressors etc...I am not sure if RR has inter-connected the two of the shafts to drive off the same turbine section (be interesting engineering I think if they did).I have not flown the RR so I am not sure on how they have the N1/N2/N3 set up...outward in or inward out.Tim__757

I didn't intend to mean that such a ratio has no value whatsoever, because it must be important somewhere, somehow...but operationally, i find that expressing it in terms of rotation speed is more PRACTICAL...since rotation speed must be in some way related to power being produced.I'm still a bit confused on the N3 issue though, but thanks for taking the time out to provide such a detailed response to both questions.

>Is the N3 the turbine that first moves when the ignition selector is placed to "GND"?Yes... or rather it drives the N3 rotor (N3 compressor and turbine). The starter motor converts APU or Ground bleed air into mechanical rotary motion which drives the N3 rotor (via a gearbox). At 50%N3, the starter is disengaged. The gearbox continues to turn with the spinning N3 rotor, however, powering the engine hydraulic pump, oil pump, elec generator, etc.Cheers.Ian R>

Thanks Ian, just drew a blank as to how RR set that up on the N3 spool with the accessory drive. Think my brain froze when I saw the pictures of the a380. I believe I dated something that resembled it.....just haven't bee "right" since.Tim__757