RB211 Info
 

Took a buddy out for the day recently on the 757 and being from
an engineering background he was very interested in the a/c's
engines themselves. When he asked how fast various stages of the engine were rotating, I as a simple driver merely pointed to the N1, N2, and N3 displays. That tells us as a percentage but
does anyone know what 100% actually means in terms of RPM for the various stages?

Just had a look at my Rolls Royce book and the only speed it gives is for a centrifugal compressor type which it says can spin at up to 1600 feet per second.

Alternatively, if you consider that the fan tips of the 211 are nominally operating at or close to the speed of sound, (hence that orrible grinding noise at take off power), then estimate the radius of the fan and with a bit of pye R squared bob's your aunties live in lover!

My maths is a bit rusty at this time of night especially after a couple of Taliskers!!!! So I'll let you work it out.

RB211-524 :

N1 100% = 3900rpm
N2 100% = 7000rpm
N3 100% = 10611rpm

Thanks guys,

We are both now enlightened.

What does 100% N1 correspond to, since T/O-thrust most of the time is more than that??

Correct me if I am wrong but dont manufacturers NOT give the RPMs for each stage because each engine is different. That is why they are rated either in EPR or as a percentage N1/N2/N3.
Each engine has a ballast resistor of differing values, so they will all give a certain thrust at a given % indicated on the flight deck instruments.

dont know about other motors but the rb211 has the ni/n2/n3 figure in rpm stamped on the engine ident tag on the engine casing

Rover,

If you're talking about the ballast resistor that I think you are, then that is just for EGT compensation. What I think you are on about is the Data plug which plugs into the wiring loom and lets the engine know what dash # it is?

:p