Jet Engine Ratings
 

Hello,,,,,,I just want to know why Jet Engines are normally rated at more than 100% N1......thanx

If you search back, you should find this subject has been covered a few times.

Basically a family of engines will be given a nominal setting for 100% fan speed (e.g. 3900 RPM on RB211-22, -524).
This is based on the original design parameters for the fan (e.g. Tip relative Mach no.). Percentage rather than actual RPM is used to give a normalisation of presentation between different aircraft/engines.

Actual RPM limits can vary with different variants but will usually be slightly higher than 100%

So if im not wrong, u mean to say that in BR 211 when u reach 3900 RPM which is going to be its maximum limit and it is rated at 100%, but due to some variables it sometime cross this limit. Correct me if im wrong does this mean that this engine is overpowered and when the conditions are not right e.g Higher outside air temp or some other variable in that case we will get its rated 100% N1 RPM atleast.............


Thanx

Operating limits for RB211-524D4:

Max take-off: 5 mins

N1 N2 N3 EGT
104.4% 106.0% 97.5% 785 deg.C

Max. overspeed: 20 seconds

N1 N2 N3
106.0% 107.0% 98.9%

Max. take-off operating limitations may be used for up to ten minutes in the event of engine failure.

Equivalent shaft speeds:

100% N1-3900 L.P. rpm
100% N2-7000 I.P. rpm
100% N3-10611 H.P. rpm.