The CMF56-7B engine and power rating

[deuzx]

I wonder about what the 100% N1 stands for? In some cases the TOGA thrust is above this? Mech. stop for thrust leaver is about 107% N1?

When you reduce the N1 with 10% does the thrust reduces with 30%?

When you buy an 20K rated engine. When does it produce 20K? At 100% N1? If you set thrust leavers to mech. stop on a 20K engine you are likely to get max thrust of the 23.5K engine as well.

I wonder

[RYR-738-JOCKEY]

I'm not a tech expert, but from what I know; The N1 percentage is the RPM of the low pressure stages of the engine, and is a reference for thrust setting/primary engine indication. But 100% N1 does not necessarily give you full rated thrust as it varies with temperature and pressure. For high temp and low pressure - the thrust output will be lower for a specific N1 percentage, and vice versa. So commanded N1 does not correlate to output thrust.
Moving the thrust lever to the forward stop will through the EEC's (Electronic engine control) give full rated thrust, f.ex. 26k, depending on engine version.

However, you have engines with an EPR (engine pressure ratio) indication (f.ex. MD80's) .A specific EPR is an indication of output thrust and will not vary with temp and press.

[BizJetJock]

The 100% point is merely a design reference, a bit like the C of G datum. It rarely has any bearing on normal ops - RR Vipers were an exception with 100% being max T/O N1. Particularly as engines are uprated and flat rated for different installations the figures vary widely. Two examples I know personally are the Challengers with the GE CF34 engines with a max APR N1 of around 98% depending on model, and the Citation Ultra with JT15D engines with a typical cruise N1 of 104%. The same applies to N2.
EPR, on the other hand, has nothing to do with speeds - it is Engine Pressure Ratio. It comes from comparing intake and exhaust pressures, and so you will get different core and fan speeds for a given EPR with different atmospheric conditions.

[barit1]

Originally Posted by deuzx

I wonder about what the 100% N1 stands for? In some cases the TOGA thrust is above this? Mech. stop for thrust leaver is about 107% ...When you buy an 20K rated engine. When does it produce 20K? At 100% N1?N1?

Within the flat-rated range of OAT, N1 varies with OAT to maintain constant rated thrust (higher temp, higher N1). This is because of the air density changing with temperature. So it's virtually never right at 100%.

Above the flat-rated OAT, N1 decreases with increasing temperature. Obviously thrust decreases in this range, but the manufacturer imposes this limit to protect the hot section parts.

When you reduce the N1 with 10% does the thrust reduces with 30%?

I don't recall just what N1-to-thrust derivative applies, but 10-to-30 seems about right.

[Old Smokey]

Originally Posted by deuzx

When you reduce the N1 with 10% does the thrust reduces with 30%?

That's about right for a low bypass engine, where the exponent for the N1 to Thrust derivative was 3.5,

i.e. Thrust is proportional to N1 ^ 3.5

This exponent is significantly reduced as bypass increases, as 'core' thrust has an exponent of 3.5, whilst the fan produced thrust, like a propeller has an exponent of 2. Thus, as bypass ratio increases, the exponent will reduce from a 'base-line' 3.5 to something above 2.

In comparing 90% N1 to 100% N1, a low bypass engine would experience a 31.7% increase in thrust, a pure fan engine would experience a 23.5% increase in thrust. The CFM-56 that you refer to would be somewhere between these two.

Regards,

Old Smokey