N1-EPR relationship for RB211 engines

[glennstevens]

I came across some footage on youtube of RB211 engines at cruise.   FL370, 265 tonnes, M0.86.  TAT -13.  At cruise EPR of 1.54 N1 is about 90 per cent.

PSX would put N1 at 81-82 per cent or thereabouts for that EPR at that FL and speed.  Im sure this must have been asked before but seems like on this model the EPR and N1 relationship is different to what we would expect.   

Sorry if this has been dealt with before or is an unfair question.

[Hardy Heinlin]

Thanks for the hint. I don't know why there's a difference of 8 %. I guess we have to live with it.

[Mariano]

Hi, Glenn.

It might depend on the variant in the video versus the variant in PSX.

The H variant had the overall pressure ratio increased from 33.0 to 34.5, while the G/H-T had a new Trent 700 core installed (hence the "T") and a bypass ratio of 4.1.

I assume that these changes (and many more) might affect the EPR-to-N1 relationship (EPR being highly dependent on overall pressure ratio increase).

Best regards,

Mariano

[Mariano]

I have more time to finish my previous post.

Also, we must take into account that PSX simulates a new engine, and the airplane in the video must be using a performance-degraded engine.

As blade tip-to-case clearance slowly increases during use, a jet engine must work harder in order to make up for the wasted energy around those tips as the air progresses through its many compressor and turbine stages.

If an engine with blade tip wear is aiming for 1.## EPR, its compressors must spin faster in order to make up for the less efficient blades. Commanded EPR is achieved, but with higher compressor speeds (I don't know if it is in the range 8%, but a difference should be noticeable).

Best regards,

Mariano

[glennstevens]

Thanks Mariano.
The link to pic attached (I hope)
Pretty sure this was a Qantas -400, which had the G or H versions I think. 

Glenn

https://1drv.ms/i/c/75b4b1f15a55f909/IQBmH60T2TICRrCa1F01nLSkAaaga1uKamk7EECmUhoRQAU?e=LW1v75

[Hardy Heinlin]

PSX simulates the RB211-524H8 for the ER models, and the RB211-524G2 for the non-ER models. The engine type number on Instructor > Model > Airframe changes accordingly.

The engine parameter data in PSX was derived from a Boeing program for dispatchers and then interpolated. I did this 20 years ago and I can't revisit this stuff, unfortunately; it's too complex, and it's too risky to modify especially when based on single real-life hints without broader context.


Regards,

|-|ardy

[Hardy Heinlin]

Also, we must take into account that PSX simulates a new engine, and the airplane in the video must be using a performance-degraded engine.

As blade tip-to-case clearance slowly increases during use ...

Good point, Mariano. Let's take blade tip abrasion as a reasoning :-)

Here's a PDF with further details.


|-|ardy

[Mariano]

Excellent article. Thanks!

Best regards,

Mariano

[jtsjc1]

Interesting because a couple days ago I was reading about National Airlines flight 27 DC-10 in 1973. The blades moved in their slots and caused an uncontained engine failure in the #3 engine. The blade tips were contacting the case and they found part of the acoustic panel in the engine.

[glennstevens]

Fair enough Hardy. 

[Mariano]

I recently operated (a 767) with one engine (CF6-80) fresh out of overhaul, and one not. The difference in parameters (other than N1, of course) was very noticeable.

It is assumed that EGT is the clearest indication of gas path health. I was able to appreciate how lower EGT margins can be a prelude of things to come.

In the late 1980s, Air New Zealand B767-200ERs with CF6-50s involved in early ETOPS were already automatically gathering EGT data throughout all phases of flight and downlinking it (via ACARS) to maintenance where a computer analyzed not only slight EGT increases in each engine throughout many flights but, more importantly, also ever so slight divergences in EGT between both engines. Once a certain threshold was reached, the system alerted maintenance and action was taken to prevent even a slight probability of an IFSD later on. Those engines did not enjoy the privileges of fault storage and faulty LRU pinpointing capabilities of today's FADECs, and the 767 did not have CMCs either.

Regards,

Mariano