Ram Effect: High Altitude / High Speed Idle Engine Parameters

[MRFarhadi]

Greetings,

Though my experience with the PSX and CF6 Engines is limited, I've found something noteworthy and odd.
During high altitude and high-speed descents near T/D, turbofan engines tend to sit at idle parameters, which are just a tad higher than ground idle parameters due to the Ram Effect of high IAS or high MACH. This would cause, in my experience with GE CF6-80C2A5F and RR Trent 772B-60, N1 to sit around 45-50% while descending above FL300-350 with Mach.80-.82 (and EPR of between 0.75 to 0.80 for the Trent). This phenomenon is not limited to the engines mentioned above and is pretty standard across all the engine types I've witnessed, being in the cockpit or through numerous videos.
In contrast, the PSX, across a multitude of engine types, does not behave the same and idle parameters seemingly stay the same as if IAS and MACH are equal to Zero (the aircraft is stationary). Leading to some vertical path excursions with zero winds and ISA conditions from the calculated VNAV PATH.
At the end, I do not believe that there's any need to emphasize that the said condition is not related to the difference between the Approach Idle (I'm not sure what Boeing calls that; "High Idle"...? probably should've done the research before making this thread  ) and Ground Idle conditions.

I hope, if there are any, said misunderstandings can be more delicately described and discussed.

Best,
Reza

[Jeroen Hoppenbrouwers]

Just adding detail: if you use the NG FMC, the (flight) idle descent path has been replaced by a just-out-of-idle descent path, which provides a bit more cushioning against unexpected tail winds by allowing the FMC to still go lower thrust if required. It does not sound like this is what you see, but just in case.


Hoppie

[MRFarhadi]

Just adding detail: if you use the NG FMC, the (flight) idle descent path has been replaced by a just-out-of-idle descent path, which provides a bit more cushioning against unexpected tail winds by allowing the FMC to still go lower thrust if required. It does not sound like this is what you see, but just in case.


Hoppie

Dear Hoppie,

Thanks for the info. Exactly, similar to the "THR IDLE" and "THR DES" on the A330s which don't have the DPO (Descent Profile Optimization function).
But as you mentioned, I just brought up the VNAV detail so as to emphasize the weirdness of the situation. 

[Hardy Heinlin]

Good morning,

guess this is a topic for the real 747 drivers (none of the experts mentioned this extra effect above FL300 to me in the past 25 years). I have no specific engine parameter data for this, so I can't comment. And I don't know if your Airbus has the same electronic engine control as the 747 anyway. But I can say something about the EPR system differences between PW and RR models on the 747. When you use the PW engines in PSX, you will see that the idle EPR will indeed go far below 1.00 at high Mach numbers -- whereas on the RR it stays close to 1.00. I think this agrees with observations on the real 747 with RR engines. I've learned this difference occurs because the PW uses core EPR, and the RR uses integrated EPR which takes the Mach influence into account for the EPR display. There are some notes in the Aerowinx manual on pages 522 and 523, in the gray text boxes at the bottom right.


Regards,

|-|ardy

[MRFarhadi]

Good morning,

guess this is a topic for the real 747 drivers (none of the experts mentioned this extra effect above FL300 to me in the past 25 years). I have no specific engine parameter data for this, so I can't comment. And I don't know if your Airbus has the same electronic engine control as the 747 anyway. But I can say something about the EPR system differences between PW and RR models on the 747. When you use the PW engines in PSX, you will see that the idle EPR will indeed go far below 1.00 at high Mach numbers -- whereas on the RR it stays close to 1.00. I think this agrees with observations on the real 747 with RR engines. I've learned this difference occurs because the PW uses core EPR, and the RR uses integrated EPR which takes the Mach influence into account for the EPR display. There are some notes in the Aerowinx manual on pages 522 and 523, in the gray text boxes at the bottom right.


Regards,

|-|ardy

Didn't know that P&W4056 and 4062 were amongst the engines that measure EPR by P7 to P2. Thanks for the info.
Indeed, my previous knowledge of EPR calculation method came from RR Trent 772B-60 which stated that it is derived from P7 to P1 for the said engine (Ref. A330 FCOM DSC-70-35-10 P 1/2 "EPR = low pressure turbine exhaust pressure / Engine air inlet air pressure.")
My point, if I was successful in making it across, is that (referring to Charles Otis' "Aircraft Gas Turbine Powerplant" § 3-3 Equation of Pt to Ps, or for this matter, Pt2 to Pambient) with the increase of Mach, the "Ram Pressure Ratio" increases which in turn would result in the increment of the static pressure of the air entering the fan. As a result, this ram compression effect rises N1 and N2 across the whole flight regime, which becomes more pronounced once the engine is put on idle thrust at high Mach.
Sure this thread can use some real-life 747 pilot expertise, but as I said, this is standard behaviour across all jet propulsion powerplants, regardless of their engine type.