Throttle HOLD does NOT hold N1 but increases in time

[jgoggi 65]

Thanks. Anyway you can check on Youtube, all takeoffs have this issue of the increasing thrust. On the other hand, I saw a video made with the old PMDG 747 and there it was correctly modelled, the thrust remained at the takeoff N1 after HOLD engagement...

[Qavion2 39]

Quote

Takeoff N1 increases from 91.4 when HOLD engages, to 93.6 when THR REF engages at 400 ft.

Just to clarify... You're talking about the thrust limit changing above 400', not the actual thrust below 400'? Sorry, I can't see any values in that video. On my computer it's just too blurry.

Quote

Charts and tables have been developed which identify the specific N1
rpm required to produce a specific thrust level for a given ambient
air temperature and pressure altitude. These engine model N1
characteristics are incorporated into the data storage of the aircraft
thrust computer and the engine power management control.

Above 400', the A/T comes alive. What is to stop the EEC or FMC adjusting target N1 based on TAT and pressure altitude? Sorry, again, I can't see anything in that video.

When you derate an engine via the assumed temperature method, you tell the aircraft that the temperature has changed and the target thrust changes accordingly. When the aircraft is flying, the engine and aircraft sensors tell the EECs/FMC that the conditions have changed. Unfortunately, most of here don't have any experience with real world values in flight. Even Level D sims probably don't provide the randomness of atmospherics in the real world. Most of us assume "correctness" based on what we see in simulators (whatever their price).

 

If someone showed us 100 real world videos in all conditions (including temperature inversions), where the thrust reference does not change above 400 feet, then we could say that that PMDG is not correct.

[jgoggi 65]

Hi John, I am talking about the actual thrust, not the thrust limit. The takeoff N1 green value is 91.4 and this value is set as soon as TOGA is pressed. Then, when HOLD engages, you see the actual thrust increasing from 91.4 to 93.6 (while the green takeoff value above remains at 91.4). The airplane has by then reached 400 ft and THR REF takes control...

[Budbud 227]

Ok, I did two flights tonight with the 400F GE variant and I actually noticed the increase of N1 during the takeoff roll. I took a video on the first flight but it is the same as the one posted so I don't feel the need to post as well.

The human mind is curious, I would have sworn that I didn't have this issue.

[Qavion2 39]

8 hours ago, jgoggi said:

Hi John, I am talking about the actual thrust, not the thrust limit. The takeoff N1 green value is 91.4 and this value is set as soon as TOGA is pressed. Then, when HOLD engages, you see the actual thrust increasing from 91.4 to 93.6 (while the green takeoff value above remains at 91.4). The airplane has by then reached 400 ft and THR REF takes control...

ah.. ok.

Are you using FSX? If the GE CF6 EEC works in a similar manner to the RB211's FAFC, the variations may be due to the FSX atmosphere not being "normal" (it isn't, I hear).

In the real world, the autothrottle is locked at 65kts, but the FAFC is not fully locked. It still applies some standard atmospheric adjustments to try to keep the thrust constant during takeoff and initial climb. However, if the FSX atmosphere is not standard or the engine bleed load changes after 65kts for some reason, then you may get the EEC not applying the correct adjustments and there will be a variation between the green value and the actual thrust.

Of course, all this assumes v3 has this level of sophistication in engine management. ;)

Anyway, I'm still trying to understand what actually goes on on the real aircraft. I don't think many people, including pilots and engineers, really know what is going on. Some things are locked in the EEC, some things aren't.

Cheers

JHW

[uhntissbaby111 58]

John,

I am in P3Dv3 and I am seeing this issue. And I dont think it has anything to do with atmosphere because this issue starts on the TO roll on the ground. And it is not just the GE, the RRs do it as well. If you look the N1 readout for the RR you will see it increase a little bit during the TO roll

[JRBarrett 3,038]

Those seeing this: are you taking off with packs "on" or "off"? I'm curious if that makes any difference. I believe BAW r/w SOP is to turn packs off at takeoff weights above 300 tons, and to leave them on below that weight. As John said, bleed load variations can affect thrust. Also, does engine derate configuration make a difference? Are you using assumed temperature + a fixed derate, or just fixed derate, or none?

[jgoggi 65]

1 hour ago, Qavion2 said:

Are you using FSX?

P3D v3.4 HF3.

 

55 minutes ago, JRBarrett said:

Those seeing this: are you taking off with packs "on" or "off"? I'm curious if that makes any difference. I believe BAW r/w SOP is to turn packs off at takeoff weights above 300 tons, and to leave them on below that weight. As John said, bleed load variations can affect thrust. Also, does engine derate configuration make a difference? Are you using assumed temperature + a fixed derate, or just fixed derate, or none?

I did not check this, will try tonight at home. Up to now I have done very light-weighted tests, with weights well below 300 tons, packs on and fixed derate only (TO2/CLB2).

 

[JRBarrett 3,038]

2 minutes ago, jgoggi said:

P3D v3.4 HF3.

 

I did not check this, will try tonight at home. Up to now I have done very light-weighted tests, with weights well below 300 tons, packs on and fixed derate only (TO2/CLB2).

 

I'm pretty sure that PMDG uses their own custom engine model, rather than relying on the default FSX/P3D turbine engine emulation - but there may be limits on how much precise control they can implement - or the degree to which external factors like bleed air loads, atmospheric conditions etc. affect engine performance in the sim.

On the other hand, data like this from end users can certainly help the developers in fine-tuning the model as much as possible. For an initial release, the QOS2 is remarkably compete and trouble free - but there are always improvements that can be made as the product matures.

[jgoggi 65]

I tried with packs off and different derates but nothing changes.

See also my reply in the topic "Thrust decrease after takeoff", that issue comes from this one.