You know, this may be viewed by some as sort of a "negative" critique of the NGX(and maybe even now the 777).  Or sort of a "how or why did they miss that" type thing. (I don't think the OP meant it that way or at least I think I remember him saying that anyway).

 

Regardless of how it's viewed, I'm reading this topic thinking, wow!  How great is the NGX and how much did this really take FS to the next level if we can be sitting here talking about a rather fine detail like this?  This type of conversation would have pretty much been unheard of before the NGX.  Maybe PMDG missed this detail or maybe they have their reasons why it's not modeled(FSX limitation or whatever), but it's still yet another example how high the bar got raised with this aircraft!

 

Thank you again PMDG for such a great aircraft!!!

Today I decided to compare the engine spool-up of the real 777 vs the PMDG 777.

 

For now I only focussed on engine spool-up time.

 

For the real-world I used multiple videos and measured it with a stopwatch app. Of course I only measured the 777's featuring GE engines. On average the spool-up from Idle to 55% N1 took 5-6 seconds.

 

Compared it to the beta video here by Kyle (skip to 25:50)

 

http://www.youtube.com/watch?v=6h_IAoL6Pik

 

13 seconds.  :bad: 

 

Not trying to be a rant to PMDG, but an engine spool-up that's twice as slow as in the real world is going to be a gamebreaker for me.

 

 

 

The only way to be accurate is to do a slam test. Find out what the correct time should be from fully idle to say 96%n2.  Model that in the sim and your normal n2 spool-up should also be quite accurate (provided that cn1 and cn2 have also been correctly modelled). The tables quoted earlier are in fact graph lines that model the relationship between cn1,n1,cn2 and n2 also corrected for mach. Some developers take it to extremes and model engine performance every 1,000 feet.

Just as an edit to my previous 777 post regarding the engine spool-up, I've just looked at a video from Luke Pabari here.

http://www.youtube.com/watch?v=XoQwdgWb3BM

In this case engine spool-up was not 13 seconds like in Kyle's video, but 7 seconds. That's very realistic. But the more important question is, why the big difference between the two? I do not get it, in Kyle's video idle to 55% took twice as long. I immediately thought about atmospheric conditions. But then compared the TAT readouts on the EICAS. In Kyle's vid it was 17C and in Luke's vid it was 30C. The higher the air tempature, the slower the spool-up, so in that case Kyle's engines should be spooling-up quicker than Luke's. And considering it's exactly the other way around, it can't be related to that.

I don't get it anymore.

Just watched at froogle's video on his twich channel.

 

The spool-up took ~8 seconds.

 

I don't get it anymore.

 

That's what I thought. 

Just as an edit to my previous 777 post regarding the engine spool-up, I've just looked at a video from Luke Pabari here.

 

http://www.youtube.com/watch?v=XoQwdgWb3BM

 

In this case engine spool-up was not 13 seconds like in Kyle's video, but 7 seconds. That's very realistic. But the more important question is, why the big difference between the two? I do not get it, in Kyle's video idle to 55% took twice as long. I immediately thought about atmospheric conditions. But then compared the TAT readouts on the EICAS. In Kyle's vid it was 17C and in Luke's vid it was 30C. The higher the air tempature, the slower the spool-up, so in that case Kyle's engines should be spooling-up quicker than Luke's. And considering it's exactly the other way around, it can't be related to that.

 

I don't get it anymore.

 

We can't tell much about the spool up without knowing how the testers moved their throttles. 

Anyway... I really don't want this to sound as an insult, but... you might want to... relax a bit... 

The 777 won't be "worse" than NGX in any aspect, I strongly believe -- so I guess we can expect a spool-up that is, at least, in the same league of realism than NGX's...

And it's in beta. Anything can change a bit or be improved until release. I'm sure the tech advisors will warn the devs if something shows up strange or wrong. 

In reading the Boeing manual, there are no set limits on N₁ time to acceleration.  The FADEC and AUTOSTART systems are in complete control.  You may be seeing an accurate engine start in both videos.

We can't tell much about the spool up without knowing how the testers moved their throttles. 

 

That's why you should try to get hold of slam tests. As regards the ambient temperature on the ground that will have little influence on the speed at which the engine starts up. But it will of course affect greatly the EGT thus risking a hot start. The ambient air pressure at the airport will have an affect on the idle percent. The ambient temperature and pressure combined will then have an effect on the throttle setting for take-off and cruise climb. This can all be modelled in the sim which I have done for the RR Conways which perform exactly as on the tin. Even the 550's risk an n1 surge above 25,000ft at temperatures below -25c. That is a risk facture explained in the engine limitations schedule which in the sim is a spin off of the airfile tables.

Arjen Vandervelde, on 23 Aug 2013 - 10:12 PM, said:

Just as an edit to my previous 777 post regarding the engine spool-up, I've just looked at a video from Luke Pabari here.

 

http://www.youtube.com/watch?v=XoQwdgWb3BM

 

In this case engine spool-up was not 13 seconds like in Kyle's video, but 7 seconds. That's very realistic. But the more important question is, why the big difference between the two? I do not get it, in Kyle's video idle to 55% took twice as long. I immediately thought about atmospheric conditions. But then compared the TAT readouts on the EICAS. In Kyle's vid it was 17C and in Luke's vid it was 30C. The higher the air tempature, the slower the spool-up, so in that case Kyle's engines should be spooling-up quicker than Luke's. And considering it's exactly the other way around, it can't be related to that.

 

I don't get it anymore.

Kyle wanted to show how the thrust should be allowed N1 to stabilise before applying TO thrust. So he left it stable while he talked, adding a few seconds. Also he then applied thrust smoothly to TO where as Luke basically slammed the levers rapidly, which could make a difference. OAT will have a small effect but thrust lever technique more so in this case.

 

Kattz, on 24 Aug 2013 - 03:28 AM, said:

In reading the Boeing manual, there are no set limits on N₁ time to acceleration. The FADEC and AUTOSTART systems are in complete control. You may be seeing an accurate engine start in both videos.

In reality all engines will start differently. FADEC is mainly monitoring the process so different engines will have different start characteristics. In the sim this variation doesn't happen as the engine model is always the same so every start should be identical under the same atmospheric conditions.

 

As for the engine acceleration time the Boeing manual won't give set limits but limits there certainly are. Minimum acceleration time is dictated by the certification rules laid down by the FAA, EASA, etc. So the FADEC is designed to allow the engine to accelerate fast enough to meet the regs without going too rapidly and risking a compressor stall or surge.

Guys,

There are a few limitations in the FSX engine model and spooling is one of them - we did our best to simulate the real spool time videos our tech advisors took on both the NGX and the 777. It's possible to get close (which we did) but it's never going to be perfect unless we were to totally take the FSX engine out of the equation and run in pseduo slew mode ala what Majestic did with their Q400. Our position on doing that is that there's way too many other issues with doing it for us to consider that currently, especially just for one minor thing like this.

One of the problems is that FSX has the engines respond differently depending on how large of a range you move the throttles in. I distinctly remember this from testing the NGX - you'd slam them forward and get one time from 0 to 40% but if you moved them just the right amount to get 40% you'd get a different time. I believe we tried to average out that effect and that's why you see the slight discrepancy in initial acceleration that's there in the first graph in the thread. (EDIT - Vangelis says we biased it toward wide open throttle - TO, GA etc where it matters for safety and performance.)

 

At any rate, it's certainly not something we missed as an oversight or anything like that. We have a ton of videos from the tech advisors of multiple spool ups in multiple phases of flight. We know exactly what it does, but this is a simulator and not the real airplane and there's always some compromises that have to be made given that fact.

 

What is surprising me though this time, is that there hasn't been a single response PMDG in this thread so far. I am a 100% sure Ryan or perhaps some other PMDG staff has read it. Perhaps that's positive sign and are re-evaluating the possibilities.

 

It really should not be surprising to you unless you've failed to notice what's going on with that other twin engine jet we're making right now... I haven't exactly had a lot of time to come read the NGX forum lately.

Thank you very much Ryan.

 

 

I think we all knew that this is a limitation of FSX and not the fault of PMDG. If I remember right, PMDG is developing an aircraft for X-Plane 10. For us it's still unknown which aircraft you're going to

 

make but does XP provide you developers a better platform to model engine spoolings more accurate than in FSX?

Getting the acceleration time at low thrust settings is what is critical - this kills more pilots than the total spool time - they pull the power back too far, then when they slam the power wide open to avoid the hill ahead........nothing happens, because the engines are so damn slow to react at the low end. This is why the engine controllers increase power when the gear is extended on the NGX. The assumption is you are low to the ground, so it forcibly takes it out of the zone of slowest response.

 

Best regards,

Robin.

Guys,

 

There are a few limitations in the FSX engine model and spooling is one of them - we did our best to simulate the real spool time ...

 

At any rate, it's certainly not something we missed as an oversight or anything like that. We have a ton of videos from the tech advisors of multiple spool ups in multiple phases of flight. We know exactly what it does, but this is a simulator and not the real airplane and there's always some compromises that have to be made given that fact.

 

 

It really should not be surprising to you unless you've failed to notice what's going on with that other twin engine jet we're making right now... I haven't exactly had a lot of time to come read the NGX forum lately.

 

Thanks, Ryan.  Strongly agree that this is a simulated aircraft and not "the real thing".  You guys did a great job with the NGX as it is.  On the other comment, I think most of us here prefer it if you hang out elsewhere and do T7 "stuff" instead of babysit us!

 

Regards,

 

Kevin