I suspect that this will be down too limitations of the underlying FSX jet model.

 

Ultimately it's a £70 simulator, there will be limitations.  Do any jet's on the market model this? 

Did you match relative humidity and density?

 

Aaron

-oOo-

Even though I do believe atmospheric conditions do influence engine spool-up rate, I do not believe it influences it to this extent, i.e. the spike that is shown in my graph. This spike is called a loop gain is in controlled by the engine management computer (or some similar system like that). This loop gain is always present regardless of the weather conditions.

EDIT:

 

Just wanted to say this. 

 

Why can't so many people resist to write off every single post they see just for the sake of it or because they can't accept that something created by PMDG shows a minor difference compared to the real-world. Surely, PMDG are the best out there, but if there would really be no differences at all then the NGX be a lot more expensive.

 

Some real-world pilot that flies for TUI also commented that some areas of the NGX are not entirely correct, such as the waypoint modelling. 

 

Still, love this thing to death. :smile:

Wait..... how do y'all notice these things????

Wait..... how do y'all notice these things????

 

If you ride an NG IRL, there is a moderate difference in engine spool up/down speeds. I personally think that it does make quite difference in the sound of the engine. 

As a real NG pilot I have to say I have never paid any attention to how the engine spools up when I set 40% for t/o, I am only interested in seeing the thrust at the right amount and increasing symmetrically.

 

I do see many differences during engine start between different aircraft and during different conditions. temperature, weak APUs, the bleed system, tailwind/headwind all affect the duration and the way the engine spools up before and after the engine start lever is moved to idle detent. I believe that each EEC is also tuned for each individual engine too, which I guess could also add differences.

 

I am absolutely certain, that all these factors would affect how the engine spools up from idle N1 too. Another factor is the software version of the EEC. Different spool up profiles are used for different conditions and phases of flight across different versions. There is a bulletin out at the moment for the current EEC version warning about slow spool up speeds in the cruise which could, remotely possible lead to a slight under speed with sudden changes of wind direction/speed.

 

I think the the attention to detail nor the desire to get NGX as close to reality as possible is admirable. However, I think the FSX environment (or for any other sim for that matter) is just too stable and canned and as such would make such replication practically impossible. As long as the engine responds characteristically like a turbofan then I think that is sufficient.

Go into the air file and look at tables 1502 to 1507

 

For example: If your 1505 doesn't look like this then it's wrong.

 

 

Go into the air file and look at tables 1502 to 1507

 

For example: If your 1505 doesn't look like this then it's wrong.

1505.jpg

Could you please define in more detail what you are referring to here? I am not fimiliar with .air files, I'm sorry.

 

 

I just wanted to add:

I did some extensive research to the engine spool-up on the 777 this time, and I noticed the exact same overshoot effect like on the 737NG. I could clearly see it on videos featuring 777's equipped with GE and PW engines. Cannot yet confirm anything about RR engines. 

Though I must say on the 777 the overshoot is a bit more subtile, just like in case of the 737NG, the effect is still present and other differences can be noticed as well: engines start to spool-up slowly, then get boost at 40%, and instantly stabilize at 55% N1, with a very slight overshoot.
Compared it to beta videos of the PMDG 777 and as far as I can tell so far, it's again completely different. In the PMDG version, initial spool up is a bit quicker, but where the real 777 would get a boost at 40%, in the PMDG version does the exact opposite happens; the engines start to spool-up regressively at this point, resulting in an endless wait for stabilization. 

Once again, PMDG probably did the best they can by at least matching the total spool up time, which in case of the 777 is around 5-6 seconds. Actual N1 stabilization though, takes quite a bit longer in the PMDG version. 

Thanks Arjen. I noticed this too.

 

The real 737NG seems to get very fast to around 40-45% N1 and the NGX seems to take a bit longer. (That doesn't mean that the NGX isn't great, it is and I still love and keep to flying it  :wub: )

This is something my experienced "jet mech" ear noticed when I first bought the NGX, and I've seen it in other products from other makers. First, let me insert the default "I love my NGX" comment, because I do.  And to the OP, this is the MOST interesting thread that I've seen on Avsim in my short history here.  I think you have done a great job with this.  Kudos.

 

I've got many years hands-on, as do others who ride these forums, with RL jet engines.  My experience is with the PW F100 and GE F110-series of engine that powers the F-15, F-16, and further developments of these engines are in the F-22 and F-35.  These engines are NOT in civilian aircraft.  I could be very wrong in what I'm going to present for thought, but I don't think so; I think it's related to what you see.  N₂ is a function of T_t2.5 or a similar temperature input and power level angle in the engines that I've worked on.  This generally means that the hydromechanical MEC or UFC (mechanical engine or unified fuel control, depending on which mfgr) is running the N₂ RPM. The RCVV and CIVV (rear compressor and compressor inlet variable vanes) are being scheduled by the MEC/UFC based on PLA, N₂, and T_t2.5 or similar functions.  This is all controlling core speed (N₂ RPM) via a hydromechanical controller (MEC/UFC) and is using fuel pressures and fuel as hydraulic fluid to move vanes in the engine to control core speed.  Fuel flow through the combustor also controls this speed.  So in summary, core RPM is controlled by airflow and fuel flow through the core.

 

Hang on, I'm going somewhere with this...

 

So, you have the core (N₂) rpm and the core is turning at X% RPM.  Now, this is completely independent of fan (N₁) speed.

 

The fans on my engines had inlet guide vanes and convergent/divergent nozzle flaps to prevent fan overspeed and control thrust.  The IGV's might exist on the commercial engines, but honestly speaking, I'm not sure.  However, N₁ speed is a direct function of N₂ speed.  When I would start up a Viper, I could hear (and feel in my butt) the core RPM come up before the fan RPM.  All turbofans I've ever worked on run in what's called a "closed loop".  The EEC or DEC would monitor the fan speed, nozzle position, IGV position, etc., and would watchdog the core operation, but the core is, again, independent.  The EEC or DEC keeps the engine running at peak parameters.  If the EEC or DEC is lost, the MEC/UFC acts as a backup and closes everything up to ensure that you can remain in the air but your max thrust and RPM is fixed at a safe setpoint.

 

Yeah, yeah, sorry, I'm getting to it.

 

So, the core is running independent of the fan, but what the core does controls the fan; i.e, airflow generated in the core runs the fan.  Now, we are on the runway and the engine is stabilized, let's call it 40%.  The stick actuator in the seat pushes the throttles forward.  The core reacts immediately, as in RIGHT NOW, because it is smaller and can spool up faster.  The fan then accelerates to catch up, and actually overshoots the target by a little.  The EEC sees this and slows the core to stabilize the fan RPM.  The fan is not directly driven by the core, it is driven by airflow, and N₁ is a direct function of N₂.  I am pretty confident that this is what you are seeing in the huge high-bypass fans on these engines in RL. 

 

No foreign objects were ingested during the formulation of this post.

Kattz,

 

Most interesting reading  .. thanks for taking the time to describe your Real World experiences, in such detail.

 

Love this technical stuff ..  and it is amazing what Real World aviation information, one can get from a Flight Sim Forum !

I love jets.  I took my daughter out to Wright Patterson AFB today where the OH ANG guys are hanging out while some repairs are done to their facility up north. This way she could see some RL F-16's doing what they do best - making noise!

Kattz,

 

Wow...and I mean it. All the appliances in my house seem infinitely easier to operate now.

Could you please define in more detail what you are referring to here? I am not fimiliar with .air files, I'm sorry.

 

The air file controls how the engines work. The relationship between n1 and n2, cn1 and cn2, egt etc etc.

How quickly they spool up normally or under slam test.

The air file will be somewhere in you aircraft directory. You will need a programme like aired to modify it. But be careful. You best bet is to obtain the engine limitations schedule for the engines that your a/c is using and work from those.

 

 

This is something my experienced "jet mech" ear noticed when I first bought the NGX, and I've seen it in other products from other makers. First, let me insert the default "I love my NGX" comment, because I do.  And to the OP, this is the MOST interesting thread that I've seen on Avsim in my short history here.  I think you have done a great job with this.  Kudos.

 

Thank you. In which add-on(s) exactly have you noticed this realistic overshoot effect? Or do you mean you have never noticed it yet, just like in the NGX?

It's an interesting post you made. I explains the overshoot effect and also proves my findings even more. It looks like it much be something that is present at all times, and doesn't magically arise when atmospheric conditions change.

I will also try to mimic the real-world engine spool-up in the NGX, I will record it and post it on YouTube, and you'll see what kind of a antics I have to make with my thrust levers to mimic it. 

The air file controls how the engines work. The relationship between n1 and n2, cn1 and cn2, egt etc etc.

How quickly they spool up normally or under slam test.

The air file will be somewhere in you aircraft directory. You will need a programme like aired to modify it. But be careful. You best bet is to obtain the engine limitations schedule for the engines that your a/c is using and work from those.

Interesting, do you think the effect I described, and as shown by the red line on my graph, can be simulated in the NGX? Or are there limiations in the .air files that don't allow for this? Just curious.