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My search ability is sub-optimal, my apologies in advance.

Am I to assume that it is a feature of FSX that when a jet engine achieves light off, it does so by spiking the thrust somewhat unrealistically? I note that there are posts regarding turboprop starts causing the aircraft to lurch forward despite the parking brake being applied. I presume this also applies to the heavy iron with CF6 and GE90 power etc. except that the parking brake is more effective (thrust/weight ratio and all that..). I also presume that PMDG & A2A products are far better behaved :cool:

I've been simming with jets for years and it never really bothered me until now LOL. Now that I've progressed into using payware more (yes, specifically the Sky Sim MD-11 lol!), I thought I would ask if there is a tweak, especially as the TSS engine sounds are quite excellent and the spiking N1 is at odds with the startup sounds.

Thanks.

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How high are the engine spikes going? 

I just tried my freeware POSKY 737-800.  I noticed at engine start, the N1 gauge surged to 84% then back to idle a 24%.  May be time for a Google search.

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Posted (edited)

Hi guys,

What you are seeing is the "normal" behavior of the default engine dynamics of FsX (and FS9 and others before). Only tweaked FDE's such as the ones you can find in usually high end Add-ons such as Leonardo Maddog or PMDG (or others) will have a realistic behavior in that matter. Anything else that uses the default engine will have this problem.

Fine tuning the engine is a very difficult in the ESP engine world (FsX, P3D) because the default is so way off.

Edited by TopGun33
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Thanks guys.:cool:

Charlie, I'll check the numbers and report back. 

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I've noticed this recently and I think I may have eliminated the startup spike by advancing my Saitek Throttle and returning it to the idle detent just prior to staring engines. I just tried it again several times and the N1 never went over 37%.  It was only there a fraction of a second and came right down.

 

Bruce

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INTERESTING! I tried a little experiment (always a good thing 😉 )

N1 spikes on the Sky Sim MD-11 depend upon when you introduce the fuel during the start procedure.

The documentation says to introduce fuel once N2 is greater than 15%. If I introduce fuel at that point, or just after, then the N1 spikes every time in excess of 80%

If I introduce the fuel after waiting for N2 to motor up to over 19% (close to maximum, I believe) then the spike is far more gentle - around 45% N1

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Posted (edited)
2 hours ago, TopGun33 said:

What you are seeing is the "normal" behavior of the default engine dynamics of FsX (and FS9 and others before). Only tweaked FDE's such as the ones you can find in usually high end Add-ons such as Leonardo Maddog or PMDG (or others) will have a realistic behavior in that matter. Anything else that uses the default engine will have this problem.

That's completely wrong. Correct startup behavior (without spikes), acceleration curve adjustment etc. can be tuned within the normal FSX/P3D SDK.

Edited by FDEdev

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Posted (edited)
48 minutes ago, HighBypass said:

The documentation says to introduce fuel once N2 is greater than 15%. If I introduce fuel at that point, or just after, then the N1 spikes every time in excess of 80%

If I introduce the fuel after waiting for N2 to motor up to over 19% (close to maximum, I believe) then the spike is far more gentle - around 45% N1

Those 'greater than xx' number are always minimum values. With most jet engines you usually wait until the spool up starts to noticable slow down.

Temperature during start will be lower as well, at least IRL.  That said, noticable spiking shouldn't occur at all, in neither case.  

If the engine has been correctly designed, this problem doesn't occur. Only in FSX turboprops you can't avoid the horrible RPM/torque overshoot (if you are staying within the SDK) 

Edited by FDEdev

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2 hours ago, FDEdev said:

That's completely wrong. Correct startup behavior (without spikes), acceleration curve adjustment etc. can be tuned within the normal FSX/P3D SDK.

Hi (first things first),

Don't make me say things I did not say.

I never said this was not possible to tune it, in fact I said the opposite since I quote examples of correct behaviors.

Perhaps the urge of showing your knowledgability has made your eyes only see what they wanted to see? I don't know.

To be sure that I make myself clear : most of the not too advanced aircraft have not been tuned in order to have an engine surge at startup, it's as simple as that. Even default planes have this flaw.

Some devs take a lot of time to refine this, while others have not. But of course, you know this already don't you? 😉

 

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Posted (edited)

Maybe this is a language problem, but to me 'only tweaked FDE's.....anything else that uses the default engine will have this problem.....because the default is so way off' implies that if you are programming without 'tweaking' it will not work. At least this is a misleading statement.

Don't know what you mean with 'even default planes have this flaw' since none of them has an (for a serious simmer) acceptable/good FDE.  

 

Edited by FDEdev

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I didn't mean to set anyone up for an argument. Thank you both for your input! Beers are on me... (virtual!) :cool:

Regardless of any tuning forthcoming, at least I have learned that the surge can be mitigated somewhat by allowing the bleed air to motor up N2 beyond 19%, at least in the Sky Sim MD-11.

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Posted (edited)

The max motoring RPM before introducing fuel also applies to turboprops. I've seen lots of 'professional' youtube reviews where the reviewer turns on the fuel immediately after pushing the start button, with the predictable outcome and the infamous 'my plane jumps forward upon engine start with the parking brake on' complaint.

Although most turboprops in FSX have this spiking problem, this rpm/torque overshoot will be usually much less with the max motoring method. 

Edited by FDEdev
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Could this be somewhat connected to reality?

In the vid below, the Captain and F/O are commenting that starter pressure is low "due to elevation (Nairobi)" then "24, it's OK" would that have been the %N2 at which the Captain introduced the fuel? at around 5 minutes 20..

https://www.youtube.com/watch?v=meY-WWCvGO4

 

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24(psi) is the bleed air pressure. The 'starter pressure low' he's talking about is either an advisory or caution message and with low pressure you can expect a lower max motoring RPM and a higher temperature during start. The possibility for a hung start also increases. 

With a low starter pressure you are more alert to start malfunctions/problems than usual 😉

edit: the 767 I flew also had PW4000 engines and the normal duct pressure for engine start was 30psi. Max motoring (less than 1% in 5sec) was 20% and minimum N2 15%.

Edited by FDEdev

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