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Von Target

Engine out, and pitching moments due to thrust...

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Two different questions, mainly addressed at those of you who have actual RL experience in the type :-)

 

1) I noticed from the first time I made my "flight tests" with the PMDG 777 that, even after disconnecting all of the "auto systems", including the thrust asymmetry compensation, that turning off one of the engines even during initial climb would have a practically unnoticeable effect in the roll and yaw. If cruising the effect was practically null.  Is this the way it is in the real thing ? Are engine failure scenarios so "easy" to cope with when manually flying the aircraft ?

 

2) I noticed there are no noticeable pitching moments due to thrust variation. Both on A/T which is how most of the time I believe the 777 is operated, or manually setting thrust, increasing or reducing thrust has no effect in pitch. I am aware the 777 is not a 747, and as far as 744 pilots comente, in that aircraft the pitching moments due to thrust variations are indeed quite noticeable, but the 777 has that "fbw-assisted component that automatically compensated for thrust and flap deployment for instance during approach, but I wonder if in reality the 777 is no "neutral" in pitch when thrust is changed?

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1)As engine thrust decreases as the airplane is faster as well as when the airplane is higher, the resulting yaw with the loss of an engine is also less then when the engine failes at V1 or Vr or V2.

How realistic this is simulated is hard to tell unless you would have had a couple of real life engine failures.

In the Level D sim I never felt the need to apply full or hard rudder to compensate for yaw with an engine failure at cruise FL. Ofcourse there is yaw at cruise level, but way less, and since you dont have any reference to ground track up there you notice much less of it either.

The bigger problem you face at cruise after an engine failure is that you cant maintain your altitude and have to start a drifdown procedure (use the FMC for this), while staying clear of other traffic (off airway heading), while communicating to ATC about this, while doing checklists!

Additionally FSX has the problem that rudder is simulated completely wrong. Just try it, kick full rudder in any airplane, hold the rudder and see how the slip indicator self centers, like an auto rudder feature.

The result is that you cant practice cross wind landings with side slip nor engine failures with use of rudder to correct for yaw.

 

2)correct, the 777 Fly by Wire system compensates for pitch up due to adding thrust and for pitch down effect due to decreasing thrust.

Quite nice during short corrective bursts.

But wha many forget...the 777 does have a (simulated) conventional trim behavior. So as speed changes and you get out of trim the nose will pitch up or down (after a 2-3 seconds) to find its in-trim speed.

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Thx Rob,

 

regarding the turn & slip gauge in FSX, and in some 3pd implementations, none does it the way it should work - even A2A and RealAir have long been trying to get it work acceptably with no complete success... This is a problem with the gauge itself rather than with the flight dynamics, which sometimes are also out of sync with reality, with either too much or less than expected yaw stability...

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Asymmetric thrust effect in FSX is always weaker than it should be. FSX doesn't model lateral-directional aerodynamics well and I suspect that thrust moments were deliberately reduced by Microsoft. So any developer wanting to get realistic engine out yaw effects would need to make the thrust moment arm much greater than the actual value.

 

In reality, with the TAC switched off, the effect of an engine failure on the 777 should be quite dramatic at takeoff.

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I thought PMDG was using an external FDM with the 777. In the NGX it is evident that it's completely out-of-sync with reality, where an egine failure at v2 causes very tame effects in yaw and associated roll, but I thought PMDG had bypassed the core FDM in their 777 ?

 

That and prop effects have always been tuned quite a bit down in MSFS...

 

Torque for instance dies as soon as you pass clean stall speed... in any prop aircraft based on the core FDM of MSFS :-(

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That is what I never understood either....why, if you model an airplane outside FSX would you not model correct rudder/yaw behavior as well?!

I guess the answere is: because the base FSX code does not make that possible.

 

Accymetrical thrust condition and crosswind landings are really great and challenging things to practice.

Too bad FSX is not good at this :-(

I heard that even more modern software like Xplane is not that good at it.

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I heard that even more modern software like Xplane is not that good at it.

 

For the 744, there's a good alternative :)

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I thought PMDG was using an external FDM with the 777. In the NGX it is evident that it's completely out-of-sync with reality, where an egine failure at v2 causes very tame effects in yaw and associated roll, but I thought PMDG had bypassed the core FDM in their 777 ?

 

That and prop effects have always been tuned quite a bit down in MSFS...

 

Torque for instance dies as soon as you pass clean stall speed... in any prop aircraft based on the core FDM of MSFS :-(

I don't think they use external FDE for the 777. There is external simulation added of course but based on what has been posted in the forum it is linked to FSX core model so it stays in sync with FSX. If they had done an external flight model the engine out behaviour would be better, wouldn't it? The FSX flight model isn't bad, it's just rather limited in some axes.

That is what I never understood either....why, if you model an airplane outside FSX would you not model correct rudder/yaw behavior as well?!

I guess the answere is: because the base FSX code does not make that possible.

 

Accymetrical thrust condition and crosswind landings are really great and challenging things to practice.

Too bad FSX is not good at this :-(

I heard that even more modern software like Xplane is not that good at it.

X Plane isn't more modern, just different. In fact the way FSX simulates flight is much closer to how full flight sims do it than X Plane is. The blade element theory stuff is just a different way to access the same data. It does however mean that flight dynamics are determined by the model shape. but that in itself makes little difference to the user. The real difference is in the detail of the flight model and the better equations of motion. You can get good flight characteristics in FSX though. Real Air, for example, are very good at it.

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