Version: 1.16.31
Frequency: Consistently
Severity: Blocker
Marketplace package name: if applicable
Context: in flight?
Bug description:
When you push full throttle the aircraft is pitching down so much, you would end up crashing.
The opposite effect is level flight full power, idle it pitches up too much.
Only solution so far, has been moving the engines lower level to the wing, but this is unrealistic, looks like this is only affecting airplanes with engines on the tail above the wing, like CJ4, Longitude, Cirrus Jet, etc.
My two upcoming jets are affected, will send videos of those private.
Repro steps:
Level flight at full power, idle, the plane pitches up like crazy.
similar at mid power, full power it pitches down like crazy.
Any advise how to tweak the effect is appreciated, my pilots relevant for these projects tells me this doesn’t happen IRL.
Best,
Raul
1 Like
Hello @SimbolFSReborn
This is mainly the result of the CG position and thrust line orientation.
Thrust line depends on engine position and thrust orientation, respectively Engine.N and ThrustAnglesPitchHeading.N in engines.cfg GENERALENGINEDATA section.
You mentioned the Vision Jet, I’m not sure how accurate this is, but I found this mention of the engine being mounted with a 8.5° pitch up angle, and the exhaust nozzle redirecting the thrust with a ~5.5° pitch down angle, such that it’s closely aligned with the CG and has therefore little thrust induced moments.
Then, this is not my area of expertise and maybe some other devs will be able to provide more details but I assume this also depends on some aerodynamics (how much drag and lift the horizontal stab produces at any given speed for example) or flight control aspects as well.
Regards,
Sylvain
I understand, but citation CJ3, and Phenoms for example have the engines trust pretty much straight and they do not suffer from this flying characteristics in real life.
A perfect trimmed aircraft with 90% thrust setting will not pitch up when you go idle..
Neither an aircraft on approach (60% thrust) will pitch down when switching to 90%.. imagine then all go around around the world? You would end up in a pretty bad situation when pushing TOGA, due to the pitch down instead of gaining direct climb due to thrust.
As I said if I put the engines level to the wings, the effect goes away, so clearly the trust vector vs CG applied to jets is too strong and unrealistic.
I have pilots of Cirrus jets, phenoms and citations on my team, they will provide their feedback on this topic during this week.
Best,
Raul
It will. It’s one of the big reasons, for example, why Boeing added MCAS to the 737, as the engines were further off the CG centerline so had more pitch up effect when changing thrust (opposite effect since the engines are on the other side of the CG compared to a bizjet). The existing 737 already had some thrust-based pitch moment, and the new engine position had more, so software was used to augment elevator during pitch changes to make the pitch moment the same as the previous 737. Else, the change in pitch moment would have required mandatory pilot retraining. Needing to adjust trim with instantaneous power changes is something that is part of jet training.
How much pitch will happen will depend on your pitch MOI and pitch stability parameters, as a higher MOI will resist the pitch changes more. We have tuned the CJ4 and Longitude in conjunction with pilots of those aircraft (a CJ4 pilot was a founder of WT, and we worked directly with Textron engineering on the Longitude) and the thrust-based pitch moments in those planes are in line with their real-world counterparts.
It may be that your pitch MOI is set too low to resist the pitch moment of the engine thrust, or you may need to raise the CG stations a bit (or both). With typical business jet wing planform with a decent amount of dihedral the center of the CG for wing fuel is higher than you would expect (i.e. not just in the middle of the wing).
Thanks,
Matt
3 Likes
Worth bringing up here the new parameter/fix empty_inertia_tensor in SU4 SU3 beta full inertial tensor fix not in SDK
Without this parameter I found it simply impossible to get correct moments and pitch stability across all phases of flight on the flight model I’m working on.
1 Like
@FlyingRaccoon @MattNischan Many thanks for all the advises and help, I have now managed to made the effect much more manegeable in line to what the pilots on type are telling me the aircraft does in real life.
I had to use a combination of things, all derived of what you guys adviced on this post, moved the CG to a better position vertically, increased MOI, adjusted pitch stability, etc. has been quite a day as not 1 single parameter helps here, but getting there.
I am now trying to understand the new inertia parameters explained by the link Steven has posted, and see how that can help me to improve things further.
Hugs to both, thank you for being to kind to put me on the right path.
All the best,
Raul
2 Likes