Boeing 777 flight control // elevator/stabilizer trim

[OE-LML]

Hallo!

I've got some questions about the (real) 777.
As we know, the 777 is using a fly-by-wire system for flight control.
But is it an Airbus-like system where the inputs of the yoke are sent to a computer, are calculated/modified and thereafter sent to the control surfaces where they indirectly move them (vertical inputs result in a change of vertical acceleration, horizontal inputs result in a change of the roll rate)?
Or is it a "conventional" system where the yoke inputs (are sent to a computer and are calculated but) directly move the control surfaces?

And what about the elevator/stabilizer trim? In FG I've recognized that pushing/pulling the yoke also slightly changes the stabilizer trim (which is in some situations quite handy but sometimes ruins a nice and steady descend/climb). Can that auto-trim be disabled?
Is it just a bug in FG or is a 777 really equipped with automatic stab/pitch trim (like an Airbus)?
BTW: Why are on Boeing planes always double trim knobs and double alternate trim knobs/levers?

Yeah, i know, lots of questions ...

As always I'm looking forward to your help and thx in advance

[Octal450]

Hi.
The 777 of FG has no FBW simulated whatsoever. It just has some hackish autotrim. It just uses an integrator attached to elevator position, so it won't be very accurate. I do not think it can be disabled unless the autopilot is switched on.

Kind Regards,
Josh

[Gijs]

If you're interested in learning about the real aircraft, I'd recommend this chapter from The Avionics Handbook: http://www.davi.ws/avionics/TheAvionics ... Cap_11.pdf

[CaptB]

https://pdfs.semanticscholar.org/d4ac/17fcc0db3396a3219785bd2b3e981c7117a9.pdf
https://en.wikipedia.org/wiki/Fly-by-wire#Airbus/Boeing
https://en.wikipedia.org/wiki/Flight_control_modes

[Soitanen]

Airbus FBW uses C* control laws, but 777 (and 787) uses C*U control law. It's speed stable mode, like in conventional airplanes, that's why 777 with FBW have trim switches on yokes. But when you apply trim switch in normal law you don't change stabilizer position directly, but change "trim speed", which affect on pitch behaviour. Elevator will control desired pitch, stabilizer will try to reduce elevator deflection after some time (like trimming).
Trim switch is double to prevent stabilizer runaway due to malfunction of single switch (stick in command position).

[CaptB]

Also this paper:

http://www.icas.org/ICAS_ARCHIVE/ICAS2012/PAPERS/605.PDF

[Octal450]

Yep, Soitanen is exactly. if you fly my Airbus for example, you can see that the system is attitude stable, in roll being a roll rate command, and pitch being a C* (pitch rate and Accel G) command. If you release the stick, he will remain attitude stable.

The 777 FBW is not this way. There is a great detailed explanation here if you don't know the technical info:
I can't post the link here for SOME REASON, but google this, and it should come up first.

Code: Select all

analysis-boeing-777-fly-by-wire-system-jaime-beneyto-g%C3%B3mez-de-barreda

Kind Regards,
Josh

[OE-LML]

Hallo!

Thx for your replies!

Small conclusion (please correct me, if I am wrong):
- Roll axis control of 777 and Airbus-FBW is the same: Generating a roll rate. Once the yoke/sidestick is released to the neutral position, the plane keeps the bank angle.
- Airbus-FBW controlls the pitch by changing vertical acceleration. Once the sidestick is released to neutral, the plane will keep the pitch angle (and is automatically trimmed).
777 controls the pitch by changing the angle of attack and has to be trimmed by the pilot. If the yoke is released to neutral, the plane will nose up/down depending on powersetting and airspeed to which the plane has been trimmed.

But I still got some questions:
What does C* and C*U stand for?

And an example: The 777's pilot (he is flying manually and without autothrottle) wants to climb. He pulls the yoke to achieve a certain pitch angle/AOA and he has to keep it pulled to maintain this attitude (while airspeed is decreasing). The FBW-system initiates the climb by deflecting the elevator. To reduce drag and load of the elevator, FBW aligns it with the stabilizer and therefore it moves the whole stabilizer to keep the plane climbing.
After some time the pilot is getting bored about gaining altitude. He stops pulling and lets the yoke go in neutral position. The 777 has not been trimmed and powersetting was not changed during climb. So the plane will level off and accelerate to the speed it had before.
But how will elevator and stabilizer react??? I guess the elevator will stay aligned with the stab. and stab. will go to the position it had before climbing!? (Pls. tell me if this example is right, and correct me if it is wrong.)

---
Now let's turn to the FG-777:

First of all I really appreciate the work "her" developers and programmers have done over the years.
But one thing that definitively upsets me is the autotrim. Was it added to simulate this described elevator-deflection-reduction to aligned-elevator-stabilizer-movement?
I have almost no knowledge about programming, but some years ago I remember I changed some parameters on the FSX-Beech-Baron to make it fly smoother.
Does anyone know how the FG-777's autotrim function is implemented and is it possible to remove it just by setting some values to 0, or by deleting some lines somewhere???


Alright ... thx in advance for your upcoming replies!

[CaptB]

Hallo!
But I still got some questions:
What does C* and C*U stand for?

C* and C*U control algorithms are described in the PDF I linked, starts at page 3.