Jump to content
Sign in to follow this  
gnomegemini

Fly-By-Wire of the 777 and it's simulation

Recommended Posts

His point might be that Boeing doesn't refer to FBW 'laws' but rather 'modes', but as you were talking about Airbusses (Airbii?) at the time I think the point was moot!


Mark Adeane - NZWN
Boeing777_Banner_BetaTeam.jpg

Share this post


Link to post
Share on other sites

(...)

Not quite sure what Capt is asking/stating? Am pretty sure he also keeps claiming to be an A320 captain :/

 

 

He claims to have been an A320 Captain for about 10 months - plus some time as an A320 F/O before that ...

Share this post


Link to post
Share on other sites

Hey gents,

check out the document which starts downloading 20sec's after entering the page.

It will answer ALL your questions concerning Fly by wire on the T7.
Boeing 777 Flight Controls Manual


Regards, always three greens and happy landings,

Max

Aerospace Engineer and Private Pilot
TriStarbanner_zpsefe0294f.jpg
Flightdiary

Share this post


Link to post
Share on other sites

Many thanks for your answers. So the 777 has no autotrim system which makes it feels more like a real aircraft. :)

Share this post


Link to post
Share on other sites

It has an autotrim system that keeps the airspeed konstant, instead of the vertical acceleration like Airbus.


Regards, always three greens and happy landings,

Max

Aerospace Engineer and Private Pilot
TriStarbanner_zpsefe0294f.jpg
Flightdiary

Share this post


Link to post
Share on other sites

Guys it really is a bit more complicated than a single sentence, and you want to be careful not to make a misleading statement by virtue of ommission of information.

The 777 FBW does have an auto trim element involved (although I don't think it's explicitly referred to as such), and C*U does take into account load factor at higher speeds.

My advice is to do some reading as per the links in this thread. You'll find it's a fascinating and worthwhile subject to look into, albeit a rather technical one. As recent events have demonstrated, this is a more complicated airplane than perhaps we give it credit for!


Mark Adeane - NZWN
Boeing777_Banner_BetaTeam.jpg

Share this post


Link to post
Share on other sites

No the 777 does NOT trim based on speed.

It trims to maintain a flight path.

 

The 777 trim system works by the pilot setting the trim speed with the trim switches.

Maybe thats what you mean.

 

So does it trim for speed or not? You want to make up your mind here?

 

So yes it trims for a given speed, but that is nothing special.

A Cessna's trim is set for a speed as well.

What is special, is that config changes/thrust changes/banking does not require you (or very little) to compensate with the yoke. The FBW system will change elevator and stabiliser for you.

 

Again, is it special or not? You correctly stated that trim on many aircraft won't account for config/thrust/attitude changes, so how can trim set speed in a Cessna without the FBW in the 777 that you say is necessary to achieve this? (It doesn't/can't)

 

I've never read a series of such confused statements before. Please, PLEASE if you don't know, or just as bad can't explain, then don't post.

 

Share this post


Link to post
Share on other sites

 

I've never read a series of such confused statements before. Please, PLEASE if you don't know, or just as bad can't explain, then don't post.

 

 

As already said, BEST is to READ carefully  the doc's given above:  :smile:

 

about the 777 FBW:

http://www.davi.ws/avionics/TheAvionicsHandbook_Cap_11.pdf  (chap 11.8.1)

 

or

about the 777 Flight Controls:

http://www.smartcockpit.com/aircraft-ressources/B777-Flight_Controls.html  (chap 9.20.10)

 

Guy

Share this post


Link to post
Share on other sites

So does it trim for speed or not? You want to make up your mind here?

 

 

 

 

Again, is it special or not? You correctly stated that trim on many aircraft won't account for config/thrust/attitude changes, so how can trim set speed in a Cessna without the FBW in the 777 that you say is necessary to achieve this? (It doesn't/can't)

 

 

 

 

 

 

I've never read a series of such confused statements before. Please, PLEASE if you don't know, or just as bad can't explain, then don't post.

 

Has it ever occured to you that maybe it is explained quite well but you yourself are the problem?!

 

First:

I did make up my mind!

The pilot needs to trim for speed.

The 777 auto corrects for config changes/thrust changes/during banks/etc

 

Second:

You seem to have no understanding of what trim is if you believe a FBW system is required to trim out a Cessna!

 

Last:

I will be glad to try to explain the system to you 10 more times in all kinds of flavours.

Realy, I dont mind.

But you are going to have to come at me a little different please!


Rob Robson

Share this post


Link to post
Share on other sites

Many thanks for your answers. So the 777 has no autotrim system which makes it feels more like a real aircraft. :)

Correct.


Rob Robson

Share this post


Link to post
Share on other sites

It has an autotrim system that keeps the airspeed konstant, instead of the vertical acceleration like Airbus.

Not correct.

Or, I would not say it like that.

 

When you say "auto trim" it gives the impression that when you add thrust and push on the yoke to maintain level while accelerating, that the "auto trim" system will then trim away those forces so you can let go of the yoke again.

(I understand this is what an Airbus does)

This is NOT what a 777 does however.

 

The pilot must retrim the aircraft for the increased speed.

The pilots trim switch input goes to the FBW system.

Inside the FBW system, the trim reference speed is increased from 210kt to 250kt for example.

The FBW system then gives commands to the elevator/stabiliser accordingly.

The pilot will feel he does not have to push on the yoke anymore (just like on a conventional plane)

 

I would not call that automatically.

That is manual trim control!

Just not like in a Cessna where you control the elevator trim tab directly with a cable.

 

When you add thrust and do not manually re-trim, then the aircraft will start a climb eventually.

Not immediately like a B737, because remember, that pitch up moment is automatically counteracted by the FBW system that wants to maintain its path.

But as airspeed increases, lift increases, and the airplane will start a climb at the trimmed airspeed.

Like I said before, just like a Cessna, there is nothing special about that part. Thats aerodynamics.

The FBW system will not stop it from climbing (again that is Airbus)!


Rob Robson

Share this post


Link to post
Share on other sites

I would say it like that, that the T7 has an autotrim system that keeps the speed constant.

The pilot chooses a reference speed with his trim switches (either on the yoke or with the trim levers). This reference speed is displayed on the bottom left corner of the PFD. 
So at constant thrust a trimmed out plane flies leveld out (this is where the T7 is pretty much like a Cessna).

Now, when the pilot increases the thrust, the plane tries to maintain the reference speed and will start a climb.

The advantage of this system is the gust compression. If the plane flies through turbulences, no matter if during climb, descent oder level flight and is trimmed out, it will automatically find back to the initial path angle due to the reference speed system. Autotrim on an Airbus tries to block acceleration. So if an Airbus flies through turbulences, it will maintain the attitude that the plane has got due to the turbulences. So if it was during climb when turbulences occurred that threw the plane in a descent, it will maintain the descent.

If there are any more questions, feel free to ask or check out my previously posted link.


Regards, always three greens and happy landings,

Max

Aerospace Engineer and Private Pilot
TriStarbanner_zpsefe0294f.jpg
Flightdiary

Share this post


Link to post
Share on other sites

Guys it really is a bit more complicated than a single sentence, and you want to be careful not to make a misleading statement by virtue of ommission of information.

The 777 FBW does have an auto trim element involved (although I don't think it's explicitly referred to as such), and C*U does take into account load factor at higher speeds.

My advice is to do some reading as per the links in this thread. You'll find it's a fascinating and worthwhile subject to look into, albeit a rather technical one. As recent events have demonstrated, this is a more complicated airplane than perhaps we give it credit for!

I am trying to explain here how the system works., as clear as I can, and so that the starter of the thread can follow along.

I am trying to stay practical too.

 

Ofcourse I ommit some/most technical details!

Heck the B777 system manuals ommit probably 80% of how the system works.

Or do you think the electrical wiring diagrams we get in the manual is all there is to it?

Why does boeing ommit?

Because we are not engineers.

We only get the info required to operate the aircraft. Thats all.

 

If I ask you: "can you explain lift to me"?

Are you going to get into a Bernoulli discussion with me from the get go?

Or will you use simplified ways to explain?

Exactly!

 

Talking about ommiting information, yee wizz!!

with "recent event" you mean San Fransisco?!

I think maybe you should wait a couple of months for the NTSB investigators to come forward with their findings before you conclude that the the 777 complexity is the cause for the crash.


Rob Robson

Share this post


Link to post
Share on other sites

I would say it like that, that the T7 has an autotrim system that keeps the speed constant.

The pilot chooses a reference speed with his trim switches (either on the yoke or with the trim levers). This reference speed is displayed on the bottom left corner of the PFD.

So at constant thrust a trimmed out plane flies leveld out (this is where the T7 is pretty much like a Cessna).

Now, when the pilot increases the thrust, the plane tries to maintain the reference speed and will start a climb.

The advantage of this system is the gust compression. If the plane flies through turbulences, no matter if during climb, descent oder level flight and is trimmed out, it will automatically find back to the initial path angle due to the reference speed system. Autotrim on an Airbus tries to block acceleration. So if an Airbus flies through turbulences, it will maintain the attitude that the plane has got due to the turbulences. So if it was during climb when turbulences occurred that threw the plane in a descent, it will maintain the descent.

If there are any more questions, feel free to ask or check out my previously posted link.

That is all fine, but the starter of the thread wanted to know how the 777 is flown/trimmed.

Or at least that was my impression.

 

Getting into how the 777 reacts to turbulence does not answere his question.

We could also get into how the 777 corrects for (some) assymetric thrust conditions with rudder input automatically but that is also not what he asked.

 

The main advantage for the pilot is not gust suppression.

The main advantage for the pilot is auto correction of config changes/thrust changes etc etc as mentioned before.

The main thing to realise is that the 777 simulates to that pilot that it flies pretty much like a conventional airplane!

 

From your explanation one can only conclude a Cessna has an autotrim system as well.

You set the trim reference speed with the wheel and the system automatically maintains speed :-)

If disturbed by turbulences, the aircraft will find its way back to the set airspeed.

Thats aerodynamic speed stability!

Oh and talking about aerodynamics;

quote

"So at constant thrust a trimmed out plane flies leveld out"

unquote

 

At trimmed airspeed the airplane can be at any flight path, not just level flight!

Climb/level/descent/!

 

 

Next.

Trim reference speed is not displayed on any of the the 777 PFD I have seen.

Could be a company option though.

Dont assume that just because one 777 displays something, that all 777 display that same info the same way or at all!

 

 

EDIT:The threads starter seems to have been helped, so thats it for me on this topic.


Rob Robson

Share this post


Link to post
Share on other sites

Ok 777simmer. Thanks for clearing everything up for us.

 

I rarely touch the forums now for reasons I'm sure are fairly self evident from this dialogue. To reiterate to the OP, and to those who want to understand the 777: don't pay too much attention to the forum 'experts', but do your own research. The necessary resources have been linked in this thread.

 

Take care!


Mark Adeane - NZWN
Boeing777_Banner_BetaTeam.jpg

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this  

  • Tom Allensworth,
    Founder of AVSIM Online


  • Flight Simulation's Premier Resource!

    AVSIM is a free service to the flight simulation community. AVSIM is staffed completely by volunteers and all funds donated to AVSIM go directly back to supporting the community. Your donation here helps to pay our bandwidth costs, emergency funding, and other general costs that crop up from time to time. Thank you for your support!

    Click here for more information and to see all donations year to date.
×
×
  • Create New...