747 v3 Trimming logic

[cagrikartal 8]

hello everyone,

 

 

Do you think 747 has a trimming logic closer to the 777 or the 737 ? or Neither  ?

 

 

 

[Andreas Stangenes 1,628]

The trim in the 777 is very different from most other ac ive flown (sim). The trim there controls speed mostly, while in the 737 it controls pitch. I think the trim is closer related tonthe 737 than the 777. 

[FlyChuck 3]

Trim in the 747 is conventional. Trimming simply moves the stabilizer.

[cagrikartal 8]

hmm how about -8 ?I heard it has new fency stuf on it   like fbw etc....

[Tabs 2,790]

hmm how about -8 ?I heard it has new fency stuf on it   like fbw etc....

 

It does not have elevator/stab FBW like the 777 or 787, the only FBW on the 748 is the roll channel. Pitch is 100% conventional just like the 400.

The trim in the 777 is very different from most other ac ive flown (sim). The trim there controls speed mostly, while in the 737 it controls pitch. I think the trim is closer related tonthe 737 than the 777. 

 

Trim in any airplane controls the speed the airplane tries to return to with your hands off the controls. If you trim out the control forces in a Cessna flying at 80 knots, then speed up or slow down, the plane will pitch and try to return to 80 (through a series of phugoid oscillations of course, but eventually it will dampen out at 80). The 777/787 trim system is essentially a software simulation of the way a normal airplane trims. The system is 100% FBW at all times, but to the pilots it behaves conventionally in that it'll return to whatever speed the airplane is trimmed for when displaced above or below it.

 

This was important to Boeing to retain physical cues that the airplane is in an out of trim situation - with the Airbus and similar systems that autotrim in their normal mode/law, you may not notice certain problems developing until they threaten the safety of the airplane. See the Perpignan A320 crash for instance - the airplane was getting progressively more and more out of trim in an unprotected FBW law and because there's no physical feedback, the pilots didn't realize it until it was too late and the system ran out of combined elevator/stab range. It would be really difficult for that to happen to a 777 or 787 because the control forces would become immense, letting them know to manually trim.

[cagrikartal 8]

It would be really difficult for that to happen to a 777 or 787 because the control forces would become immense, letting them know to manually trim.

 

This is why in the sim we get the FBW in blue in the pfd to compansate the backforce of yoke as most of us dont have a motorised joystick or yoke.

 

Now, tell me i dont deserve a free 747v3 copy !!!  

[skelsey 765]

 

 

This is why in the sim we get the FBW in blue in the pfd to compansate the backforce of yoke as most of us dont have a motorised joystick or yoke.

 

However, if you're having to maintain backpressure (or forward pressure) then you are out of trim... and you should be able to feel if you are holding pressure on the yoke/stick, regardless of whether you have force feedback or not

 

As Ryan says, it's easy to overthink the 777 trim - it's a lot of complicated software to essentially make it behave almost exactly how an aeroplane with conventional controls does by itself. Just fly the aeroplane and trim to relieve the force on the controls, exactly like a C172.

[Michael Moe 495]

However, if you're having to maintain backpressure (or forward pressure) then you are out of trim... and you should be able to feel if you are holding pressure on the yoke/stick, regardless of whether you have force feedback or not

 

As Ryan says, it's easy to overthink the 777 trim - it's a lot of complicated software to essentially make it behave almost exactly how an aeroplane with conventional controls does by itself. Just fly the aeroplane and trim to relieve the force on the controls, exactly like a C172.

Yeah i still do over complicate it i guess :wink:

 

 

Ryan said 

"Trim in any airplane controls the speed the airplane tries to return to with your hands off the controls. If you trim out the control forces in a Cessna flying at 80 knots, then speed up or slow down, the plane will pitch and try to return to 80 (through a series of phugoid oscillations of course, but eventually it will dampen out at 80)"

 

I think why i complicate it is becuase i saw a video were the captain was using the trim on a 777 to change flight level i think

 

Michael Moe

[ph-cxz 28]

 

 

I think why i complicate it is becuase i saw a video were the captain was using the trim on a 777 to change flight level i think

 

You cont change flight level with trim only a decrease or increase in altitude window allows that or the pilot was flying manual.

[Michael Moe 495]

 

I think why i complicate it is becuase i saw a video were the captain was using the trim on a 777 to change flight level i think

 

You cont change flight level with trim only a decrease or increase in altitude window allows that or the pilot was flying manual.

he was flying manually - 

 

Michael Moe

[whoispankaj 51]

 

I think why i complicate it is becuase i saw a video were the captain was using the trim on a 777 to change flight level i think

He was demonstrating how trim works in 777.. Not using trim to change flight level....

 

You cont change flight level with trim only a decrease or increase in altitude window allows that or the pilot was flying manual.

[downscc 1,327]

 

I think why i complicate it is becuase i saw a video were the captain was using the trim on a 777 to change flight level i think

 

You cont change flight level with trim only a decrease or increase in altitude window allows that or the pilot was flying manual.

I think all flight instructors teach their students to fly the trim...., this doesn't mean you use trim instead of elevator pressure but that you learn to fly attitude and trim the control forces.  Even with manual trim in a Cessna, one hand on the yoke and the other on the trim wheel. It's becomes second nature and occurs without thinking about it.