Elevator Trim? The sense this is making is NOT!

[Octal450]

I did notice alot of planes use elevator trim not elevator with autopilot, I find this slower, and very nonsense! When the autopiltot disengage you might need to revert the trim to center, which is not easy to do, considering it goes very slowly with keyboard. Also I know sometime the plane tries to pitch up once the rear gear has touchdown, due to the trimming. Don't it make more sense you use the eleavtor????

The sense this is making is NOT!

Is there any reason behind this, or just some developer randomly decided that and people picked up on it???

411DEV

PS: Israel: I know I f***ed up the DC-10-30 one, only one file I changed got commited! (I really need to check my commits )

[KL-666]

Hello it0uchpods,

There are 3 moving parts on the horizontal stabilizer in modern jetliners:

1) The whole wing (true trim putting the stabilizer wing nicely in the airflow)
2) The elevator
3) The elevator trim tab (trims by holding the elevator in a certain position)

Which one is powered by autopilot can be different between planes and must be read in the aircraft's manual.

The most desired situation on long flights is to have the whole wing nicely in the airflow, so drag and fuel consumption are low. For that kind of trim the autopilot does not necessarily have to power the whole wing directly. On planes with autotrim the autopilot may move the elevator just like the pilot does. Then the autotrim kicks in and alleviates the elevator to neutral.

So in a sense the autopilot indirectly causes the trim to be set. Being in trim is important too when disengaging the autopilot. Else you may find yourself pitching when disengaging.

The pitch problem you describe is typically caused by an aircraft without autotrim. With autopilot on, the stabilizer wing has been out of trim, and the plane's flight path was determined by the elevator fighting the wrong trim setting. When the autopilot is disengaged the elevator is not held in position anymore, and you will find the plane going in the direction where the trim has been all the time.

All in all, an autopilot should trim the whole stabilizer wing, either direct or indirect via autotrim. Holding the elevator in a deflected position for a long period, either direct or indirect via the trim tab is always incorrect.

[edit]
On older jetliners and smaller aircraft nowadays, you may find an autopilot that does not trim. Yet it warns the pilot saying the like of: "Hello pilot, i am deflecting the elevator for a while now. If you care about your fuel consumption, then please trim the stabilizer for me in order to get the elevator neutral."
[/edit]

Kind regards, Vincent

[IAHM-COL]

Hi Itouchpods
Yes. Revise the whole elvator -> elevator trim change.
I still believe the autopilot should modify the trim in this case, and I still believe that there is not a "elevator" property set. But you will need to verify and make sure; so you are confident the changes proposed are comprehensive, and the planes flies as it should (and potentially better than before) before committing.

But another thing I wanted to mention somewhere. Commit title.
Commit title should be informative of the change made. Some sort of the best possible summary of changes

example

"Changes the autopilot behavior to respond to elevator, not trim"

is a better title -aka more informative than

"who keeps doing this?"

Just food for though, and conseils for good git practice

[Octal450]

Yeah I realize how stupid I am after KL-666 explained to me yesterday, I have reverted this on all my planes. Sorry about that...

[KL-666]

I object to calling yourself stupid. Intelligence is not in knowing everything, but in being open to learn things you do not know yet. So by my definition you justnow showed intelligence.

Kind regards, Vincent

[IAHM-COL]

I cant but agree with KL-666. You demostrate yourself to be a very intelligent and keen to learn type of a guy. I'm glad you learned the elevator-trim trick and can move forward

[legoboyvdlp]

+999

[Octal450]

Thanks for the kind words, does motivate me. I will edit the title.

Have a nice day!

Cheers!

[Octal450]

Actually it IS wrong.

The pitch should be set by the autopilot with ELEVATOR, and then the aircraft should trim itself. This is what the modern IT-AUTOFLIGHT autopilots do, and this is realistic.

Kind Regards,
Josh