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-   -   Questions about elevator-trim 737 (https://www.pprune.org/tech-log/643972-questions-about-elevator-trim-737-a.html)

herculesthepilot 29th Nov 2021 09:38

Questions about elevator-trim 737
 
Hi all,

i am a bit confused about the way the elevator-trim effects the neutral position of the control-columns.

As far as i understood the stabilizer works independent from the elevator....isnt it so?

During trim the stabilizer moves, but the elevators stays in position. So i assume the neutral-position of the columns doesn't changes, during trim operates!?

In other words/ questions: Let's say i pull the columns, so the elevators moves downwards, i use trim Up to compensate the forces needed to keep the pitch.
Will the columns stays in that position, even when the forces trimmed away, or will the columns move back step by step to the previous neutral position while trimming?

I remember that speed-trim/ mach-trim moves the columns a bit, but not totally sure to be honest:-) But that's another story

Hope someone can help me out with informations.

vilas 29th Nov 2021 11:42


In other words/ questions: Let's say i pull the columns, so the elevators moves downwards, i use trim Up to compensate the forces needed to keep the pitch.
Will the columns stays in that position, even when the forces trimmed away, or will the columns move back step by step to the previous neutral position while trimming?
you pull column elevator moves up not down. You trim the stab to align with elevator i.e you use trim switch up but actually the stab moves down and then release the column. The neutral position of the yoke remains same.

Uplinker 29th Nov 2021 12:51

You pull or push the yoke to achieve the pitch you want and hold that. Then you operate the trim switch or wheel in the appropriate direction, whilst simultaneously returning the yoke to neutral. If you get it right, you will get the yoke back to neutral; the THS where you wanted it, and the elevators neutral in line.

Bergerie1 29th Nov 2021 13:59

Read this, it explains it all very elegantly:-

https://skybrary.aero/sites/default/...shelf/2627.pdf

FlyingStone 29th Nov 2021 14:19

737, as most jets, doesn't have an elevator trim, it has a stabilizer trim. You trim the stabilizer, not the elevator, hence elevator (control column) neutral position is always constant.

Uplinker 29th Nov 2021 22:24

Not sure about 'elegantly', Bergerie - 5 pages to describe what took me just 5 lines to say ! :)

MechEngr 30th Nov 2021 00:25

I believe that for the elevator to see no net force it needs to be aligned with airflow across the stabilizer, so it makes sense that the neutral coordinate system for the elevator is with the stabilizer. The problem is that changing the trim changes the trim speed of the plane, so if there was elevator used to compensate for a mismatch between the current trim angle and the correct angle for that speed, then for level flight with changing airspeed the pilot would either have to add or release a load on the elevator which means a change in stick position. OTOH, if the stabilizer trim is coordinated with the speed change, then there would be no need to even touch the control wheel, which would remain at the neutral position. In all other cases, where the trim was incorrect for the airspeed, then correctly trimming will allow the pilot to release the load on the control wheel whereupon it will return to the neutral position.

If the wheel is left free, then changing the air speed or moving the trim from the trim-speed position will cause the elevator to move relative to the stabilizer and the control wheel will move with it. I don't recommend testing this in flight; instead hold the wheel position and feel the change in force as an indicator.

ImbracableCrunk 30th Nov 2021 11:46

I didn't really get the point of the article.


For years Boeing manuals have said flatly that the control wheel cannot be moved opposite to the direction of trimming motion (the trim motors cut out if it is).... Wrong, it can, and indeed has to, be moved in the opposite direction every time the trim is used; the action is achieved by just relaxing the pressure on the column and allowing to drift back to neutral. It is true that if pressure is applied to the column opposite to the direction of trim, then the trim cuts out
I thought Boeing says you can't trim opposite the direction of yoke movement (from neutral).

vilas 30th Nov 2021 16:59

Trimming the stab and return of yoke to neutral should be taught to pilots. Because after trimming if the yoke is not returned to neutral it becomes additional elevator input to trimmed position. Rostov on don accident the pilot on a go around as expected pushed the yoke forward to control the pitch up but he used a prolonged forward trim of some 12or 14 secs and held the yoke in the same position putting the aircraft in a 65 dive from which recovery wasn't possible.

safetypee 30th Nov 2021 17:48

vilas, a basis of the certification of conventional aircraft is the relationship of stick force with speed; and for manoeuvre, stick force with acceleration (g).

A pilot can detect zero force, but not ‘neutral’ stick position without a reference point or scale.
A ‘neutral’ position (constant speed, zero force, in trim, no manoeuvre) can vary with speed range, configuration, cg, etc.

Uplinker 30th Nov 2021 19:12

I agree, vilas, and we were taught exactly that as part of our Jet LOFT ; (737 sim), at the end of our ATPL course. As someone else said, when you trim, you trim the THS, not the elevators, hence you have to return the elevators to neutral in-line, as you trim, by returning the yoke to neutral.

I personally don't like that design from an operational point of view, and can fully see why Airbus in their wisdom designed it out :)


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