metamarty
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737 Trim And Yoke Center Position

Tue Apr 02, 2013 1:00 pm

Hello all,

It is my understanding that airplanes are trimmed by using the pitch trim to remove force from the yoke. This scheme is usable in light aircraft where trim tabs are used on the elevators. With this method, the center position of the yoke moves when you move the trim.

Airliners generally have movable stabilizers and it is my understanding that they have a centering system allowing the pilot to trim the aircraft the same way as small aircraft: by applying trim until the force on the yoke is reduced to 0. However, I've heard from various sources now that aircraft like the 737 have a fixed pitch center. With a fixed pitch center, you would need to hold the trim button and simultaneously release to yoke back to its center position, which seems unnatural to me.

Does anyone know what the relationship is between the stabilizer trim and the center position of the yoke?

Thanks,

Marty
 
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CCA
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RE: 737 Trim And Yoke Center Position

Wed Apr 03, 2013 2:00 pm

Yes the elevator control is always zeroed, and as you say you realease the column back to centre while trimming.

Trimming the stabilizer has no effect on the position of the elevator.
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AAR90
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RE: 737 Trim And Yoke Center Position

Wed Apr 03, 2013 3:38 pm

Quoting metamarty (Thread starter):
However, I've heard from various sources now that aircraft like the 737 have a fixed pitch center. With a fixed pitch center, you would need to hold the trim button and simultaneously release to yoke back to its center position, which seems unnatural to me.

Seems unnatural to me as well.

Quoting CCA (Reply 1):

Yes the elevator control is always zeroed, and as you say you realease the column back to centre while trimming.

I do not recall ever releasing "the column back to centre while trimming." AFTER trimming, yes. That is the whole point of aircraft trim... stabilized flight with NO flight control inputs.

Excellent reference: the Boeing 737 Technical Site
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CCA
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RE: 737 Trim And Yoke Center Position

Wed Apr 03, 2013 5:12 pm

Quoting AAR90 (Reply 2):
I do not recall ever releasing

You are if you're flying a Boeing, the elevators are rigged to neutral so the column always returns to neutral, the more you trim out the force the more you realease the column towards neutral.

During level flight the column isn't forward during high speed and aft during slow speed it's always neutral, otherwise an elevator input is being put in.

Watch the A/P slow from 320 to 220 in level flight the column remains in the neutral position.

I can't see why the 737 would be different to the 747.

[Edited 2013-04-03 10:14:55]
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Starlionblue
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RE: 737 Trim And Yoke Center Position

Thu Apr 04, 2013 1:17 am

Quoting metamarty (Thread starter):
. With a fixed pitch center, you would need to hold the trim button and simultaneously release to yoke back to its center position, which seems unnatural to me.

I don't see how it is unnatural. If you release the control column, the elevator will go to neutral and, assuming you have feedback to the column, take the column with it. This is whether the aircraft is in trim or not, and of course disregarding a stalled stabilizer condition.

What you describe is exactly how you trim a Cessna 172 with electric trim. You'd hold the trim switches with your left thumb while simultaneously moving the yoke in the opposite direction until it reaches center. Alternatively, you'd use the trim wheel in one direction while releasing the yoke in the other. Takes a bit of practice but that examiner expects you to hold altitude; you can't trim, then release the yoke.

Quoting metamarty (Thread starter):
It is my understanding that airplanes are trimmed by using the pitch trim to remove force from the yoke. This scheme is usable in light aircraft where trim tabs are used on the elevators. With this method, the center position of the yoke moves when you move the trim.

Does it? My memory could be playing tricks on me but I'd think the elevator on a Cessna 172 is centered unless you exert force on the control column, and regardless of whether the aircraft is in trim. Quite a bit of air flowing by to center it.
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roseflyer
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RE: 737 Trim And Yoke Center Position

Thu Apr 04, 2013 9:04 am

Quoting CCA (Reply 3):
You are if you're flying a Boeing, the elevators are rigged to neutral so the column always returns to neutral, the more you trim out the force the more you realease the column towards neutral.

During level flight the column isn't forward during high speed and aft during slow speed it's always neutral, otherwise an elevator input is being put in.

Watch the A/P slow from 320 to 220 in level flight the column remains in the neutral position.

I can't see why the 737 would be different to the 747.

Just a comment. The column is rigged to neutral, but that doesn't mean the elevators are neutral relative to stabilizer. Depending on model there is an offset so that neutral elevator relative to the stabilizer is not necessarily column neutral. For example the 747 is bias rigged one degree nose up (elevator trailing edge up) for neutral column. Also, the inboard and outboard elevators are rigged 1 degree offset of each other.

In my experience, Elevator rigging on a Boeing airplane is just about the most complex rigging job you get outside of trying to correct a 737 that has its steering slightly offset.
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CCA
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RE: 737 Trim And Yoke Center Position

Thu Apr 04, 2013 2:15 pm

Quoting Roseflyer (Reply 5):
Just a comment. The column is rigged to neutral, but that doesn't mean the elevators are neutral relative to stabilizer. Depending on model there is an offset so that neutral elevator relative to the stabilizer is not necessarily column neutral. For example the 747 is bias rigged one degree nose up (elevator trailing edge up) for neutral column. Also, the inboard and outboard elevators are rigged 1 degree offset of each other.

True this was discovered during flight test and now the new neutral position is where the elevator index plate is on the fuselage, and then the O/B rigged the set dimension below the I/B.

Agree rigging on Boeing is intense
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737tdi
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RE: 737 Trim And Yoke Center Position

Thu Apr 04, 2013 5:03 pm

Not to be argumentative but Boeing rigging is quite simple IMO. McD. could be much more time consuming. Just replaced a 737 classic RH horiz. stab. and elevator last week. Rigging the elevator is straight forward, obtain your throws/neutral by adjustment of the PCU rods, adjust the elevator tab to the one removed, test fly for power off and done if it passes. I know I have simplified it but thats the gist.
 
AAR90
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RE: 737 Trim And Yoke Center Position

Sun Apr 07, 2013 7:29 pm

Quoting CCA (Reply 3):
You are if you're flying a Boeing, the elevators are rigged to neutral so the column always returns to neutral, the more you trim out the force the more you realease the column towards neutral.

As you write slightly differently, I think our disagreement is with the original semantics (the use of the words). Yes, I "trim out the pressure" which to me is different than "release the column." I do not "release" anything until I am DONE trimming the acft. Just a different way of saying the same thing.  
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metamarty
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RE: 737 Trim And Yoke Center Position

Mon Apr 08, 2013 5:55 pm

Many thanks for your great input so far! As I understand it now, the acting of trimming implies letting the yoke fall back to its neutral position. I believe the amount of trim required is proportional to the displacement of the yoke from its center position (measured in degrees) and is not proportional to the amount of force that is felt in the yoke.
 
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Starlionblue
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RE: 737 Trim And Yoke Center Position

Tue Apr 09, 2013 12:13 am

Quoting metamarty (Reply 9):
As I understand it now, the acting of trimming implies letting the yoke fall back to its neutral position.

Indeed.

Quoting metamarty (Reply 9):
I believe the amount of trim required is proportional to the displacement of the yoke from its center position (measured in degrees) and is not proportional to the amount of force that is felt in the yoke.

For the controls themselves, that is true. However the way the pilot gauges how much trim is needed is by using his built-in force gauges, that is his arms, to feel how much trim is needed. I don't look at the yoke displacement to trim; I feel how much force I need to exert on the yoke.
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roseflyer
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RE: 737 Trim And Yoke Center Position

Tue Apr 09, 2013 12:17 pm

Quoting metamarty (Reply 9):

Many thanks for your great input so far! As I understand it now, the acting of trimming implies letting the yoke fall back to its neutral position. I believe the amount of trim required is proportional to the displacement of the yoke from its center position (measured in degrees) and is not proportional to the amount of force that is felt in the yoke.


The 737 uses a feel and centering unit that is pneumatically and spring controlled. Force does not go up directly with displacement. Trim is in units and is constant with displacement, so you are correct. Trim is related to column position, s
although a pilot rarely will trim based on displacement, but rather their force they exert.
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