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Fly a Boeing? Why is right rudder trim needed in cruise?

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Fly a Boeing? Why is right rudder trim needed in cruise?

Old 2nd Apr 2013, 10:10
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Fly a Boeing? Why is right rudder trim needed in cruise?

Something that I've wondered often but never received a definitive explanation to. Why in the cruise with balanced thrust and fuel do Boeings need a touch of right rudder trim? My experience is on the B757 and B777 but all the variants of the two types are the same. Answers on a post card...

Last edited by Straight & Level; 2nd Apr 2013 at 10:10.
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Old 2nd Apr 2013, 10:32
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One theory (not mine) was that solar heating on one side of the aircraft had a slightly distorting effect on the fuselage
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Old 2nd Apr 2013, 10:50
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I've not heard that one. What about a night flight
Other theories are that the cargo doors on the 757 and 777 are on the right side, and in some way this affects the torsional stiffness of the fuselage, hence the need for rudder trim. What about jet efflux? Could that be an explanation. Aerodynamicists?
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Old 2nd Apr 2013, 11:24
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Is it the same for different engine types?
Rollers turn one way, GE/PW the other.
I would expect diferent trim for different engines if it was linked.
Just a thought.
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Old 2nd Apr 2013, 11:48
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Good point Turin

RB211/RR Trent engines turn clockwise as viewed from the front, whereas GE's turn anti-clockwise.

All the types I've flown generally need a squeeze of right trim, so you are probably right, it's not down to the engines.

Last edited by Straight & Level; 2nd Apr 2013 at 12:05. Reason: Typo
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Old 2nd Apr 2013, 12:01
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I have noticed the same on the B737 NG. Always a touch of right rudder trim, some more pronounced than others, but always to the right. So its not just 777 and 757's.

If you look at a 737 from behind on the taxi there is quite a noticeable castering effect. Where the aircraft appears to be crabbing slightly to one side. Don't know if that has anything to do with it??
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Old 2nd Apr 2013, 12:03
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That's to help you out in a cross wind Jim.
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Old 2nd Apr 2013, 12:06
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Hmm, I never noticed any tendency towards either the right or the left, on the 737s I have flown some don't need any trim, some a unit left, some half a unit right...
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Old 2nd Apr 2013, 12:34
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One feasable explanation is it counters the torque of the anti-collision lights
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Old 2nd Apr 2013, 12:35
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On the B742 and B744 and especially on the B748 I believe it has to do with the differing thermal effects on the steel control rod holding the rudder surface in position vs. the aluminium rib structure. Generally as the aircraft structure cools in climb and cruise it pulls the rudder slightly left requiring correction with right rudder trim, and the opposite occurs in descent.

If the rudder control architecture is similar on all Boeings then this could be it?
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Old 2nd Apr 2013, 12:46
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As main dog said, I've got the Boeing notice on my ipad it's called thermal rudder drift.

Last edited by SMOC; 2nd Apr 2013 at 12:54.
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Old 2nd Apr 2013, 13:09
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I've noted from time to time that the Captain often sets a little too much left trim during preflight due to the paralax error on the rudder trim indicator as seen from the captain's chair (737).
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Old 2nd Apr 2013, 13:20
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A theory I developed with a friend of mine who is a 737NG FO:

The cargo doors on a 737 are on the right hand side. The wear and tear reduces the smoothness of the seal, as well as possibly some of the paint work on that side of the aircraft resulting in the boundary layer becoming more turbulent over that section of the fuselage, creating the small yaw effect.

Could be wrong, don't shoot me down
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Old 2nd Apr 2013, 13:33
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Bearcat, I've heard something similar myself, but was wondering if there was anything definitive written by Boeing about this. So far I think SMOC and main_dog have the most plausible explanation Any chance of a copy SMOC or a link perhaps?
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Old 2nd Apr 2013, 13:44
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Here's an excerpt from a B747-8 rudder system thermal effects presentation:

During climb as ambient the temperature decreases, the aluminum ribs contract faster and to a greater degree than the steel actuator/tab rod.

The aluminum contraction pulls the rudder hinge pivot point closer to the fin rear spar and the rudder surface is held by the steel actuator and therefore pushed to the left.

Rudder Surface will move approximately 0.3 to 0.5 degrees Left depending on conditions during climb.

Tab rod will also experience drift caused by the same phenomena. Lower tab drift is a sum of the main drift and tab drift.

As the aluminum and steel parts stabilize at sub zero cruse temperatures the offset decreases to approximately 0.2 to 0.3 degrees (Rudder Left).

In descent as the ambient temperature increases, the aluminum warms and expands faster than the steel resulting the rudder being pushed to the right.

Rudder Surfaces will move approximately 0.3 to 0.5 degrees Right depending on conditions during descent.
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Old 2nd Apr 2013, 14:14
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Straight & Level, yeah and I also found a flaw in my assumptions. If the cargo doors are on the right it should cause a right yaw and hence a LEFT trim input to cancel.
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Old 2nd Apr 2013, 14:18
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I've always heard it was to offset the weight of either the captains wallet, or his watch? Not so?
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Old 2nd Apr 2013, 14:23
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Is this just Northern hemisphere by chance? Surely you would need to trim to the right in order to produce the turn? Anyone from 'downunder' awake yet?
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Old 2nd Apr 2013, 14:23
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Has this got anything to do with it....

Link
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Old 2nd Apr 2013, 14:54
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Good point Bearcat! Thanks for the excerpt Main_dog, I think that explains it reasonably well

BOAC: Most of my flying is in the Northern Hemisphere. Care to expand on your question? I'm not sure I follow your logic
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