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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...
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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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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? |
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. |
Good point Turin :D
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. |
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?? |
That's to help you out in a cross wind Jim.
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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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One feasable explanation is it counters the torque of the anti-collision lights:ok:
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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? |
As main dog said, I've got the Boeing notice on my ipad it's called thermal rudder drift.
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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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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 :ok: |
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:D Any chance of a copy SMOC or a link perhaps? :O
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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. |
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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I've always heard it was to offset the weight of either the captains wallet, or his watch? Not so? :E
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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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Good point Bearcat! Thanks for the excerpt Main_dog, I think that explains it reasonably well :ok:
BOAC: Most of my flying is in the Northern Hemisphere. Care to expand on your question? I'm not sure I follow your logic :O |
I know you're all talking Boeing, but the Airbus FCOM says to expect rudder trim to reside between 2.3 units Left and 1.0 units Right in the cruise. I rarely see it, if ever, sitting to the right. It's nearly always left trim.
I mention it because the cargo doors are on the right side of the A320 too so you wouldn't expect left rudder trim if this theory was true. |
Care to expand on your question? I'm not sure I follow your logic |
Coriolis, old chap. I use to notice the amount of trim reducing as I neared Banjul in the old days. |
Jim smitty.
The 737 crabbing is because the landing gear design includes a shimmy damper that permits the mlg to castor left or right. |
Since nobody knows if this is even real and probably isn't, A
OA changes some at cruise so the slight power reduction at cruise or AOA change is the only thing different. P factor on a prop airplane required right rudder in a climb and not in cruise. If the engine rotates the counterclockwise direction I guess the opposite would be true but since it goes out the same exhaust the only effect would be the intake pulling air further out on one engine vs the other. |
CRABBED TAXYING 737s.
The reason for the B737 appearing to taxy with drift is because the MLG torque links have a damped mechanism to allow limited castoring of the mainwheels.
If you are lucky enough to get close to one, check the "knee-joint" at the front of the MLG and you'll see the shiny portion of the damper, a bit like an oleo. I try to reassure my F/Os when they're getting twitched about X-wind landings, that the airframe owns the first 15 knots of crosswind, as per autolands, and they've just got to sort the other 18/20 depending on winglets! So far it seems to reassure them and me................:ok: |
Is this the exact same on the 737-200 series.
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Right rudder trim north of the equator. Left rudder trim south of the equator.
Been that way ever since they discovered the equator and modern plumbing. |
None of us believe that because once established in flight we all know no rudder is required no matter what the wind is doing. Coriolis effect only affects how the atmosphere circulates in the northern and southern hemisphere, not how you have to fly your airplane. Maybe you are just kidding but if you aren't get a book out and read how it works.
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Does the amount of trim required get any worse at the very beginning of April?
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None of us believe that because once established in flight we all know no rudder is required no matter what the wind is doing. Coriolis effect only affects how the atmosphere circulates in the northern and southern hemisphere, not how you have to fly your airplane. Maybe you are just kidding but if you aren't get a book out and read how it works. |
So, this thread is taking an interesting 'turn'. Pun intended ;)
I still maintain that the likely culprit is uneven cooling in the rudder actuator mechanism, as do Boeing it seems (thanks SMOC and main_dog). Now granted, it's been a while since I studied my ATPL theory subjects but I'm struggling to understand how Coriolis affects the rudder trim required in steady-state cruise. |
once established in flight we all know no rudder is required no matter what the wind is doing. |
He's actually wrong about coriolis and P factor on a turbo prop.
Anyway, there is a simple reason for this rudder trim phenomenon and that is because the right hand engine is started first the right hand thrust pixies are that little bit more warmed up than lefties and, therefore, produce that little bit more thrust, hence yaw, hence trim required. Easy. :ok: |
Ah yes, the pixies!
I don't know the answer to the coriolis question, but if a mass of air moving within the atmosphere above the planet is affected by the coriolis force and veers slightly, then why not an aircraft also moving in the atmosphere above the planet? Indeed - I have just checked my dictionary, and its' definition of coriolis is; "A hypothetical force used to explain the deflection in the path of a body moving relative to the earth" (If aircraft are affected by this, I would not have thought that rudder trim is the answer - the aircraft will account for it by comparing its required track with its actual track and adjust heading accordingly. Rudder trim is used to adjust for any slight variations in the airframe - one wing fractionally more draggy than the other - asymmetric fuselage drag etc.) |
I find that every one of our -800s needs a bit of left in the climb and back to 0 in cruise.
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That's because of the reduced demand on each sides' pixies allowing them to provide exactly the same thermal efficiency and, therefore, thrust. :ok:
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P factor was taught when I was learning and the same laws apply now. Trim once in an ocean of air doesn't compensate for wind, only for misrigging or assymetrical thrust. A jet engine, if it has any AOA, would have slightly more intake on the downward spinning fan but I have never noticed enough to care. I think a lot of folks here are pulling our leg to see if we will bite. Then we have a few that....never mind.
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I guess it is their head they can put it anywhere they want.
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Boeing Flight Operations Review 737-22
An airplane in steady cruising flight may require some degree of
lateral and/or directional trim inputs to minimize drag. The trim requirement can arise from improper rigging of flight controls after maintenance, internal load distribution, fuel unbalance, and engine thrust setting imbalance. The degree of trim inputs required will depend on two basic factors: 1) The degree of controls misrig, lateral unbalance, etc., and 2) Trim technique employed |
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