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Unpainted Rudder=Unbalanced Rudder?

Sat Jan 31, 2004 6:46 am

Correct me if I'm wrong, but because the rudder has to be precisely balanced in the tail when it is attached, and hundreds (?) of pounds of paint can throw it off, the rudder is painted before it is attached to an aircraft.

So, what's the deal with this A320? Unless the rudder can be re-balanced, wouldnt stripping that part of its paint throw it off?

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RE: Unpainted Rudder=Unbalanced Rudder?

Sat Jan 31, 2004 8:03 am

In most every case where a rudder is hydraulically actuated, like here with the UAL A320, balancing of the surface is not as critical as say a cable pulley system. The rudder actuator and damper systems compensate for any flutter.

In the case above the rudder is also most likely brand new and is 'pre' balanced in it's bare metal form. When painted, it would then need to be re-balanced IF it was required by the manuals.
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RE: Unpainted Rudder=Unbalanced Rudder?

Sun Feb 01, 2004 12:11 am

Can't speak for the A320 specifically but Airbus does not balance their primary control surfaces.

Boeing and Douglas balance most of their primary control surfaces.

For instance the DC10 & MD11;

Inbd ailerons - not balanced

Outbd ailerons - balanced

O/B elevator - balanced

I/B elevator - not balanced

Upper rudder - balanced -

Lower rudder - not balanced

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RE: Unpainted Rudder=Unbalanced Rudder?

Tue Feb 03, 2004 12:29 am

It's most likely a loaner after a defect was discovered in the old one.
Or considering the operator maybe it was more "cost effective" to defer the paint until the plane goes in for check.
Actually if you'll look you'll see that the rudder is painted. If it weren't it'd be an entirely different color and wouldn't last more than a week without significant water infiltration into the composite layers.
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RE: Unpainted Rudder=Unbalanced Rudder?

Thu Feb 05, 2004 4:33 am

I bet that's the a/c that I saw at DCA underging rudder replacement after the original was damaged in a tail-door strike in the UA hangar.
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RE: Unpainted Rudder=Unbalanced Rudder?

Thu Feb 05, 2004 9:54 am

Usually a rudder has a mass balance moment with the basic factory finish.

You can add weight (either paint or repair) to the surface up to a certain limit, then the surface needs to be balanced off the aircraft. You can usually add several coats of paint to a rudder before it becomes out of limits. In any case you can remove old paint to recover the weight of an additional paint coat.

Because paint is usually applied evenly it is rarely a problem.

The problem comes when you carry out a repair. For example you can add a lot of repair weight at the hinge line of the rudder with little affect or you can add a small amount of repair weight at the trailing edge with a BIG affect.

As implied above, it is only manual flight controls that you need to really worry about.
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RE: Unpainted Rudder=Unbalanced Rudder?

Sat Feb 07, 2004 5:37 pm

3 questions. 1) why must a rudder be balanced? 2) how does one go about balancing one? and 3) do they have to take the rudder off to balance it (i.e. when applying a new paint scheme to an a/c or repainting a current one)?

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RE: Unpainted Rudder=Unbalanced Rudder?

Sat Feb 07, 2004 10:01 pm

1. Because even a powered rudder stand a chance of vibrating in flight if un-balanced. Ref B767 AMM:

You can balance the rudder on or off the aircraft depending on how far out of balance it is. The only way to balance, according to the above SRM ref is as follows:
. Serviceability can be regained by removing the paint buildup from Repaints and Repairs that were performed without paint removal, and refinishing the rudder.

In a nutshell, the SRM tells you to maintain a record of repair. Keeping track of material removed and installed. Using simple math, determine what the new balance moment is. If it falls within the limit; do nothing. Between the lower and upper limit; remove paint. Above the upper limit, remove the rudder and re-balance. In fact the SRM says:
Ask The Boeing Company to analyze the Rework Record to determine the rudder's further serviceability.

I try not to use the actual limits in my replies, so as not to confuse. Always check the applicable SRM or AMM. (Standard disclosure)
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RE: Unpainted Rudder=Unbalanced Rudder?

Sat Feb 07, 2004 10:05 pm

I have no idea why "AN / ZGGG), China">" was inserted in the above post.
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RE: Unpainted Rudder=Unbalanced Rudder?

Sun Feb 08, 2004 9:59 am


I'll do my best to answer all your questions........as I've balanced a few rudders in my life.....!!

1).....Not all rudders require balancing, and as said above..'if and when' is driven by the aircraft SRM. Most all hydraulically driven rudders do not require balancing as the actuators will dampen any flutter that may occur. That will bring us to WHY. Flutter will occur to a rudder that is out of balance, and flutter is a BAD thing. It causes undo wear and tear on parts, and will also induce a vibration....much like having a wheel out of balance on your car.
2)..... Rudders are balanced using two formulas........ 'Hinge Line'....and 'Water Line'. Hinge Line is just as the name implies...on its hinge access...Most cases the rudder is placed on its side supported at its hinge points with 2 knife edges, then a weight scale is placed at the trailing edge, the rudder placed parallel to the ground and a reading is taken to the weight applied to the scale at the trailing edge...A formula is then used to calculate if the rudder is within its balance range. With the 'Water Line', 2 weight scales are placed on a table in line with the rudders hinge points, and the rudder is then placed on the scales where readings are taken......again a formula is used to calculate if the rudder falls with its balance range.
3)....Well........Maybe....?? If the SRM requires balancing, the rudder is removed and the above formulas are used........if not required. Some manufactures provide standard tables to adjust any weights.

Some manufactures may use different procedures, but this is the one I know the best.

Hope this helps..........

[Edited 2004-02-08 02:05:07]
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RE: Unpainted Rudder=Unbalanced Rudder?

Sun Feb 08, 2004 7:02 pm

thanks air2gxs and embqa. most helpful. i have a bit of an understanding now.  Smile

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