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What Is "moment Arm?"

Mon Nov 05, 2007 4:43 am

As I was reading a book on the 747SP I read that the whole tailplane area was increased in size due to it having a different moment arm compared to the 747 classic. Can someone explain to me what does that mean? Thanks a lot
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RE: What Is "moment Arm?"

Mon Nov 05, 2007 4:46 am

Moment=Force X Distance

Moment Arm is the Distance.

The tail was redesigned because the forward fuselage was shortened (reducing weight forward of the wing and the center of gravity) and the old tail wasn’t capable of generating enough down force to keep the aircraft balanced.

Best wishes,

[Edited 2007-11-04 20:47:45]
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RE: What Is "moment Arm?"

Mon Nov 05, 2007 4:52 am

Moment arm is a physics concept to do with levers (for example crowbars, wrenches, shovels etc...) You know how with a long lever you can move more than with a short lever? That's because the moment arm is longer. Force applied to an object with a lever is proportional to lever length AND force at the end of the lever.

With me so far? If we are still having language issues the pics here should be helpful:

So anyway, back to the SP. The fin holds the plane stable in yaw and the rudder actually yaws on command. With longer fuselage, the rudder and fin are further from the center of yaw and can be made smaller since the moment arm (length of the lever) is longer. The SP is shorter than the garden variety 747, so the moment arm is shorter. Thus more force need to be applied. So the fin and rudder were sized up.

The SP is to the plain vanilla 747 as if you were digging with a shovel and all of a sudden had to use a 20% shorter handle. You would need to apply more force to the handle at your end to lift the same amount.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
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RE: What Is "moment Arm?"

Mon Nov 05, 2007 4:53 am

Think about it this way. When you are trying to undo a nut, is it easier to use a short wrench or a long wrench? It's easier with a long wrench because the moment arm(a.k.a. the length of the wrench) is bigger. With the shorter wrench you would need to use more muscles. Same concept with a 747, the longer fuselage on a 747 classic means less power is needed to counter the moment(power in this case being tail downforce, and moment arm being the length of the fuselage from the center of gravity). Less power means smaller tail.
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RE: What Is "moment Arm?"

Mon Nov 05, 2007 4:53 am

Since the SP is shortened, the rudder and tailplane are less effective, so it is enlarged. There is much more to this, someone will explain better than I can. I just can't think of the correct words.
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RE: What Is "moment Arm?"

Sat Nov 10, 2007 7:58 am

"Moment Arm represents the magnitude of force applied to a rotational system"
A torque wrench is a good example,how a longer moment arm helps.
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