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What Is "moment Arm?"  
User currently offlineAR385 From Mexico, joined Nov 2003, 7517 posts, RR: 42
Posted (8 years 6 months 4 weeks 13 hours ago) and read 24657 times:
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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

5 replies: All unread, jump to last
User currently offlineF86sabre From United States of America, joined Oct 2001, 92 posts, RR: 0
Reply 1, posted (8 years 6 months 4 weeks 13 hours ago) and read 24656 times:

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]

User currently offlineStarlionblue From Greenland, joined Feb 2004, 17663 posts, RR: 65
Reply 2, posted (8 years 6 months 4 weeks 13 hours ago) and read 24649 times:

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: http://en.wikipedia.org/wiki/Lever

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."
User currently offlineSovietjet From Bulgaria, joined Mar 2003, 2734 posts, RR: 15
Reply 3, posted (8 years 6 months 4 weeks 13 hours ago) and read 24649 times:
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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.

User currently offlineDakar From United States of America, joined Nov 2005, 71 posts, RR: 0
Reply 4, posted (8 years 6 months 4 weeks 13 hours ago) and read 24649 times:

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.

User currently offlineHAWK21M From India, joined Jan 2001, 31875 posts, RR: 54
Reply 5, posted (8 years 6 months 3 weeks 2 days 10 hours ago) and read 24405 times:


"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.

Think of the brighter side!
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