An-124 and wing dihedral

Date:         17 May 98 00:43:27 
From:         kls@ohare.Chicago.COM (Karl Swartz)
Organization: Chicago Software Works, Menlo Park, California
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Last Thursday there were a couple of Antonov An-124s parked at Moffet
Field.  (I've seen one there before.  Anyone know what they might be
doing there?)  They reminded me of a question I've had about wing
dihedral.

The dihedral angle of an aircraft's wings refers to the inclination of
the wings from horizontal.  When the 777 first appeared, many people
noted that it seemed to have greater wing dihedral than many other
designs, and indeed if you see one parked next to a 747 it's clear that
the 747's wings are relatively close to horizontal while the 777's wing
tips are far above the 747's.  (Obviosuly you'd better be confident that
the wings of one of the planes aren't sagging from a full load of fuel
if you make this comparison!)

The An-124 also has substantial wing dihedral, but its wing tips are
*lower* than its wing roots, just the opposite of the 777.  Various US
cargo transports with high wings (C-141, C-5, C-17) are similar, though
not as pronounced as the An-124.

I can see how "positive" dihedral (like the 777) might help cancel out
unwanted roll.  This might also explain why the 777 has greater dihedral
than the 747, since an engine failure on a 777 would produce greater yaw
which in turn would trigger a roll.  (The same sort of yaw-induced roll
which is one of the theories for the USAir 427 crash.)

I can't see what "negative" dihedral (like the An-124) would accomplish,
or why it would even be desireable.

Would someone care to enlighten me on the aerodynamics of wing dihedral?

--
Karl Swartz	|Home	kls@chicago.com
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"The average dog is a nicer person than the average person." - Andrew A. Rooney