Re: An-124 and wing dihedral

Date:         18 May 98 16:02:56 
From: (Merlin Dorfman)
Organization: Netcom On-Line Services
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Karl Swartz (kls@ohare.Chicago.COM) wrote:
: 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?)

     I believe they are under contract to Lockheed Martin to fly the
commercial boosters and satellites to the launch sites.

: 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.

     Negative dihedral...known as "anhedral."

: 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?

     Again this is way back to things I haven't used for years, but
a low wing aircraft will roll "into" a sideslip because the side flow
around the fuselage will push down on the wings; (positive)
dihedral prevents this.  A high-wing aircraft will roll out of the
sideslip, but perhaps too fast, and anhedral will slow it down.
                                   Merlin Dorfman