Re: An-124 and wing dihedral

Date:         18 May 98 16:03:03 
From:         Don Stauffer <stauffer@htc.honeywell.com>
Organization: honeywell
References:   1
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Karl Swartz wrote:
> 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.

All modern transports have VERY flexible wings.  What is important is
the dihedral in flight, not on the ground.

> 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?

Dihedral is an extremely complicated subject that must be analyzed with
3 D geometry.  Any attempt at a 2D explanation is doomed to fail.  Some
explanations are just plain wrong, others highly misleading.  What you
must do is look at the angle of attack of the air on the two wings as a
slight sideslip developes.  By definition, with a sideslip the angle of
attack is NOT parallel to the aircraft longitudinal axis.

Now, sweepback has a very similar function to dihedral, so planes with
sweptback wings need less dihedral than straight wings.

--
Don Stauffer in Minneapolis
home web site- http://home1.gte.net/stauffer/
home email- stauffer@gte.net
work email- stauffer@htc.honeywell.com