From: reich@nosc.mil (Ronald S. Reich)
Subject: Re: TURNING
Date: Mon, 25 Oct 1993 17:02:41 GMT

>From: swann_m@filton.enet.dec.com
>Question and comments deleted...
>         I appreciate it's not quite so simple as "this  is  the 
>answer", but my brain's going fuzzy!
        
     Hi   Mike,

I hope this extemely brief presentation sheds some light on  your 
interest.  There is a lot more to it than is presented here,  but 
this should give you the general idea.

Unbalanced  FORCES of the wind on the dihedral sail  surfaces  of 
the kite will cause the kite to turn.

                                               A=left anhedral
                                               B=left dihedral
               /\            /\                C=right dihedral           
          A  /    \B     C /    \ D            D=right anhedral
           /        \    /        \            L=left fly line
         /  \         \/            \          R=right fly line
       /      \     spine             \        1&2=line of wind
                \                                  direction for
           *      \               *                right turn.
           |        \             | \
           |          \           |   \
           L            \         R     \
                          \               \
                            \               2
                              \ 
                                \
                                  1

1.   When the kite is square to the wind, the forces are
balanced about the center spine.  The effective wall of wind
strikes the sail parallel to the fly lines L and R.

2.   When the kite is tilted by pulling on the right fly line,
The effective wall of wind now strikes surfaces A and C more
than their counterparts D and B.

3.   The  lever arm, from tow point (*R) of the wind force  (line 
1)  acting on surface A is longer than the lever arm of the  wind 
force acting on surface C.

4.   The  angle  of the surface A from the trailing edge  to  the 
nose is greater than that of the angle of the surface C.

The  cumulative effect of facts 3 and 4 cause the kite to  rotate 
about the line 2-*R.

     Set up your kite and take a look at it from the angles  that 
the wind would be coming from under different conditions.  Notice 
how  much  of each surface you can actually see.  These  are  the 
surfaces  that  the force of the wind will be  pushing  on.   The 
anhedral  surfaces  have  a different angle of  attack  that  the 
dihedral  surfaces.  The dihedral surface is flatter to the  wind 
and  will therefore move slower than the anhedral  surface.   The 
result  is that the anhedral surface will dominate and cause  the 
kite to turn.

     A similar analysis can be applied to the actions created  by 
pulling on the lines of a Flexifoil.  The curved Flexifoil shifts 
the  effective  lever arm of the wind force to the far  side  and 
causes the kite to turn.

     Incidently; the word "anhedral" is a manufactured word to be 
used  for  descriptive purposes.  All sloped  surfaces,  inclined 
upward or downward, are actually "dihedral."  I started using the 
word "anhedral" about 7 year ago when it seemed to make it easier 
for the attendees of the syminars to understand.

                  The wind is Up and I am Out,

                            Ron Reich