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Standoff placement -how does it influence performance?

To: kites@das.harvard.edu
~Date: Wed, 26 Jan 1994 21:50:44 GMT
~From: cora@quads.uchicago.edu (Andre The Great)
~Subject: Standoff placement -how does it influence performance?

Andre The Great, writes:
>A  friend of mine is considering building a kite-  the  original 
>plan  is  for a kite without standoffs- however,  he  wishes  to 
>modify the design so as to incorporate a pair. The question is : 
>where  along the trailing edge should one attach the  standoffs? 

     The rest deleted:
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Here is a Theoretical Analysis for the placement of the Standoffs 
for a Delta shaped Dual Line Kite.

     Assumptions:

          1.   Nose angle is about 90 degrees.
          2.   Wing Span is about 8 feet.
          3.   Depth of Billow is about 10 inches.
          4.   There is a ridged center spine.

     Purpose of the Standoffs:

     The primary purpose of the standoff is to reduce the  amount 
of  time  required for the Sail to acquire the  Billow  position.  
This is a definate advantage in light and gusty winds when flying 
the kite near the edge of the wind window.

     Primary Balanced Placement of Standoff:

     Hang  the  kite from the ceiling with the  bridle  side  up.  
Adjust the angle such that the back side of the sail is  parallel 
to  the floor. (frame will be tilted nose down)  Lay a  dowel  or 
kite  stick on each side of the center spine.  The  dowel  should 
create  the natural billow. (valley from nose to  trailing  edge)  
The  point  at which the valley meets the trailing  edge  is  the 
Natural Balanced Position for a Standoff.  The center of pressure 
of the sail should be very near the line created at the bottom of 
the  valley.  The flight characteristics will be different for  a 
center of pressure on the outboard side of the valley then for  a 
center of pressure on the spine side of the valley.  But that  is 
a whole other design consideration.

     Theory of Turning:                       Top View
                        ___               /\            /\              
                       /.|.\            /    \        /    \            
                     / . | . \        / A    B \    / C    D \        
                   /  .  |  .  \    /------------\/------------\   
                 /   .   |   .   \                             
               /  A .  B | C  . D  \                 __
             /     .     |     .     \     Line of  | \    Wind
           /------*------|------*------\   Sight        \
                  S             S
               Standoff      Standoff

     Considering the active surfaces for a Right Turn.  The  Line 
of Sight represents the effective wind direction when you pull on 
the right line and tilt the right side of the kite into the wind.  
The surfaces B and D are positioned "Edge to the Wind" and there-
fore have limited effect on the turn.  However, surfaces A and  C 
have been positioned to become the active surfaces with more wind 
pushing on them.  Notice that surface A is further away from  the 
center  spine than surface C and that the Plane of surface A  has 
increased its angle of attack due to its slope from the  trailing 
edge  toward  the  nose of the kite.  Conversely,  the  plane  of 
surface C has become almost vertical.(flat to the wind) This is a 
stalled  position.   Therefore surface A advances and  surface  C 
slows down.  Consequently the kite turns to the right.


Effect of Standoff Position:

     When you move the standoff out toward the leading edge,  the 
effect  is to reduce the area of surface A and increase the  area 
of  Surface  C.  At some point, Surface C will dominate  and  the 
kite will have difficulty turning.  However,  The position of the 
bridle pick point (Tow Point) will also play a part in the  kites 
ability  to  turn.  The placement of the Pick Point  is  a  whole 
other design consideration.

     When  you move the standoff in toward the center spine,  the 
effect is to increase the area of Surface A and decrease the area 
of  Surface C.  Turns should become faster and tighter.   An  ex-
treme  case would be to Standoff the center spine from the  cross 
bar.   (No  central Dihedral.)  The Speed Wing is an  example  of 
this concept. (Though it has no center spine)

I hope that this has shed some light on the effects of the place-
ment of the standoffs for a delta dual line kite.

                            Ron Reich

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