Date:	Mon, 30 Jan 1995 21:27:39 -1000
From:	lord@eskimo.com (David Lord)
Message-Id: <lord.7.001776A6@eskimo.com>
Organization: Eskimo North (206) For-Ever
Subject: 1.5 wrap 

Andrew
    With the method you describe for a 1.5 wrap spar or the varying pitch 
spiral, you would only end up with a single wrap wall thickness. All such 
spars would have near the same stiffness. Stiffness or Moment of Inertia is 
characterised by the equation:  M.O.I.= K(I.D.^4-O.D.^4).  So you see the two 
factors that control stiffness is I.D. and wall thickness. Cross wraps would 
help resist torque which is not very important in most kite designs. Most 
wrapped spars are made from a single piece cut the length of the spar and 
having a width that just wraps around the mandrel the required number of 
times for the type of spar desired. As you can see with this type of 
construction a 1/2 wrap leaves an unbalanced spar that would break at roughly 
the same stress as a 1 wrap.  The problem with 1 wrap spars is that there is 
always small voids in the epoxy. A second wrap virtually assures that two 
voids wont line up.  I am satisfied, as someone else has commented that 
Advantage just makes a smaller I.D. two wrap spar and call it a 1.5 wrap for 
some inexplicable reason.
                                                    Dave Lord 


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