Re: [RCSE] Majestic 110" RES from Laser Arts

[Ed Jett]

Let's assume that we are talking about spar systems where there are shear 
webs involved.  What we have is a beam in this case. If this beam is 
rectangular in shape or symmetrical about it's longitudinal centerline, then 
the top half of the spar assembly is basically in compression and the bottom 
half is basically in tension.  The highest levels of stress occur at the 
outer surfaces of the spar (i.e., the top and bottom surfaces).

It is a fact that CF (spruce, balsa, basswood, for that matter) is roughly 
twice as strong in tension than it is in compression.  It is also a fact 
that CF is by far stronger in tension AND compression than any of the other 
materials mentioned here. Thus the reason for putting the CF on the top of 
the top spar is that this is where the best use of its superior compressive 
strength can be utilized.

If the CF is put on the bottom of the top spruce (or balsa or basswood) spar 
stock, the spruce just adds weight without adding any significant strength; 
it might as well then be balsa, foam or lightweight spackle for that matter. 
You should put the material with the highest compressive strength at the 
spot where the highest compressive stress will occur and the material with 
the highest tensile strength  at the point where the highest tensile stress 
occurs.  For wing spars or other beams, this is the top and the bottom of 
the fabrication and the material that has the highest compressive strength 
and tensile strength (currently available to modelers) is CF.

The shear webbing in the spar system prevents inward buckling of the spar 
caps and the wrapping of  the spar with glass cloth, or CF tow, or Kevlar 
tow CF braid prevents the sparcaps from delaminating or buckling outward. 
Using a FG or kevlar or CF braided tube over the spar and vacuum bagged is a 
very good way of constructing a spar system.  Since, I don't have a 
Vacbagging system, I have to wrap my spars by hand.

Your comments about
----- Original Message ----- 
From: "Kurt W. Zimmerman" <[EMAIL PROTECTED]>
To: <Soaring@airage.com>
Sent: Thursday, January 06, 2005 11:02 AM
Subject: Re: [RCSE] Majestic 110" RES from Laser Arts


Greetings guys....
There has been a lot of discussion on this reflector about how and
where one should attach carbon onto spars to make them effective.
If I were building a woody that required some additional strength to
a wing I would put carbon on the bottoms of both upper and lower spars. 
The
reasons are this, first, carbon fiber is stronger under tension and not
compression.  Second I would be concerned of the carbon delaminating if it
was on the upper surface of a spar.  The other thing that I'd consider 
doing
is adding the carbon to the spars prior to assembly.  I'd even go as far 
as
using a vacuum bag to apply enough consistent pressure along the entire
surface of the spar & carbon.

Just throwing my 2 cents in....
Kurt
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