From: (Brad Gillies)
Date:         25 Jan 96 00:54:59 
Organization: Internex Online, Toronto, Ontario, Canada (416 363 3783)
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The theory behind winglets is to INCREASE the EFFECTIVE length of the wing
without increasing the SPAN.
In order to effectively achieve this goal the winglet must be rather large in
size.  The A310 and A320 winglets are too small to be really effective
although they do give a small increase in efficiency.
The winglet itself (on a 747-400) is relatively light as it is made of carbon
fibre and graphite composites.  So weight is not really the issue.
The 777 has no real need for the winglet as the wing design is very efficient
and you will notice it is also long.  Effectively achieving the same goal only
with an increase in span.  This is why they came up with the folding wing
option.  This was to accomodate the aircraft on gates spaced for smaller
aircraft (757,767 etc).  The need never (not yet) arose for the option due to
the availability of wider spaced gates designed for the 747 and such.  The
only hitch is gate scheduling.
I recently (2years ago) had the opportunity to get involved with an aircraft
(727) which was modified with winglets (Valsan Modification) The aircraft
achieved a 5% decrease in fuel burn and a faster time to cruise altitude. I
hardly think that is anything to scoff at.
  There are people who think it is not worth it and there are people who think
it works great.  I (obviously) am a proponent of the winglet and its
advantages. I have seen it work with my own eyes in real life flying not in
some wind tunnel or computer. Flying is the true test.  They work.

These opinions are mine and mine alone.
I am not affiliated with any of the above mentioned companies or their