At 17:18 30 November 2003, Eric Greenwell wrote:

And what produces the 'thrust' that moves your sailboats?
Gravity?

This may be a simple confusion over the word 'thrust',
which I used (as
did the original question) in the sense of 'force in
the direction of
motion'. Both the glider and the sailboat are propelled
in a forward
direction by lift from the wing or sail.

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Eric Greenwell
Washington State
USA

Eric,

The 'thrust' for a sailboat comes from the wind.
No wind, no go. The 'thrust' for a glider is from
gravity. No gravity, no go. A good clue to this is
that sail boats have their wings built vertically and
gliders have theirs horizontal.

The observation that the airofoil (wing, sail or winglet)
of a non powered vehicle can have a forward pointing
component to its lift vector cannot seriously be argued
to represent thrust by anyone with any notion as to
cause and effect. Your definition of thrust (as applied
to a glider) as a 'force in the direction of motion'
is fundamentally flawed as the 'force' cannot exist
without an external source of power i.e gravity dragging
the glider downwards. The thrust for a glider is always
directed vertically downwards. Conveniently for us
there is a viscous medium between the glider and the
ground and clever design of the lift and drag aspects
of the glider shape allows a resolution of the lift
and drag vectors such that the glider slides forwards
as it falls. Winglets have a favourable reducing effect
on the overall drag at certain speeds by reducing the
induced drag.

In the case of a glider the thrust or force that powers
it only acts in the direction of flight when the glider
is pointing vertically downwards.

John Galloway