John Galloway wrote:

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.

We are becoming almost philosophical here, but I'll give it a go. I'm
not seriously arguing it is "thrust" as the term is typically used by
aeronautical engineers. Bob Salvo asked "Do winglets produce thrust?",
and most of us knew exactly what he was asking: "Do winglets produce a
force in the forward direction?".

I continued to use in that manner in what I hoped was a light-hearted
way to get people to think about what does move a glider through the air.

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.

I agree the force of gravity is needed. I don't agree it is "the" source
of power. See below.

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.

What if the glider isn't falling, but moving at a constant altitude
while ridge or wave soaring? What powers it then? I think it's the wind,
like a sailboat.

We also get our power (it might be better to say "energy") from the
towplane, that first pulls us up, then from the thermals that let us
climb on our own.

Winglets have a favourable reducing effect
on the overall drag at certain speeds by reducing the
induced drag.

Agreed.

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Eric Greenwell
Washington State
USA