motorgliders as towplanes

Derek Copeland wrote:
In free unaccelerated flight with no thrust, i.e. no aerotow, winch, or
turbo, a glider IS essentially gravity powered. The resultant force of
gravity plus wing lift, angled very slightly forward, opposes drag. Thus a
glider runs down a very slight slope through the air. The less drag there
is, the flatter the glide angle becomes.

Nicely worded answer.

On several occasions I've had (non glider pilot) friends ask me why does
it help when we make our gliders heavier with water ballast. Seems
counter-intuitive.

I'm thinking that a proper explanation is in terms of the gravitational
force in a similar fashion to what you describe. Higher mass = higher
gravitational force (F=MA). Hence the glider is "pulled down the slope"
by a higher force. The glide angle is no better, but we can glide
faster at essentially the same glide angle which is an advantage (normal
caveats about thermal climb ability trade-off). A more complete answer
might also discuss the higher drag at higher speed interplay, but that
could probably be left out as a simplification. Perhaps a further
discussion of the classic experiment where in a vacuum a feather and a
rock will fall to earth at the same rate because the acceleration of
gravity is a constant (I know, but within limits it *is* a constant).
But in the presence of air the "air-drag" on the feather is relatively
high compared to the relatively low gravity "down-pull" due to its low mass.

Comments on this explanation are welcomed/sought. I thought I would
find a well worded description of this in Reichmann but it isn't there
that I can see. TIA

Regards,

-Doug