As best I can tell, it seems for a given configuration air speed for
best glide angle (I will define best as most shallow angle) changes
with weight, but the angle itself doesn't change much. Hi performance
gliders carry water to increase penetration into headwinds, dump it
to increase time aloft, but still go down about foot for every 20 some
they move thru the air, don't they?

I need, for a fictional piece I'm writing, an efficient long range low
power consumption airplane and don't want it to fail because of
technical errors.

Thanks for your comments