Alan Baker wrote:
In a glide in a low wing aircraft:

Total aerodynamic force (lift and drag!)
^
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|
M (Centre of Mass)
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C (Centre of Aerodynamic Pressure)
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Weight (no down arrow head... ...sorry)

It is late and I'm about to go to bed, but after seeing your force diagrams
like the one above I wrote a private e-mail to the original poster with the
advice that while you get many concepts correct you also appear to apply
them incorrectly on occasion, so your advice should be disregarded as
suspect. Nothing personal - but someone's time, money, and life are
involved. Getting things wrong is pretty common - we all do it - even Nobel
prize winners in physics have had their physical intuition fail them
(Feynman wrote of one such case in one of his books.)

Anyway, all the force diagrams you drew appear to have important errors.

And for what it is worth, I have a BSc in physics and was just reviewing a
textbook on basic mechanics a week ago (J. P. Den Hartog's "Mechanics" -
inexpensive and worth buying by the way) but even so I would have to think
the problem through carefully before attempting even a qualified answer.