<font face="Verdana, Arial, Helvetica" size="2">I maintain that the statement that weight does not affect the glide angle is an oversimplification. This is true if one considers only induced drag and the vector diagrams. However, this is only part of the story and does not take the increase in profile drag (form, skin friction, interference drag) into account at all.</font>
It
is, as I pointed out previously, a slight oversimplification, but it certainly does take into account profile drag (the term I'm more familiar with for form, skin friction, interference is 'parasite drag' but that may be a US/UK difference).
The important characteristic of drag is that, for a particular angle of attack, the drag is proportional to the speed squared: i.e.
D = 1/2 rho v^2 Cd(a)
where Cd is the drag coefficient which may be a function of angle of attack, a.
This is true of the induced drag, where Cd is approximately proportional to a squared, but equally true of parasite drag, where Cd is approximately independent of a.
We've also got lift:
L = 1/2 rho v^2 Cl(a)
where Cl is approximately proportional to a.
So L/D = Cl(a)/Cd(a) which depends entirely on the angle of attack a.
Whatever angle of attack maximises Cl(a)/Cd(a) is the best glide speed. The speed that corresponds to this depends on the weight: the higher the weight the more lift is required and therefore the higher the v required for a given a.
Only if the D = 1/2 rho v^2 Cd(a) relationship breaks down does the angle of attack for best glide vary with speed. In the case I alluded to in a previous reply, Cd itself turns out to be weakly dependent on v, so the v terms don't cancel when you divide L/D.
But that may not cut much ice with the students unless they have degrees in physics.
