G'day,
There's a couple of good ways I would explain it. One way is to fly a heavy acft then a light acft and note the difference, but that's not really practical
. Fortunaely, the theory is pretty straight forward too......
(1) Explain how angle, rate of descent and TAS/GS effect each other. (e.g. rate increases with an increase in TAS for a given angle etc. etc.)
(2) Draw a diagram of the forces acting on the aircraft during the glide; include resolving the Weight Vector into a vertical and horizontal component.
(3) Prove / Explain to the student that the angle between the Weight Vector and the Vertical Component of Weight = the Angle of Descent. (if you get my meaning, it's kind of hard to show it without a diagram).
(4) It should now be straight forward enough to show to the student that as the total weight increases, the gliding angle remains unchanged. You could do it vectorally (i.e. with pictures on the board
) or by simple explaining it to her/him.(The patter would go something like this: "Well, if we increase the weight of the aircraft, we also need increased lift to support the aircraft. This increased lift is created by flying slightly faster, but still at the same AoA [and thus the same L/D ratio]. The extra "Apparent Thrust" needed to fly faster is provided by the extra weight we flying at, so note that our gliding angle has remained unchanged provided we glide a little faster to acheive our best L/D AoA" )
If you have a Flight Manual for a higher performance aircraft available, show the student how the Best Glide Speed reduces as the aircraft's weight reduces. This kinda backs up the theory I just outlined above.
As for explaining that L/D ratio does not change with an increase in weight, the best way I can think of to do this would be to develop a graph of L/D at various AoA's and show to the student what actually happens!
HTH and Hope it is what you were after,
Cheers,
Grade 3