If I had my books and my scanner, I would post here the L/D curve, and a graph of an airplane gliding angle.
If an airplane has a Lift of 10 tons, and a drag of 1 ton, it has a L/D of 10 and it glides at an gradient of 10%. That is a fact. It takes some graphics and formulas, though.
The same airplane weighing 20 tons and with a drag of 2 tons, has the same L/D ratio, and therefore glides exactly the same, at a 10% gradient.
What's more. A 1 ton cessna with a 100 kg drag will have the same L/D ratio of 10 and glide at 10% gradient.
L/D ratio determines glide angle.
The L/D ratio is equivalent to the CL/CD ratio. CL and CD only depend on AoA, therefore L/D only depends on AoA, for a given configuration. Not on weight, not on speed. Only on AoA.
For 1g flight at a given weight, there is a relationship between speed and AoA, so that to each speed corresponds an AoA. Therefore, to each speed during a 1g glide at a given weight corresponds a L/D ratio.
Normally airplanes fly at AoAs smaller than max L/D AoA. When flying at AoAs higher than that, we are said to be in the "reverse command region", or the "back side of the curve". So an increase in speed is always decreasing the AoA and the L/D ratio, and therefore increases the glide angle (reduces the gliding distance).
The AoA-Speed relationship at 1g flight varies with weight. At increased weights, increased AoA are necessary to increase Lift in the same amount for a constant speed.
If you glide at the same speed but higher weight, your AoA is higher, and your L/D ratio increases, and so your glide angle reduces (gliding distance increases).
Now think of two big football balls (football balls are spheric, ok? I don't know nor I want to know what "soccer" is...) One filled with lead, the other with air. Which one will travel faster on a given slope? And Which one will need less slope to maintain a given speed? Well, the heavier needs a shallower slope because its weight can overcome more drag than the heavy one, right?.
Now, from the energy point of view you might have trouble understanding, because here there is a lot of energy dissipation. Actually, descent is all about energy dissipation (you glide at quite a constant speed and yet lose altitude, right?). You can't use energy conservation here...
finally, two airplanes of same model flying at maximum glide speed (min drag speed or max L/D speed) will glide the same, irrespective of the weight. Only the heavier will fly faster.
Last edited by Microburst2002; 23rd Nov 2012 at 06:39.