I will assume that you know that the "best glide for range" occurs at an angle of attack for best L/D ratio. It is the L/D ratio, and nothing else, that determined the glide angle and hence the glide range in still air.

For most training aircraft the L/D ratio will be a max at an AoA of around 4 degrees; it will depend on the wing profile but the best L/D will occur at a fixed AoA.

Now, consider two identical aircraft at different weights on a glide at an AoA of 4 degrees. Both aircraft will follow the same glide path as both aircraft will have the same L/D ratio.

The heaver aircraft will need to generate more lift than the lighter aircraft. If the AoA is fixed at 4 degrees, the only way to generate that extra lift is for the heavier aircraft to fly at a higher speed. We now have two aircraft on the same glide slope flying at two different speed. Which aircraft will reach the ground first?

The answer is, the aircraft with the greatest airspeed - the heavy aircraft. Given that they both start off at the same altitude, the heavy aircraft will have the higher rate of descent.

Just something to get you thinking.

[attachment=1670]glide-distance-small.jpg[/attachment]

what if I glide on the speed higher than the best lift/drag ratio speed?

what will happen to my range? it will increase or decrease? thats where I'm stuck..

or what if I glide on the speed below the best gliding speed?

That's the point I'm trying to understand

[color=blue]what if I glide on the speed higher than the best lift/drag ratio speed?[/color]
Then you are not flying at the L/D[sub]max[/sub] speed and range will be reduced

[color=blue]what if I glide on the speed below the best gliding speed?[/color]
Then you are not flying at the best L/D[sub]max[/sub] speed and range will be reduced

[u]Here is some background theory that might help.[/u]
On a glide descent, the forward component of weight (FCW), is equal the aircraft drag. But the FCW is a function of the glide angle. The steep the glide angle the greater the FCW and the lower the glide range.

So if you want max range on the glide, you need to fly at the speed for min drag - ie the speed at which the L/D ratio is at a max.

If Drag is at a min; the FCW is at a min; the glide angle is at a min; and range is at a max.

You will find this explanation in any text book

[attachment=1672]Forces on Descent.PNG[/attachment]

[attachment=1671]Best Glide.jpg[/attachment]

Thanks alot..now it's clear