The physics of a gliding airplane are simple.
There is potential energy, proportional to height above the ground.
There is also kinetic energy, proportional to speed squared.

First, understand the speed.
If the plane isn't slightly nose-heavy, it will fly a scalloped up-down cycle.
If it does that, add a little weight to the nose, or distribute the wing area more toward the rear.
Assuming you've done that, you control the speed by turning up the trailing edges. The more they are turned up, increasing the angle of attack, the slower it flies.
(Up to a maximum angle of attack, at which the wings stop working, or "stall".)

Back to energy.
If there were no drag, the plane would never come down.
Since there is drag, the drag tends to slow the plane down, decreasing its kinetic energy.
Countering that is the plane's tendency to maintain constant speed and kinetic energy, so it descends, turning potential energy into kinetic energy, just like a ball rolling down a slope.
So the more drag, the more quickly it descends, the less drag, the more slowly it descends.

A way to minimize drag is to minimize speed, because drag force is proportional to speed squared.

So the speed you trim it for depends on what you want to maximize:

- To maximize gliding range, you trim for a speed which is slow enough to have low drag, but not so slow that you don't cover much ground.

- To maximize time aloft, you trim for an even slower speed with has even lower drag, thus minimizing the sink rate. This speed is roughly half way between the speed for maximum range and the even slower stall speed $V_S$.

Check these links: [V-speeds][1], and [Gliding flight][2].


  [1]: http://en.wikipedia.org/wiki/V-speed
  [2]: http://en.wikipedia.org/wiki/Gliding_flight