Why throwing a glider harder does not make it go further

Why launch speed above a certain point is wasted, and what sets that point.

A glider that is not being powered settles into whatever speed and angle its trim, meaning the position of its centre of gravity and the setting of its wing and tail, dictates it should fly at, so launching it faster than that speed only buys a brief climb before it slows back down to the same trimmed glide it would have flown anyway, and the extra speed spent getting there is largely wasted.

The physics of coasting speed

Once a glider leaves the hand, gravity and drag are the only forces still acting on it besides lift, and there is no engine left to sustain any speed above what the trim naturally settles into. Launching it well above that trimmed speed does not extend the glide, it converts the excess speed into height instead, since a wing meeting the air faster than its trimmed condition produces more lift than is needed to simply hold the aircraft up, and that surplus lift pushes the nose up and the model climbs. As it climbs it slows down, trading speed for height exactly as a ball thrown straight up trades speed for height, and once its speed has fallen back to the trimmed value the climb stops and the glider settles into descending at the same angle and the same speed it would have flown at from a gentle launch. The zoom and settle costs energy along the way, since drag is working on the aircraft throughout the climb, and drag grows quickly with speed, so the fastest part of the flight is also the part losing the most energy to the air for the least useful return. None of this is unique to gliders released by hand, either, since a full-size sailplane released from a tow at too high a speed goes through exactly the same zoom-and-settle sequence before it ever reaches its own best gliding speed, the physics caring nothing about the size of the aircraft involved, only about how far its actual speed sits from the speed its trim is built around.

The skateboard comparison

Give a skateboard a hard shove at the top of a gentle slope and it accelerates briefly, but within a short distance it settles into coasting at very nearly the same speed a board simply released from a standstill at the same spot would reach by that point. The reason is that rolling resistance and air resistance both grow quickly as speed increases, so the extra speed from the shove is fighting a rapidly rising penalty and bleeds off fast, while the slope itself, not the shove, is what determines the steady coasting speed the board eventually settles into either way. A glider's trimmed speed plays the same role the slope's angle plays for the skateboard: it sets the equilibrium the aircraft is always heading toward, regardless of how energetically it started.

The one number worth remembering

Drag rises with the square of speed, so doubling the excess speed above trim at launch does not merely double the energy that gets wasted fighting it, it roughly quadruples the drag force resisting that excess speed for as long as it lasts. A launch only modestly faster than trimmed speed costs proportionally far less than a launch thrown as hard as an arm can manage, which is why a gentle, well-aimed launch at close to the trimmed speed usually produces a longer, flatter flight than an aggressive one.

What follows from this

Because the trimmed glide is where the aircraft ends up regardless of launch speed, the part of the flight actually worth optimising is the launch height and the launch angle, not the launch speed. A glider released cleanly at close to its trimmed speed, aimed directly along the path it will eventually settle into, wastes almost nothing on a climb-and-recover cycle and spends its whole flight descending efficiently, while the same model thrown as hard as possible spends its first moments burning speed against drag for a brief, showy zoom that adds little to the distance covered and can just as easily overload the wing and stall it if the excess is too great. The one situation where extra launch speed genuinely helps is when it is converted deliberately into height that the glider would not otherwise have reached, a controlled zoom timed and aimed on purpose rather than an accidental byproduct of an over-hard throw, since height gained this way still has to be paid for out of the same launch energy either way.