Confusion about lifting force and the work exerted by the engine?

Question

Answertariat:

Why can glider's glide with minimal altitude loss?

Here is my flawed reasoning for why they cannot!
Lets say we have a glider that weights 100kg flying through the air at a speed of 10m/s. Traveling at this speed, it generates a lift of ~1 kN (F = mg so it stays at constant altitude). The kinetic energy of the glider is 5kJ. Without energy entering the system, the kinetic energy should be converted into lifting force. By this reasoning, the glider should reach zero velocity in just a few meters.

Basically, it seems like the force required to keep a plane in the air is less than the force driving it at a constant velocity.

Answer 1

As the previous answer says, the glider is falling (in the absence of an updraft). You erred in thinking that the fact that F(lift) = mg means level flight; it only means no vertical acceleration. So in equilibrium conditions the glider descends at a constant rate, speed and kinetic energy are constant, and the drag energy is supplied by decreasing potential energy. Incidentally, the glide ratio is also the lift-to-drag ratio L/D and equals the tangent of the glide slope. Some sailplanes manage an L/D ratio of 50 or more, and some WWII gliders reached 20.
Yours, kirchwey
(3rd Assistant to the 2nd Adjutant Auxiliary Underanswertary)

Answer 2

You're looking from the wrong frame of reference.

Consider the glider from the point of view of a bird off to the side. As the glider goes forward it trades altitude for speed (which gives it lift) so the plane is falling as it goes forward. This 'fall' is the source of the energy to overcome the drag losses and generate the speed needed to create lift.

The ratio of distance traveled to the altitude lost is the "glide ratio" -- the better the glider the higher the ratio.

Another way gliders generate lift is to ride thermals -- pockets of rising warm air. If the air is rising faster than the plane "falls" it becomes an elevator for the glider.