This is a good question.

CL/CD is best described as what you stated above. This means that as energy is introduced into the system, the greatest amount of energy possible is going to create lift and not to the creation of drag.

Energy is introduced into the airplane system through the powerplant and wind currents / thermals.

So, to fly the greatest distance (Max Range) on a tank of gas, it would be prudent to fly at the speed referenced by CL/CD.

What happens when the engine quits and no more energy is being introduced into the airplane system? The airplane is now converting potential energy (altitude) to kinetic energy (airspeed). To decrease this conversion rate to a minimum, the pilot should also fly at CL/CD. This is known as best glide speed. The speed which will allow the pilot to fly the farthest under any circumstance.

Glider pilots are able to stay in the air a long time by climbing in thermals (adding potential energy) then gliding at best glide speed till the next thermal.

So the speed referenced under CL/CD will allow the pilot the greatest range as well as the greatest glide.

As the airplane slows below CL/CD (L/Dmax) more induced drag is introduced into the system. To maintain altitude at slower and slower speeds; power will have to be added. The region of speeds below CL/CD are known as the "Region of Reverse Command"

Some references for this information are:

• Pilots Handbook of Aeronautical Knowledge page 4-6, 10-8, 10-10
• Airplane Flying Handbook page 3-16