Glide ratio is the ratio of the distance a glider can travel horizontally to the altitude lost in transit. For instance, if a glider can travel 40 miles horizontally while losing one mile of altitude, the glide ratio is 40:1 (typical for a medium-high performance glider). The best glide ratio or maximum glide ratio is simply the best ratio a glider can achieve. Best glide ratio is achieved at a specific airspeed, which varies depending on the glider type. One flies at best glide speed in order to maximize the distance covered. When gliders are flying in a straight line and want to go as far as possible, they fly at best glide speed (typically 55-65kts).
Minimum sink is the minimum vertical speed the glider is capable of flying at in still air (typically 100-200ft/min). Minimum sink is also achieved at a specific airspeed, depending on the glider type (typically 40-50kts). One flies at minimum sink speed to maximize altitude gain when flying in lift. If you're circling in a thermal that is going up at 500ft/min, and your minimum sink speed is 100ft/min, then the glider will go up at 400ft/min.
To review, the key differences are:
- Glide ratio specifies how "flat" the glide angle is
- Minimum sink specifies the minimum vertical speed the glider can fly at
- One flies at best glide speed to maximize the distance covered vs altitude lost
- One flies at minimum sink speed to maximize altitude gain in lift
- Best glide ratio is achieved at a higher airspeed than minimum sink
Any aircraft can be a glider with its engines out, so this applies to all aircraft.
However, I don't understand how the lowest sink rate you can get isn't the same as the shallowest angle you can glide at.
Remember that minimum sink comes at a lower airspeed than best glide. At minimum sink speed, the glide ratio is worse because even though the aircraft is sinking slower, it is also moving forward slower.