Glide ratio =  Lift  : 1 Drag  Glide ratio is the number of feet a glider travels horizontally  in still air for every foot of altitude lost. If a glider has a 50:1  glide ratio, then it travels 50 feet for every foot of altitude lost.  This explains why minimizing drag is so critically important.  Because drag varies with airspeed, the glide ratio must also  vary with airspeed. A glide polar shown in  Figure 3–15  is  a graph, normally provided in a glider’s flight manual, that  details the glider’s still air sink rate at airspeeds within its  flight envelope. The glide ratio at a particular airspeed can  be estimated from the glide polar using: Airspeed and sink rate must both be in the same units. The  example in  Figure 3-14  uses knots. The minimum sink speed  is the airspeed at which the glider loses altitude at the lowest  rate. It can be determined from the polar by locating the point  on the graph with the lowest sink rate and reading off the  corresponding airspeed.  [Figure 3-16] The best glide speed is the airspeed at which, in still air, the  glider achieves its best glide ratio. It is also known as the  best lift/drag (L/D) speed. This can be determined from the  polar by drawing a line from the origin that is tangential to  the curve (e.g., just touching).  [Figure 3-17]  The point of  contact is the best glide speed; the glide ratio at this speed  can be calculated as previously described. In still air, the  glider should be flown at this speed to get from A to B with  minimum height loss. Increasing the mass of a glider by adding water ballast,  for example, shifts the glide polar down and to the  right.  [Figure 3-18]  The minimum sink rate is therefore  increased, so as expected, the extra weight makes it harder  to climb in thermals. However, the best glide ratio remains  approximately the same, but now occurs at a higher airspeed.  Therefore, if the thermals are strong enough to compensate  for the poor climb performance, then water ballast allows a  faster inter-thermal cruise. This results in greater distances  being traveled per time interval. Glide Ratio Glide ratio =  Airspeed : 1 Sink rate Figure 3-16.  Minimum sink speed can be found using the glide  polar graph. Figure 3-17.  The glider polar graph helps determine the glider’s  best glide speed. Figure 3-15.  Glide polar graph. Airspeed (knots) Sink rate (knots) 0 25 50 75 0 –1 –2 –3 –4 –5 –6 Glide ratio at 75 knots = :1  75 3.4 = 22:1 Airspeed (knots) Sink rate (knots) Best lift/drag (L/D) Tangent Glider  sink rate 0 25 50 75 0 –1 –2 –3 –4 –5 –6 46 62 = :1  62 1.6 = 38:1 Best lift/drag (L/D) ratio – 1.6 Airspeed (knots) Sink rate (knots) Minimum sink speed 0 25 50 75 0 –1 –2 –3 –4 –5 –6 46 = :1  46 1.3 = 35:1 Minimum sink glide ratio Glider sink rate Minimum flying speed – 1.3 3-8