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## Glide Ratio 

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. 

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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: 

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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. 

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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. 

