The goal of this undergraduate thesis is to analyse flight performance of range and endurance of an engineless sailplane. The model for which analysis was made is similar to the ASG-32 sailplane of the German company Alexander Schleicher. From the model created in that way, the geometric features necessary for the evaluation of the zero drag coefficient of the sailplane were extracted. The zero drag coefficient was calculated for each segment of glider separately and then the sum of the zero coefficient of drag of each segment and it was assumed to be constant for the entire range of flight speed. The induced drag was determined using the Weissinger lifting line model and finally the total drag was determined and the glider polar is also shown. The maximum coefficient of lift is estimated by using the procedure for aircraft with high aspect ratio of wings. The range of the sailplane was determined according to the flight regime of minimum force required and it was proved how the range changes linearly depending on the available flight altitude. The required flight speed and sink speed for such a regime are graphically represented. The influence of flight altitude increase, the influence of non-standard atmosphere and influence of ballast mass increase are shown. The endurance of the sailplane was determined according to the flight regime of minimum power required, and it was also shown how an endurance changes with respect to the flight altitude and this dependence is slightly parabolic. Furthermore, it was shown how an non-standard atmosphere (hot and cold atmosphere) affects endurance performance and it could be concluded that the colder one is more favorable than the standard one, while the warmer one downgrades endurance and provides the worst results. In addition, it was also observed how the endurance changes for a certain flight altitude depending on how much water ballast mass we put in the sailplane. It was concluded that the additional mass negatively affects the endurance and percentage difference between an empty and a full sailplane could be up to 10 %. Finally, the minimum flight speed and minimum sink speed (vertical speed) for the various flight altitudes are represented and additionally how the non-standard atmosphere affects them. Also, it was briefly commented the dependence of the minimum flight speed and minimum vertical speed (sink speed) depending on mass of water ballast putted in the sailplane.