Although appearing as a normal glider when viewed from the outside, DLR's glider DG 300-17 is a special one: Its performance is accurately measured and noted. With this knowledge, it can be used as a calibration standard for measuring the speed polar of gliders. The speed polar shows the descent rate versus horizontal airspeed which is the main criterion for rating the performance of gliders. Simply spoken: the lesser the descent rate at a given airspeed the better. An important benchmark in judging the performance of gliders is the best gliding ratio which can be obtained from the speed polar. The best gliding ratio equals the minimum ratio of vertical speed versus horizontal airspeed and thus defines the angle at which the aircraft glides back to the earth. For the DG 300-17, this angle equals approximately 1:40.
Since all vertical movements of the atmosphere will influence the results when measuring vertical speed versus horizontal speed directly, determining the speed polar requires more laborious methods. Using a relative measuring technique produces remedy: By measuring the relative motion of the glider in relation to another calibrated glider instead of the absolute motion, the large scale atmospheric influence can be eliminated because both aircraft fly in the same air mass. But even this procedure requires calm atmospheric conditions free of turbulence to achieve sufficient accuracy. Another condition for reliable results is a small performance differential between both gliders. In order to find a good compromise suiting the large performance spectrum of all types of gliders, the DG 300-17 has an enlarged wingspan of 17 m instead of 15 m of the standard version. This is where the type designator "-17" comes from.
Whereas in former times the relative motion between both gliders was determined from photos in a quite complex manual procedure, nowadays Differential-GPS-based methods are used. The principle of measurement still is the same.
In order to make the DG 300-17 capable as a calibration aircraft it not only is equipped with the necessary instrumentation system. It is being serviced with special care. Even small scratches in the surface will alter the performance and therefore will make the aircraft useless as a calibration standard. For this reason, the DG 300-17 is employed in calibration flights exclusively in order to avoid any unnecessary damage. In continuously repeated control surveys every change in performance will be detected and considered. The DG 300-17 is being gauged in a continuous process.