The particular aircraft is a glider fitted with a retractable engine. Shortly after takeoff for a soaring flight, the pilot noticed that the airspeed indicator (ASI) appeared to be operating erratically. The flight was continued with the pilot estimating the airspeed, but no thermals were found and the pilot decided to return to the departure point using engine power. The aircraft stalled during the landing flare and the left wing struck the ground. The aircraft slewed to the left and touched down while travelling sideways. The fuselage was fractured during the ensuing ground slide. The approach was flown with landing flap and a substantial degree of air-brake deployed. With the engine also extended the glider was in an extremely high drag situation. Although the flight manual is specific about the degradation in performance of the glider with the engine extended, the pilot reported that his endorsement training failed to adequately address this aspect. Investigation also revealed that some dirt and insects were present in the pitot/static plumbing connected to the ASI. These may have caused the reported erratic readings during the initial stages of the flight.