In recent years systematic flyover noise measurements of propeller driven general aviation airplanes were performed. This paper presents a brief description of the measurement and data reduction techniques together with some results. Computation methods are given to estimate the effect of powersetting on the noise level and to approximate the shape of the noise field around these aircraft. They are used to compute Leq-contours around general aviation airports. Contour plots are used to compare the relative effects of several measures for noise reduction on the ground. Noise reduction at the source is the most effective way to achieve this goal.