HOW MUCH PERFORMANCE IS DESIRED FOR A SPORT MOTOR GLIDER?
The parameters here are glide ratio and sink rate. Because of its inherently higher drag, the traditional fixed engine motor glider is almost certain to suffer a performance loss over a pure sailplane of comparable size, or retractable engine sailplane. Back in the 1950s, the international soaring community chose a 15-meter (49.2 ft.) span as a practical size for competition and sport sailplanes. Thus, the majority of single seat sailplanes today are 15-meter ships, and they have become the standard of comparison. Recently, we have seen the establishment of an 18meter (59 ft) class, largely inhabited by self-launch sailplanes. The added span helps carry the weight of the retractable engine and still offer very good minimum sink rates. Thus, lets think in terms of a minimum 15-meter span as a basis for motor glider design concept. The best 15-meter fiberglass sailplanes achieve L/D ratios of 45 or slightly higher. The best of the 15 meter aluminum sailplanes can attain around 40:1 L/D.
