Glider pilots are quite familiar with the effects of weight on glider performance, i.e., increasing weight increases stall speed, minimum sink speed, best L/D speed, and all Mac-Cready speeds ... and, unfortunately degrades climb performance. The 64-thousand-dollar question is "When is the lift strong enough to justify adding ballast to improve net average speed?" QUESTION 1: Your girlfriend is an outstanding glider pilot and a member of your soaring club. She is planning a 50 miles cross-country, on a glorious Saturday afternoon, in the club's PW-6 glider. The weather is perfect. Blue sky, scattered clouds, and very light winds aloft. Her plan is to fly out 25 nm to a nearby uncontrolled airport and back. Her goal is to break her record time for this flight. She plans on taking a 3,000 ft tow above your home airport (elevation 1,500 ft MSL, pattern altitude 2,500 ft MSL) before starting her clock. The ther-mals have been reported as standard summer thermals, i.e., standard British thermals with 4.2 kt of airmass lift at the core decreasing parabolically to zero at a 1,000 ft radius. She asks you the following question: "Assuming I fly perfectly, that is, perfectly centered in all thermals at the optimum airspeed and bank angle to maximize net climb rates, and I cruise between thermals at the optimum MacCready speed between thermals, should I fly solo at minimum weight, or take you along in the backseat plus a little ballast in the front seat to fly at gross weight? Which way will give me a higher average speed?" The PW-6 polars for minimum weight (900 lb) and gross weight (1,220 lb) are shown in Figure 1.
展开▼
