34 34 Takeoff and climb-out: lightweight glider Takeoff and climb-out: heavy glider Figure 5-7.  Effect of weight on takeoff distance and climb-out rate  and angle. Wind velocity 18 mph Wind velocity 16 mph Wind velocity 14 mph Wind velocity 12 mph Wind velocity 10 mph Figure 5-6.  Effect of wind velocity gradient on a glider turning into  the wind. Stronger airflow over higher wing causes bank to steepen  when close to the surface where surface friction slows winds. 5-6 the heavier glider has more mass to lift to altitude than does  the lighter glider (whether ground launch, aerotow launch,  or self-launch).  [Figure 5-7] The heavy glider can fly faster than the light glider while  maintaining the same glide ratio as the light glider. The  advantage of the heavier weight becomes apparent during  cruising flight. The heavy glider can fly faster than the light  glider and still retain the same lift-to-drag (L/D) ratio. When circling in thermals to climb, the heavy glider is at a  disadvantage relative to the light glider. The increased weight  of the heavy glider means stall airspeed and minimum sink  airspeed are greater than they would be if the glider were  operating at a light weight. At any given bank angle, the  heavy glider’s higher airspeeds mean the pilot must fly larger  diameter thermalling circles than the pilot of the light glider.  Since the best lift in thermals is often found in a narrow  cylinder near the core of the thermal, larger diameter circles  generally mean the heavy glider is unable to exploit the strong  lift of the thermal core, as well as the slower, lightweight  glider. This results in the heavy glider’s inability to climb as  fast in a thermal as the light glider.  [Figure 5-8] The heavy glider has a higher stall speed and a higher  minimum controllable airspeed than an otherwise identical,  but lighter, glider. The  stall speed of a glider increases with  the square root of the increase in weight. If the weight of  the glider is doubled (multiplied by 2.0), then the stall speed  increases by more than 40 percent (1.41 is the approximate  square root of 2; 1.41 times the old stall speed results in  the new stall speed at the heavier weight). For example, a  540-pound glider has a stalling speed of 40 knots. The pilot  adds 300 pounds of water ballast making the new weight 840  pounds. The new stalling speed is approximately 57 knots  (square root of √ 300 + 40 = 57).