Sink rate (knots) Airspeed (knots) 1 2 3 4 5 6 7 8 9 10 11 12 0 10 20 30 40 50 60 70 80 90 100 110 120 130 Best glide speed Headwind change { Sink rate (knots) Lift rate (knots) Airspeed (knots) 5 4 3 2 1 0 –1 –2 –3 –4 –5 –6 –7 10 20 30 40 50 60 70 80 90 100 110 120 130 Best speed to fly is 60 knots Total sink { Figure 5-13.  Best speed to fly in a 20-knot headwind. Figure 5-14.  Best speed to fly in sink. Weight and Balance Information Limitations and occurs at a higher speed.  [Figure 5-15]  With ballast, it  would be more difficult to work small, weak thermals. The  best glide ratio is the same, but it occurs at a higher speed. In  addition, the sink rate at higher speeds is lower with ballast.  From the polar, then, ballast should be used under stronger  thermal conditions for better speed between thermals. Note  that the stall speed is higher with ballast as well. Flaps with a negative setting as opposed to a 0 degree setting  during cruise also reduce the sink rate at higher speeds, as  shown in the polar.  [Figure 5-16]  Therefore, when cruising at  or above 70 knots, a –8° flap setting would be advantageous  for this glider. The polar with flaps set at –8° does not extend  to speeds lower than 70 knots since the negative flap setting  loses its advantage there. Wingtip extensions also alter the polar, as shown in  Figure 5-17 . The illustration shows that the additional  3 meters of wingspan is advantageous at all speeds. In  some gliders, the low-speed performance is better with the  tip extensions, while high-speed performance is slightly  diminished by comparison. The GFM/POH provides information about the weight and  balance of the glider. This information is correct when the  glider is new as delivered from the factory. Subsequent  maintenance and modifications can alter weight and balance  considerably. Changes to the glider that affect weight and  balance should be noted in the airframe logbook and on  appropriate cockpit placards that might list, for example,  “Maximum Fuselage Weight: 460 pounds.” Weight is a major factor in glider construction and operation;  it demands respect from all pilots. The pilot should always be  aware of proper weight management and the consequences  of overloading the glider. Whether the glider is very simple or very complex, designers  and manufacturers provide operating limitations to ensure  the safety of flight. The VG diagram provides the pilot with  information on the design limitations of the glider, such as  limiting airspeeds and load factors (L.F. in  Figure 5-18 ).  5-10