to overcome that unnecessary drag. The energy that produces  the vortices is wasted energy. The object of glider design is  to convert all of the energy into useful lift and the necessary  thrust. Any wasted energy translates into poorer performance.  [Figure 3-12]  Glider designers attempt to reduce drag by  increasing the aspect ratio of the glider. The greater the aspect  ratio of the wing is, the lower the induced drag is. Wingtip  devices, or winglets, are also used to improve the efficiency  of the glider. There are several types of wingtip devices and,  though they function in different manners, the intended effect  is always to reduce the aircraft’s drag by altering the airflow  near the wingtips. Such devices increase the effective aspect  ratio of a wing, without materially increasing the wingspan. The shape, or planform, of the wings also has an effect on  the amount of lift and drag produced. The four most common  wing planforms used on gliders are elliptical, rectangular,  tapered, and swept forward.  [Figure 3-14] Total drag on a glider is the sum of parasite and induced drag.  The total drag curve represents these combined forces and is  plotted against airspeed.  [Figure 3-13] L/D$_{MAX}$ is the point at which the lift-to-drag ratio is greatest. At  this speed, the total lift capacity of the glider, when compared  to the total drag of the glider, is most favorable. In calm air,  this is the airspeed used to obtain maximum glide distance. An elliptical wing is a wing planform shape that minimizes  induced drag. Elliptical taper shortens the chord near the  wingtips in such a way that all parts of the wing experience  equivalent downwash, and lift at the wing tips is essentially  Figure 3-13.  Total drag from the sum of parasite and induced drag. Wing Planform Elliptical Wing Total Drag Drag  Speed  I n d u c e d d r a g P a r s i t e d r a g T o t a l d r a g (L/ D$_{MAX}$ ) Minimu m Drag Vertical lift Total lift Average relative wind 2 Wingtip vortices develop. 4 The average relative wind is inclined downward  and rearward, and lift is inclined aft. The rearward  component of lift is induced drag. 3 The downwash increases behind the wing. 1 High pressure air joins low pressure air at the trailing edge of the wing and wingtips. Low pr essur e Low pr essur e H$_{igh pr}$ $^{essur}$e $_{H}$igh pr essur e Atmospheric pressure Atmospheric pressure Figure 3-12.  Induced drag is that part of total drag created by the production of lift, occurring only when lift is being developed. 3-6