Glider pilots generally want a glider that can make use of one knot thermals that may be only about 400 feet in diameter and would like to be able to glide as far as they can see with a minimum loss of height. The contest pilot wants the glider to have good performance at ever increasing speeds and good handling characteristics. The instructor may want an aircraft that sacrifices higher speed performance and glide ratio and allows for lower stalling speeds, more resistance to spinning, and better thermalling characteristics. Once the criteria have been set, the resultant glider will be generally characterized by a graph showing the sink rate at various speeds, better known as the polar. The key points to glean from this are:
- The level flight stall speed. A trainer should be a lower energy aircraft and exhibit a lower wing loading so that they can be landed in a low energy configuration. This usually translates into a stall speed of 40 kt or lower (35 kt would be ideal).
- The minimum sink speed which is generally used for thermalling. Modern trainers should be able to thermal at around 45 kt to give the novice the best chance of developing their soaring skills. Practical design rules indicate that the minimum sink rate is approximately 76% of the best L/D speed.
- The best Lift/Drag or glide ratio. Something around 28:1 would be ideal. This allows for lots of margin with low time pilots.
- The best glide ratio speed (minimum drag speed). Something over 50 kt is usually an indication of better performance, and allows for some safety margin when headwinds are encountered.
- The 20:1 glide ratio speed. This is a measure how much the aircraft can be pushed in dealing with headwinds or sink. This would represent the minimum speed to be flown under such circumstances.
- The 10:1 glide ratio speed. This is useful generally only for covering territory under strong conditions. Not usually applicable to low time pilots.