content_image/568522.jpg
This can be manipulated into V$_{S}$  times the square root of the load factor (which equals weight × G loading). 

For example, if a glider stall speed is 34 knots, consider the following formula (34 × √1.2 (load factor) = 34 × 1.10 = 37 at 30° ) for thermalling: 

• In a 30° banked turn, load factor is 1.2 Gs. The approximate square root of 1.2 is 1.1. Now multiply 34 knots times 1.1 yields a 37-knot stall speed. Since the minimum sink speed is 40 which is still above the stall speed but by only approximately 3 knots, the margin of safety is decreasing and the pilot should consider increasing the minimum airspeed by a factor proportionate to the stall speed increase, in this case 44 knots (40 × √1.2 (load factor) = 40 × 1.10 = 44 at 30°). 

• In a 45° banked turn, load factor is 1.18 (34 × √1.4 = 34 × 1.18 = 40.12 knots at 45°). Now multiply 34 knots times 1.18 which yields a 41-knot stall speed. The minimum sink speed of 40 knots is now below the stall speed. The pilot should increase the minimum airspeed proportionately to the stall airspeed, and the new speed would be 48 knots (40 × √1.4 (load factor) = 41 × 1.18 = 48 at 45°), a 7-knot safety factor. 

• In a 60° banked turn, load factor is 2.0 Gs. The approximate square root of 2.0 is 1.4 (34 × √2.0 = 34 × 1.41 = 48 knots at 60°). Now multiply 34 knots times 1.4 which yields a 48-knot stall speed. The minimum sink speed of 40 knots is now below the stall speed. The pilot should increase the minimum airspeed proportionately to 57 knots, yielding an 9-knot safety (40 × √2.0 (load factor) = 40 × 1.41 = 56.4 at 60°). 

Minimum sink airspeed is always lower than best L/D airspeed at any given operating weight. If the operating weight of the glider is noticeably less than maximum gross weight, then the actual minimum sink airspeed at that operating weight is lower than that published by the manufacturer. 

Common errors regarding minimum sink airspeed include: 

• Improper determination of minimum sink speed. 

• Failure to maintain proper pitch attitude and airspeed control. 

## Best Glide Airspeed 

Best glide (L/D) airspeed is defined as the airspeed that results in the least amount of altitude loss over a given distance. This allows the glider to glide the greatest distance in still air. This performance is expressed as glide ratio. The manufacturer publishes the best glide airspeed for specified weights and the resulting glide ratio. For example, a glide ratio of 36:1 

means that the glider loses 1 foot of altitude for every 36 feet of forward movement in still air at this airspeed. The glide ratio decreases at airspeeds above or below best glide airspeed. The best glide speed can be found from the glider polars in Chapter 5, Performance Limitations. 

Common errors regarding best glide airspeed include: 

• Improper determination of best airspeed to fly and not factoring in lift and headwinds. 

• Failure to maintain proper pitch attitude and airspeed control. 

## Speed to Fly 

Much is said about the importance of maintaining the best gliding speed, but what is important is to maintain an optimum glide speed, a penetration speed that takes atmospheric conditions into account (e.g., sinking air or a headwind). The gliding community refers to this as the speed to fly. The normal recommendation for countering a headwind is to add one-third to one-half of the estimated wind speed to V$_{BG}$ , which increases the rate of sink but also increases the ground speed. For a tailwind, deduct one-third to one-half the estimated wind speed from V$_{BG}$, which reduces both the rate of sink and the groundspeed. Bear in mind that, for safety, it is better to err towards higher rather than lower airspeeds. 

To illustrate this, the polar curve in Figure 7-35  indicates the optimum glide speed when adjusted for headwind, tailwind or sinking air. For a tailwind (A), the starting point on the horizontal scale has been moved a distance to the left corresponding to the tailwind velocity. Consequently, the black tangential line contacts the curve at an optimal glide speed that is lower than the best L/D no-wind glide (B), with a slightly lower rate of sink. This is the opposite for a headwind (C), shown by the purple line, and sinking air (D), shown 

<img src='content_image/568522.jpg'>

