## Minimum Sink Airspeed

Minimum sink airspeed is defined as the airspeed at which the glider loses the least altitude in a given period of time. Minimum sink airspeed varies with the weight of the glider. Glider manufacturers publish altitude loss in feet per minute or meters per second (e.g., 122 ft/min or 0.62 m/sec) at a specified weight. Flying at minimum sink airspeed results in maximum duration in the absence of convection in the atmosphere.

The minimum sink airspeed given in the GFM/POH is based on the following conditions.

- The glider is wings level and flying a straight flightpath; load factor is 1.0 G.
- The glider flight controls are perfectly coordinated.
- Wing flaps are set to zero degrees and air brakes are closed and locked.
- The wings are free of bugs or other contaminants.
- The glider is at a manufacturer-specified weight.

While flying in a thermaling turn, the proper airspeed is the minimum sink airspeed appropriate to the load factor, or G-load, that the glider is undergoing. The glider’s stall speed increases with load factor. The minimum sink speed needs to be increased with an increase in load factor. Another factor that always needs to be considered is that the gross weight and stall speed of the glider can vary if equipped with water ballast, which affects the minimum sink airspeed. For most gliders, the following weights will be of greatest interest:

- Maximum gross weight with full water and both seats occupied
- Maximum gross weight with both seats occupied and no water ballast
- One pilot on board

**Flight Literacy Recommends**

**Rod Machado's How to Fly an Airplane Handbook**– Learn the basic fundamentals of flying any airplane. Make flight training easier, less expensive, and more enjoyable. Master all the checkride maneuvers. Learn the "stick and rudder" philosophy of flying. Prevent an airplane from accidentally stalling or spinning. Land a plane quickly and enjoyably.

The effect of only weight on stall speed can be expressed by using the basic lift formula that is manipulated and simplified to derive the necessary information. The modified lift formula is:

V_{S}1 = stall speed corresponding to some gross weight, W1

V_{S}2 = stall speed corresponding to a different gross weight, W2

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 by the yellow line. For sinking air, the starting point on the vertical scale has been moved up a distance corresponding to the vertical velocity of the air. Consequently, the red tangential line contacts the curve at a glide speed higher than best glide speed.

Speed to fly depends on:

- The rate of climb the pilot expects to achieve in the next thermal or updraft.
- The rate of ascent or descent of the air mass through which the glider is flying.
- The glider’s inherent sink rate at all airspeeds between minimum-sink airspeed and never-exceed airspeed.
- Headwind or tailwind conditions.

The object of speed to fly is to minimize the time and/or altitude required to fly from the current position to the next thermal and to minimize time in sink and maximize time in lift. Speed-to-fly information is presented to the pilot in one or more of the following ways:

- By placing a speed-to-fly ring (MacCready ring) around the variometer dial.
- Using the appropriate table or chart.
- Using an electronic flight computer that displays the current optimum speed to fly.

The pilot determines the speed to fly during initial planning and then constantly updates this information in flight. The pilot must be aware of changes in the flying conditions in order to be successful in conducting cross-country flights or during a soaring competition.

Common errors regarding speed to fly are:

- Improper determination of speed to fly.
- Failure to maintain proper pitch attitude and airspeed control.
- Not transitioning from cruise speed to climb speed in lift as needed and not changing to cruise speed when leaving lift in a prudent manner.