The GFM/POH provides the pilot with the necessary performance information to operate the glider safely. A GFM/ POH may include the following information:
- Description of glider primary components
- Glider assembly
- Weight and balance data
- Description of glider systems
- Glider performance
- Operating limitations
Cockpit placards provide the pilot with readily available information that is essential for the safe operation of the glider. All required placards are located in the GFM/POH.
The amount of information that placards must convey to the pilot increases as the complexity of the glider increases. High performance gliders may be equipped with wing flaps, retractable landing gear, a water ballast system, drogue chute for use in the landing approach, and other features that are intended to enhance performance. These gliders may require additional placards. [Figure 5-10]
The GFM/POH is the source provided by the manufacturer for glider performance information. In the GFM/POH, glider performance is presented in terms of specific airspeed, such as stall speed, minimum sinking airspeed, best L/D airspeed, maneuvering speed, rough air speed, and the never exceed speed (VNE). Some performance airspeeds apply only to particular types of gliders. Gliders with wing flaps, for instance, have a maximum permitted flaps extended airspeed (VFE).
Manuals for self-launching gliders include performance information about powered operations. These include rate of climb, engine and propeller limitations, fuel consumption, endurance, and cruise.Manuals for self-launching gliders include performance information about powered operations. These include rate of climb, engine and propeller limitations, fuel consumption, endurance, and cruise.
In addition, the manufacturer provides information about the rate of sink in terms of airspeed, which is summarized in a graph called a polar curve, or simply a polar. [Figures 5-11]
The vertical axis of the polar shows the sink rate in knots (increasing sink downwards), while the horizontal axis shows airspeed in knots. Every type of glider has a characteristic polar derived either from theoretical calculations by the designer or by actual in-flight measurement of the sink rate at different speeds. The polar of each individual glider varies (even from other gliders of the same type) by a few percent depending on relative smoothness of the wing surface, the amount of sealing around control surfaces, and even the number of bugs on the wing’s leading edge. The polar forms the basis for speed to fly and final glide tools that will be discussed in Chapter 11, Cross-Country Soaring.
Minimum sink rate is determined from the polar by extending a horizontal line from the top of the polar to the vertical axis. [Figure 5-12] In this example, a minimum sink of 1.9 knots occurs at 40 knots. Note that the sink rate increases between minimum sink speed and the stall speed (the left end point of the polar). The best glide speed (best L/D) is found by drawing a tangent to the polar from the origin. The best L/D speed is 50 knots with a sink speed of 2.1 knots. The glide ratio at best L/D speed is determined by dividing the best L/D speed by the sink rate at that speed, or 50/2.1, which is approximately 24. Thus, this glider has a best glide ratio in calm air (no lift or sink and no headwind or tailwind) of 24:1 at 50 knots.
The best speed to fly for distance in a headwind is easily determined from the polar. To do this, shift the origin to the right along the horizontal axis by the speed of the headwind and draw a new tangent line to the polar.
From the new tangent point, read the best speed to fly. An example for a 20-knot headwind is shown in Figure 5-13. The speed to fly in a 20-knot headwind is found to be 60 knots. By repeating the procedure for different headwinds, it is apparent that flying a faster airspeed as the headwind increases results in the greatest distance over the ground. If this is done for the polar curves from many gliders, a general rule of thumb is found: add half the headwind component to the best L/D for the maximum distance. For tailwinds, shift the origin to the left of the zero mark on the horizontal axis. The speed to fly in a tailwind lies between minimum sink and best L/D, but is never lower than minimum sink speed.
Sinking air usually exists between thermals, and it is most efficient to fly faster than best L/D in order to spend less time in sinking air. How much faster to fly can be determined by the glider polar, as illustrated in Figure 5-14 for an air mass that is sinking at 3 knots. The polar graph in this figure has its vertical axis extended upwards. Shift the origin vertically by 3 knots and draw a new tangent to the polar. Then, draw a line vertically to read the best speed to fly. For this glider, the best speed to fly is found to be 60 knots. Note, the variometer shows the total sink of 5 knots (3 knots for sink and 2 knots for the aircraft) as illustrated in the figure.
If the glider is equipped with water ballast, wing flaps, or wingtip extensions, the performance characteristics of the glider is depicted in multiple configurations. [Figures 5-15, 5-16, and 5-17] Comparing the polar with and without ballast, it is evident that the minimum sink is higher 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.
Weight and Balance Information
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).
Pilots should become familiar with all the operating limitations of each glider being flown. Figure 5-18 shows four different possible conditions and the basic flight envelope for a high performance glider.