Glider Flight Handbook Part 2 PDF

Airspeed (knots) 0 10 20 30 40 50 10 10 20 30 40 50 Airspeed (knots) 60 70 60 70 80 90 100 110 120 130 80 90 100 110 120 130 (knots) SinkSink raterate (knots) 21 32 Dua l (h e avi Dua l (h e e r ) avie Sro) lo So (ligh t lo (lig er) hte r) 43 54 65 76 87 98 109 11 10 12...

Airspeed (knots) 0 10 20 30 40 50 10 10 20 30 40 50 Airspeed (knots) 60 70 60 70 80 90 100 110 120 130 80 90 100 110 120 130 (knots) SinkSink raterate (knots) 21 32 Dua l (h e avi Dua l (h e e r ) avie Sro) lo So (ligh t lo (lig er) hte r) 43 54 65 76 87 98 109 11 10 12 11 12 Figure 5-11. Dual and solo polar performance curves for a two-seat glider. Airspeed (knots) 0 10 20 30 40 50 10 10 20 30 40 50 (knots) SinkSink raterate (knots) 21 32 43 54 65 76 Minimum sink airspeed 40 knots sink airspeed Minimum 40 knots Airspeed (knots) 60 70 60 70 80 90 100 110 120 130 80 90 100 110 120 130 L/D maximum 50 L/Dknots maximum 50 knots 87 98 109 11 10 12 11 12 {{ Headwind Figure 5-12. Minimumchange sink airspeed and maximum L/D speed. Airspeed (knots) Headwind change (knots) 0 is determined 10 20 30 the 40 60 70speed 80 L/D speed by dividing best L/D50speedAirspeed by The to 0 10 speed,20 30 which 40is approximately 50 60 the sink 1rate at that or 50/2.1, 24. Thus, this glider has a best glide ratio in calm air (no lift 21 32 no headwind or tailwind) of 24:1 at 50 knots. or sink and (knots) SinkSink raterate (knots) 43 best54 speed Best glide speed Best glide speed The to fly for distance in a headwind is easily 65 determined from the polar. To do this, shift the origin to the 76 right along the horizontal axis by the speed of the headwind 87 and draw98 a new tangent line to the polar. 109 11 From the 10 new tangent point, read the best speed to fly. An 12 11for a 20-knot headwind is shown in Figure 5-13. example 12 to fly in a 20-knot headwind is found to be 60 The speed 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. 100 lies 110 130 sink fly in90a tailwind between120 minimum 80 but is90 100 110minimum 120 sink 130 and70best L/D, never lower than 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 5-9 11 12 { Headwind change 0 10 20 Airspeed (knots) 30 40 50 60 70 80 90 100 110 120 130 80 90 100 110 120 130 1 2 Sink rate (knots) 3 Best glide speed 4 5 6 7 8 9 10 11 12 Figure 5-13. Best speed to fly in a 20-knot headwind. 3 2 1 0 –1 –2 –3 –4 { Sink rate (knots) 4 Total sink Lift rate (knots) Airspeed (knots) 5 10 20 30 40 50 60 70 Best speed to fly is 60 knots –5 –6 –7 Figure 5-14. Best speed to fly in sink. 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. 5-10 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. Limitations 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). Airspeed (knots) 0 10 20 30 40 50 60 70 80 90 100 110 120 130 90 100 110 120 130 100 110 120 130 1 70 knots 2 Sink rate (knots) 3 4 50 knots 5 6 7 Notice higher airspeed with ballast for same sink rate 8 9 = performance without water ballast = performance with 300 lb water ballast 10 11 12 Figure 5-15. Effect of water ballast on performance polar. Airspeed (knots) 0 10 20 30 40 50 60 70 80 85 knots 1 2 56 knots Sink rate (knots) 3 4 5 With flaps set to –8 degrees, sink rate does not change significantly but airspeed increases. 6 7 8 9 = performance, flaps set to 0° = performance, flaps set to −8° 10 11 12 Figure 5-16. Performance polar with flaps at 0° and –8°. Airspeed (knots) 0 10 20 30 40 50 43 knots 60 70 80 90 1 2 Sink rate (knots) 3 40 knots 4 5 6 7 8 9 10 11 = 15-meter wingspan performance = 18-meter wingspan performance 12 Figure 5-17. Performance polar with 15-meter and 18-meter wingspan configurations. 5-11 Problems Associated With CG Forward of Forward Limit 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. If the CG is within limits, pitch attitude control stays within acceptable limits. However, if the glider is loaded so the CG is forward of the forward limit, handling is compromised. The glider is said to be nose heavy. Nose heaviness makes it difficult to raise the nose on takeoff and considerable back pressure on the control stick is required to control the pitch attitude. Tail stalls occur at airspeeds higher than normal and are followed by a rapid nose-down pitch tendency. Restoring a normal flight attitude during stall recoveries takes longer. The landing flare is more difficult than normal, or perhaps even impossible, due to nose heaviness. Inability to flare could result in a hard nose-first landing. Weight and Balance Center of Gravity Longitudinal balance affects the stability of the longitudinal axis of the glider. To achieve satisfactory pitch attitude handling in a glider, the CG of the properly loaded glider is forward of the center of pressure (CP). When a glider is produced, the manufacturer provides glider CG limitations, which require compliance. These limitations are generally found in the GFM/ POH and may also be found in the glider airframe logbook. Addition or removal of equipment, such as radios, batteries, or flight instruments, or airframe repairs can have an effect on the CG position. Aviation maintenance technicians (AMTs) must record any changes in the weight and balance data in the GFM/POH or glider airframe logbook. Weight and balance placards in the cockpit must also be updated. The following are the most common reasons for CG forward of forward limit: • Pilot weight exceeds the maximum permitted pilot weight. • Seat or nose ballast weights are installed but are not required due to the weight of the pilot. SGS 1-35 Basic Flight Envelope High Performance Class +6.0 Cond I Pos. Man. L.F. = 5.33 III I +5.0 SIT PO +3.0 PS 4F 2 IVE TOR AC F UST G +2.0 +1.0 0.0 −1.0 NEG ATI −2.0 Cond II Neg. Man. L.F. = 2.67 −3.0 VE 24 F PS II GU ST TOR IV 0 20 40 60 80 100 120 Load Factor Negative Positive Feet per second FAC −4.0 140 V = Velocity (mph) Figure 5-18. Typical example of basic flight envelope for a high-performance glider. 5-12 L.F. – Neg. – Pos. – FPS – VG = 154 MPH N Limit wing load factor (“G” units) +4.0 Cond III Pos. Gust L.F. = 3.55 160 Cond IV Neg. Gust L.F. = 3.55 180 200 220 Problems Associated With CG Aft of Aft Limit The fundamental problem with a CG aft of the aft limit is the designed function of the horizontal stabilizer and elevator. Fixed wing aircraft are generally designed so that the horizontal stabilizer and elevator provide a down force to counter the slightly nose forward CG such that the aircraft tend to resume a level pitch attitude after an upset about the lateral axis. As the airspeed changes, the pilot changes the trim or trims the aircraft so the down force exactly balances the forward CG within limits. Should the aircraft be upset and the nose pitches upward, the resultant slower airspeed results in less down force produced by the horizontal stabilizer and elevator. This decreased down force lets the nose lower so the airspeed retains to the pre-upset value. This is called positive stability. Conversely, if the upset places the aircraft in a nose down attitude, the increased airspeed will increase the down force and raise the nose to the pre-upset balanced condition. However, if the control surface is in a stalled condition, this stabilizing action will not begin until the control surface regains un-stalled airflow and begins producing down force again. The following are the most common reasons for flight with CG located behind permissible limits: • Pilot weight is less than the specified minimum pilot seat weight and trim ballast weights necessary for the lightweight pilot are not installed in the glider prior to flight. • Tailwheel dolly is still attached, far aft on the tailboom of the glider. • Foreign matter or debris (water, ice, mud, sand, and nests) has accumulated in the aft fuselage of the glider and was not discovered and removed prior to flight. • 225 Rear seat pilot weight (lb) If the glider is loaded so the CG location is behind the aft limit, handling is compromised. The glider is said to be tail heavy. Tail heaviness can make pitch control of the glider difficult or even impossible. 200 150 100 0 125 In Figure 5-19, the chart indicates that the minimum weight for the front seat pilot is 125 pounds, and that the maximum is 250 pounds. It also indicates that the maximum rear seat pilot weight is 225 pounds. If each pilot weighs 150 pounds, the intersection of pilot weights falls within the envelope; 150 175 200 225 250 Front seat pilot weight (lb) Within weight and balance limits Out of weight and balance limits Figure 5-19. Weight and balance envelope. the glider load is within the envelope and is safe for flight. If each pilot weighs 225 pounds, the rear seat maximum load is exceeded, and the glider load is outside the envelope and unsafe for flight. The CG position can also be determined by calculation using the following formulas: • Weight × Arm = Moment • Total Moment ÷ Total Weight = CG Position (in inches aft of the reference datum) The computational method involves the application of basic math functions. The following is an example of the computational method. Given: Maximum gross weight....................1,100 lb Empty weight.......................................600 lb CG range....................................14.8–18.6 in Front seat occupant..............................180 lb A heavy, non-approved tailwheel or tail skid was installed on the aft tail boom of the glider. Sample Weight and Balance Problems Some glider manufacturers provide weight and balance information in a graphic presentation. A well designed graph provides a convenient way to determine whether the glider is within weight and balance limitations. 50 Rear seat occupant...............................200 lb To determine the loaded weight and CG, follow these steps. 1. List the empty weight of the glider and the weight of the occupants. 2. Enter the moment for each item listed. Remember, weight × arm = moment. To simplify calculations, the moments may be divided by 100. 3. Total the weight and moments. 4. To determine the CG, divide the moments by the weight. 5-13 NOTE: The weight and balance records for a particular glider provide the empty weight and moment, as well as the information on the arm distance. [Figure 5-20] Item Weight (pounds) Arm (inches) Empty weight 600 +20 12,000 Front seat pilot 180 +30 +5,400 Rear seat pilot Moment (inch·pounds) 200 −5 −1,000 980 total weight +16.73 +16,400 total moment Figure 5-20. Weight and balance: front and rear seat pilot weights and moments. In Figure 5-20, the weight of each pilot has been entered into the correct block in the table. For the front seat pilot, multiplying 180 pounds by +30 inches yields a moment of +5,400 inch·pounds. For the rear seat pilot, multiplying 200 pounds by –5 inches yields a moment of –1,000 inch·pounds. The next step is to find the sum of all weights (980 pounds) and record it. Then, find the sum of all moments (+16,400 inch·pounds) and record it. Now, find the arm (the CG position) of the loaded glider. Divide the total moment by the total weight to discover the CG of the loaded aircraft glider in inches from the datum: +16,400 inch·pounds ÷ 980 pounds = +16.73 inches The final step is to determine whether total weight and CG location values are within acceptable limits. The GFM/ POH lists the maximum gross weight as 1,100 pounds. The operating weight of 980 pounds is less than the 1,100 pounds maximum gross weight. The GFM/POH lists the approved CG range as between +14.80 inches and +18.60 inches from the datum. The operating CG is +16.73 inches from the datum and is within these limits. The weight and balance are within operating limits. Ballast Ballast is nonstructural weight that is added to a glider. In soaring, ballast weight is used for two purposes. Trim ballast is used to adjust the location of the CG of the glider so handling characteristics remain within acceptable limits. Performance ballast is loaded into the glider to improve high-speed cruise performance. Removable trim ballast weights are usually made of metal and are bolted into a ballast receptacle incorporated in the glider structure. The manufacturer generally provides an attachment point well forward in the glider cabin for trim ballast weights. These weights are designed to compensate for a front seat pilot who weighs less than the minimum permissible front seat pilot weight. The ballast weight mounted well forward in the glider cabin helps place the CG within permissible limits, which allows the maximum shift in CG with the minimum addition of weight. 5-14 Some trim ballast weights are in the form of seat cushions, with sand or lead shot sewn into the unit to provide additional weight. This type of ballast, which is installed under the pilot’s seat cushion, is inferior to bolted-in ballast because seat cushions tend to shift position. Seat cushion ballast should never be used during acrobatic or inverted flight. Sometimes trim ballast is water placed in a tail tank in the vertical fin of the fuselage. The purpose of the fin trim ballast tank is to adjust CG location after water is added to, or drained from, the main wing ballast tanks. Unless the main wing ballast tanks are precisely centered on the CG of the loaded aircraft glider, CG location shifts when water is added to the main ballast tanks. CG location shifts again when water is dumped from the main ballast tanks. Adjusting the amount of water in the fin tank compensates for CG shifts resulting from changes in the amount of water ballast carried in the main wing ballast tanks. Water weighs 8.35 pounds per gallon. Because the tail tank is located far aft, it does not take much water to have a considerable effect on CG location. For this reason, tail tanks do not need to contain a large volume of water. Tail tank maximum water capacity is generally less than two gallons of water. Although some older gliders employed bags of sand or bolted-in lead weights as performance ballast, water is used most commonly to enhance high-speed performance in modern sailplanes. Increasing the operating weight of the glider increases the optimum speed to fly during wings-level cruising flight. The resulting higher groundspeed provides a very desirable advantage in cross-country soaring and in sailplane racing. Water ballast tanks are located in the main wing panels. Clean water is added through fill ports in the top of each wing. In most gliders, the water tanks or bags can be partially or completely filled, depending on the pilot’s choice of operating weight. After water is added, the filler caps are replaced to prevent water from sloshing out of the filler holes. Drain valves are fitted to the bottom of each tank. The valves are controlled from inside the cockpit. The tanks can be fully or partially drained while the glider is on the ground to reduce the weight of the glider prior to launch, if the pilot so desires. The ballast tanks also can be partially or completely drained in flight—a process called dumping ballast. The long streaks of white spray behind a speeding airborne glider are dramatic evidence that the glider pilot is dumping water ballast, most likely to lighten the glider prior to landing. The filler caps are vented to allow air to enter the tanks to replace the volume of water draining from the tanks. It is important to ensure that the vents are working properly to prevent wing damage when water ballast is drained or jettisoned. [Figures 5-21 and 5-22] freezes, dumping ballast is difficult or impossible. If water in the wings is allowed to freeze, serious wing damage is likely to occur. Damage occurs because the volume of water expands during the freezing process. The resulting increased volume can deform ribs and other wing structures or cause glue bonds to delaminate. When weather or flight conditions are very cold, do not use water ballast unless antifreeze has been added to the water. Prior to using an antifreeze solution, consult the GFM to ensure that antifreeze compounds are approved for use in the glider. A glider carrying large amounts of water ballast has noticeably different handling characteristics than the same glider without water ballast. Water ballast: Figure 5-21. Water ballast tank vented filler cap. It is important to check the drain valves for correct operation prior to flight. Water ballast should drain from each wing tank at the same rate. Unequal draining leads to a wing-heavy condition that makes in-flight handling, as well as landings, more difficult. If the wing-heavy condition is extreme, it is possible the pilot will lose control of the glider. Ballast drains should also be checked to ensure that water ballast drains properly into the airstream, rather than leaking into the fuselage and pooling in the bottom of the fuselage. Water that is trapped in the fuselage may flow through or over bulkheads, causing dislocation of the CG of the glider. This CG dislocation can lead to loss of control of the glider. The flight manual provides guidance regarding the length of time it takes for the ballast tanks to drain completely. For modern gliders, it takes about 3 to 5 minutes to drain a full tank. When landing is imminent, dump ballast early enough to give the ballast drains sufficient time to empty the tanks. Use of water ballast when ambient temperatures are low can result in water freezing the drain valve. If the drain valve Drain valve closed • Reduces the rate of acceleration of the glider at the beginning of the launch due to the increased glider weight. • Increases the length of ground roll prior to glider liftoff. • Increases stall speed. • Reduces aileron control during the takeoff roll, increasing the chance of uncontrolled wing drop and resultant ground loop. • Reduces rate of climb during climb-out. • Reduces aileron response during free flight. The addition of large amounts of water increases lateral stability substantially. This makes quick banking maneuvers difficult or impossible to perform. Water ballast is routinely dumped before landing to reduce the weight of the glider. Dumping ballast: • Decreases stall speed. • Decreases the optimum airspeed for the landing approach. • Shortens landing roll. Drain valve open Figure 5-22. Water ballast drain valve handles. 5-15 • Reduces the load that glider structures must support during landing and rollout. The performance advantage of water ballast during strong soaring conditions is considerable. However, there is a down side. The pilot should be aware that water ballast degrades takeoff performance, climb rate, and low-speed handling. Before committing to a launch with water ballast aboard, the pilot should review operating limitations to ensure that safety of flight is not compromised. 5-16