Glider Performance PDF

Chapter 5 Glider Performance Introduction Glider performance during launch phase, landing, and free flight phase depends on many factors: design, weather, wind, and other atmospheric phenomena. 5-1 Factors Affecting Performance High and Low Density Altitude Conditions 13 Altimeter setting ("H...

Chapter 5 Glider Performance Introduction Glider performance during launch phase, landing, and free flight phase depends on many factors: design, weather, wind, and other atmospheric phenomena. 5-1 Factors Affecting Performance High and Low Density Altitude Conditions 13 Altimeter setting ("Hg) 13 ,00 0 e al tit ud e 11 ,00 0 Pr es su r 10 ,00 0 9,0 00 10 6,0 00 7 re 7,0 8 00 8,0 00 9 peratu 5,0 00 6 5 4,0 00 Approximate density altitude (thousand feet) 11 (fe et ) 12 ,00 0 12 rd tem 00 4 3,0 3 5-2 1,630 28.3 1,533 28.4 1,436 28.5 1,340 28.6 1,244 28.7 1,148 28.8 1,053 28.9 957 29.0 863 29.1 768 29.2 673 29.3 579 29.4 485 29.5 392 29.6 298 29.7 205 29.8 112 29.9 20 00 0 30.1 −165 30.2 −257 30.3 −348 30.4 −440 30.5 −531 C-18° -12° -7° -1° 4° 10° 16° 21° 27° 32° 38° 30.6 −622 30.7 −712 30.8 −803 1 ve le a Se 1,0 00 l 2 0 Due to changing weather conditions, atmospheric pressure at a given location changes from day to day. The following is the METAR report for Love Field observed on the 23rd at 21:53Z (GMT) which indicates a local pressure of A2953, or altimeter setting of 29.53 "Hg. When barometric pressure drops, air density decreases. The reduced density of the air results in an increase in density altitude and decreased glider performance. This reduces takeoff and climb performance and increases the length of runway needed for landing. 1,727 28.2 −73 00 Atmospheric Pressure 1,824 28.1 30.0 -1, Most performance charts do not require a pilot to compute density altitude. Instead, the computation is built into the performance chart itself. A pilot needs only the correct pressure altitude and the temperature. Some charts, however, may require computing density altitude before entering them. Density altitude may be computed using a density altitude chart or by using a flight computer. 28.0 29.92 2,0 One way to determine density altitude is to use charts designed for that purpose. [Figure 5-1] For example, you plan to depart an airport where the field elevation is 1,600 feet MSL. If the altimeter setting is 29.80, and the temperature is 85 °F, what is the density altitude? First, correct for nonstandard pressure (29.8 "Hg) by referring to the right side of the chart and adding 112 feet to the field elevation. The result is a pressure altitude of 1,712 feet. Then, enter the chart at the bottom, just above the temperature of 85 °F (29.4 °C). Proceed up the chart vertically until intercepting the diagonal 1,712-foot pressure altitude line, then move horizontally to the left and read the density altitude of approximately 3,500 feet. This means a self-launching glider or towplane will perform as if it were at 3,500 feet MSL on a standard day. 14 a Stand Every pilot must understand the terms “high density altitude” and “low density altitude.” In general, high density altitude refers to thin air, while low density altitude refers to dense air. Those conditions that result in a high density altitude (thin air) are high elevations, low atmospheric pressure, high temperatures, high humidity, or some combination thereof. Lower elevations, high atmospheric pressure, low temperatures, and low humidity are more indicative of low density altitude (dense air). However, high density altitudes may be present at lower elevations on hot days, so it is important to calculate the density altitude and determine performance before a flight. 14 ,00 0 15 Pressure altitude conversion factor Density Altitude Chart Glider performance during launch depends on the power output of the launch mechanism and on the aerodynamic efficiency of the glider itself. The four major factors that affect performance are density altitude, weight, design, and wind. S.L. F 0° 10° 20° 30° 40° 50° 60° 70° 80° 90° 100° Outside air temperature Figure 5-1. Density altitude chart. KDAL 232153Z 21006KT 7SM -RA BKN025 BKN060 OVC110 12/11 A2953 RMK AO2 PRESFR SLP995 P0005 T01220106 The 12/11 notation of this report indicates the reported temperature and dewpoint for Love Field. When barometric pressure rises, air density increases. The greater density of the air results in lower density altitude. Thus, takeoff and climb performance improves, and the length of runway needed for landing decreases. Altitude Takeoff and climb-out: no wind 34 As altitude increases, air density decreases. At altitude, the atmospheric pressure that acts on a given volume of air is less, allowing the air molecules to space themselves further apart. The result is that a given volume of air at high altitude contains fewer air molecules than the same volume of air at lower altitude. As altitude increases, density altitude increases, and glider takeoff and climb performance is reduced. Takeoff and climb-out: 25-knot headwind Temperature Temperature changes have a large effect on density altitude. When air is heated, it expands—the molecules move farther apart, making the air less dense. Takeoff and climb performance is reduced, while the length of runway required for landing is increased. KDAL 240453Z 21007KT 10SM CLR 25/15 A3010 RMK AO2… KDEN 240453Z 24006KT 10SM FEW120 SCT200 25/15 A3010 RMK AO2… The computed density altitude for Love Field is 1,774 feet; for Denver, 7,837 feet—almost twice the density altitude increase compared to the increase for Love Field. The effects of attitude and temperature are significant, a fact pilots must consider when computing aircraft performance. Wind Wind affects glider performance in many ways. Headwind during launch results in shorter ground roll, while tailwind causes longer ground roll before takeoff. [Figure 5-2] Crosswinds during launch require proper crosswind procedures or control input to track along the runway. During cruising flight, headwinds reduce the groundspeed of the glider. A glider flying at 60 knots true airspeed into a headwind of 25 knots has a groundspeed of only 35 knots. Tailwinds, on the other hand, increase the groundspeed of the glider. A glider flying at 60 knots true airspeed with a tailwind of 25 knots has a groundspeed of 85 knots. Crosswinds during cruising flight cause glider heading (the direction in which the glider nose is pointed) and glider track (the path of the glider over the ground) to diverge. In a glider, it must be remembered that crosswinds have some head or tailwind component that results in a lower or higher true groundspeed. When planning the landing, the wind effects must be factored into the landing pattern sight picture 34 Consider the following METAR for two airports with same altimeter setting, temperature, and dewpoint. Love Field (KDAL) airport elevation of 487 feet versus Denver International (KDEN) at 5,431 feet. d W in Figure 5-2. Apparent wind effect on takeoff distance and climb-out angle. and allowances must be made for the winds, indicated or expected. It is a lot easier to lose altitude than it is to make up altitude when the glider is down low. When gliding toward an object on the ground in the presence of crosswind, such as on final glide at the end of a cross-country flight, the glider pilot should keep the nose of the glider pointed somewhat upwind of the target on the ground. For instance, if the crosswind is from the right during final glide, the nose of the glider is pointed a bit to the right of the target on the ground. The glider’s heading is upwind (to the right, in this case) of the target, but if the angle of crab is correct, the glider’s track is straight toward the target on the ground. [Figure 5-3] Headwind during landing results in a shortened ground roll, and tailwind results in a longer ground roll. Crosswind landings require the pilot to compensate for drift with the proper flight control input, such as a sideslip or a crab. Glider pilots must be aware of the apparent angle versus the rate of descent. The glider descends at a constant rate, but the different groundspeeds will result in different approach angles. These different approach angles require specific techniques to ensure safe touchdowns in the landing zone. For example, if landing in a strong headwind, the glider pilot should plan for a closer base leg to allow for the apparent steeper approach due to the slower ground speed. Another technique would be the delayed extension of spoilers or drag brakes and accepting the faster airspeed to counter the headwind component. In the case of a tailwind and an apparent lower angle of approach, the glider pilot can use more spoiler or drag brake extension or slipping or a combination as allowed by the GFM to touchdown in the landing zone. In any condition, the glider pilot must make allowances for the current conditions of wind and density altitude to ensure a safe landing in the landing area. A glider pilot must respond to the current conditions and amend the traffic pattern and/or modify their procedures to compensate 5-3 Glider Airspeed – 60 Knots Wind – 10 Knots and 30° off the nose Glider Ground Speed – 51 Knots Crab angle of about 4° Target He a din g Target Glider Airspeed – 60 Knots Wind – 10 Knots and 45° off the nose Glider Ground Speed – 52 Knots Crab angle of about 6° gle b an Track Wind at 10 knots He ad ing Cra Track Glider Airspeed – 60 Knots Wind – 10 Knots and 90° off the nose Glider Ground Speed – 59 Knots Crab angle of about 9.5° Final glide Final glide No wind Heading = Track Crosswind from the right Heading is crabbed upwind of track Figure 5-3. Crosswind effect on final glide. for the conditions and land safely. The different groundspeeds also result in different touchdown points unless the pilot takes some kind of action. In any case, the pilot should aim for the touchdown zone markers and not for the end of the runway. [Figure 5-4] Touchdown point Landing: 25-knot headwind d W in 34 Touchdown point Landing: 15-knot tailwind W in d 34 5-4 34 During landing in windy/gusty conditions, there is a tendency to lose airspeed (flying speed) and increase sink. The friction between the ground and the air mass reduces the wind strength. The glider may be flying into a strong headwind at one moment; a few seconds later, the windspeed diminishes to nearly zero. The pilot landing into a headwind can usually expect to lose some of that headwind while approaching the surface due to surface friction slowing the wind. When the change is abrupt, the pilot experiences a loss of airspeed because it requires some small time and loss of altitude to accelerate the inertia of the glider up to the airspeed previously displayed when into the stronger headwind. This takes on the appearance of having to dive at the ground to maintain flying speed. Fly a faster approach to ensure staying above stalling speed. Depending on the wind change, a longer ground roll may be the result. If this occurs near the ground, the glider loses speed, and there may be insufficient altitude to recover the lost speed. This is called “wind gradient.” Consideration Landing: no wind Touchdown point Figure 5-4. Wind effect on final approach and landing distance. of wind gradient during a ground launch is important, as a sudden increase in windspeed could result in exceeding the designed launch speed. [Figure 5-5] The pilot landing with a tail wind has a higher groundspeed for an indicated airspeed. As the surface friction slows the winds, the pilot may see an increase in airspeed before the higher inertia-induced airspeed is dissipated, which may increase the ground roll distance to touchdown. This may be experienced with a downdraft in the vicinity of obstructions upwind of the runway as the winds curl down and wrap under the obstructions. This effect can lead to major undershoots of the approach path and landing short if the winds are strong enough. Local pilots can be a rich source of information about local wind currents and hazards. Glider pilots must understand that wind near the ground behaves differently higher up. Atmospheric conditions, such as thermal formation, turbulence, and gust and lulls, change the in-flight behavior of the glider significantly. As the wind flows over the ground, ground obstructions, such as buildings, trees, hills, and irregular formations along the ground, interfere with the flow of the wind, decreasing its velocity and breaking up its smooth flow as occurs in wave and ridge flying. Wind gradient affects a pilot turning too steeply on final approach at a very low airspeed and at low altitudes near to the surface. There is less wind across the lower wing than across the higher wing. The rolling force created by the wind gradient affects the entire wing area. This can prevent the pilot from controlling the bank with the ailerons and may roll the glider past a vertical bank. [Figure 5-6] Be cautious with any bank angle and at any airspeed while close to the ground when transitioning a wind gradient. When approaching to land during windy and gusty conditions, add half of the wind velocity to the approach speed to ensure adequate speed for a possible encounter with a wind gradient. During landing under these conditions, it is acceptable to allow the glider to touch down a little faster than normal instead of holding the glider off the ground for a low kinetic energy landing. Upon touchdown during these landings, extending the air brakes fully prevents the glider from becoming airborne through a wind gust during the landing roll. Some self-launching gliders are designed for extended periods of powered cruising flight. For these self-launching gliders, maximum range (distance) for powered flight and maximum duration (elapsed time aloft) for powered flight are primarily limited by the self-launching glider’s fuel capacity. Wind has no effect on flight duration but does have a significant effect on range. During powered cruising flight, a headwind reduces range, and a tailwind increases range. The Glider Flight Manual/Pilot’s Operating Handbook (GFM/POH) provides recommended airspeeds and power settings to maximize range when flying in no-wind, headwind, or tailwind conditions. Weight Glider lift, drag, and glide ratio characteristics are governed solely by its design and construction, and are predetermined at takeoff. The only characteristic the pilot controls is the weight of the glider. In some cases, pilots may control glider configurations, as some high-performance gliders may have a wing extension option not available on other models. Increased weight decreases takeoff and climb performance, but increases high-speed cruise performance. During launch, a heavy glider takes longer to accelerate to flying speed. The heavy glider has more inertia, making it more difficult to accelerate the mass of the glider to flying speed. After takeoff, the heavier glider takes longer to climb out because Airspeed 0 7 0 knots Wind 20 knots Airspeed 0 6 0 knots Wind 10 knots Wind gradient 57.5 MPH (50 knots) Figure 5-5. During gusting conditions, the pilot must monitor the pitch during the tow. 5-5 mp h W i 12 nd v m elo ph c ity ve Wi l n 10 oci d m ty ph Takeoff and climb-out: heavy glider 34 Wi nd ve loc ity 14 mp h 16 elo city Win dv 34 Wind veloci ty 18 mp h Takeoff and climb-out: lightweight glider Figure 5-7. Effect of weight on takeoff distance and climb-out rate and angle. The heavy glider has a higher stall speed and a higher minimum controllable airspeed than an otherwise identical, but lighter, glider. The stall speed of a glider increases with the square root of the increase in weight. If the weight of the glider is doubled (multiplied by 2.0), then the stall speed increases by more than 40 percent (1.41 is the approximate square root of 2; 1.41 times the old stall speed results in the new stall speed at the heavier weight). For example, a 540-pound glider has a stalling speed of 40 knots. The pilot adds 300 pounds of water ballast making the new weight 840 pounds. The new stalling speed is approximately 57 knots (square root of √ 300 + 40 = 57). When circling in thermals to climb, the heavy glider is at a disadvantage relative to the light glider. The increased weight of the heavy glider means stall airspeed and minimum sink airspeed are greater than they would be if the glider were operating at a light weight. At any given bank angle, the heavy glider’s higher airspeeds mean the pilot must fly larger diameter thermalling circles than the pilot of the light glider. Since the best lift in thermals is often found in a narrow cylinder near the core of the thermal, larger diameter circles generally mean the heavy glider is unable to exploit the strong lift of the thermal core, as well as the slower, lightweight glider. This results in the heavy glider’s inability to climb as fast in a thermal as the light glider. [Figure 5-8] Figure 5-6. Effect of wind velocity gradient on a glider turning into the wind. Stronger airflow over higher wing causes bank to steepen when close to the surface where surface friction slows winds. the heavier glider has more mass to lift to altitude than does the lighter glider (whether ground launch, aerotow launch, or self-launch). [Figure 5-7] 5-6 The heavy glider can fly faster than the light glider while maintaining the same glide ratio as the light glider. The advantage of the heavier weight becomes apparent during cruising flight. The heavy glider can fly faster than the light glider and still retain the same lift-to-drag (L/D) ratio. glider to climb reasonably well, the heavy glider’s advantage during the cruising portion of flight outweighs the heavy glider’s disadvantage during climbs. Water is often used as ballast to increase the weight of the glider. However, the increased weight requires a higher airspeed during the approach and a longer landing roll. Once the cross-country phase is completed, the water ballast serves no further purpose. The pilot should jettison the water ballast prior to entering the traffic pattern. Reducing the weight of the glider prior to landing allows the pilot to make a normal approach and landing. The lighter landing weight also reduces the loads that the landing gear of the glider must support. Lightweight glider thermal circle Heavy (ballasted) glider thermal cycle Figure 5-8. Effect of added weight on thermaling turn radius. If the operating weight of a given glider is increased, the stall airspeed, minimum controllable airspeed, minimum sink airspeed, and the best L/D airspeed are increased by a factor equal to the square root of the increase in weight. [Figure 5-9] Glide ratio is not affected by weight because, while a heavier glider sinks faster, it does so at a greater airspeed. The glider descends faster, but covers the same horizontal distance (at a higher speed) as a lighter glider with the same glide ratio and starting altitude. Operating Weight Stall Airspeed Minimum Sink Best L/D Airspeed 800 pounds 36 knots 48 knots 60 knots 1,200 pounds 44 knots 58 knots 73 knots 1,600 pounds 50 knots 68 knots 83 knots With the glide ratio data available to the pilot, provided by the charts/graphs located in the GFM/POH for the glider, the pilot can review or plot any specific combination of airspeed and glide ratio/lift to drag ratio (L/D). The resulting plot of L/D with airspeed (angle of attack) shows that glide ratio increases to some maximum at the lowest airspeed. Maximum lift-to-drag ratio (L/DMAX)/glide ratio, occurs at one specific airspeed (angle of attack and lift coefficient). If the glider is operated in a steady flight condition, total drag is at a minimum. This is solely based on airspeed. Any airspeed (angle of attack lower or higher) than that for L/DMAX/ glide ratio reduces the L/DMAX/glide ratio and consequently increases the total drag for a given glider’s lift. Note that a change in gross weight would require a change in airspeed to support the new weight at the same lift coefficient and angle of attack. This is why the glider GFM/POH has different speeds for flying with or without ballast. The configuration of a glider during flight has a great effect on the L/D. One of the most important of which is the glider’s best L/D/glide ratio. Figure 5-9. Effect of added weight on performance airspeeds. Rate of Climb To help gliders fly faster, some gliders have tanks that can hold up to 80 gallons of water. Higher speeds are desirable for cross-country flying and racing. The disadvantages of these ballasted gliders include reduced climb rates in thermals and the possibility that suitable lift cannot be located after tow release. To prevent this, the water ballast can be jettisoned at any time through dump valves, allowing the pilot to reduce the weight of the glider to aid in increased climb rates. Rate of climb for the ground-launched glider primarily depends on the strength of the ground-launching equipment. When ground launching, rates of climb generally are quite rapid, and can exceed 2,000 feet per minute (fpm) if the winch or tow vehicle is very powerful. When aerotowing, rate of climb is determined by the power of the towplane. It is important when selecting a towplane to ensure that it is capable of towing the glider, considering the existing conditions and glider weight. The addition of ballast to increase weight allows the glider to fly at increased airspeeds while maintaining its L/D ratio. Figure 5-9 shows that adding 400 pounds of water ballast increases the best L/D airspeed from 60 knots to 73 knots. The heavy glider has more difficulty climbing in thermals than the light glider, but if lift is strong enough for the heavy Self-launching glider rate of climb is determined by design, powerplant output, and glider weight. The rate of climb of self-launching gliders may vary from as low as 200 fpm to as much as 800 fpm or more in others. The pilot should consult the GFM/POH to determine rate of climb under the existing conditions. 5-7 Flight Manuals and Placards 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 Placards 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] 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. Glider Polars 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. Performance Information 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 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 Glider Self-Launching Glider Minimum pilot seat weight—154 lb (70 kg) Maximum pilot seat weight—264 lb (120 kg) INTENTIONAL SPINNING PROHIBITED EMERGENCY CANOPY JETTISON Stall speed—35 knots Maximum ground launch speed—74 knots Maneuvering speed—86 knots Maximum aerotow speed—94 knots VNE—136 knots Figure 5-10. Typical placards for nonmotorized and self-launching gliders. 5-8 DO NOT EXCEED 6,400 After inflight engine shutdown, feather propeller Engine rpm OFF