Glider Flight Manual PDF

Glider wings have a high aspect ratio, as shown in Figure 3-19. High aspect ratio wings produce a comparably high amount of lift at low angles of attack with less induced drag. Airspeed (knots) 0 25 50 52 0 70 75 Weight Weight is the third force that acts on a glider in flight. Weight oppose...

Glider wings have a high aspect ratio, as shown in Figure 3-19. High aspect ratio wings produce a comparably high amount of lift at low angles of attack with less induced drag. Airspeed (knots) 0 25 50 52 0 70 75 Weight Weight is the third force that acts on a glider in flight. Weight opposes lift and acts vertically through the CG of the glider. Gravitational pull provides the force necessary to move a glider through the air since a portion of the weight vector of a glider is directed forward. Sink rate (knots) –1 –2 Tangent –1.9 Ballasted lift/drag (L/D) –3 –4 Ballasted lift/drag (L/D) –5 = 70 :1 1.9 Thrust Thrust is the forward force that propels a self-launching glider through the air. Self-launching gliders have engine-driven propellers that provide this thrust. Unpowered gliders have an outside force, such as a towplane, winch, or automobile, to launch the glider. Airborne gliders obtain thrust from conversion of potential energy to kinetic energy. = 36:1 –6 Glider sink rate Sink rate with water ballast Figure 3-18. Calculating glide speed with water ballast. Aspect Ratio The aspect ratio is another factor that affects the lift and drag created by a wing. Aspect ratio is determined by dividing the wingspan (from wingtip to wingtip), by the average wing chord. Three Axes of Rotation The glider is maneuvered around three axes of rotation: yaw (vertical), lateral, and longitudinal. They rotate around one 51' wing span Chord lines 4.3 feet Chord lines 4.3 feet Wing area = 219.5 ft 2 Maximum gross weight = 1,040 lb Aspect ratio = 11.85:1 Glide ratio = 22:1 Aspect ratio is determined by dividing the wingspan (from wingtip to wingtip), by the average chord. 86' 11.3" wing span Chord lines 2.22 feet Chord lines 2.22 feet Wing area = 193.32 ft 2 Maximum gross weight = 1,808 lb Aspect ratio = 39:1 Glide ratio = 60:1 Figure 3-19. Aspect ratio. 3-9 central point in the glider called the CG. This point is the center of the glider’s total weight and varies with the loading of the glider. Yaw is movement around the vertical axis, which can be represented by an imaginary straight line drawn vertically through the CG. Moving the rudder left or right causes the glider to yaw the nose to the left or right. Moving the ailerons left or right to bank moves the glider around the longitudinal axis. This axis would appear if a line were drawn through the center of the fuselage from nose to tail. Pulling the stick back or pushing it forward, raising or lowering the nose, controls the pitch of the glider or its movement around the lateral axis. The lateral axis could be seen if a line were drawn from one side of the fuselage to the other through the CG. [Figure 3-20] Stability neutral static stability have neither the tendency to return to equilibrium nor the tendency to continue displacement. Dynamic stability describes a glider’s motion and time required for a response to static stability. In other words, dynamic stability describes the manner in which a glider oscillates when responding to static stability. A glider that displays positive dynamic and static stability reduces its oscillations with time. A glider demonstrating negative dynamic stability is the opposite situation; its oscillations increase in amplitude with time following a displacement. A glider displaying neutral dynamic stability experiences oscillations, which remain at the same amplitude without increasing or decreasing over time. Figure 3-21 illustrates the various types of dynamic stability. A glider is in equilibrium when all of its forces are in balance. Stability is defined as the glider’s ability to maintain a uniform flight condition and return to that condition after being disturbed. Often during flight, gliders encounter equilibrium-changing pitch disturbances. These can occur in the form of vertical gusts, a sudden shift in CG, or deflection of the controls by the pilot. For example, a stable glider would display a tendency to return to equilibrium after encountering a force that causes the nose to pitch up. Both static and dynamic stability are particularly important for pitch control about the lateral axis. Measurement of stability about this axis is known as longitudinal stability. Gliders are designed to be slightly nose heavy in order to improve their longitudinal stability. This causes the glider to tend to nose down during normal flight. The horizontal stabilizer on the tail is mounted at a slightly negative AOA to offset this tendency. When a dynamically stable glider oscillates, the amplitude of the oscillations should reduce through each cycle and eventually settle down to a speed at which the downward force on the tail exactly offsets the tendency to dive. [Figure 3-22] Static stability and dynamic stability are two types of stability a glider displays in flight. Static stability is the initial tendency to return to a state of equilibrium when disturbed from that state. The three types of static stability are positive, negative, and neutral. When a glider demonstrates positive static stability, it tends to return to equilibrium. A glider demonstrating negative static stability displays a tendency to increase its displacement. Gliders that demonstrate Adjusting the trim assists in maintaining a desired pitch attitude. A glider with positive static and dynamic longitudinal stability tends to return to the trimmed pitch attitude when the force that displaced it is removed. If a glider displays negative stability, oscillations increase over time. If uncorrected, negative stability can induce loads exceeding the design limitations of the glider. YAW La ter al ax is l axis Longitudina 3-10 PITCH ROLL Figure 3-20. Three axes of rotation. Vertical axis CG Neutral dynamic stability Positive dynamic stability Negative dynamic stability Figure 3-21. Three types of dynamic stability. Center of lift Negative AOA Center of gravity (Exaggerated in this illustration) Figure 3-22. Use of the horizontal stabilizer angle to offset the natural tendency of a glider to nose down. Another factor that is critical to the longitudinal stability of a glider is its loading in relation to the CG. The CG of the glider is the point at which the total force of gravity is considered to act. When the glider is improperly loaded so it exceeds the aft CG limit, it loses longitudinal stability. As airspeed decreases, the nose of a glider rises. To recover, control inputs must be applied to force the nose down to return to a level flight attitude. It is possible that the glider could be loaded so far aft of the approved limits that control inputs are not sufficient to stop the nose from pitching up. If this were the case, the glider could enter a spin from which recovery would be impossible. Loading a glider with the CG too far forward is also hazardous. In extreme cases, the glider may not have enough pitch control to hold the nose up during an approach to a landing. For these reasons, it is important to ensure that the glider is within weight and balance limits prior to each flight. Proper loading of a glider and the importance of CG is discussed further in Chapter 5, Performance Limitations. Flutter Another factor that can affect the ability to control the glider is flutter. Flutter occurs when rapid vibrations are induced through the control surfaces while the glider is traveling at high speeds. Looseness in the control surfaces can result in flutter while flying near maximum speed. Another factor that can reduce the airspeed at which flutter can occur is a disturbance to the balance of the control surfaces. If vibrations are felt in the control surfaces, reduce the airspeed. 3-11 Lateral Stability Another type of stability that describes the glider’s tendency to return to wings-level flight following a displacement is lateral stability. When a glider is rolled into a bank, it has a tendency to sideslip in the direction of the bank. For example, due to a gust of wind, the glider wing is lifted and the glider starts to roll. The angle of attack on the downward going wing is increased because the wing is moving down and now the air is moving up past it. This causes the lift on this wing to increase. On the upward going wing, the opposite is occurring. The angle of attack is reduced because the wing is moving up and the air is moving down past it. Lift on this wing is therefore reduced. This does produce a countertorque that damps out the rolling motion, but does not roll the glider back to wings level as the effect stops when the glider stops. [Figure 3-23] To obtain lateral stability, dihedral is designed into the wings. Dihedral is the upward angle of the wings from a horizontal (front/rear view) axis of the plane. As a glider flies along Angle of attack reduced, lift reduced n tion of rota tio rec i D Angle of attack increased, lift increased Figure 3-23. Lateral stability. 3-12 and encounters turbulence, the dihedral provides positive lateral stability by providing more lift for the lower wing and reducing the lift on the raised wing. As one wing lowers, it becomes closer to perpendicular to the surface and level. Because it is closer to level and perpendicular to the weight force, the lift produced directly opposes the force of weight. This must be instantly compared to the higher and now more canted wing referenced to the force of weight. The higher wing’s lift relative to the force of weight is now less because of the vector angle. This imbalance of lift causes the lower wing to rise as the higher descends until lift equalizes, resulting in level flight. [Figure 3-24] Turning Flight Before a glider turns, it must first overcome inertia, or its tendency to continue in a straight line. A pilot creates the necessary turning force by using the ailerons to bank the glider so that the direction of total lift is inclined. This divides the force of lift into two components; one component acts vertically to oppose weight, while the other acts horizontally to oppose centrifugal force. The latter is the horizontal component of lift. [Figure 3-25] To maintain attitude with the horizon during a turn, glider pilots need to increase back pressure on the control stick. The horizontal component of lift creates a force directed inward toward the center of rotation, which is known as centripetal force. [Figure 3-26] This center-seeking force causes the glider to turn. Since centripetal force works against the tendency of the aircraft to continue in a straight line, inertia tends to oppose centripetal force toward the outside of the turn. This opposing force is known as centrifugal force. In reality, centrifugal force is not a true aerodynamic force; it is an apparent force that results from the effect of inertia during the turn. Load Factors The preceding sections only briefly considered some of the practical points of the principles of turning flight. However, with the responsibilities of the pilot and the safety of passengers, the competent pilot must have a well-founded concept of the forces that act on the glider during turning flight and the advantageous use of these forces, as well as the operating limitations of the particular glider. Any force applied to a glider to deflect its flight from a straight line produces a stress on its structure; the amount of this force is called load factor. Dihedral angle Dihedral angle Figure 3-24. Dihedral angle. Lift Vertical component of lift (effective lift) Centrifugal force R nt fo rc e e c or lf ta Weight ta pe tri en ul C es Figure 3-25. Forces in a banked turn. Figure 3-26. Centripetal force is a force that makes a body follow a curved path. 3-13 It is interesting to note that in subjecting a glider to three Gs in a pullup from a dive, the pilot is pressed down into the seat with a force equal to three times the person’s weight. Thus, an idea of the magnitude of the load factor obtained in any maneuver can be determined by considering the degree to which the pilot is pressed down into the seat. Since the operating speed of modern gliders has increased significantly, this effect has become so pronounced that it is a primary consideration in the design of the structure for all gliders. If attempting to improve turn performance by increasing angle of bank while maintaining airspeed, pay close attention to glider limitations due to the effects of increasing the load factor. Load factor is defined as the ratio of the load supported by the glider’s wings to the actual weight of the aircraft and its contents. A glider in stabilized, wings-level flight has a load factor of one. Load factor increases rapidly as the angle of bank increases due to increase wing loading. [Figure 3-27] With the structural design of gliders planned to withstand only a certain amount of overload, knowledge of load factors has become essential for all pilots. Load factors are important to the pilot for two distinct reasons: 1. It is possible for a pilot to impose an obviously dangerous overload on the glider structures. 2. Increased load factor increases the stalling speed, making stalls possible at seemingly safe flight speeds due to increased wing loading. In a turn at constant speed, the AOA must be increased to furnish the extra lift necessary to overcome the centrifugal force and inertia opposing the turn. As the bank angle increases, AOA must also increase to provide the required lift. The result of increasing the AOA is a stall when the critical AOA is exceeded in a turn. [Figure 3-28] 3-14 Percentage increase in stall speed A load factor of three means that the total load on a glider’s structure is three times its gross weight. Gravity load factors are usually expressed in terms of “G”—that is, a load factor of three may be spoken of as three Gs, or a load factor of four as four Gs. A load factor of one, or 1 G, represents conditions in straight-and-level flight, in which the lift is equal to the weight. Therefore, two Gs would be two times the normal weight. Gliders may be designed to withstand stress of up to nine Gs. 180 160 140 120 100 80 60 40 20 0 0° 10° 20° 30° 40° 50° 60° 70° 80° 90° 100° Angle of bank (degrees) Figure 3-27. A glider’s stall speed increases as the bank angle increases. For example, a 60° angle of bank causes a 40 percent increase in the glider’s stall speed. 9 8 7 Load factor (Gs) A load factor is the ratio of the total air load acting on the glider to the gross weight of the glider. A glider in flight with a load factor of one does not mean the glider is accelerating; it means the lift on the aircraft is the same as in straightand-level flight. Load factor may be positive or negative, dependent on the current flightpath. 6 5 4 3 2 1 0 0° 10° 20° 30° 40° 50° 60° 70° 80° 90° 100° Angle of bank (degrees) Figure 3-28. The loads placed on a glider increase as the angle of bank increases. Rate of Turn Rate of turn refers to the amount of time it takes for a glider to turn a specified number of degrees. If flown at the same airspeed and angle of bank, every glider turns at the same rate. If airspeed increases and the angle of bank remains the same, the rate of turn decreases. Conversely, a constant airspeed coupled with an angle of bank increase results in a higher rate of turn. Radius of Turn The amount of horizontal distance an aircraft uses to complete a turn is referred to as the radius of turn. The radius of turn at any given bank angle varies directly with the square of the airspeed. Therefore, if the airspeed of the glider were doubled, the radius of the turn would be four times greater. Although the radius of turn is also dependent on a glider’s airspeed and angle of bank, the relationship is the opposite of rate of turn. As the glider’s airspeed is increased with the angle of bank held constant, the radius of turn increases. On the other hand, if the angle of bank increases and the airspeed remains the same, the radius of turn is decreased. [Figure 3-29] When flying in thermals, the radius of turn is an important factor as it helps to gain the maximum altitude. A smaller turn radius enables a glider to fly closer to the fastest rising core of the thermal and gain altitude more quickly. Turn Coordination It is important that rudder and aileron inputs are coordinated during a turn so maximum glider performance can be maintained. If too little rudder is applied, or if rudder is applied too late, the result is a slip. Too much rudder, or rudder applied before aileron, results in a skid. Both skids and slips swing the fuselage of the glider into the relative wind, creating additional parasite drag, which reduces lift and airspeed. Although this increased drag caused by a slip can be useful during approach to landing to steepen the approach path and counteract a crosswind, it decreases glider performance during other phases of flight. When rolling into a turn, the aileron on the inside of the turn is raised and the aileron on the outside of the turn is lowered. The lowered aileron on the outside wing increases lift by increasing wing camber and produces more lift for that wing. Since induced drag is a byproduct of lift, the outside wing also produces more drag than the inside wing. This causes adverse yaw, a yawing tendency toward the outside of the turn. Coordinated use of rudder and aileron corrects for adverse yaw and aileron drag. Adverse yaw in gliders can be more pronounced due to the much longer wings as compared to an airplane of equal weight. The longer wings constitute longer lever arms for the adverse yaw forces to act on the glider. Therefore, more rudder movement is necessary to counteract the adverse yaw and have a coordinated turn. Slips A slip is a descent with one wing lowered and the glider’s longitudinal axis at an angle to the flightpath. It may be used for one or both of two purposes: to steepen the approach path without increasing the airspeed, as would be the case if a dive were used, or used to make the glider move sideways through the air to counteract the drift that results from a crosswind. Formerly, slips were used as a normal means of controlling landing descents to short or obstructed fields, but they are now primarily used in the performance of crosswind and short-field landings. With the installation of wing flaps and effective spoilers on modern gliders, the use of slips to steepen or control the angle of descent is no longer the only procedure available. However, pilots still need skill in the performance of forward slips to correct for possible errors in judgment of the landing approach. The shape of the glider’s wing planform can greatly affect the slip. If the glider has a rectangular wing planform, the slip has little effect on the lift production of the wing other than the wing area being obscured by the fuselage vortices. The direction of the relative wind to the wing has the same effect on both wings so no inequalities of lift form. However, if the wing is tapered or has leading edge aft sweep, then the relative wind has a large effect on the production of lift. Level stall airspeed at gross weight is 38 knots Stall speed: 41 knots Stall speed: 45 knots 30° 45° Stall speed: 53 knots 60° TAS 40 MPH Turn radius 1 8 5 feet Turn radius 1 0 7 feet Turn radius 0 6 2 feet TAS 60 MPH Turn radius 4 1 7 feet Turn radius 2 4 0 feet Turn radius 1 3 9 feet TAS 80 MPH Turn radius 7 4 0 feet Turn radius 4 2 8 feet Turn radius 2 4 7 feet Figure 3-29. A glider’s radius of turn as compared to angle of bank. 3-15 If a glider with tapered wings, as shown in Figure 3-14, were to begin a slip to the left with the left wing lower, the left wing will have a relative wind more aligned with its chord line and effectively higher airflow (airspeed) that generates more lift as compared to the higher right wing with angled relative wind, resulting in lower effective airflow (airspeed) over that wing. This differential in airflow or relative airspeed of the wings when taken to the extremes of the flight envelope results in the higher wing stalling and often an inverted spin. the pressure on the rudder is released abruptly, the nose swings too quickly into line and the glider tends to acquire excess speed. Depending on the exact wing shape, an elliptical wing can have characteristics more like a tapered wing. [Figure 3-14] Pilots should always consult the GFM and know what the gliders limitations are concerning slips. Forward Slip The use of slips has limitations. Some pilots may try to lose altitude by violent slipping, rather than by smoothly maneuvering, exercising good judgment, and using only a slight or moderate slip. In short-field landings, this erratic practice invariably leads to trouble since enough excess speed may prevent touching down anywhere near the proper point, and very often results in overshooting the entire field. If a slip is used during the last portion of a final approach, the longitudinal axis of the glider must be aligned with the runway just prior to touchdown so that the glider touches down headed in the direction in which it is moving over the runway. This requires timely action to modify the slip and align the glider’s longitudinal axis with its direction of travel over the ground at the instant of touchdown. Failure to accomplish this imposes severe sideloads on the landing gear and imparts violent ground looping tendencies. Discontinuing the slip is accomplished by leveling the wings and simultaneously releasing the rudder pressure, while readjusting the pitch attitude to the normal glide attitude. If Forward Slip Figure 3-30. A comparison of a forward slip to a sideslip. 3-16 Because of the location of the pitot tube and static vents, airspeed indicators in some gliders may have considerable error when the glider is in a slip. The pilot must be aware of this possibility and recognize a properly performed slip by the attitude of the glider, the sound of the airflow, and the feel of the flight controls. The forward slip is a slip in which the glider’s direction of motion is the same as before the slip was begun. [Figure 3-30] The primary purpose of a forward slip is to dissipate altitude without increasing the glider’s speed, particularly in gliders not equipped with flaps, or if the spoilers are inoperative. There are many circumstances requiring the use of forward slips, such as a landing approach over obstacles and shortfield landings, in which it is always wise to allow an extra margin of altitude for safety in the original estimate of the approach. In the latter case, if the inaccuracy of the approach is confirmed by excess altitude when nearing the boundary of the selected field, slipping can dissipate the excess altitude. If there is any crosswind, the slip is much more effective if made toward the wind. Assuming the glider is originally in straight flight, the wing on the side toward which the slip is to be made should be lowered by use of the ailerons. Simultaneously, the airplane’s nose must be yawed in the opposite direction by applying opposite rudder so that the glider’s longitudinal axis is at an angle to its original flightpath. The degree to which the nose is yawed in the opposite direction from the bank should be such that the original ground track is maintained. The nose should also be raised as necessary to prevent the airspeed from increasing. Slideslip