PEMP ACD2501. M.S. Ramaiah School of Advanced Studies, Bengaluru

Transcription

1 Aircraft Performance, Stability and Control Session delivered by: Mr. Ramjan Pathan 1

2 Session Objectives Aircraft Performance: Basicsof performance (t (steadystateand tt d accelerated) Performance characteristics of aircraft for (Civil passenger, cargo, Military- fighter, bomber) Range, Endurance, Rate of climb, maximum Mach number Stability and Control Basics of stability : CG location, AC, limits Longitudinal, Lateral control 2

3 Performance Basics Speeds : Maximum and Stall, Range, and Rate of Climb 3

4 Performance Performance: A measure of how well a device does its job Airplane Performance Examples Speed -> how fast/slow can it go? Rate of Climb -> how fast can it go up? Ceiling -> how high can it go? Range -> how far can it go? Endurance -> for how long can it fly? Takeoff/Landing -> how much runway does it need? Turning -> what is the minimum turn radius? 4

5 Performance Helicopter Performance Examples Hover Capability->how much weight ihtcan it lift vertically? Speed -> how fast can it go? Rate of Climb -> how fast can it go up? Ceiling -> how high can it go? Range -> how far can it go? Endurance -> for how long can it fly? 5

6 Performance Aircraft performance is determined by following Mathematical Modeling Computational Fluid Dynamics Classical l aero/propulsive/mass / analyses Ground Testing Wind tunnel testing Static engine testing Flight Testing 6

7 Steady level Flight 7

8 Performance 8

9 Performance 9

10 Performance 10

11 Performance 11

12 Performance 12

13 Stall Speed 13

14 Stall Speed 14

15 Stall Speed : Example 15

16 V max : Maximum Speed 16

17 V max : Maximum Speed 17

18 V max : Maximum Speed 18

19 V max : Maximum Speed 19

20 V max : Maximum Speed 20

21 V max : Maximum Speed 21

22 Rate of Climb (ROC) 22

23 ROC 23

24 ROC 24

25 ROC 25

26 Typical ROC numbers Ref : Performance Stability Dynamics and Control by Bandu Pamadi 26

27 Turn Performance Airplanes turn by tilting the Lift vector, to give : Horizontal component, L * Sin ( µ), µ bank angle Sharper the turn, larger the needed Lift Vector 27

28 Turn Performance 28

29 Turn Performance 29

30 Range : How Far 30

31 Range 31

32 Range 32

33 Range 33

34 Range 34

35 Range 35

36 Range 36

37 Range 37

38 Stability Longitudinal: Static, Dynamic Lateral 38

39 Stability Analysis Elevator, Aileron & Rudder Fixed These are at a fixed angle during the motion Elevator, Aileron & Rudder Free They are free to adjust as the motion goes on 39

40 Otto Lilienthal Determined of prop of cambered wings. Recorded materials, construction tech, handling char, aerodynamics from over 2000 models His models were statically stable but with negligible control Octave Chanute Biplane and multiplane wings Wing controls and vertical tail Samuel Pierpont Langley Concluded that heavier than air flight was possible Analysed existing data and developed eloped his own from his own experiments Developed and perfected unpiloted powered models All these people had influence on the Wright Brothers design 40

41 The Wright Brothers finally succeeded where others failed because of their dedicated scientific and engg efforts. Some of their major accomplishments are; They designed and built a wind tunnel and balance system to conduct aerodynamic tests. They developed a systematic airfoil aerodynamic database They developed a flight control system with adequate control capability They designed a lightweight engine and efficient propeller They designed an airplane with sufficient strength-to- weight ratio, capable of sustained powered flight 41

42 Equilibrium, Stability and Control Equilibrium : When all forces (Lift, Weight, Drag, Thrust ) and moments about the c.g cancel out Stability : An airplane is said to be statically stable if, following a disturbance, forces and moments are produced by the airplane which tend to reduce the disturbance by itself. Control : Forces and moments produced by pilot inputs to bring the airplane back to equilibrium after disturbance. 42

43 Equilibrium, Stability and Control Stability and controllability are different Stability : If a system is in equilibrium, i ability to maintain i that state Controllability : the ability to change the equilibrium state Very stable airplane will resist changes in it s attitude and hence, will be difficult to control. Military airplanes, for which maneuverability is one of the requirements, have lower levels of stability than civil airplanes. Stability is desirable but not necessary in piloted planes 43

44 Stability An airplane may be stable under some conditions of flight and unstable under other conditions. For instance, an airplane which is stable during straight and level flight may be unstable when inverted, and vice versa. This stability is sometimes called inherent stability. Modern combat aircraft are deliberately made to be inherently unstable, as this increases their manoeuvrability (Eg TEJAS) This requires a sophisticated automatic artificial stabilisation i system, which h has to be totally reliable. 44

45 Static Stability 1 DOF Statically stable. If the forces and moments on the body caused by a disturbance tend initially to return the body toward its equilibrium position, the body is statically stable. Statically unstable. If the forces and moments are such that the body continues to move away from its equilibrium position after being disturbed, the body is statically unstable. Neutrally stable. If the body is disturbed but the moments remain zero, the body stays in equilibrium and is neutrally stable 45

46 Dynamic Stability 1. Dynamic stability deals with the time history of the vehicle s motion after it initially responds to its static stability. 2. Consider an airplane flying at an angle of attack (AOA) such that the moments about the center of gravity (cg) are zero. 3. The aircraft is therefore in equilibrium at α e and is said to be trimmed, and α e is called the trim angle of attack. 4. Now imagine that a wind gust disturbs the airplane and changes its angle of attack to some new value α. Hence, the plane was pitched through a displacement (α - α e ) 5. Three responses are possible Aperiodic Damped Damped Oscillatory Un-damped d Divergent 46

47 Stability For a successful flight : Airplane must be able to achieve equilibrium i flight It must be manoeuvrable for wide range of velocities and altitudes For these, aircraft must possess aerodynamic and propulsive p controls Stability and control characteristics of an airplane are referred to as handling characteristics 47

48 Longitudinal Stability and Control Wing Contribution Aft Tail Contribution Canard Configuration Fuselage Contribution Power Effects Elevator Effectiveness Elevator Trim Hinge Moment 48

49 Pitching Moment Vs CL 49

50 Degree of Longitudinal Stability 50

51 Effect of CG movement 51

52 Directional Stability and Control Following components contribute to instability Wing Contribution ti Fuselage Contribution Nacells Following components contribute to instability Vertical Tail Contribution Rudder 52

53 Roll Stability and Control Wing Dihedral Wing Sweep Position of Wings on Fuselage Vertical Tail Ailerons Spoiler 53

54 FLIGHT CONTROLS Flight controls and instrument panels vary, but have the same basic functions. 54

55 FLIGHT CONTROLS Turning Left Turning Right Moving the yoke LEFT or RIGHT moves the ailerons on the wings in opposite directions. One moves UP as the other goes DOWN. 55

56 FLIGHT CONTROLS Pulling back on the yoke moves the elevator on the tail UP, moving the airplane nose UP to climb. 56

57 FLIGHT CONTROLS Pushing forward moves the elevator DOWN, moves the nose DOWN to descend. 57

58 FLIGHT CONTROLS Brakes are located at the top or toe of the pedal Pilots use rudder pedals on the floor to move the rudder LEFT or RIGHT to help the airplane turn. 58

59 Stick Force Force exerted by pilot to move the control surface Stick Force Gradients Trim Tabs 59

60 Stability and Control Inherently stable airplane returns to its original condition after being disturbed. Requires less effort to control Center of Gravity concerns: Unable to compensate with elevator in pitch axis Weight and Balance becomes critical taught in a coming lecture 60

61 Control Surfaces and their Function 61

62 Control Surfaces and their Function 62

63 Subjects involved in Flight Dynamics 63

64 Inverted Flight 64

65 Aerodynamic Surfaces 65

66 Aerodynamic Surfaces B727 Spoilers 66

67 Stability & Control The 3 axes of motion: roll, pitch, yaw Yaw Pitch Roll yaw 67

68 Roll Control 68

69 Phugoid Motion Phugoid mode is a lightly damped long period oscillation. The incidence is almost constant and the aircraft varies altitude at constant energy, trading potential for kinetic and back again 69

70 Trim and Stability 70

71 Stability and Moment coefficient variation How the moment coefficient CM varies with angle-of-attack determines the stability of the aircraft 71

72 Effect of C. G. Position on Stability If the c.g. is forward of the aerodynamic centre, dmcg/dα will be negative and the aircraft will therefore be statically stable. If the c.g. is aft of the aerodynamic centre, dmcg/dα will be positive and the aircraft will therefore be statically unstable. If the c.g. isattheaerodynamic centre, dmcg/dα will be zero and the aircraft will therefore be neutrally stable. 72

73 Forces and Locations Conventional A/c Assuming T, D and Z T and Z D are small 73

74 Forces and Locations Conventional A/c L is bigger than D and T so it is a fair to drop the last 2 terms giving. Using non-dimensional coefficients and defining tail volume ratio as Typical values for Tail volume ratio 74

75 Typical Values for Tail Volume Ratio Note : For Trim, LHS should be zero This equation can answer the questions What is the lift required at the tailplane for trim. or Calculate theelevator anglerequired for trim. 75

76 Longitudinal Stability down lif ft weig ght lift Static stability (tendency to return after control input) up elevator increases downward lift, angle of attack increases; lift increases, drag increases, aircraft slows; less downward d lift, angle of attack decreases (nose drops). 76

77 Aside: CG and Center of Pressure Location down lif ft lift Aft CG increases speed: the tail creates less lift (less drag); the tail creates less down force (wings need to create less lift). This also decreases stall speed (lower angle of attack req d). 77

78 Longitudinal Modes 78

79 Lateral Stability Lateral static stability : refers to the ability of the aircraft to generate a yawing moment to cancel disturbances in sideslip V Question : Which direction should the yawing moment act to align the aircraft with the velocity vector? Original Flight Direction 79

80 Typical Experimental results 80

81 Lateral Stability It is the ability of the aircraft to recover from a roll without pilot s intervention. Dihedral is good for lateral stability. If the wing is tilted upwards from root to tip, it has a dihedral. 81

82 Anhedral Anhedral is bad for lateral stability. If the wing dips down from root to tip, it has an anhedral. 82

83 Not all of Dihedral!! Note the large anhedral on IL76 IAF Gajaraj; j; this is common on high wing planes 83

84 What happens when the aircraft undergo a roll? Lift Lift A portion of the lift is pointed sideways. The vehicle moves laterally. This is called sideslip. 84

85 During sideslip, a relative wind flows from right to left A downwash occurs on the left wing, reducing lift. This wind has a component normal to the wing on the right, viewing from the front. This is an upwash. The upwash increases lift on the right wing. As a result, the aircraft rights itself, and recovers from the roll. 85

86 Effect of Dihedral 86

87 Effect of Side force 87

88 Dutch Roll 88

89 Effect of Vertical Fin on Rolling Vertical Fin is used to stabilize the aircraft s direction This is often referred to as weather cock behaviour This force however induces a rolling movement as acts F f over the c.g location 89

90 Load Factors due to Banking Figure reveals an important fact about turns that the load factor increases at a terrific rate after a bank has reached 45 or 50. If the load factor at 60 bank is 2 G s, then at 80 bank it is more than 5G s. The wing must produce lift equal to these load factors if altitude is to be maintained. 90

91 V-g Diagram Aircraft s operational envelope is presented as V-g diagram 91

92 The price paid for a large static stability margin The aircraft may become sluggish, hard to maneuver. The tail will resist the pilot s attempt to change the aircraft angle of attack. A large tail adds to aircraft weight, and cost. A smaller tail will require a long fuselage( g( a long enough crowbar! ) to generate enough of a pitching moment to bring the nose up or down. Tail generates drag, including wave drag! 92

93 Horizontal Tail in Steady Level Flight needs to produce a download to balance all moments. Aircraft c.g. Tail Lift The wing produces a counterclockwise moment about the c.g. The tail will have to produce a clockwise moment about the c.g. These two moments (i.e. force times distance) must roughly balance. The wing has to generate enough lift to overcome the weight + Tail lift 93

94 Relaxed Static Stability For improved maneuverability, some fighter aircraft sacrifice the static stability margin. Some fighter aircraft are statically unstable. Their nose will continue to pitch up, the lift will continue to go up when a upward gust is encountered. Result: A/C will stall, flip over. These aircraft must be actively controlled by the pilot, or an onboard computer. Redundant computer systems are present in case a computer based flight control fails. 94

95 Directional Stability Freestream comes from pilot s right side, due to cross wind. It causes nose to rotate to left viewed from the top. The force on the tail causes the aircraft to rotate back to original direction. A cross wind may cause the nose to rotate about the vertical axis, changing the flight direction. The vertical tail behaves like a wing at an angle of attack, producing a side force, rotates the aircraft to its original direction. All of this occurs without pilot action or intervention. 95

96 Why twin tail? Some fighter aircraft have twin tails. Each of thetails maybesmall, reducing radar cross section. Alternatively, ti l twice the surface means twice the amount of side force that can be generated, giving good directional control. Disadvantage: Cost of manufacturing, weight go up. 96

97 Twin Fins for Directional Stability 97

98 Directional Control Adverse aileron yaw The aileron that moves downward creates lift and induced drag. Induced drag pulls the nose of the airplane around in the direction opposite the way the airplane should turn. 98

99 Directional Control Rudder Rotates the airplane about its vertical axis (Yawing) Also provides a form of roll control because the application of rudder causes yaw which will induce a roll. 99

100 Lateral Stability If one wing is lowered (e.g. by turbulence), the airplane sideslips. The lower wing has a greater angle of attack (more lift). This raises the lower wing

101 Directional Stability As the airplane turns to the left (e.g. in turbulence), the vertical stabilizer creates lift toward the left. The airplane turns to the right

102 Session Objectives Following Topics were covered in this lecture AircraftPerformance: Basics of performance (steady state and accelerated) Performance characteristics ti of aircraft for (Civil il passenger, cargo, Military- fighter, bomber) Range, g, Endurance, Rate of climb, maximum Mach number Stability and Control Basics of stability : CG location, AC, limits Longitudinal, Lateral control 102

103 Thank you! 103