Aircraft Control Devices and Systems

aerodynamic reduction of hinge moments • Elevator hinge moment Helevator = CHelevator 1 2 ρV 2Sc 27 Control Surface Dynamics Linear dynamic models of control surface motion 28 δ !! E = CH δ !E δ !E + CH δ E δ E + CH αδ E α + CHC δ E ( )qSc ⎡ ⎣ ⎤ ⎦ I δ E δ !!A = CH δ !A δ !A + CH δ A δ A + CH βδ A β + CHC δ A ( )qSb ⎡ ⎣ ⎤ ⎦ I δ A δ !!R = CH δ !R δ !R + CH δ R δ R + CH βδ R β + CHC δ R ( )qSb ⎡ ⎣ ⎤ ⎦ I δ R CH ! δ : aerodynamic/mechanical damping CH δ : aerodynamic/mechanical spring CH α , CH β : floating tendency CHC : pilot or autopilot input 15 Angle of Attack and Control Surface Deflection • Horizontal tail at positive angle of attack • Horizontal tail with elevator control surface • Horizontal tail with positive elevator deflection 29 Floating and Restoring Moments on a Control Surface • Positive elevator deflection produces a negative (
restoring) moment, Hδ, on elevator due to aerodynamic or mechanical spring • Positive angle of attack produces negative moment on the elevator • With
stick free, i.e., no opposing command torques, elevator
floats up due to negative Hδ 30 16 Horn Balance CH ≈ CH α α + CH δ E δ E + CH pilot input • Stick-free case – Control surface free to
float CH ≈ CH α α + CH δ E δ E • Normally CH α < 0 : reduces short-period stability CH δ E < 0 : required for mechanical stability NACA TR-927, 1948 31 Elevator Horn Balance CH α CH δ E • Static elevator effects Horn Balance • Inertial and aerodynamic effects • Control surface in front of hinge line – Increasing elevator improves pitch stability, to a point • Too much horn area – Degrades restoring moment – Increases possibility of mechanical instability – Increases possibility of destabilizing coupling to short- period mode € CH α 32 Fokker dr.1 Martin B-57E 17 Overhang or Leading-Edge Balance • Area in front of the hinge line • Effect is similar to that of horn balance • Varying gap and protrusion into airstream with deflection angle CH ≈ CH α α + CH δ δ + CH pilot input NACA TR-927, 1948 33 CH α CH δ E Trailing-Edge Bevel Balance • Bevel may have strong effect on aerodynamic hinge moments • See discussion in Abzug and Larrabee CH ≈ CH α α + CH δ δ + CH pilot input 34 CH α CH δ E 18 Internally Balanced Control Surface § NACA Report 868 § Control-surface fin with flexible seal moves within an internal cavity in the main surface § Differential pressures reduce control hinge moment CH ≈ CH α α + CH δ δ + CH pilot input 35 Control Tabs • Trim tab – Control surface and tab commanded separately • Geared tab – Tab leverage reduces commanded control surface torque • Servo tab – Tab produces only commanded control surface torque • Spring tab – Command deflects both surface and tab with airspeed-dependent ratio • Stability tab – Tab is separate mechanical system that augments aircraft stability 36 19 B-52 Rudder Control Linkages 37 Nonlinear Control Mechanization Effects 38 20 Control Mechanization Complications • Fabric-covered control surfaces (e.g., DC-3, Spitfire) subject to distortion under air loads, changing stability and control characteristics • Control cable stretching • Elasticity of the airframe changes cable/pushrod geometry • Nonlinear control effects – friction – breakout forces – backlash Douglas DC-3 Supermarine Spitfire 39 Mechanical Nonlinearity • Friction between surfaces • Dead zone (backlash) due to loose mechanical connection 40 21 • Breakout force • Force threshold 41 Mechanical Nonlinearity Rudder Lock • Rudder deflected to stops at high sideslip; aircraft trims at high β • 3 necessary ingredients – Low directional stability at high sideslip due to stalling of fin – High (positive) hinge moment- due-to-sideslip at high sideslip (e.g., B-26) – Negative rudder yawing moment • Problematical if rudder is unpowered and requires high foot-pedal force (
rudder float of large WWII aircraft) • Solutions – Increase high-sideslip directional stability by adding a dorsal fin (e.g., B-737-100 (before), B-737- 700 (after)) – Hydraulically powered rudder Boeing 737-100 42 Martin B-26 Boeing 737-700 22 Instabilities Due To Control Mechanization • Aileron buzz (aero-mechanical instability; P-80) • Rudder snaking (Dutch roll/mechanical coupling; Meteor, He-162) • Aeroelastic coupling (B-47, Boeing 707 yaw dampers) 43 Yaw Damping 44 23 Boeing B-47 Yaw Damper • Yaw rate gyro drives rudder to increase Dutch roll damping • Comment: The plane wouldn
t need this contraption if it had been designed right in the first place. [WRONG] • Most jet aircraft have yaw dampers 45 Yaw rate washout to reduce opposition to steady turns (TBD) Boeing B-47 B-52 Mechanical Yaw Damper • Combined stable rudder tab, low-friction bearings, small bobweight, and eddy-current damper for B-52 • Advantages – Requires no power, sensors, actuators, or computers – Simple mechanical components • Problems – Misalignment, need for high precision – Friction and wear over time – Jamming, galling, and fouling – High sensitivity to operating conditions, design difficulty 46 Boeing B-52 24 Flight Control Systems 47 Mechanical and Augmented Control Systems • Mechanical system – Push rods, bellcranks, cables, pulleys • Power boost – Pilot's input augmented by hydraulic servo that lowers manual force • Fully powered (irreversible) system – No direct mechanical path from pilot to controls – Mechanical linkages from cockpit controls to servo actuators 48 25 Stability Augmentation System (SAS) for Northrop YB-35/49 Flying Wing Bombers • Northrop B-35/49 flying wing bombers motivated significant SAS development • Complications – Pneumatic/hydraulic logic – Primitive electronic analog computation – No digital computation – Unreliable and inaccurate sensors and actuators ("servo-actuators") – Limited math models of system components – “Seat-of-the-pants” design and implementation • Northrop among first to take systematic approach to SAS design Northrop XB-35 Northrop YB-49 Advanced Control Systems • Artificial-feel system – Restores control forces to those of an "honest" airplane – "q-feel" modifies force gradient – Variation with trim stabilizer angle – Bobweight responds to gravity and to normal acceleration • Fly-by-wire/light system – Minimal mechanical runs – Command input and feedback signals drive servo actuators – Fully powered systems – Move from hydraulic to electric power 50 26 Next Time: Linearized Equations and Modes of Motion Reading: Flight Dynamics 234-242, 255-266, 274-297, 321- 325, 329-330 51 Develop linear equations to describe small perturbational motions Apply to aircraft dynamic equations Learning Objectives Supplemental Material 52 27 B-52 Control Compromises to Minimize Required Control Power • Limited-authority rudder, allowed by – Low maneuvering requirement – Reduced engine-out requirement (1 of 8 engines) – Crosswind landing gear • Limited-authority elevator, allowed by – Low maneuvering requirement – Movable stabilator for trim – Fuel pumping to shift center of mass • Small manually controlled "feeler" ailerons with spring tabs – Primary roll control from powered spoilers, minimizing wing twist 53 Rudder Snaking • Control-free dynamics – Nominally symmetric control position – Internal friction – Aerodynamic imbalance • Coupling of mechanical motion with Dutch roll mode Douglas DC-2 • Solutions – Trailing-edge bevel – Flat-sided surfaces – Fully powered controls 54 28 Roll/Spiral Limit Cycle Due to Aileron Imbalance • Unstable nonlinear oscillation grows until it reaches a steady state • This is called a limit cycle Lockheed P-38 55 Control Surface Buzz North American FJ-4 • At transonic speed, normal shocks may occur on control surface – With deflection, shocks move differentially – Possibility of self-sustained nonlinear oscillation (limit cycle) ARC R&M 3364 • Solutions – Splitter-plate rudder fixes shock location for small deflections – Blunt trailing edge – Fully powered controls with actuators at the surfaces 56 29 The Unpowered F4D Rudder • Rudder not a problem under normal flight conditions – Single-engine, delta-wing aircraft requiring small rudder inputs • Not a factor for upright spin – Rudder was ineffectual, shielded from flow by the large delta wing • However, in an inverted spin – rudder effectiveness was high – floating tendency deflected rudder in a pro-spin direction – 300 lb of pedal force to neutralize the rudder • Fortunately, the test aircraft had a spin chute 57 Powered Flight Control Systems • Early powered systems had a single powered channel, with mechanical backup – Pilot-initiated reversion to "conventional" manual controls – Flying qualities with manual control often unacceptable • Reversion typically could not be undone – Gearing change between control stick and control to produce acceptable pilot load – Flying qualities changed during a high- stress event • Hydraulic system failure was common – Redundancy was needed • Alternative to eject in military aircraft Douglas A4D Douglas A3D Boeing B-47 58 30
Classical Lateral Control Logic for a Fighter Aircraft (c.1970) MIL-DTL-9490E, Flight Control Systems - Design, Installation and Test of Piloted Aircraft, General Specification for, 22 April 2008 Superseded for new designs on same date by SAE-AS94900 http://www.sae.org/servlets/works/documentHome.do?comtID=TEAA6A3&docID=AS94900&inputPage=dOcDeTaIlS 59 Boeing 767 Elevator Control System Abzug & Larrabee, 2002 60 31 Boeing 777 Fly-By-Wire Control System 61 Control-Configured Vehicles • Command/stability augmentation • Lateral-directional response – Bank without turn – Turn without bank – Yaw without lateral translation – Lateral translation without yaw – Velocity-axis roll (i.e., bank) • Longitudinal response – Pitch without heave – Heave without pitch – Normal load factor – Pitch-command/attitude-hold – Flight path angle Princeton Variable-Response Research Aircraft 62 USAF F-15 IFCS USAF AFTI/F-16 32 Direct Lift and Propulsion Control 63 Direct-Lift Control-Approach Power Compensation • F-8 Crusader – Variable-incidence wing, better pilot visibility – Flight path control at low approach speeds • requires throttle use • could not be accomplished with pitch control alone – Engine response time is slow – Flight test of direct lift control (DLC), using ailerons as flaps • Approach power compensation for A-7 Corsair II and direct lift control studied using Princeton’s Variable- Response Research Aircraft Princeton VRA Vought A-7 Vought F-8 64 33 Direct-Lift/Drag Control • Direct-lift control on S-3A Viking – Implemented with spoilers – Rigged up during landing to allow
lift. • Speed brakes on T-45A Goshawk make up for slow spool-up time of jet engine – BAE Hawk's speed brake moved to sides for carrier landing – Idle speed increased from 55% to 78% to allow more effective modulation via speed brakes Lockheed S-3A Boeing T-45 65 United Flight 232, DC-10 Sioux City, IA, 1989 • Uncontained engine failure damaged all three flight control hydraulic systems (http://en.wikipedia.org/wiki/United_Airlines_Flight_232) 66 34 United Flight 232, DC-10 Sioux City, IA, 1989 • Pilot maneuvered on differential control of engines to make a runway approach • 101 people died • 185 survived 67 Propulsion Controlled Aircraft Proposed backup attitude control in event of flight control system failure Differential throttling of engines to produce control moments Requires feedback control for satisfactory flying qualities NASA MD-11 PCA Flight Test NASA F-15 PCA Flight Test Proposed retrofit to McDonnell-Douglas (Boeing) C-17 68