Aircraft Flight Dynamics Robert F. Stengel Lecture16 Aircraft Control Devices and Systems

Aircraft Control Devices and Systems 
 Robert Stengel, Aircraft Flight Dynamics, MAE 331, 2012" Copyright 2012 by Robert Stengel. All rights reserved. For educational use only.! http://www.princeton.edu/~stengel/MAE331.html ! http://www.princeton.edu/~stengel/FlightDynamics.html ! •  Control surfaces" •  Control mechanisms" •  Flight control systems" Design for Control" •  Elevator/stabilator: pitch control" •  Rudder: yaw control" •  Ailerons: roll control" •  Trailing-edge flaps: low-angle lift control" •  Leading-edge flaps/slats: High-angle lift control" •  Spoilers: Roll, lift, and drag control" •  Thrust: speed/altitude control" Critical Issues for Control" •  Effect of control surface deflections on aircraft motions" –  Generation of control forces and rigid-body moments on the aircraft" –  Rigid-body dynamics of the aircraft"   δ E is an input for longitudinal motion"  θ =  Mechanical, Power-Boosted System" Grumman A-6! McDonnell Douglas F-15! Critical Issues for Control" •  Command and control of the control surfaces" –  Displacements, forces, and hinge moments of the control mechanisms" –  Dynamics of control linkages included in model"   δ E is a state for mechanical dynamics" δ  E =  Control Surface Dynamics and Aerodynamics Aerodynamic and Mechanical Moments on Control Surfaces" •  Increasing size and speed of aircraft leads to increased hinge moments" •  This leads to need for mechanical or aerodynamic reduction of hinge moments" •  Need for aerodynamically balanced surfaces" •  Elevator hinge moment" H elevator = C H elevator 1 2 ρ V 2 Sc Aerodynamic and Mechanical Moments on Control Surfaces" C H surface = C H  δ  δ + C H δ δ + C H α α + C H command + C H  δ : aerodynamic/mechanical damping moment C H δ : aerodynamic/mechanical spring moment C H α : floating tendency C H command : pilot or autopilot input •  Hinge-moment coefficient, C H " –  Linear model of dynamic effects" 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" Floating and Restoring Moments on a Control Surface" •  Positive elevator deflection produces a negative (restoring) moment, H δ , 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 torques, elevator floats up due to negative H δ " Dynamic Model of a Control Surface Mechanism"  δ − H  δ  δ − H δ δ = H α α + H command + mechanism dynamics = external forcing •  Approximate control dynamics by a 2 nd - order LTI system" •  Bring all torques and inertias to right side"  δ E = H elevator I elevator = C H elevator 1 2 ρ V 2 Sc I elevator = C H  δ E  δ E + C H δ E δ E + C H α α + C H command + $ % & ' 1 2 ρ V 2 Sc I elevator ≡ H  δ E  δ E + H δ E δ E + H α α + H command + Dynamic Model of a Control Surface Mechanism" I elevator = effective inertia of surface, linkages, etc. H  δ E = ∂ H elevator I elevator ( ) ∂  δ ; H δ E = ∂ H elevator I elevator ( ) ∂δ H α = ∂ H elevator I elevator ( ) ∂α •  Stability and control derivatives of the control mechanism" Coupling of System Model and Control Mechanism Dynamics " •  2 nd -order model of control-deflection dynamics" –  Command input from cockpit" –  Forcing by aerodynamic effects" •  Control surface deflection" •  Aircraft angle of attack and angular rates" •  Short period approximation" •  Coupling with mechanism dynamics" Δ  x SP = F SP Δx SP + G SP Δu SP = F SP Δx SP + F δ E SP Δx δ E Δ  q Δ  α $ % & & ' ( ) ) ≈ M q M α 1 − L α V N $ % & & & ' ( ) ) ) Δq Δ α $ % & & ' ( ) ) + M δ E 0 − L δ E V N 0 $ % & & & ' ( ) ) ) Δ δ E Δ  δ E $ % & ' ( ) Δ  x δ E = F δ E Δx δ E + G δ E Δu δ E + F SP δ E Δx SP Δ  δ E Δ  δ E # $ % % & ' ( ( ≈ 0 1 H δ E H  δ E # $ % % & ' ( ( Δ δ E Δ  δ E # $ % & ' ( + 0 −H δ E # $ % % & ' ( ( Δ δ E command + 0 0 H q H α # $ % % & ' ( ( Δq Δ α # $ % % & ' ( ( Short Period Model Augmented by Control Mechanism Dynamics " •  Augmented dynamic equation" •  Augmented stability and control matrices" F SP/ δ E = F SP F δ E SP F SP δ E F δ E " # $ $ % & ' ' = M q M α M δ E 0 1 − L α V N − L δ E V N 0 0 0 0 1 H q H α H δ E H  δ E " # $ $ $ $ $ $ % & ' ' ' ' ' ' Δx SP ' = Δq Δ α Δ δ E Δ  δ E $ % & & & & & ' ( ) ) ) ) ) Δ  x SP / δ E = F SP / δ E Δx SP / δ E + G SP / δ E Δ δ E command State Vector! G SP / δ E = 0 0 0 H δ E " # $ $ $ $ % & ' ' ' ' Roots of the Augmented Short Period Model " •  Characteristic equation for short-period/elevator dynamics" Δ SP/ δ E s ( ) = sI n − F SP/ δ E = s − M q ( ) −M α −M δ E 0 −1 s + L α V N ( ) L δ E V N 0 0 0 s −1 −H q −H α −H δ E s − H  δ E ( ) = 0 Δ SP / δ E s ( ) = s 2 + 2 ζ SP ω n SP s + ω n SP 2 ( ) s 2 + 2 ζ δ E ω n δ E s + ω n δ E 2 ( ) Short Period" Control Mechanism" Roots of the Augmented Short Period Model " •  Coupling of the modes depends on design parameters" M δ E , L δ E V N , H q , and H α •  Desirable for mechanical natural frequency > short-period natural frequency" •  Coupling dynamics can be evaluated by root locus analysis" Horn Balance" C H ≈ C H α α + C H δ E δ E + C H pilot input •  Stick-free case" –  Control surface free to float " C H ≈ C H α α + C H δ E δ E •  Normally " C H α < 0 : reduces short-period stability C H δ E < 0 : required for mechanical stability NACA TR-927, 1948! 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" € C H α 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" C H ≈ C H α α + C H δ δ + C H pilot input NACA TR-927, 1948! All-Moving Control Surfaces" •  Particularly effective at supersonic speed (Boeing Bomarc wing tips, North American X-15 horizontal and vertical tails, Grumman F-14 horizontal tail)" •  SB.4s aero-isoclinic wing" •  Sometimes used for trim only (e.g., Lockheed L-1011 horizontal tail)" •  Hinge moment variations with flight condition" Shorts SB.4! Boeing ! Bomarc! North American X-15! Grumman F-14! Lockheed L-1011! Control Surface Types Elevator" •  Horizontal tail and elevator in wing wake at selected angles of attack" •  Effectiveness of low mounting is unaffected by wing wake at high angle of attack" •  Effectiveness of high-mounted elevator is unaffected by wing wake at low to moderate angle of attack" Ailerons" •  When one aileron goes up, the other goes down" –  Average hinge moment affects stick force" Compensating Ailerons" •  Frise aileron" –  Asymmetric contour, with hinge line at or below lower aerodynamic surface" –  Reduces hinge moment" •  Cross-coupling effects can be adverse or favorable, e.g. yaw rate with roll" –  Up travel of one > down travel of other to control yaw effect" Abzug & Larrabee, 2002! Spoilers" •  Spoiler reduces lift, increases drag" –  Speed control" •  Differential spoilers" –  Roll control " –  Avoid twist produced by outboard ailerons on long, slender wings" –  free trailing edge for larger high-lift flaps" •  Plug-slot spoiler on P-61 Black Widow: low control force" •  Hinged flap has high hinge moment" North American P-61! Abzug & Larrabee, 2002! Elevons" •  Combined pitch and roll control using symmetric and asymmetric surface deflection" •  Principally used on" –  Delta-wing configurations" –  Swing-wing aircraft" Grumman F-14! General Dynamics F-106! Canards" •  Pitch control" –  Ahead of wing downwash" –  High angle of attack effectiveness" –  Desirable flying qualities effect (TBD)" Dassault Rafale! SAAB Gripen! Yaw Control of Tailless Configurations" •  Typically unstable in pitch and yaw" •  Dependent on flight control system for stability" •  Split ailerons or differential drag flaps produce yawing moment" McDonnell Douglas X-36! Northrop Grumman B-2! Rudder" •  Rudder provides yaw control" –  Turn coordination" –  Countering adverse yaw" –  Crosswind correction" –  Countering yaw due to engine loss" •  Strong rolling effect, particularly at high α " •  Only control surface whose nominal aerodynamic angle is zero" •  Possible nonlinear effect at low deflection angle" •  Insensitivity at high supersonic speed" –  Wedge shape, all-moving surface on North American X-15" Martin B-57! Bell X-2! Rudder Has Mechanical As Well as Aerodynamic Effects " !  American Airlines 587 takeoff behind Japan Air 47, Nov. 12, 2001" !  Excessive periodic commands to rudder caused vertical tail failure" Japan B-747!American A-300! http://www.usatoday.com/story/travel/flights/2012/11/19/airbus-rudder/1707421/! NTSB Simulation of American Flight 587 " !  Flight simulation derived from digital flight data recorder (DFDR) tape" Control Mechanization  Effects Control Mechanization Effects" •  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! Nonlinear Control Mechanism Effects" •  Friction" •  Deadzone" Control Mechanization Effects" •  Breakout force" •  Force threshold" 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" Internally Balanced Control Surface" !  B-52 application" !  Control-surface fin with flexible seal moves within an internal cavity in the main surface" !  Differential pressures reduce control hinge moment" C H ≈ C H α α + C H δ δ + C H pilot input Boeing B-52! B-52 Rudder Control Linkages" 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" –  May involve 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" [...]... T-45! Next Time: Flight Testing for Stability and Control Reading Flight Dynamics, 419-428 Aircraft Stability and Control, Ch 3 Virtual Textbook, Part 17 Trailing-Edge Bevel Balance " Supplementary! Material! •  Bevel has strong effect on aerodynamic hinge moments" •  See discussion in Abzug and Larrabee! C H ≈ C Hα α + C Hδ δ + C H pilot input Control Tabs " Control Flap Carryover Effect on Lift Produced... Fortunately, the test aircraft had a spin chute" 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 Powered Flight Control Systems " •  Early powered systems had a single... load factor" Pitch-command/attitude-hold" Flight path angle" Princeton Variable-Response Research Aircraft! USAF AFTI/F-16! 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" Propulsion Controlled Aircraft " •  •  •  Proposed backup attitude control in event of flight control system failure"... path control at low approach speeds " •  requires throttle use " •  could not be accomplished with pitch control alone " Vought A-7! –  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 VariableResponse Research Aircraft" Princeton VRA! Direct-Lift/Drag Control. .. "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 highstress event" •  Hydraulic system failure was common" •  Alternative to eject in military aircraft" –  Redundancy was needed" Advanced Control Systems "... Model)* Flight Path Angle! Pitch Rate! Pitch Angle! Angle of Attack! * p 524, Flight Dynamics" Root Locus Analysis of Angular " Feedback to Thrust (4th-Order Model) Flight Path Angle! Pitch Angle! Pitch Rate! Angle of Attack! Direct-Lift Control- Approach Power Compensation " •  F-8 Crusader " Direct Lift and Propulsion Control Vought F-8! –  Variable-incidence wing, better pilot visibility" –  Flight. .. 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" B-47! –  Minimal mechanical runs" –  Command input and feedback signals drive servo actuators" –  Fully powered systems" –  Move from hydraulic to electric power" United Flight 232,... 2002! 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" " Boeing 777 Fly-By-Wire Control System "... hydraulic to electric power" United Flight 232, DC-10
 Sioux City, IA, 1989 " Control- Configured Vehicles " •  •  Command/stability augmentation" Lateral-directional response" –  –  –  –  –  •  USAF F-15 IFCS! •  Uncontained engine failure damaged all three flight control hydraulic systems (http://en.wikipedia.org/wiki/United_Airlines _Flight_ 232)" Bank without turn" Turn without bank" Yaw without lateral translation"... geared tabs" –  Tab is linked to the main surface in opposition to control motion, reducing the hinge moment with little change in control effect" from Schlichting & Truckenbrodt! •  Flying tabs" –  Pilot's controls affect only the tab, whose hinge moment moves the control surface" •  Linked tabs" –  divide pilot's input between tab and main surface" •  Spring tabs " C Lδ E C Lα vs cf xf + cf –  put . only.! http://www.princeton.edu/~stengel/MAE331.html ! http://www.princeton.edu/~stengel/FlightDynamics.html ! •  Control surfaces" •  Control mechanisms" •  Flight control systems& quot; Design for Control& quot; •  Elevator/stabilator: pitch control& quot; • . F-15! Critical Issues for Control& quot; •  Command and control of the control surfaces" –  Displacements, forces, and hinge moments of the control mechanisms" –  Dynamics of control linkages. for mechanical dynamics& quot; δ  E =  Control Surface Dynamics and Aerodynamics Aerodynamic and Mechanical Moments on Control Surfaces" •  Increasing size and speed of aircraft leads