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