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• stick shaker, stick pusher
• individual actuator loop closure - Force Fight
• Instead of designing Band-Aids to make it possible for the
pilot to live with the vagaries in the system, the FBW system
should eliminate these vagaries (and Band-Aids)
Major FBW Design Issues
• Controllers - Column & Wheel versus Sidestick
• Feel system - Passive (e.g. spring) or Active (expensive !)
• Control augmentation - Algorithm response type
• simple or none - little or no HQ advantages
• stability/command – substantial benefits possible
• more complex/costly – many issues
• Handling Qualities: what HQ, how best achieved
• envelope protection - major safety benefit !
• Good design enhances pilot control authority
• mode changes takeoff/landing
• Actuators: loop closure, e.g. central or remote loop closure
• Redundancy architecture & component reliability
Handling Qualities
Definition: The conglomerate of characteristics and features
that facilitate the execution of a specific flight
control task; includes display and feel characteristics
• good HQ requires design attributes appropriate to control
task (e.g. pitch attitude, FPA, or altitude control)
• each task has a finite time allotment or expectation for
its completion (bandwidth requirement)
• direct control of “slow variables” requires special design
attributes (e.g.FPA response augmentation & display)
• control harmony is achieved when the pilot can execute
the task without undue stress and conscious effort
FBW Control
Response Attributes for Good HQ
Desired Attributes:
• “K/S”- like response
• low response lag τ
• correct sensitivity K
• good damping
• no overshoot
• control harmony with
other variables (θ, γ, nz)
• consistency between
flight conditions
Time
Response
Theta, FPA
Input δcol
Sensitivity (slope)
Lag τ
KS
NOTE: signal δcol can serve as the cmd reference KS
FBW Control Algorithm Types
• Pitch:
• pitch attitude rate command (+ pitch attitude hold)
• nz-command
• proportional angle of attack (AOA) command
• C*= nz command / Vertical Speed hold
• FPA rate command / FPA hold
• Roll: roll rate command / roll attitude hold
+ heading or track hold for bank angle < Xo
• Yaw: sideslip command proportional to pedal
Given sufficient know-how, all of these concepts can be
made to perform well : the devil is in the details!
Basic FBW System Example
Embraer RJ-170 / DO-728 concept
stick
δe
Air Data
Actuator Actuator
Electronics
Pos sensor
Modular
Avionics
Units
Passive Feel
Default
Gains
IRU
Airspeed Gain Sched
AOA limiting
default
Autopilot
servo
clutch
Autopilot cmds
Raytheon Low Cost GA FBW Concept
Bonanza Flight Demo System
throttle
servo
engine
δ e
Airpl
FCC ΔT
display
stick
Decoupled
Control
Algorithm
sensor
sensor
clutch
clutch
servo
Vcmd
FPAcmd
• Stick commands proportional FPA; Throttle commands speed
Trim
Down
Up
Rationale For Low End GA FBW
• Eliminate most low pilot skill related accidents
• stall, spin
• Loss of Control due to spatial disorientation
• Lack of IMC flight skills (inadvertent weather)
• Accept new FBW system related accidents, but lower overall rate
Approach:
• embedded envelope protection functions
• low cost FBW design strategy:
• simple control algorithm
• simple high reliability components
• dual sensor set, computer and data bus
• basic redundancy and FDIR strategies, e.g.
• single servo on split surfaces
C* Design Concept
KS
KI
S
act.
engine
Vc o
g
throttle
+_
nz pilot
q
++
stick
FF
shaping
δe
ΔT
Airplane
ACE
FCC
comp
C*
C*cmd
C* Morphed into FPA rate cmd/hold
• responses identical to original C*, if gains are equivalent
• fewer, simpler sensors
• no pilot-out-of-the-loop control reference drift
• still need extensive flight condition tuning
• missing: integral control of γ-error
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