A COMPUTER BASED AUTOROTATION TRAINER

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A COMPUTER BASED AUTOROTATION TRAINER

• Edward Bachelder, Ph.D.
• Bimal L. Aponso
• Dongchan Lee, Ph.D.
• Systems Technology, Inc.
• Hawthorne, CA
• Presented at
• 2005 International Helicopter Safety Symposium
• September 26-29, 2005, Montreal, Quebec, Canada

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OVERVIEW

• Motivation and concept
• Technical approach
• Testing and validation
• Example autorotations
• Computer Based Autorotation Trainer concept

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MOTIVATION

• For a safe outcome, helicopter autorotation
  requires precise and time-critical maneuvering in
  multiple axes.
• Consequences of inappropriate timing and
  magnitude of control inputs can be fatal.
• An autorotation trainer that could demonstrate
  proper control technique would be beneficial for
  pilot training and safety.
• An autorotation trainer should allow pilots to
  preview and rehearse autorotations from entry
  conditions throughout the flight envelope.

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AUTOROTATION SEQUENCE(Entry from Hover)
a.) Entry b.) Stabilization c.) Maximum Flare
d.) Touchdown.
a.
b.
c.
d.
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THE HUMAN ELEMENT

• Humans prefer to operate linear, decoupled
  systems to nonlinear, coupled systems
• Human improvisation to unfamiliar conditions is
  relatively easy
• Human response is
• More repeatable
• Less prone to operator noise

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THE HUMAN ELEMENT

• Helicopter dynamics during autorotation are
  highly nonlinear and coupled
• Nonlinear examples
• Lift vs rotor speed
• Lift vs airspeed
• Coupling examples
• Rotor speed and airspeed both affect lift
• Collective affects rotor speed, cyclic both
  airspeed and rotorspeed
• Scanning technique critical for coordinating
  proper controls sequence
• During glide Airspeed, Nr, ball, radalt
• In flare Nr, pitch, radalt

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AUTOROTATIONITS LIKE HERDING CATS
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TECHNICAL APPROACHTHE OPTIMAL PILOT CONCEPT

• Apply optimal control theory to compute optimal
  trajectories and control inputs required for safe
  autorotation or one-engine inoperative (OEI)
  situations the Optimal Pilot.
• The Optimal Pilot will demonstrate autorotation
  trajectories over a broad range of initial and
  final conditions and rotorcraft configurations.
• Visually integrate and display optimal inputs
  with the helicopters critical states and outside
  (OTW) view to provide a sight picture.
• Preview and practice autorotations in a flight
  simulator using a Flight Director type display to
  advise the pilot of the optimal control inputs.

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TECHNICAL APPROACHOPTIMIZATION METHOD

• Two-point boundary value problem minimize
  objective (cost) function.
• Transformation to parameter optimization problem
  using Direct-Collocation.
• Continuous solution discretized in time using
  nodes.
• Rotorcraft equations-of-motion and other
  non-linear constraints applied at each node.
• Parameter optimization problem was solved using a
  commercially available Sequential Quadratic
  Programming (SQP) algorithm -- SNOPT
• SNOPT is very well suited for near real-time
  generation of control commands, exhibiting stable
  and robust behavior for numerous entry conditions
  and roughly-estimated starting trajectories.

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TECHNICAL APPROACHPROBLEM FORMULATION

• Cost function includes
• Sink-rate and forward speed at touchdown
• Desired touchdown distance or flight time
  minimization (for OEI situation only)
• Weightings on penalty terms were tuned to provide
  robust solutions across a wide range of
  autorotation entry conditions.
• Longitudinal only, controls were collective and
  pitch attitude.
• Constraints
• Rotorcraft equations-of-motion (represented by
  non-linear point-mass model).
• Rotor speed overspeed and droop limits.
• Pitch and collective control limits.
• Maximum achievable sink rate.
• Maximum pitch rate
• Touchdown pitch attitude (to prevent tail strike)

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TECHNICAL APPROACHINTEGRATED DISPLAY FLIGHT
DIRECTOR
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TRAINING METHOD

• Compensatory tracking
• Compensatory tracking with feedforward cues
• Precognitive

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TESTING VALIDATIONREAL-TIME IMPLEMENTATION
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TESTING VALIDATIONFLIGHT TRAINING DEVICE

• Testing performed on a fixed-base FTD by Frasca
  International.
• Wide field-of-view visual display.
• High-fidelity cockpit controls and instrument
  panel.
• Simulated helicopter was a Bell-206L-4.
• Rotorcraft mathematical model with adequate
  fidelity for pilot training throughout the flight
  envelope including autorotation.
• FAA approved under 14 CFR Parts 61 and 141.

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TESTING VALIDATIONDEVELOPMENT PROCESS

• Point-mass model parameters were identified to
  match the flight simulation model during
  autorotation.
• Primarily scaling of pitch and collective from
  optimal solution to longitudinal cyclic and
  collective on the simulator.
• Validated using fully-coupled autorotations
• A flare law was added to take over from optimal
  guidance during final flare and landing.
• Simple lateral feedback control system was
  implemented to maintain heading and roll attitude.

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TESTING VALIDATIONEVALUATION METHOD

• Optimizer continuously updates optimal solution
  based on rotorcraft states obtained from
  simulator.
• Update is stopped when engine is failed.
• Procedure
• Fly to required entry condition.
• Stabilize and wait for a stable optimal solution.
• Fail engine and enter automated autorotation.
• Autorotation trajectory flown is based on the
  solution just prior to engine failure.
• Safe or crash landing determined by the FTD
  simulation model.

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TESTING VALIDATIONEVALUATED ENTRY CONDITIONS
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TESTING VALIDATIONFULLY-COUPLED
AUTOROTATIONS(400 Ft and 100 Ft Hover Entry)
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TESTING VALIDATIONCONCLUSIONS

• Optimal pilot concept was validated on the Frasca
  FTD.
• Optimal guidance allowed safe autorotation from
  well within the avoid regions of the
  Height-Velocity envelope.
• Ability to train a pilot on autorotation
  technique using the flight director display was
  also demonstrated (results presented at AHS Forum
  61, Grapevine, TX).
• Incorporate Optimal Pilot concept in a CBT to
  allow pilots to preview autorotations.

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COMPUTER BASED AUTOROTATION TRAINEREXAMPLE
AUTOROTATIONS

• Autorotations flown by the optimal pilot (optimal
  commands are coupled to rotorcraft controls).
• Show extreme entry conditions to illustrate the
  effectiveness of the concept.
• Time history data altitude (H, ft), airspeed (V,
  kts), pitch attitude (?, degrees), vertical
  velocity (w, fpm), rotor speed (?, ), collective
  (?c, ).
• Bell 206 Model Power failure at time 0.
• Video clips show OTW sight picture and optimal
  pitch attitude/collective commands.

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AUTOROTATION CBTCONTROL INPUT PREVIEW

• Example cueing display for an autorotation from a
  200ft hover entry
• Pitch attitude preview on right, collective on
  left.
• Tick marks show 1 second time intervals.
• Pitch attitude cue indicates immediate pitch down
  followed by a steep pitch up with a final
  nose-over to avoid tail strike.
• Collective cueing indicates immediate lowering of
  collective with collective pull at the end of the
  maneuver.

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EXAMPLE AUTOROTATIONSENTRY CONDITIONS

• H-V flight envelope shows avoid regions for
  Bell 206L-4.
• Example autorotations shown for
• Heavy weight (4500 lbs), 400 ft hover entry
  (within avoid region).
• Heavy weight (4500 lbs), 80 ft/60 kts entry (knee
  point of avoid region).
• Medium weight (3600 lbs), 200 ft hover entry
  (within avoid region).
• Medium weight (3600 lbs), 20 ft/40 kts entry
  (outside avoid region).

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EXAMPLE AUTOROTATION TIME HISTORY(HEAVY WEIGHT,
400 FT HOVER ENTRY)(Touchdown 18 kts, 248 fpm)
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EXAMPLE AUTOROTATION VIDEO(HEAVY WEIGHT, 400 FT
HOVER ENTRY)(Touchdown 18 kts, 248 fpm)
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EXAMPLE AUTOROTATION TIME HISTORY(HEAVY WEIGHT,
80 FT/60 KT ENTRY)(Touchdown 19 kts, 221 fpm)
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EXAMPLE AUTOROTATION VIDEO (HEAVY WEIGHT,
80FT/60KT ENTRY)(Touchdown 19 kts, 221 fpm)
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EXAMPLE AUTOROTATION TIME HISTORY(MEDIUM WEIGHT,
200 FT HOVER ENTRY)(Touchdown 20 kts, 369 fpm)
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EXAMPLE AUTOROTATION VIDEO (MEDIUM WEIGHT, 200
FT HOVER ENTRY)(Touchdown 20 kts, 369 fpm)
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EXAMPLE AUTOROTATION TIME HISTORY(MEDIUM WEIGHT,
20FT/40KT ENTRY)(Touchdown 20 kts, 211 fpm)
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EXAMPLE AUTOROTATION VIDEO (MEDIUM WEIGHT,
20FT/40KT ENTRY)(Touchdown 20 kts, 211 fpm)
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AUTOROTATION CBT CONCEPT

• Objectives
• Provide pilots with a preview of the control
  inputs and trajectory required for safe
  autorotation from entry conditions across the
  flight envelope.
• Provide pilots with an OTW sight picture of the
  autorotation.
• Allow pilots to rehearse autorotations in an
  interactive environment.
• CBT configuration
• Preset rotorcraft model parameters (for specific
  rotorcraft) or allow user to setup the rotorcraft
  model.
• User sets up entry flight condition (speed,
  altitude, weight, wind, etc).
• Allow user to adjust cost and constraint
  parameters (allowable rotor droop, for example)?
• CBT Output
• OTW scene with or without superimposed optimal
  trajectory information.
• Other external views to demonstrate trajectory
  and rotorcraft state information
• Time history information

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AUTOROTATION CBTNEXT STEPS

• Evaluate Industry interest and required
  functionality and features for
• PC based CBT (preview autorotations on the
  desktop).
• PC based flight simulation training aid (provide
  cueing during flight simulation).
• Refine optimal pilot algorithm
• Automatic point mass parameter estimation
• Winds
• Develop a graphical user interface.
• Validate further using high-fidelity moving-base
  flight simulator.