Transcript Document

RWDC
Background Discussion
Harold Youngren
AeroCraft
129 Pitt St
Portland, ME
207-671-7350 cell 207-871-0552
[email protected]
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Background
 Aerospace engineer (MIT, Lockheed-Martin, consultant)
– Work on design of aircraft, wings, propellers, helicopters, CFD, aeroelastic analysis
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RWDC Background Talk
 Overview of design problem (how the pieces relate)
–
Aero
–
Structure
–
Aeroelastic
–
Optimization
 Wing aerodynamics
–
lift, drag as they affect wings
–
airfoils and compressible drag
–
vortex drag due to lift
–
drag reduction approaches for wing
 Structure
–
basic info on materials
–
beams and torsion boxes
–
wing structures, examples of likely wing structures
 Aeroelasticity what this is, static aeroelastics vs flutter
–
static deflection with load mass distribution
–
flutter
–
how to design a wing to eliminate flutter
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RWDC Key Design Issues
 Overall Problem
– Focus is transonic airliner wing with specified flight conditions and load
– Metric (objective function) for wing optimization driven by weight and drag
– Design for cruise metric and max loading condition at 3.75g
 Aerodynamics -> lift and drag
– Challenging operating point (Mach=0.7, CL~0.7) will involve transonic effects
– Lift must support the aircraft weight
– Design to reduce drag rise using combination of sweep and airfoil selection or thickness
 Structure -> weight
– Structure must be optimized for high load condition (avoid static divergence)
– Minimize weight for objective function metric
– Flutter free to max velocity (mostly involves control of stiffness and mass centers)
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RWDC - Resources
 Aircraft Design Resources
– Online material on aircraft design (free!)
– Aircraft Design, Synthesis and Analysis http://adg.stanford.edu/aa241/AircraftDesign.html
 Aerodynamics Design Resources
– Online aero design textbook (free!)
http://www.desktop.aero/appliedaero/preface/welcome.html
– Somewhat aircraft design oriented, but has technical focus
– Very good introduction for aerodynamics with lots of background, plots, examples
– Discussion of background issues for wing, airfoil design
 Structural Design Resources
– Limited simple resources
– Online beam calculation http://www.engapplets.vt.edu/statics/BeamView/BeamView.html
– Online structural mechanics material http://web.mst.edu/~mecmovie/
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RWDC Aero – Flight Conditions
Flight condition nomenclature
– Altitude - sets temperature (T), density (r), pressure (p),
speed of sound (a)
– Speed = V
– Mach number, M = V/a = ratio of speed to speed of sound
– Dynamic pressure, q = ½ * r * V2, is pressure of
oncoming “wind”
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RWDC Aero – Aircraft Nomenclature
Aircraft nomenclature
– Wingspan
– Leading edge (LE)
– Trailing edge (TE)
– LE sweep
– Root chord, tip chord
– ¼ chord sweep
– Dihedral
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RWDC Aero – Wing Nomenclature
Wing outer mold line (outer
shape) specified by:
– Span
– Chord (root, tip)
– Sweep
– Taper
– Dihedral
– Airfoils and twist
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RWDC Aero - Airfoils
Airfoil nomenclature
– Chord length
– Leading edge
– Trailing edge
– Thickness
– Camber
– Angle of attack
Airfoils come in
thousands of shapes
for special purposes
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RWDC Aero – Airfoil Forces
Airfoils refer to the 2D sections of a wing
Airfoil Force Nomenclature
– CL = lift coefficient
– CD = drag coefficient
– CM = pitching moment coefficient
– Angle of attack = a determines forces CL,CD,CM
– Characteristics are also a function of Mach number and
other factors to a smaller degree
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RWDC Aero – Airfoil Aerodynamics
Airfoil aerodynamics
– Lift (CL) is linearly proportional to
angle of attack (CL~2p*a) with lift
slope Cla=2p until stall
– Drag (CD) is low up until stall
– Moment (CM) about ¼ chord is
nearly constant
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RWDC Aero – Airfoil Aerodynamics
Airfoil drag comes from three
sources:
– Viscous drag CDv
– Pressure drag CDp
– Compressible drag CDc
– Viscous and pressure drag lumped
into CDo
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RWDC Aero - Airfoil Transonic Drag Rise
Airfoils develop strong shock waves with increasing
speed (NACA 0012 12% thick airfoil shown)
Mach 0.6
Mach 0.7
Shock Wave
Mach 0.8
Strong Shock
Wave
Higher drag
Pressure indicated by color
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RWDC Aero - Airfoil Compressible Drag
Airfoil drag increases rapidly beyond critical Mach
number, Mcrit~0.65 for this airfoil
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RWDC Aero - Wing Transonic Drag Rise
Optimized wing at
transonic speed
operates with
(mild) shock wave
Thicker airfoils
and/or higher lift
increase shock
strength and drag
Drag Rise
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RWDC Aero – Wing Aerodyamics
Wings characteristics
include lift, drag and
moment (about aircraft
CG)
Wing drag includes
airfoil drag across wing
– Airfoil section drag
Cdv+Cdp+Cdc
Induced drag Cdi
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RWDC Aero – Wing Aerodyamics
Induced drag is a
function of loading of
wing along span
– Loading goes to zero at tips
Optimal spanloading is
elliptical (minimum Cdi)
– Small changes from
elliptical loading possible
without excessive penalty
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RWDC Aero – Wing Spanwise Load Distribution
Wing loads are
modified by airfoil
incidence angles
Lower outboard loading and root
bending moment with tip wash-out
– Tip wash-out decreases
outboard loading
– Lower outboard loading
reduces high bending
moments at wing root
Wing angle adjusted for
constant total lift
Tip wash-in (higher incidence)
Tip wash-out (lower incidence)
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RWDC Structure – Example Wing Structure
RWDC rules specify wing
box for structural elements
(mid-chord region of wing)
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RWDC Structure – Wing Box
Wing box is key element of structural
design, also holds fuel mass
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RWDC Structure – Wing Box Layout
Wing box is made up of:
– Skin
– Spars (spar cap + web)
– Stringers
– Ribs
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RWDC Structure – Basic Loads
Loads are forces applied to
structure
Loads take three main
forms:
– Tension (pulling)
– Compression (pushing)
– Shear (sideways forces)
– Torsion (another shear force)
Materials are measured
and specified with respect
to these three loadings
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RWDC Structure – Aero Loads
Loads on wing consist
primarily of:
– Lift loads
– Torsion loads
– Inertial loads or gravity (weight)
Key focus in design is
controlling the spanwise
loading of the wing with
incidence angles of the airfoil
sections
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RWDC Structure – Beams
Beams resist bending
and shear forces
– Caps resist bending by
tension and compression
– Webs resist shear forces
from side loads and bending
– “I” beam shape comes from
large caps needed to resist
bending of long, slender
wings
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RWDC Structure – Elastic Axis
Beams have a natural axis
for bending without twist –
the elastic axis
– Forces applied at the elastic axis
will not twist the beam
Forces applied away from
the elastic axis will cause the
beam to twist
– Aerodynamic forces act at AC
– Inertial forces (due to
acceleration at at CG
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RWDC Structure – Material Properties
Materials have strength
properties specified by
stress (force/unit area)
and strain (relative
change in size or length)
– Modulus “springiness” of
material (how much it
moves with applied stress)
– Yield stress (when material
starts to fail)
– Ultimate stress (failure and
beyond)
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RWDC Structure – Materials
Materials list for RWDC (gives material properties)
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RWDC Structure – Material Properties
Materials for RWDC include knocked down properties for strength
– Aluminum (traditional aerospace material, also lists steel, titanium)
– Glass/epoxy <- small improvement from aluminum
– Carbon/epoxy <- best overall for weight and strength
– Kevlar/epoxy <- good in tension, poor in compression
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RWDC Structure – Composite Materials
Composites (carbon, glass,
kevlar)
– Fibers embedded as layers in
epoxy matrix
– Can be tailored by fiber orientation
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RWDC Structure – Finite Element Analysis
Structure is analyzed
with finite elements using
NASTRAN (or similar)
computer code
– Requires division of
structure into small “bricks”
or plates or rods
– Each element is assigned
properties (material,
thickness, etc)
– Computer solves for
stresses in elements so that
designer can check to
ensure that stresses do not
exceed material limits
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RWDC Optimization
Optimization for wing design seeks best solution for
weight and drag using objective function that blends
these to produce a “psuedo weight” number
OF = [145,360 + Wwing ] + 19 * q * S * [ 0.01819 + CDwing ]
Where q = 162.92 lb/ft^2, and S = 1,400 ft^2
Weight of wing strongly driven by sweep, airfoil thickness and
loading (airfoil incidence)
Drag of wing driven by sweep, thickness at specified Mach 0.7
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RWDC Optimization – Objective Function Space
Optimization by changing
design parameters to find
lower OF values in design
space
– Direction from gradient search or
steepest descent
– Design space may not be as simple
as this, it have local minima!!!
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RWDC Optimization - Wing Design Parameters
Key wing design
parameters
– Sweep (more sweep
reduces effective Mach
number)
– Airfoil thickness (thicker
wing is lighter)
– Structural stiffness for
bending and torsion
(tailors twist under load
to shift loading inboard
and reduce structural
weight)
RWDC rules specify:
Span
Area
Taper Ratio (tip chord/root chord)
Dihedral
Material properties
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RWDC Aeroelastics – Background
Aeroelastic effects are static (aerostatic) or dynamic (flutter)
Aerostatic deflections are due to airloads deflecting the
structure in bending and twist
– Changes to structural shape (particularly twist) will change airloads
– Predicting aerostatic deflections may require iteration of aero loads
analysis and structural analysis
Flutter is a dynamic instability where the airloads force
motions of the structure that grow with time
– Indicated by dynamic analysis of aero/mass/structural system (one or
more characteristic roots go unstable)
– Flutter is almost always destructive and is avoided by design
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RWDC Aeroelastics – Aerostatic Divergence
Example of aerostatics - NASA
HELIOS Solar Airplane
– Very flexible structure
NASA Helios Solar Airplane (2003)
Encounter with gust drastically
deflected wing upwards leading to
failure of flight control system (was
not designed to cope with highly
bent wing)
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RWDC Aeroelastics – Aerostatic Divergence
NASA HELIOS Solar Airplane
Gust caused tips to rise over 50 ft (increasing dihedral) leading
to loss of control and catastrophic overload of structure
Gust raised wingtips 50’ and twisted wing
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RWDC Aeroelastics – Flutter
Flutter of swept flying wing
– Unstable pitching and bending motions of wing at critical
speed
– Torsion no involved with this flutter (unusual)
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RWDC Aeroelastics – Flutter
Flutter is found by examining
behavior of dynamic system
– Root locus shows behavior of
system roots (characteristic
modes, such as wing 1st bending)
as velocity of aircraft is increased
– When complex root crosses real
axis then system becomes
unstable
– Modal analysis like this done by
NASTRAN using structural FEM
and mass distribution with
unsteady airloads from ZAERO
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RWDC Aeroelastics – Flutter
Wing flutter can be cured (delayed) by:
Increasing wing stiffness (adds weight)
– This could mean increasing bending stiffness and/or torsional
stiffness
Moving mass center closer to or forward of the elastic axis
Moving elastic axis of the wing closer to the mass centers
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RWDC Aero – Blank Slide
Blank slide
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