aerodynamics seminar
TRANSCRIPT
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Aerodynamics 101
How do those things really fly?
Dr. Paul Kutler
Saturday, March 31, 2007
Monterey Airport
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Airbus 380
An aerodynamics challenge
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FA-18 Condensation Pattern
Aerodynamics involves multiple flow regimes
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Legacy Aircraft
Aerodynamics is a maturing science
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Outline
Terms and Definitions
Forces Acting on Airplane
Lift
DragConcluding remarks
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Terms and Nomenclature
Airfoil Angle of attack
Angle of incidence
Aspect Ratio
Boundary Layer
Camber
Chord
Mean camber line
Pressure coefficient
Leading edge
Relative wind Reynolds Number
Thickness
Trailing edge
Wing planform
Wingspan
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Force Diagram
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Airfoil Definitions
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Definition of Lift, Drag & Moment
L = 1/2 V2CLSD = 1/2 V2CDS
M = 1/2 V2CMS c
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A Misconception
A fluid element that splits at the leading edge andtravels over and under the airfoil will meet at the
trailing edge.The distance traveled over the top is greater than over thebottom.
It must therefore travel faster over the top to meet at thetrailing edge.
According to Bernoullis equation, the pressure is lower onthe top than on the bottom.
Hence, lift is produced.
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How Lift is Produced
Continuity equation
Bernoullis equation
Pressure differential
Lift is produced
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The Truth
A fluid element moving over the top surface leavesthe trailing edge long before the fluid elementmoving over the bottom surface reaches the
trailing edge.
The two elements do not meet at the trailing edge.
This result has been validated both experimentallyand computationally.
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Airfoil Lift Curve (clvs. )
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Lift Curve - Cambered &Symmetric Airfoils
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Slow Flight and Steep TurnsL = 1/2 V2CLS
Outcome versus Action
Slow Flight
Lift equals weightVelocity is decreased
CLmust increase
must be increased on the lift curve
Velocity can be reduced until CLmaxisreached
Beyond that, a stall results
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Slow Flight and Steep TurnsL = 1/2 V2CLS
Outcome versus Action(Concluded)
Steep Turns (Bank, yank and crank)
Lift vector is rotated inward (bank) by the bankangle reducing the vertical component of lift
Lift equals weight divided by cosine
Either V (crank), CLor both must be increased to
replenish liftTo increase CL, increase (yank)on the lift curve
To increase V, give it some gas
More effective since lift is proportional to the velocity
squared
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Stalling Airfoil
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Effect of Bank Angle on StallSpeed
L = 1/2 V2CLS
equals the bank angle
At stall CLequals CLmaxL = W / cos ThusV
stall= [2 W / (C
L maxS cos )] 1/2
Airplane thus stalls at a higher speed
Load factor increases in a bankThus as load factor increases, Vstallincreases
This is whats taught in the Pilots Handbook
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Effect of CG Location on StallSpeed
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Surface Oil Flow - Grumman Yankee= 40,110 , &240
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Airfoil Pressure Distribution
NACA 0012, M = 0.345, = 3.930
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Supercritical Airfoil &Pressure Distribution
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Drag of an Airfoil
D = Df+ Dp+ Dw
D = total drag on airfoilDf= skin friction drag
Dp= pressure drag due to
flow separationDw = wave drag (for transonic
and supersonic flows)
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Skin Friction Drag
The flow at the surface of the airfoil adheres tothe surface (no-slip condition)
A boundary layer is created-a thin viscousregion near the airfoil surface
Friction of the air at the surface creates ashear stress
The velocity profile in the boundary layer goes
from zero at the wall to 99% of the free-stream value
= (dV/dy)wall
is the dynamic viscosity of air [3.73 (10) -7
sl/f/s]
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The Boundary LayerTwo types of viscous flows
Laminar
Streamlines are smooth and regular
Fluid element moves smoothly along streamline
Produces less drag
TurbulentStreamlines break up
Fluid element moves in a random, irregular andtortuous fashion
Produces more dragw laminar< w turbulent
Reynolds Number
Rex= Vx /
Ratio of inertia to viscous forces
B d L Thi k
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Boundary Layer Thickness(Flat Plate)
Laminar Flow= 5 x / Rex
1/2
Turbulent Flow= 0.16 x / Rex
1/7
Turbulent Flow-Tripped B.L.= 0.37 x / Rex
1/5
Example: Chord = 5 f, V= 150 MPH, Sea
LevelRex= 6,962,025
= 0.114 inches Laminar B.L.
= 1.011 inches Turbulent B.L.
= 7.049 inches Tripped Turbulent B.L.
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Infinite vs. Finite Wings
AR = b2/ S
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Finite Wings
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The Origin of Downwash
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The Origin of Induced Drag
Di= L sin i
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Elliptical Lift Distribution
CD,I= CL2/ (e AR)
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Change in Lift Curve Slope
for Finite Wings
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Ground Effect
Occurs during landing and takeoff
Gives a feeling of floating or riding on acushion of air between wing and ground
In fact, there is no cushion of air
Its effect is to increase the lift of the wing andreduce the induced drag
The ground diminishes the strength of the wing
tip vortices and reduces the amount ofdownwash
The effective angle of attack is increased andlift increases
G o nd Effect
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Ground Effect(Concluded)
Mathematically SpeakingL= 1/2
V
2S CL
An increased angle of attack, increases CL
Hence L is increased
D= 1/2 V
2S [CD,0+ CL2/(e AR)]
CD,0is the zero lift drag (parasite)
CL2/(e AR)is the induced drag
e is the span efficiency factor = (16 h / b)2/ [1 + (16 h / b)2]
b is the wingspan
h is the height of the wing above the ground
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Wing Dihedral ()
Wings are bent upwardthrough an angle , calledthe dihedral angle
Dihedral provides lateralstability, i.e., an airplane ina bank will return to itsequilibrium position
This is a result of the lift onthe higher wing being lessthan the lift on the lowerwing providing a restoringrolling moment
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Drag of a Finite Wing
D = Df+ Dp+ Dw + Di
D = total drag on wing
Df= skin friction dragDp= pressure drag due to
flow separation
Dw = wave drag (for transonicand supersonic flows)
Di= Induced drag (drag due to
lift)
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Drag of a Wing
(Continued)
Induced drag - drag due tolift
Parasite drag - drag due tonon-lifting surfacesProfile drag
Skin frictionPressure drag (Form drag)
Interference drag (e.g., wing-fuselage, wing-pylon)
Flaps
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FlapsA Mechanism for High Lift
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Effect of Flaps on Lift Curve
High Lift Devices
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High Lift Devices
1. No flap2. Plain flap3. Split flap4. L. E. slat5. Single slotted flap
6. Double-slotted flap7. Double-slotted flap
with slat8. Double-slotted flap
with slat andboundary layersuction
9. Not shown - Fowlerflap
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Shape Comparison
Modern vs. Conventional Airfoils
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Maximum Lift Coefficient ComparisonModern vs. Conventional Airfoils
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Whats Next on the AgendaBoeing 787 Dreamliner
Boeing 787
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Whats Next on the Agenda
Boeing Blended Wing-Body Configuration
Boeing 797
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Concluding Remarks
What was not discussedTransonic flow
Drag-divergence Mach numberSupersonic flow
Wave drag
Swept wings
Compressibility effectsBoundary layer theory
The history of aerodynamics
b
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Airbus 380 Interior
Good aerodynamics results in improved creature comforts
Q i d
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Questions and Answers
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Backup Slides
Wi l t
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WingletsReduced induced drag
Equivalent to extendingwingspan 1/2 of wingletheight
Less wing bending momentand less wing weight thanextending wing
Hinders spanwise flow and
pressure drop at the wingtip
Looks modern/esthetically
pleasing
Boeing 737 Winglet
V t G t
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Vortex Generators
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Swept-Wing Principle
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Wave Drag
H d J t
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HondaJet
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HondaJet
Engine Position
The Sweet SpotLocation where the engine coexists with the wing
and enjoys favorable interference effects
The reason -Transonic Area RuleRichard Whitcomb - NASA Scientist
The total cross-sectional area must vary smoothlyfrom the nose to tail to minimize the wave drag
Wave drag is created by shock waves that appearover the aircraft as a result of local regions of
embedded supersonic flow
HondaJet
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HondaJetAerodynamics
Engine inlet is positioned at 75% chordAs the cross-sectional area decreases at the trailing
edge of the wing, the engine adds area thusyielding a smooth area variation
This engine position also slows the flow anddecreases the wing-shock strength
The critical Mach number is thus increased from.70 to .73
The pylon is positioned near the outer portion ofthe nacelle and cambered inward to follow the flowdirection
During stall, separation starts outboard of thepylon; separation does not occur between the
lon and fusela e
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HondaJet
Aerodynamics(Continued)
Natural laminar flow fuselage nose
Following the area rule, the nose expandsfrom its tip and then contracts as the
windshield emerges.As the wing is approached, the fuselage
cross-sectional area increases smoothly;
this helps maintain the laminar flow
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HondaJet
Aerodynamics(Concluded)
Natural laminar flow wing
Utilizes integral, machined panels that
minimizes the number of parts for smootherflow when mated together
Employs winglets to reduce induced drag
30% more efficient than other business jets
E l i Fli ht
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Eagle in Flight
Winglets
Elastic Flaps
Minimized Noise& Detectability
Variable
Camber
Retractable Landing Gear
STOL/VTOLCapabilities
Smart Structures
Tilting
Control
CenterSmooth
Fairings
Variable
Twist
Adaptive
Dihedral
Turbulator
Tail ?
b/2
c
cd,i= cl2/
AR
cl= 2 L/V2S