Aerodynamics Seminar.ppt
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Transcript of Aerodynamics Seminar.ppt
DATE-8 FEB 13’
Aerodynamics
How do those things really fly?
ABHISHEK DANGIROLL NO.
1006447001
Airbus 380
An aerodynamics challenge
FA-18 Condensation Pattern
Aerodynamics involves multiple flow regimes
Legacy Aircraft
Aerodynamics is a maturing science
Outline
Terms and DefinitionsForces Acting on Airplane
LiftDragConcluding remarks
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 platform Wingspan
Force Diagram
Airfoil Definitions
Definition of Lift, Drag & Moment
L = 1/2 V2 CL SD = 1/2 V2 CD S
M = 1/2 V2 CM S c
A Misconception
A fluid element that splits at the leading edge and travels over and under the airfoil will meet at the trailing edge. The distance traveled over the top is greater than over the bottom.
It must therefore travel faster over the top to meet at the trailing edge.
According to Bernoulli’s equation, the pressure is lower on the top than on the bottom.
Hence, lift is produced.
How Lift is Produced
• Continuity equation
• Bernoulli’s equation
• Pressure differential
• Lift is produced
The Truth A fluid element moving over the top surface leaves the trailing edge long before the fluid element moving over the bottom surface reaches the trailing edge.
The two elements do not meet at the trailing edge.
This result has been validated both experimentally and computationally.
Airfoil Lift Curve (cl vs. )
Lift Curve - Cambered & Symmetric
Airfoils
Slow Flight and Steep Turns
L = 1/2 V2 CL SOutcome versus Action
Slow FlightLift equals weightVelocity is decreasedCL must increase must be increased on the lift curveVelocity can be reduced until CL max is reached
Beyond that, a stall results
Slow Flight and Steep TurnsL = 1/2 V2 CL S
Outcome versus Action(Concluded)
Steep Turns (“Bank, yank and crank”)Lift vector is rotated inward (“bank”) by the bank angle reducing the vertical component of lift
Lift equals weight divided by cosine Either V (“crank”), CL or both must be increased to replenish lift
To increase CL, increase (“yank”) on the lift curve
To increase V, give it some gasMore effective since lift is proportional to the velocity squared
Stalling Airfoil
Effect of Bank Angle on Stall Speed
L = 1/2 V2 CL S equals the bank angleAt stall CL equals CLmax
L = W / cos Thus
Vstall = [2 W / ( CL max S cos )] 1/2
Airplane thus stalls at a higher speedLoad factor increases in a bank
Thus as load factor increases, Vstall increasesThis is what’s taught in the “Pilot’s Handbook”
Surface Oil Flow - Grumman Yankee = 40, 110 , & 240
Drag of an AirfoilD = Df + Dp + Dw
D = total drag on airfoilDf = skin friction dragDp = pressure drag due to flow separationDw = wave drag (for transonic and supersonic flows)
Skin Friction Drag The flow at the surface of the airfoil adheres to the surface (“no-slip condition”)
A “boundary layer” is created-a thin viscous region near the airfoil surface
Friction of the air at the surface creates a shear 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]
The Boundary LayerTwo types of viscous flows
LaminarStreamlines are smooth and regularFluid element moves smoothly along streamlineProduces less drag
TurbulentStreamlines break upFluid element moves in a random, irregular and tortuous fashion
Produces more dragw laminar < w turbulent
Reynolds NumberRex = V∞ x / Ratio of inertia to viscous forces
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.
Infinite vs. Finite Wings
Finite Wings
The Origin of Downwash
The Origin of Induced Drag
Di = L sin i
Elliptical Lift Distribution
CD,I = CL2/ (e AR)
Change in Lift Curve Slope
for Finite Wings
Ground EffectOccurs during landing and takeoffGives a feeling of “floating” or “riding on a cushion of air” between wing and ground
In fact, there is no cushion of airIts effect is to increase the lift of the wing and reduce the induced drag
The ground diminishes the strength of the wing tip vortices and reduces the amount of downwash
The effective angle of attack is increased and lift increases
Ground Effect(Concluded)
Mathematically SpeakingL = 1/2 ∞ V∞
2 S CL
An increased angle of attack, increases CL
Hence L is increasedD = 1/2 ∞ V∞
2 S [CD,0 + CL2/( e AR)]
CD,0 is the zero lift drag (parasite) CL
2/( e AR) is the induced drage is the span efficiency factor = (16 h / b)2 / [1 + (16 h / b)2 ]b is the wingspanh is the height of the wing above the ground
Wing Dihedral ()Wings are bent upward through an angle , called the dihedral angle
Dihedral provides lateral stability, i.e., an airplane in a bank will return to its equilibrium position
This is a result of the lift on the higher wing being less than the lift on the lower wing providing a restoring rolling moment
Drag of a Finite Wing
D = Df + Dp + Dw + Di
D = total drag on wingDf = skin friction dragDp = pressure drag due to flow separationDw = wave drag (for transonic and supersonic flows)Di = Induced drag (drag due to lift)
Drag of a Wing(Continued)
Induced drag - drag due to lift
Parasite drag - drag due to non-lifting surfacesProfile drag
Skin frictionPressure drag (“Form drag”)
Interference drag (e.g., wing-fuselage, wing-pylon)
FlapsA Mechanism for High Lift
Effect of Flaps on Lift Curve
High Lift Devices
1. No flap2. Plain flap3. Split flap4. L. E. slat5. Single slotted flap6. Double-slotted flap7. Double-slotted flap
with slat8. Double-slotted flap
with slat and boundary layer suction
9. Not shown - Fowler flap
Shape ComparisonModern vs. Conventional
Airfoils
Maximum Lift Coefficient Comparison
Modern vs. Conventional Airfoils
What’s Next on the AgendaBoeing 787 Dreamliner
Boeing 787
What’s Next on the AgendaBoeing Blended Wing-Body Configuration
Boeing 797
Concluding RemarksWhat was not discussed
Transonic flowDrag-divergence Mach numberSupersonic flowWave dragSwept wingsCompressibility effectsBoundary layer theoryThe history of aerodynamics
Airbus 380 Interior
Good aerodynamics results in improved creature comforts
Winglets
Reduced induced dragEquivalent to extending wingspan 1/2 of winglet height
Less wing bending moment and less wing weight than extending wing
Hinders spanwise flow and pressure drop at the wing tip
Looks modern/esthetically pleasing Boeing 737 Winglet
HondaJet
HondaJetEngine
PositionThe “Sweet Spot”
Location where the engine coexists with the wing and enjoys favorable interference effects
The reason - “Transonic Area Rule”Richard Whitcomb - NASA ScientistThe total cross-sectional area must vary smoothly from the nose to tail to minimize the wave drag
Wave drag is created by shock waves that appear over the aircraft as a result of local regions of embedded supersonic flow
HondaJet Aerodynamics Engine inlet is positioned at 75% chord
As the cross-sectional area decreases at the trailing edge of the wing, the engine adds area thus yielding a smooth area variation
This engine position also slows the flow and decreases the wing-shock strength
The critical Mach number is thus increased from .70 to .73
The pylon is positioned near the outer portion of the nacelle and cambered inward to follow the flow direction
During stall, separation starts outboard of the pylon; separation does not occur between the pylon and fuselage
HondaJetAerodynamics
(Continued)
Natural laminar flow fuselage noseFollowing the area rule, the nose expands from 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
HondaJetAerodynamics (Concluded)
Natural laminar flow wingUtilizes integral, machined panels that minimizes the number of parts for smoother flow when mated together
Employs winglets to reduce induced drag30% more efficient than other business jets
Eagle in Flight
Winglets
Elastic Flaps
Minimized Noise & Detectability
VariableCamber
Retractable Landing Gear
STOL/VTOLCapabilities
Smart Structures
Tilting Control CenterSmooth
Fairings
VariableTwist
AdaptiveDihedral
Turbulator
Tail ?
b/2c
cd,i = cl2 /
π AR
cl = 2 L/ V2 S
Questions and Answers