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# Aerodynamics 101 How do those things really fly? - PowerPoint PPT Presentation

Aerodynamics 101 How do those things really fly?. Dr. Paul Kutler Saturday, March 31, 2007 Monterey Airport. Airbus 380. An aerodynamics challenge. FA-18 Condensation Pattern. Aerodynamics involves multiple flow regimes. Legacy Aircraft. Aerodynamics is a maturing science. Outline.

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### Aerodynamics 101How do those things really fly?

Dr. Paul Kutler

Saturday, March 31, 2007

Monterey Airport

An aerodynamics challenge

Aerodynamics involves multiple flow regimes

Aerodynamics is a maturing science

• Terms and Definitions

• Forces Acting on Airplane

• Lift

• Drag

• Concluding remarks

• Airfoil

• Angle of attack

• Angle of incidence

• Aspect Ratio

• Boundary Layer

• Camber

• Chord

• Mean camber line

• Pressure coefficient

• Relative wind

• Reynolds Number

• Thickness

• Trailing edge

• Wing planform

• Wingspan

L = 1/2  V2 CL S

D = 1/2  V2 CD S

M = 1/2  V2 CM S c

• 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.

• Continuity equation

• Bernoulli’s equation

• Pressure differential

• Lift is produced

• 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.

Slow Flight and Steep Turns L = 1/2  V2 CL SOutcome versus Action

• Slow Flight

• Lift equals weight

• Velocity is decreased

• CL must increase

•  must be increased on the lift curve

• Velocity can be reduced until CLmax is reached

• Beyond that, a stall results

Slow Flight and Steep TurnsL = 1/2  V2 CL SOutcome 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 gas

• More effective since lift is proportional to the velocity squared

• L = 1/2  V2 CL S

•  equals the bank angle

• At stall CL equals CLmax

• L = W / cos 

• Thus

• Vstall = [2 W / ( CL max S cos )] 1/2

• Airplane thus stalls at a higher speed

• Load factor increases in a bank

• Thus as load factor increases, Vstall increases

• This is what’s taught in the “Pilot’s Handbook”

NACA 0012, M ∞ = 0.345,  = 3.930

D = Df+ Dp + Dw

D = total drag on airfoil

Df = skin friction drag

Dp = pressure drag due to

flow separation

Dw = wave drag (for transonic

and supersonic flows)

• 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]

• Two types of viscous flows

• Laminar

• Streamlines are smooth and regular

• Fluid element moves smoothly along streamline

• Produces less drag

• Turbulent

• Streamlines break up

• Fluid element moves in a random, irregular and tortuous fashion

• Produces more drag

• w laminar < w turbulent

• Reynolds Number

• Rex =  V∞ x / 

• Ratio of inertia to viscous forces

Boundary Layer Thickness(Flat Plate)

• Laminar Flow

•  = 5 x / Rex1/2

• Turbulent Flow

•  = 0.16 x / Rex1/7

• Turbulent Flow-Tripped B.L.

•  = 0.37 x / Rex1/5

• Example: Chord = 5 f, V∞ = 150 MPH, Sea Level

• Rex = 6,962,025

•  = 0.114 inches Laminar B.L.

•  = 1.011 inches Turbulent B.L.

•  = 7.049 inches Tripped Turbulent B.L.

AR = b2 / S

Di = L sin i

CD,I = CL2/ (e AR)

• Occurs during landing and takeoff

• Gives a feeling of “floating” or “riding on a cushion of air” between wing and ground

• In fact, there is no cushion of air

• Its 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 Speaking

• L = 1/2  ∞V∞2S CL

• An increased angle of attack, increases CL

• Hence L is increased

• D = 1/2 ∞ V∞2 S [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

• 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

D = Df+ Dp + Dw + Di

D = total drag on wing

Df = skin friction drag

Dp = pressure drag due to

flow separation

Dw = 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 surfaces

• Profile drag

• Skin friction

• Pressure drag (“Form drag”)

• Interference drag (e.g., wing-fuselage, wing-pylon)

FlapsA Mechanism for High Lift

• No flap

• Plain flap

• Split flap

• L. E. slat

• Single slotted flap

• Double-slotted flap

• Double-slotted flap with slat

• Double-slotted flap with slat and boundary layer suction

• Not shown - Fowler flap

Shape ComparisonModern vs. Conventional Airfoils

Maximum Lift Coefficient ComparisonModern vs. Conventional Airfoils

• Boeing 787 Dreamliner

Boeing 787

• Boeing Blended Wing-Body Configuration

Boeing 797

• What was not discussed

• Transonic flow

• Drag-divergence Mach number

• Supersonic flow

• Wave drag

• Swept wings

• Compressibility effects

• Boundary layer theory

• The history of aerodynamics

Good aerodynamics results in improved creature comforts

• Reduced induced drag

• Equivalent 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

HondaJetEngine Position

• The “Sweet Spot”

• Location where the engine coexists with the wing and enjoys favorable interference effects

• The reason - “Transonic Area Rule”

• Richard Whitcomb - NASA Scientist

• The 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

HondaJetAerodynamics

• 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 nose

• Following 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 wing

• Utilizes integral, machined panels that minimizes the number of parts for smoother flow when mated together

• Employs winglets to reduce induced drag

• 30% more efficient than other business jets

cl = 2 L/

 V2 S

Turbulator

STOL/VTOL

Capabilities

Winglets

Variable

Twist

Smart Structures

b/2

c

Dihedral

Tail ?

Variable

Camber

Elastic Flaps

Tilting Control Center

cd,i = cl2 /

π AR

Minimized Noise

& Detectability

Smooth

Fairings

Retractable Landing Gear