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