ME403 Chapter 22D Airfoil Aerodynamics

Lift is mainly provided by the wing with an airfoil

cross-section shape

Airfoil Geometry

An airfoil is the 2D cross-section shape of the wing,

which creates significant lift but minimal drag because of

this aerodynamic shape

Historical Airfoils

Historical Airfoils

Typical Streamlines

Angle of Attack

chord lineV

Pressure Distribution

99500

99550

99600

99650

99700

99750

99800

99850

99900

99950

100000

0 0.2 0.4 0.6 0.8 1

Chordwise Distance, x, m

Su

rfa

ce P

ress

ue

, P, N

/sq

m

Net Normal Force

Upper Surface Pressure

Lower Surface Pressure

n P P dxl

c

u ( )0

Pressure Coefficient Distribution

02

2

1

V

ppcp

2

2

1

V

ppcp

In the uniform free-stream:

At the stagnation point

(at which velocity V=0): 12

2

1

2

2

1

2

2

10

0

V

V

V

ppcp

Positive Cp means the pressure is higher than the free-stream (atmospheric) pressure, and negative Cp means suction relative to free-stream pressure. The maximum, which occurs at the stagnation point, is always 1.

Viscous Boundary Layer

Transition Separation

1 23

4

V Edge of boundary layer

Velocity profile creates skin friction (shear) drag on surface

Curve fit formula for turbulent boundary layer (Re > 500,000):

Flat Plate Skin Friction Drag Coefficient

Evolution of Airfoil Design

Delaying transition point from Laminar to Turbulent boundary layer reduces skin

friction drag

Boundary Layer Flow Separation

When flow separation occurs, there is also pressure drag.

100% Pressure Drag

Pressure (Form) Drag due to Flow Separation

Total Profile Drag= Skin Friction Drag

+ Form Drag

Resultant Aerodynamic Force

Airfoil

Total Aerodynamic Force(Sum of Pressure and Shear)

Lift

Drag

V

Lift & Drag Coefficients

Chord Line

normal forcelift

V

drag

chordwise force

cV

l

cbV

Lcl 2

2

12

2

1

cV

d

cbV

Dcd 2

2

12

2

1

Center of PressureThe resultant aerodynamic force acts at the Center of

Pressure (c.p.), about which the moment is zero.

Open-Circuit Wind Tunnel

Wind Tunnel Tests

Force transducer behind model senses lift, drag and pitching moment directly.Motor-controlled mechanism adjusts the model’s angle of attack.

Closed-Circuit Wind Tunnel

Wing Section Models

Model for measuring lift, drag and pitching moment

Model for measuring surface pressure distribution

There is a maximum Lift-to-Drag ratio (L/D).

Location of Center of Pressure (c.p.) varies

with

NACA 0006 Dataat Re = 3,180,000

NACA 2312 Data at Re = 3,120,000

Lift decreases and drag increases sharply beyond the stall (max. Cl) point, due to boundary layer separation.

NACA Airfoils and Test Data

4-Digit Series

5-Digit Series

6 Series

http://naca.larc.nasa.gov/reports/1945/naca-report-824/

Stalled Airfoil

Reynolds Number Effect

Since the c.p. varies with , it is more desirable to use a fixed Aerodynamic Center (a.c.) as the point of action of the lift and drag. The pitching moment about this point can be calculated, and is found insensitive to . For most

airfoils, the a.c. locates at around quarter chord (x=c/4).

Aerodynamic Center

222

1 cV

mcm

Pitching Moment Coefficient:

Typical Non-Cambered AirfoilLift Curve & Drag Polar

NACA 0006

Typical Cambered AirfoilNACA 2412

Lift Curve & Drag Polar

Typical Airfoil Aerodynamic Characteristicsat Re = 6 million

NACA 0006 NACA 2412

Zero-Lift Angle of Attack (deg.) 0 -2

Stall Angle of Attack (deg.) 9 16

Maximum Lift Coefficient 0.9 1.7

Lift Curve Slope (/deg.) 0.1 0.108

Moment Coefficient (before stall) 0 -0.05 to -0.02

Minimum Drag Coefficient 0.005 0.006

Max. Lift-to-Drag Ratio (L/D) 0.7/0.0076 = 92.1 1.0/0.0088 = 113

Computation Fluid Dynamics Simulation

CFD Simulation: Near stall

CFD Simulation: Fully Stalled

Airfoil Generator at http://www.ae.su.oz.au/aero/info/index.html

Airfoil Analysis Code at http://www.ae.su.oz.au/aero/info/index.html