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 sufficient lift
with minimal drag
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 free-stream:
At stagnation point (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
Laminar boundary layer creates less 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
SV
lcl 2
2
1
SV
dcd 2
2
1
Center of PressureThe resultant aerodynamic force acts at the Center of
Pressure (c.p.), about which the moment is zero.
NACA Airfoils and Test Data
4-Digit Series
5-Digit Series
6 Series
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.
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
cm
V Scm
1
22
Pitching Moment Coefficient:
Typical Non-Cambered AirfoilLift Curve & Drag Polar
NACA 0006
Typical Cambered AirfoilNACA 2412
Lift Curve & Drag Polar
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 (1/deg.) 0.1 0.108
Moment Coefficient (before stall) 0 0.05
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