Measurement of Pressure Distribution
and Lift for an Airfoil
Purpose
Test
design
Measurement system and Procedures
Instrumentation
 Data reduction
 Data acquisition

Uncertainty Analysis
Purpose
 Examine
the surface pressure distribution on a Clark-Y airfoil
 Compute the lift and drag forces acting on the airfoil
 Specify the flow Reynolds number
 Compare the results with benchmark data
 Uncertainty analysis for
 Pressure
coefficient
 Lift coefficient
Test Design
Facility consists of:

Closed circuit vertical
wind tunnel.
 Airfoil
 Load cell
 Temperature sensor
 Automated data acquisition
system
Test Design (contd.)
Airfoil (=airplane surface: as wing) is placed in test
section of a wind tunnel with free-stream velocity
of 15 m/s. This airfoil is exposed to:
Forces acting normal to free stream = Lift

Forces acting parallel to free stream = Drag
Only two dimensional airfoils are considered:
Top of Airfoil:

The velocity of the flow is greater than the free-stream.

The pressure is negative
Underside of Airfoil:

Velocity of the flow is less than the the free-stream.

The pressure is positive
This pressure distribution contribute to the lift

Measurement systems
Software
- Surface
Pressure
- Velocity
- WT Control
Instrumentation







Protractor – angle of attack
Resistance temperature detectors
(RTD)
Pitot static probe – velocity
Scanning valve – scans pressure
ports
Pressure transducer (Validyne)
Digital Voltmeter (DVM)
Load cell – lift and drag force
Digital
i/o
PC
A/D
Boards
Serial
Comm.
(COM1)
Metrabyte
M2521
Signal
Conditioner
Scanivalve
Position
Circuit (SPC)
Scanivalve
Controller
(SC)
RTD
Pressure
Input
Digital
Voltimeter
(DVM)
Scanivalve
Pitot Tube
(Free
Stream)
Pressure
Transducer
(Validyne)
Scanivalve
Signal
Conditioner
(SSC)
Pressure Taps
Airfoil Model
Bundle of
tubes
AOA, and Pressure taps positions
Data reduction
In this experiment, the lift
force, L on the Airfoil will be
determined by integration of
the measured pressure
distribution over the Airfoil’s
surface. The figure shows a
typical pressure distribution
on an Airfoil and its
projection .
Data reduction
Cp 
Calculation of lift and drag forces



The lift force L is determined by integration of the measured
pressure distribution over the airfoil’s surface.
It is expressed in a dimensionless form by the pressure
coefficient Cp where, pi = surface pressure measured, = P
pressure in the free-stream
The lift force is also measured using the load cell and data
acquisition system directly.
U = free-stream velocity, r = air density ( temperature),
pstagnation = stagnation pressure measured at the tip of the
pitot tube, L = Lift force, b = airfoil span, c = airfoil chord
2 pstagnation  p 
U 
r
CL 
2L
rU 2 bc
L    p  p sin  ds
s
 p

2D
CD D, r , U  , b, c 
rU 2 bc
pi  p
1
rU 2
2
CL 
 p sin  ds
s
1
rU 2 c
2
Calibration of load cell
mass (kg)
Volts
0
-0.021
0.295
-0.1525
0.415
-0.203
0.765
-0.3565
1.31
-0.5935
1.635
-0.7385
Program output
Lift
Curve fitting method
Mass
Calibration program
-0.6
-0.4
-0.2
Volts
y = -3.9781x - 0.0792
R2 = 0.9992
2
Fzavg
1.5
1
0.5
Linear
0
(Fzavg)
-0.5 0
Data acquisition
Setting up the initial motor speed
Visualization of wind tunnel conditions
Data acquisition (contd.)
Data needed:
 Observation point list
 Sampling Rate
 Settling Time
 Length of each Sample
 Angle of attack
Airfoil pressure visualization
Calculation of lift force
Program to measure lift force in volts
Uncertainty analysis
Uncertainty analysis
Pressure coefficient
C p  f ( pi  p  , r , U  )
2
2
U Cp
 BCp
 PCp2
B   B  
i 1
p _ p  
i

2
i
2
i
2
( pi  p )
C p
  pi  p  
PCp  2S Cp
2
( pi  p )
B

Cl  f ( pi  p  ,  i , r , U  , c)
2
2
U CL
 BCL
 PCL2
j
2
Cp
Lift coefficient
2
rU 2
M
j
B  i2 Bi2   (2pi  p ) B(2pi  p )
2
CL
i 1
PCL  2SCL
M
Benchmark data
Benchmark data for pressure coefficient for AOA = 6
Benchmark data for pressure coefficient for AOA = 0
Benchmark data for lift coefficient
2
2
AOA = 6
0
0
20
40
60
80
100
-1
-2
-3
1.8
1
1.6
0
0
20
40
60
80
100
-1
-2
Lift coefficient (Cl)
1
Coeffcient of pressure (Cp)
Coeffcient of pressure (Cp)
AOA = 0
-3
1.4
1.2
1
0.8
0.6
0.4
0.2
0
-4
-4
0
x/c
x/c
5
10
15
20
25
30
35
Angle of attack (AOA)
Benchmark data for pressure coefficient for AOA = 13
2
2
AOA = 16
AOA = 13
1
0
0
20
40
60
-1
-2
-3
80
100
Coeffcient of pressure (Cp)
1
0
0
20
40
60
80
100
-1
Benchmark data for drag coefficient
-2
0.4
-3
0.35
-4
x/c
-4
x/c
a) Distribution of the pressure coefficients for
= 0, 6, 13, 16 and Re = 300,000; , Benchmark data
Drag coefficient (Cd)
Coeffcient of pressure (Cp)
Reference data for CL
Benchmark data for pressure coefficient for AOA = 16
0.3
0.25
0.2
0.15
0.1
0.05
0
0
5
10
15
20
25
Angle of attack (AOA)
Reference data for CD
30
35