Airfoil lift Measurement by

Wake Survey

10/11/2011

Calvin Lau

#34165140

Group 3

ABSTRACT:

Objective of the lab was to determine the drag of an airfoil through the means of a Hot

film Anemometer. By varying the angle of attack the air foil creates a fluctuation at the wake.

The anemometer would have a voltage change due to the variation of the cooling in the heated

element inside the wake.

EXPERIMENTAL APPARATUS AND PROCEDURE:

The procedure of the lab incorporates a low speed aerodynamic tunnel of an eighteen

inch square plexi-glass section that is eight feet long. The suction type motor was a 30HP

centrifugal blower powered by a drive belt. The airfoil needed for the lab was a NACA 0012,

which is attached to a beam and a device at which a student was able to easily change the angle

of attack by a labeled readout. An anemometer is positioned behind the air foil and is freely

adjustable in the vertical direction. This methodology would allow us to measure the velocity

fluctuations behind the wake under different attack angles.

Using the setup provided (also shown in figure below), while the wind tunnel is on and a

specific angle of attack is chosen, one should obtain the voltage output through the anemometer

by labview (setup is made by the teaching assistant). Through an oscilloscope which displays a

approximate line during free stream, we would need to change the distance of the anemometer at

which the probe passes through the edge of the wake (turbulent signal lines). Once data has been

recorded, repeat the steps with a different angle of attack. A calibration data would be given for

the anemometer at which a fourth order polynomial of the velocity can be determined from the

voltage. Another main equation to use is the Drag force:

D'=ρ∫b

h

u2 (u1−u2 )dy

Figure 1: 2-D view of the airfoil and wind tunnelRESULTS AND DISCUSSIONS:

Plotted below are the velocity profiles in the wake for each attack angle. The small

dipping variation is the velocity defect at which turbulence occurs behind the airfoil.

0 20 40 60 80 100 120 14017.6

17.8

18

18.2

18.4

18.6

18.8

19

19.2

19.4

19.6

Velocity vs Distance (y) of 0 attack angle

Distance (mm)

Velo

city

Plot 1: Velocity vs Anemometer distance at 0 attack angle

0 20 40 60 80 100 120 140 16017

17.5

18

18.5

19

19.5

20

Velocity vs Distance (y) of 3 attack angle

Distance (mm)

Velo

city

Plot 2: Velocity vs Anemometer distance at 3 attack angle

0 20 40 60 80 100 120 140 16016

16.5

17

17.5

18

18.5

19

19.5

20

Velocity vs Distance (y) of 9 attack angle

Distance (mm)

Velo

city

Plot 3: Velocity vs Anemometer distance at 9 attack angle

0 20 40 60 80 100 120 14015.5

16

16.5

17

17.5

18

18.5

19

19.5

20

Velocity vs Distance (y) of 10 attack angle

Distance (mm)

Velo

city

Plot 4: Velocity vs Anemometer distance at 10 attack angle

0 20 40 60 80 100 120 14015.5

16

16.5

17

17.5

18

18.5

19

19.5

20

Velocity vs Distance (y) of 11 attack angle

Distance (mm)

Velo

city

Plot 5: Velocity vs Anemometer distance at 11 attack angle

0 50 100 150 200 250 30015

15.5

16

16.5

17

17.5

18

18.5

19

19.5

20

Velocity vs Distance (y) of 12 attack angle

Distance (mm)

Velo

city

Plot 6: Velocity vs Anemometer distance at 12 attack angle

With these plots, we then would take the average of the velocities in each attack angle in order to

calculate the drag force and the drag coefficient.

Cd=D

ρ∞∗A∗V2

2

Angle of attack Drag Drag Coefficient0 17.84680348 0.0718368733 19.30862471 0.0785379969 20.24786362 0.082788929

10 22.17857761 0.0913699411 23.28736359 0.09650332312 30.02335688 0.12956815

Table 1: Drag forces and coefficients of different attack angles

0 2 4 6 8 10 12 140.06

0.07

0.08

0.09

0.1

0.11

0.12

0.13

0.14

Drag Coefficient vs Attack angle

Angle of attack

Drag

coeffi

cient

Plot 7: Drag Coefficient vs Attack angle

Here we find that the drag coefficient increases with an increasing attack angle. This is

actually true in its terms where an increasing attack angle creates more lift (without changing the

free stream velocity). In order to generate more lift, the airfoil must also generate more drag to

create and upward force to counteract the weight more. The velocity defects shown give us a

general idea of where the turbulence of the wake is located. By using these defects, we can

determine that the attack angle of twelve degrees is our stall angle or close to it. The reason for

such is that based on the graph, we find turbulence within the free stream (jumps at all distances)

and behind the wake of the airfoil. While increasing the distance of the anemometer, we find that

the velocity defect is extended a little longer compared to the rest and its location is further

down.

Below is a generated plot of Group 5 at which we are to compare our data to.

Attack angle Drag Drag Coefficient0 70.30130389 0.3864903883 70.89436204 0.3915686939 70.56564854 0.388747304

10 71.2346319 0.3945068711 71.62801593 0.39792627512 72.59292101 0.406418218

Table 2: Drag forces and coefficient of Group 5

0 2 4 6 8 10 12 140.375

0.38

0.385

0.39

0.395

0.4

0.405

0.41

Drag Coefficient vs Attack Angle (Group 5)

Attack angle

Drag

Coe

fficie

nt

Plot 8: Drag coefficient vs attack angle of group 5 data

Here we find that group 5 had a similar to better data distribution than we had. For

characteristics, we both find an increasing drag coefficient for an increasing attack angle. Even

though their drag forces are higher than ours, there is still an increasing drag force. Below is a

figure of the datasheet for the NACA 0012 airfoil. Using the plots generated form our velocity

profiles and the respective drag coefficient, we find that we have very similar characteristics

displayed, though our velocity may be slower in the wind tunnel compared to group 5.

0 20 40 60 80 100 120 140 160 18014

14.5

15

15.5

16

16.5

17

Velocity vs Distance of 12 attack angle (Group 5)

Distance

Velo

city

Plot 9: Velocity vs Distance of 12 attack angle from Group 5

Figure 2: NACA 0012 airfoil

Here we can also view that with an increasing attack angle; we have an increasing drag

coefficient on the left plot. In relation to our theory of increased drag coefficient, should result in

an increase in lift coefficient if free stream velocity is constant, the plot on the right clearly

shows such a case for and increasing attack angle.

Based on the right plot of the figure, we find that the stall angle is around sixteen degrees

of the attack angle. This shows us that near twelve degrees, at which turbulent flows were shown

in our velocity profile and group 5’s velocity profile, our acquired measurements are fairly

accurate results. To explain the difference in the numerical drag forces and drag coefficients, the

only explanations are a change in free stream velocity from the wind tunnel and calibration data

from the anemometer (possible degeneration of current in the system).

2960000 2980000 3000000 3020000 3040000 3060000 3080000 31000000.06

0.07

0.08

0.09

0.1

0.11

0.12

0.13

0.14

Cd vs Re

Reynolds Number

Cd

Plot 10: Drag coefficient vs Reynolds number

Shown is a plot of our drag coefficient vs Reynolds number which holds the concept of

sensitivity to Reynolds number. The reason to this is that the drag coefficient is a function of the

Reynolds number. By increasing the Reynolds number, one also increases the drag coefficient, in

relation to the equation:

Cd=Fd∨D12ρV 2 A

ℜ= ρVDμ

V=ℜ∗μρ∗D

CONCLUSIONS:

In conclusion, we find that the characteristics shown in the datasheet of NACA 0012

figures represent our data (and also Group 5’s data). The velocity profiles showcase very similar

velocity defects and turbulent locations in the wake until the airfoil reaches an attack angle of

twelve degrees. There were no deviations from theory. Our data proves that the drag forces

increases with increasing attack angles. In terms of equations, an airfoil generates lift in two

factors, increasing the velocity or increasing the attack angle. By doing so, our data also

represents a decreasing Reynolds number while the drag coefficient increases due to the equation

above. Discrepancies shown in our results are only the drag forces. This may be due to a

decreased free stream velocity at which the wind tunnel was possibly set for the lab. Even with

the discrepancy, it rarely affects our data’s characteristics which still represent the NACA

0012data sheet.

REFERENCES:

1) http://www.eng.buffalo.edu/Departments/mae/madnia/Teaching/mae424/Laboratory/labnotes2.pdf

2)Fundamentals of Aerodynamics, 5th edition, John D. Anderson, Jr.

3) http://www.eng.buffalo.edu/Departments/mae/madnia/Teaching/mae424/Laboratory/naca-

airfoils.pdf

APPENDIX: