water tunnel airfoil analysis

AEROSPACE 305W AERODYNAMMICS LABORATORY

Water Channel Airfoil Analysis

Lab 2: Section 18

February 26, 2014 Performed in Room 44 Hammond Building

Peter B. Jackson

Lab Partners Names:

Paige Dumke and David Spadaro

Lab TA: Christy Lihn

Course Instructor: Richard Auhl

Abstract

In this experiment, a 3D airfoil SM701 wing was attached to a force balance in various

configurations and tested in the water channel. The chord length of the SM701 was 6 and had a

span of 18. The force balance was used to measure the aerodynamic forces and pitching

moments acting on the wing in the water tunnel. The water channel speed was calibrated as a

function of the dial setting, an approximate flow velocity was then determined for different dial

settings. The aerodynamic forces were determined by the voltage readings from the force balance

and corrected for the forces exerted on the mounting strut by the water flow. The increase in the

dial settings displayed a smaller velocity for the foam disk. The increased velocity showed lift

and drag forces to increase while the pitching moments to decrease. In lab results showed that

the best angle of attack for the SM701 was approximately 12.


I. Introduction

In this laboratory, the water channel airfoil analysis was introduced in three main phases.

The experiment included the calibration of the water channel balance, water channel velocity and

the aerodynamic properties of the SM701 airfoil.

The aerodynamics forces were determined using the force balance instrument acting on

the 3D wing model. In the water channel the force balance was used to measure the main

components of the lift and drag forces, and pitching moments. Equation (1) was used to calculate

the aerodynamic forces and moments.

= 1 1 12 2 2

3 3 3

1

1 1 2 23 3

(1)

The balance load cells were used to determine the change in output voltages (E). The

change in output voltages was determined by the balance loads cells, which were multiplied by

the inverse of the influence coefficient matrix (1) to obtain the corresponding aerodynamic

forces and pitching moments.

Secondly was the water channel flow speed as a function of the dial settings. This was

accomplished by using a round foam disk, which was carefully deposited in the center of the

contraction section, and the time it took the disk to travel the 7 foot distance between two marks

on the test section in the water tunnel was recorded.

The final part was to determine the aerodynamic properties of the SM701. The 3D wing

model had an aspect ratio of 3 calculated using Equation (1) and the measurements of chord

length of 6 and a span of 18.


= 2

(1)

Equation (2) was used to calculate the Reynolds number to be of approximately 62,009.4. = vc

= vc

(2)

The maximum lift force was calculated to be 2.289 lbs using the Equation (3)

= 12v2CL (3)

The maximum drag force of 0.4906lbs using Equation (4)

= 12v2CD (4)

The wing model was mounted on a strut and placed in the water channel at different angles of attack and the data was measured and record.

II. Experimental Procedure

The laboratory involved measuring the aerodynamic forces, moments and other

properties of a #D SM701 airfoil wing. To calibrate the force balance, the settings for the

instruments were determined by an initial loading with the strut alone and then with the airfoil.

The data was collected by applying known masses and measuring the resultant voltages in the

force balance load cells. Readings were taken for three components: the lift forces, drag forces,

and pitching moments. The angle of attack was measured from the bottom surface of the wing as

the center of the chord was used as the reference location. Corrections were made to account for

the +2.5 of error involved with this method. The experiment started at an angle of attack of

11.8 and finished at -25.9. The wing on the strut was carefully lowered into the water channel

about halfway. All three channels of the balance amplifier were zeroed by trimming the value to


near zero volts and the computer was used to record the velocity voltages from all the channels

of the balance. Figure 1 displays the water channel balance and its components that were used to

measure the aerodynamics forces and moments acting on the 3D wing model.

Figure 1: Three Component Water Channel Balance (All dimensions in inches)

The water channel speed was calibrated by the dial settings that were done at 1.0, 1.5,

2.0, 2.5, 3.0, 3.5, 4.0, 4.5 and 5.0 on the dial setting. And the times it took the round foam disk to

travel the 7 foot distance between two marks on the test section at different angles of attack were

recorded. The water speed was increased to the dial setting 2.8 and again the computer began to

record the voltages after the velocity had stabilized. After, the angle of attack was continually


adjusted two turns in the negative direction for approximately 40 turns. At every two turns the

force balance voltages recorded.

Once that process was finished the SM701 was dismounted and the voltages from the

balance were recorded from the strut alone. The voltages were recorded from the dial settings

that ranged from 1.0 to 4.0. Using the above recorded data, the lift, drag, and pitch were

calculated. Figure 2 displays the SM701 3D wing airfoil mounted on the force balance and

submerged in the water channel during testing.

Figure 2: The SM701 3D Wing mounted on the force balance

Figure 3, shows how the lab was setup in the water channel including the computer setup

and the arrows displaying where the start and end points of the distance the foam disk had to


travel in the test section. The water channel speed calibration was calibrated after the distance

traveled and time was recorded and a graph that represents the relationship displayed in Figure 4.

Figure 3: The Water Channel Setup Experiment

III. Results & Discussion

IV.

The Table below represents the calculated coefficients of matrix (A) and the inverse

matrix (A-1) during the pre-lab assignment, and their units are volts/lb.

Table 1: Coefficients of Matrix A and A-1

Coefficients of matrix A (volts/lbs) Coefficients of matrix A-1(volts/lbs) -1.0835 2.1805 -1.0542 0.1113 2.6265 0.0004 -0.0246 -2.3991 1.1471

-0.905 -0.008 -0.832 0.038 0.381 0.035 0.061 0.797 0.927

Start point End point

Computer setup


Table 2 represents the percent errors calculated during the simulated loading procedure.

Table 2: Percentage Errors of Stimulated Load

Figure 4 displays the velocity verses dial setting for the morning 2 lab period. The dial

settings were in increments of 0.5 and the velocity ranged from 0 to 2.5 with slope of

approximately 0.57.

Figure 4: Velocity vs. Dial Setting Graph

Figure 5 is a graph of the forces and moments vs. velocity for the strut only where the

blue is the lift, red is the drag, and the green is the pitching moment. . The graph displays the

relationship between the different channels and the velocity with the three respective equations.

y = 0.5708x - 0.0498

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

0 1 2 3 4 5

velo

city

(ft/s

)

dial setting

Percent Errors CH 1 CH 2 CH 3

6.7% 1.4% 21.0% 3.8% 1.0% 8.2%

1.5% -1.0% 1.5% 0.7% -0.5% 0.0%

1.0% -6.0% 0.3% 0.4% -2.1% 0.9%


Figure 5: Graph of Forces vs Dial Setting for the Strut Alone

Figure 6 is a graph CL vs. alpha for wing alone. This shows that the highest angle of

approximately 12o for a CLmax of approximately 1.8. In comparison the Clmax for lab 2 was

approximately 1.3 which is slightly less and the CL for lab 3.

y = 0.0001x6 - 0.0014x5 + 0.0071x4 - 0.0168x3 + 0.0174x2 - 0.0072x + 0.0022 R = 0.9992

y = -0.0009x6 + 0.0136x5 - 0.0755x4 + 0.1867x3 - 0.1847x2 + 0.0617x - 0.0021 R = 0.9923

y = -0.0006x6 + 0.0089x5 - 0.0493x4 + 0.1212x3 - 0.1147x2 + 0.0396x + 7E-05 R = 0.9994

-0.02

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

Forc

e (lb

s)

Dial Setting


Figure 6: A Graph of CL vs. Alpha

Figure 7 is a graph of CD vs. alpha for the wing alone showing that the best sink angle with this

airfoil for a CD of 0.0344 is was -7o corresponding to the least drag force.

Figure 7: A Graph of CL vs. Alpha

Figure 8, is a graph of CM vs. alpha showing that the pitching moment coefficient CM

does not vary with angle of attack, or at least it did not vary significantly over the operating

range of angle of attack of the airfoil.

-1.5

-1

-0.5

0

0.5

1

1.5

2

-30 -20 -10 0 10 20Cl

Alpha (deg)

-0.1

0

0.1

0.2

0.3

0.4

0.5

-30 -20 -10 0 10 20

Cd

Alpha (deg)


Figure 8: A Graph of CM vs. Alpha

Figure 9 is a graph of CL vs. CD is polar curve showing variations CL as a function of CD. The

CDmin is approximately 0.003 for this Wing model. CDmin for lab 2 was approximately 0.008

which higher than lab 3, meaning that SM701 airfoil has more lift to drag ratio than the S805

airfoil

Figure 9: A Graph of CL vs. CD

-0.04

-0.02

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

-30 -20 -10 0 10 20

Cm

Alpha (deg)

-0.06

-0.04

-0.02

0

0.02

0.04

0.06

0.08

0.1

-0.01 0 0.01 0.02 0.03 0.04

Cl

Cd


Figure 10 illustrates the variations of the coefficients lift-to-drag ratio versus angle of

attack. As it is noted, this graph has one maximum point where the value of the lift-to-drag ratio

is the highest at this point. The angle of attack corresponding to this point is an optimum angle

attack. This angle was approximately 6o. For lab 2 that angle was approximately 6 as well.

Figure 10: A Graph of CL/CD vs. Alpha

In this lab, hysteresis was observed when decreasing the angle of attack of the wing after

stall at around -25.9o, the angle of attack when the flow reattached was generally lower than the

angle of attack where the flow separated during the angle of attack increase.

-8

-6

-4

-2

0

2

4

6

8

-30 -20 -10 0 10 20Cl/

Cd

Alpha (deg)


V. Conclusion

This experiment illustrated the technique of using a force balance to measure

aerodynamic forces on the SM701 wing in the water channel. It was seen that although the force

balance is a beneficial tool in the testing process, it was important to adjust the data to construct

an accurate portrayal of the forces on the wing in flight. An adjustment made was for the

mounting strut. It was observed that the water flow exerted a large drag force on the strut, which

affected the readings on the wing.

The second objective displayed the water channel velocity calibration. In terms of a graph

this relationship should be a linear as expected showing a linear relationship between Velocities

and dial setting as displayed in Figure 4. Since the dial was increased it caused the water to flow

faster and the foam disk to move faster as well.

The aerodynamic properties of the SM701 were concluded that the best scenario for the

wing was at the angles of attack of around 12. From the data this concluded that the ideal CL

was around -0.02, the ideal CD was around 0.03 as in Figure 9 and the ideal CM was about -0.03

as Figure 8.

The facility for the most part was well laid out. I believe testing different airfoils could be

an area of improvement. Most errors that were observed were the due to human error and this

particular method; therefore, this method has room for improvement for example, streamlining

the strut to reduce drag that causes increased forces and pitching moments.

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