Week 3:  Lift and Drag Measurements

 

Abstract

     This exercise was designed to measure the lift and drag forces acting upon the "General Lee", a 1969 Dodge Charger.  A dynamometer was used to measure the forces acting on the car while it was placed in a wind tunnel and subjected to settings ranging from 10 to 26 Hz.  (~ 7-21 m/sec)  The results, shown below, show that the design of this vehicle has undesirable lift and drag effects acting upon it.  The lift coefficient remained relatively constant throughout the entire velocity range, but had a large uncertainty an lower velocities.  The Coefficient of Drag changed as the flow over the car changed from laminar to turbulent.

 

Testing Apparatus/Methods

     The apparatus used for measuring the forces on the model consisted of the wind tunnel, and a dynamometer.  The car was pre-mounted on the dynamometer upon arriving in the lab, and the equipment was pre-calibrated.  A schematic of the testing apparatus is shown below.  The wind tunnel was set to 10 Hz, and the flow allowed to stabilize.  Data were obtained using Microsoft Excel, software provided by the manufacturer of the equipment.  This software takes the necessary readings, and inputs them into the spreadsheet.  The wind tunnel was then increased in increments of 2 Hz, and the procedure repeated until a maximum setting of 26 Hz was reached.  The results were then taken and analyzed.

Figure #1:  Testing Apparatus

Results

Wind Tunnel Speed [Hz] Cd Cl Drag Uncertainty Lift Uncertainty
10 -0.07725 0.0142 0.0361 1.081
12 -0.08051 0.0292 0.0300 0.552
14 -0.08656 0.0297 0.0269 0.318
16 -0.08958 0.0294 0.0238 0.201
18 -0.09170 0.0294 0.0214 0.134
20 -0.10520 0.0292 0.0218 0.094
22 -0.11234 0.0288 0.0208 0.068
24 -0.12629 0.0293 0.0214 0.052
26 -0.12511 0.0292 0.0195 0.040

 

 

 

 

Discussion/Conclusion

     The results above show that the General Lee experiences unwanted lift and drag forces.  It can clearly be seen that there is virtually no change in the lift coefficient with an increase in velocity, although the uncertainty does decrease.  The drag coefficient drops significantly as the flow switches from laminar to turbulent.  This was the same phenomena discussed by Dr. Shapiro in the videos watched in class.  The drag forces will increase over an object until the flow becomes turbulent.  While the transition is occurring, the drag forces are unpredictable, until they again begin to increase.