Race Car Aerodynamics Part 2: Lift and Drag - Union College

Union College Week 8 Mer331 – Fluid Mechanics

Race Car Aerodynamics Part 2: Lift and Drag

The purpose of this week’s lab exercise is to measure the lift and drag forces acting on the radio
controlled vehicle that your group is studying. Drag is the force that acts opposite to the path of the
vehicle’s motion. Drag is detrimental to vehicle performance because it limits the top speed of a
vehicle and increases the fuel consumption, both of which are negative consequences for race
vehicles. Low drag vehicles usually have one or some combination of the following characteristics:
streamlined shape, low frontal area, and minimal openings in the bodywork for windows or cooling
ducts. The drag performance of vehicles is characterized by the drag coefficient (CD) which is
defined as:

CD = FD / ( ½ * ρ * V2 * AF) (1)

Where FD is the drag force, ρ is the air density, V is the free stream velocity, and AF is the frontal
area of the vehicle. This non-dimensional coefficient allows the drag performance between different
vehicles and different setups of the same vehicle to be compared directly.

Lift is the other of the two main aerodynamic forces imposed on a race vehicle, but unlike drag, lift
can be manipulated to enhance the performance of a racecar and decrease lap times. Lift is the force
that acts on a vehicle normal to the road surface that the vehicle rides on. As its definition implies,
lift usually has the effect of “pulling” the vehicle upwards - away from the surface it drives on.
However, by manipulating the racecar geometry it is possible to create negative lift, or down-force.
Down-force enhances vehicle performance by increasing the normal load on the tires. This increases
the potential cornering force which results in the ability of the vehicle to go around corners faster
and reduce lap times. The lift of the vehicle is characterized by the lift coefficient (CL) and is defined
as:

CL = FL / ( ½ * ρ * V2 * AT) (2)

Where FL is the lift force, AT is the area of the top surface of the vehicle (see Table 1), and the other
variables are as defined above. A negative lift coefficient means that a vehicle is experiencing down
force (Note: See last week’s lab handout for frontal and top area information on your vehicle).

Procedure

The race car body will be pre-mounted in the wind tunnel for you and the pressure transducer has
been pre-calibrated (use the same calibration that you used for the surface pressure measurements).
You will be provided with calibration data for the dynamometer.

REMEMBER TO RECORD ALL IMPORTANT INFORMATION IN YOUR LAB
NOTEBOOK!

1. Note any experimental observations about how the car is mounted in the wind tunnel. Note
the room temperature for your density calculation.

2. Make sure that the pressure transducer output is connected to channel 1, the dynamometer
lift (blue/black, blue wire) is connected to channel 2 and the dynamometer drag

Winter 2013 Professor Anderson

Union College Week 8 Mer331 – Fluid Mechanics

(green/black, white wire) is connected to channel 3 on the Daq connector box and that the
box is connected to the computer.
3. Check the connections on the wind tunnel pitot probe. The stagnation pressure (vertical tap)
should be connected to the “total” connection on the back of the pressure transducer and
the static pressure (horizontal tap) should be connected to the “static” tap on the back of the
pressure transducer and the pressure selector switch should be set to channel 0.
4. Start Excel and make sure the Daq software is running. Check that the data acquisition is set
to read channels 1, 2 and 3 (-10to +10V) and record 100 readings at a rate of about 6 hz.
5. With the wind tunnel off, start the data acquisition program and read the data on channels 1,
2 and 3. Since there is no flow all three should be close to zero. Confirm this before
proceeding. If the lift and drag values are not zero (or less than .05 V) you will need to re-
zero the lift and drag system using the two brass thumb wheels mounted directly on the
dynamometer to account for the weight of your car model. DO NOT adjust the span or
zero dials for the lift or drag on the wind tunnel instrumentation box. These are the dials
that are helpfully labeled “do not touch.”
6. Set the wind tunnel speed to 10 Hz and turn on the wind tunnel.
7. Acquire and save your data. Note: a negative Lift value implies downforce and a positive
Drag value implies Drag (in direction of flow).
8. Increment the wind tunnel speed by 4 Hz, and repeat step 7 until the wind tunnel speed is
54 hz. Be sure to allow the system stabilize for a minute or so after you change each wind
speed
9. Save your output file and move to another computer to perform your data reduction.

Data Reduction

a) Convert your pressure transducer voltages to Pressure (using calibration for transducer provided
last week) and calculate wind tunnel speed for each motor frequency setting. Check these
numbers against the data you acquired two weeks ago when you calibrated the wind tunnel.

b) Review the lift and drag calibration information (see Figure 1 and 2 below). Note: To acquire
this data we removed the dynamometer from the wind tunnel and mounted it on a calibration
test stand. We then hung calibrated weights in the range from 10 to 1000 g in lift and drag
configurations and recorded the voltage output of the dynamometer. Use the information
provided to convert your dynamometer voltage output to lift and drag forces. The uncertainty
estimate given in each figure is a combination of SLF and calibration accuracy. You will need to
add the effects of random variations in your measurements (using the stdev) in your uncertainty
analysis.

c) Calculate lift and drag coefficient at each velocity. Calculate the uncertainty in lift and drag
coefficient at each velocity and make a plot of CD and CL versus Re with error bars. Note: For
the Reynolds number you should use car length as your length scale.

Report

Prepare a memo report on the results (including the uncertainty estimates with sufficient DETAIL
in an appendix) of your lift and drag data. The intention of the memo is to relay your results to me.
Include a description of the experiment, your results and a short discussion. Include tables of data
as an attachment. Draft memo reports are due one week from the day that you performed the lab.
The final group report will be due Friday Mar 8.

Winter 2013 Professor Anderson

Union College Week 8 Mer331 – Fluid Mechanics

Figure 1. Dynamometer Drag Force Calibration (performed 2/22/13).

Figure 2. Dynamometer Lift Force Calibration (performed 2/22/13).

Winter 2013 Professor Anderson