Modifying an RC Car: Optimizing Down and Drag Forces

Modifying an RC Car: Optimizing Down and Drag Forces
Final Project Report
ME 241 – Fluids Lab
Professor Gans
April 27th, 2011
Group #6
Matthew Vallone
Aleandro Carisch
Matthew Guisbond
Abstract
The objective of this project is to address the lack of aerodynamic design in RC
cars capable of operating at high speed. We tested and evaluated the effects that a body,
an underbody, a rear wing, and a rear diffuser have on the drag and down forces on an
RC car. The goal was to attain a relatively high down force to drag force ratio, but also
have a down force value that compressed the suspension system between 50% and 70%
at a maximum speed of 65 mph. We compared two body types, chose the body that
resulted in less drag force at maximum speed, and added each additional component and
the combinations to improve the down force to drag force ratio. We arrived at the result
that the optimum ratio occurred when a wheel-encasing body, a rear wing, and a diffuser
led to a down force to drag force ratio of 1.16 and 41.3% compression of the suspension.
Introduction
In recent years, developments have led to RC cars with advanced power
technology, giving them the ability to operate at very high speeds. Unfortunately, these
advancements have not been matched by aerodynamic improvement. This has led to
unstable RC cars with a high potential to perform uncontrollably1. As a response, this
project purposes to increase the stability of modern RC cars by finding a way to increase
the down force exerted on them. In addition, we will optimize the down force to drag
force ratio through experimentation. We will experiment with a modified body design, an
underbody, a rear diffuser, and a rear wing, and test these modifications in the wind
tunnel.
There are many reasons why RC cars are capable of operating at high speeds,
despite weighing approximately 2 pounds2. Advancements in motor technology have led
to new brushless motors that allow RC cars to operate at over 60 mph3. Also, lithiumpolymer batteries provide voltages as high as 3.7 volts per cell and deliver charge at rates
close to 100 amperes, all in very small packages4.
The manner in which we will deal with this high power is by implementing
components that add down force. Down force is important because it presses the car to
the surface on which it drives, increasing its stability at higher speeds5. The down force
equals the normal force, which causes the friction force between the tires and the ground
to increase. This increased friction causes the RC car to track better while traveling along
the road without increasing the car’s weight, making the car more stable. Unfortunately,
any design that adds down force also adds drag force. Drag force is an unwanted force
1
that acts in the direction opposite of motion6. Therefore, a design that adds a large
amount of down force relative to the drag force it adds is most desirable.
Procedure
The first step in addressing this issue was to define what the optimum down force
to drag force ratio will mean. We defined the optimum ratio as the highest ratio that also
has a down force that keeps the suspension system between 50% and 70% compression at
maximum speed. This will stabilize the car by lowering its center of gravity, but will also
allow for unaffected performance when the surface with which the car is in contact is not
perfectly smooth.
Because the compression of the suspension system is important in the
optimization of the RC car stability, we took data on the force required to compress the
suspension a certain amount. We first measured the linearity of the front and rear springs
by detaching them from the car and, using a scale and a level, compressed each spring
from 0 cm to 3 cm in 0.5 cm increments, and recorded the force required at each point.
After this test, we re-attached the springs and measured the force required to compress
both the front and rear of the car to 100% compression, using the same scale and level.
From these values and the linear nature of the springs, we were able to determine the
force required to compress the entire suspension system between 50% and 70%.
With the force-to-compression data collected, we moved on to preparation for
wind tunnel testing. We first focused our efforts on designing an attachment for the RC
car to the load cell. The first step in doing so was to prepare the RC car. We removed the
roll cage, wheelie bar, transmission, and front and rear drive shafts to make the
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attachment possible. We then attached a 3/8” threaded rod to the metal skid plate on the
underside of the RC car through a system of nuts and lock washers. At this point, we
machined a 1”×1”×1.25” aluminum block to have four holes that matched the locations
of the set screws on the load cell, as well as a 3/8-16 threaded hole in the center of the
face that is orthogonal to the holes for the set screws, shown in Figure 1. Finally, we
assembled the attachment and mounted it into the wind tunnel, as seen in Figure 2.
At this point, we began wind tunnel testing of the drag forces and down forces
exerted on the RC car. In the first step, the control was created by testing the RC car in its
stock form, without a body or any components attached as shown in Figure 3. We
introduced the car to air speeds ranging from 0 mph to 65 mph, and measured the drag
forces and down forces by attaching one load cell to the RC car along the center of its
length, as shown in Figure 4. We attached the three-axis load cell fixture to an existing
metal skid plate on the underside of the RC car. The attachment was robust enough to
measure both drag and down forces along two different axes, and also helped estimate the
location of the down force through the moment axis. The down force, as shown in Figure
5, did not always act along the axis of the attachment rod to the load cell, and, in this
case, caused a measurement by the moment axis. The drag force also inherently caused a
moment to be measured, but this was accounted for by knowing the magnitude of the
drag force and the distance of the attachment rod from the RC car’s center of gravity to
the load cell. Therefore, by knowing the magnitude of the down force and the corrected
moment magnitude, we estimated the location of the down force along the longitudinal
axis of the body.
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The next step in the process was to modify the stock design, which is bodiless, to
help stabilize the car. The first modification was the addition of a body. We decided that
we would construct two bodies: one that was designed to cover the frame of the RC car
only, and one that was designed to cover the frame and the wheels of the RC car. We
made these bodies out of modeling foam with a fiberglass coating. Originally, we
attempted to glue the foam sheets together, but realized that the glue dried too quickly.
Therefore, we rethought our idea, purchased spray adhesive and foam glue, and glued the
foam together. We also purchased an electric carving knife, which helped when we began
to shape the foam. The design of the bodies roughly modeled an aerodynamic automobile
that performs at high speeds. With both designs available, we determined the drag and
down forces exerted on each and chose the design with less drag force to continue testing.
After we chose the more effective body design, we added an underbody, a rear
wing and a rear diffuser, as well as the possible combinations of these to test the drag and
down forces of each. The underbody modeled the design in Figure 6, and was made of a
thin sheet of acrylic plastic. This underbody created down force by increasing the air
speed along the bottom of the car and, therefore, reducing the pressure7. In a stationaryground wind tunnel, a boundary layer can build up on the underside of the car that may
interfere with the boundary layer of the lower components of the car8. To correct for this
effect, we slightly elevated the RC car in the wind tunnel to simulate an elevated ground
plane that more closely approximates the velocity profile of a car traveling on the ground,
as shown in Figure 7. Using the Blasius solution for the boundary layer over a flat plate,
we found that the nominal boundary layer thickness in the wind tunnel at 65 mph was
approximately 2 mm. This value is less than the elevated height of the bottom of the RC
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car in the wind tunnel. The rear wing was a semi-symmetrical airfoil with constant chord
length, and the rear diffuser reduced the air turbulence exiting the underbody, and
therefore the drag force, in the rear of the car9.
To construct the bodies and the rear wing, we used 5 sheets of 24”×18”×2”
modeling foam, available through McMaster-Carr at $26.14 per sheet. We strengthened
the foam with fiberglass from Tower Hobbies, which cost $49.94. The underbody
required a 12”×12”×0.1” acrylic sheet that cost $3.86 and foam glue at a price of $28.37,
available through McMaster-Carr and Tower Hobbies. With the additional purchase of
Elmer’s spray adhesive for $4.99, and an electric carving knife for $9.99 through Black
and Decker, our total project cost was brought to $227.85, as shown in Table 1.
We were able to test each component and examine their combinations with
specific design elements and a detailed experimental process. We arrived at a solution
that best stabilized the RC car despite its high speed and power capabilities by
implementing components that added down force without substantial drag force.
Results
As a first step toward completing the project, we verified the linearity of the
springs in the suspension system of the RC car. Using the method outlined in the
procedure, we found data for both the front and rear springs, shown in Figures 8 and 9,
respectively. From these results, we found it clear that the suspension system is linear.
This allowed us to find that, for the front of the car, 1200 grams were required to fully
compress the suspension, while 1550 grams were needed to compress the rear
suspension. Because these springs are in parallel, we knew that 2750 grams would be
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needed to compress the car to its maximum allowable point. This meant that an optimum
design would produce between 1375 grams and 1925 grams, or 13.5 N and 19 N, of
down force at the center of gravity of the RC car..
With this data organized, we originally compared the control data to the data from
each of the bodies. As Figure 10 shows, the chassis-and-wheel-encasing body, known as
the full body, provided more down force on the RC car for the majority of the testing
range than did the partial body that covered only the chassis. Both body designs resulted
in less down force at maximum speed than did the control test, but this value could be
improved with the addition of the rear wing, underbody, and diffuser. Figure 11 shows
that the drag force increased as air speed increased, and that both of the bodies yielded
higher drag force values than did the RC car as a chassis. These initial results provided us
with down force to drag force ratios of 0.73, 0.40, and 0.27 for the control, partial body,
and full body, respectively.
We predicted that the additional components would help increase down force, so
we chose to continue testing with the body type that resulted in less drag force.
Therefore, we chose the full body, as its lower drag force values would result in a higher
ratio with the added components. Once the full body was chosen as the more effective
design, we gathered results from the addition of components to the RC car. Figure 12
shows that four of the configurations resulted in more down force at maximum speed
than did the control:
•
Full Body + Rear Wing
Full Body + Rear Wing + Underbody
•
•
Full Body + Rear Wing + Diffuser
6
•
Full Body + Rear Wing + Underbody + Diffuser
While the control test yielded 5.8 N of down force at maximum speed, the highest value
shown for the Full Body + Rear Wing + Diffuser configuration was 11.1 N.
Figure 13 shows that these four configurations, along with the full-body-only test,
also resulted in higher drag force at maximum speed than did the control test. In the
control case, the maximum drag force was 8.0 N, while the maximum values of these
configurations ranged from 9.3 N to 10.6 N. The remaining tests yielded approximately
the same drag force as the control test, varying by only 0.1 N.
In a comparison of the ratio of down force to drag force, Figure 14 shows that the
above mentioned configurations resulted in higher ratios that the control test ratio of 0.73.
The highest of these values resulted from the Full Body + Rear Wing + Diffuser
configuration with a ratio of 1.16. This indicates that, of the available configuration, the
optimum design is the Full Body + Rear Wing + Diffuser with a maximum down force
value of 11.1 N and a down force to drag force ratio of 1.16.
Discussion
As the results in Figure 10 show, the down force increased as air speed increased
despite the addition of a body. This is due to the fact that, at low wind speeds, the added
weight of the bodies increased the down force reading on the load cell. This trend
occurred until the wind speed increased to the point where the bodies began to display
tendencies of an airfoil. This caused a lifting phenomenon on the RC car, therefore
reducing the down force reading by the load cell.
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The results also show that, of the added components, the rear wing had the most
dramatic effect on down force. In each configuration that included the rear wing, the
down force was higher than all other configurations without the rear wing. Figure 12
illustrates this point, also showing that the configurations with the underbody and diffuser
resulted in less down force at maximum speed than the control test.
The magnitude of the down force and drag force on the RC car was inherently
important in increasing the stability of the car, but the location of the down force along
the longitudinal axis of the car also played an influential role. As Figure 15 shows, it was
a general trend that the down force was located closer to the center of gravity of the RC
car when the magnitude of the down force was higher. The exception to this was in the
control case, where the down force was nominally less than 1 cm from the center of
gravity axis despite the relatively low down force. Next to the control, the down force in
the Full Body + Rear Wing + Diffuser arrangement was closest to the axis of center of
gravity, with a nominal distance also less than 1 cm.
Although none of the configurations achieved 50%-70% compression of the
suspension system at maximum speed, the Full Body + Rear Wing + Diffuser design
resulted in 11.1 N of down force at an air speed of 65 mph, compressing the suspension
system 41.3%. The down force was most closely located at the center of gravity among
the component configurations, and it resulted in the highest down force to drag force ratio
with a value of 1.16. These factors make the Full Body + Rear Wing + Diffuser the
optimum available configuration.
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References
1. Beeby, Dave. 15 February 2011.
<.http://fastestrc.blogspot.com/2007/08/aerodynamics-for-high-speed-rccars.html>.
2. Losi Company. 15 February 2011.
<http://www.losi.com/Products/Features.aspx?ProdID=LOSB0217LE>.
3. Castle Creations, Inc. 15 February 2011.
<http://www.castlecreations.com/support/documents/Mamba%20Setups.pdf>.
4. Thunder Power RC. 15 February 2011.
<http://thunderpowerrc.com/PDF/ThunderPowerRC-Pricing.pdf#page=4>.
5. Yager, Bryan. NAS Organization. 15 February 2011.
<http://www.nas.nasa.gov/About/Education/Racecar/physics.html>.
6. Yager, Bryan. NAS Organization. 15 February 2011.
<http://www.nas.nasa.gov/About/Education/Racecar/glossary.html>.
7. Beeby, Dave. 15 February 2011.
<http://fastestrc.blogspot.com/2007/08/aerodynamics-for-high-speed-rccars.html>.
8. HEV Team. San Diego State University. 15 February 2011.
<http://engineering.sdsu.edu/~hev/aerodyn.html>.
9. APR Performance, Inc. 15 February 2011.
<http://www.aprperformance.com/index.php?option=com_content&task=view&i
d=69&Itemid=47>.
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Figures
Figure 1 Aluminum block, 1” ×1” ×1.25”, with four holes in location of set
screws on load cell, and 3/8-16 threaded rod.
Figure 2 RC car in the wind tunnel with attachment to load cell.
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Figure 3 Aerial view of RC car.
Figure 4 Schematic of load cell attachment.
11
Figure 5 Free body diagram of forces on RC car that the load cell will measure.
The red arrow indicates down force, the blue arrow indicates drag force, and the
green arrow shows the moment about the axis perpendicular to the page.
Figure 6 Design for underbody of RC car. Dark section will be acrylic sheet, and
lighter sections will be foam reinforced with fiberglass.
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Velocity Profile in Wind Tunnel
Velocity Profile on Ground
Modified Velocity Profile in Wind Tunnel
Figure 7 Schematic showing difference in velocity profile in a wind tunnel and
along the ground. An elevated ground plane minimizes this difference.
Mass (g)
Front Spring Mass vs. Displacement
1100
1000
900
800
700
600
500
400
300
200
100
0
y = 280.51x - 9.9571
R2 = 0.9993
0
0.5
1
1.5
2
2.5
Displacement (cm)
Figure 8 Plot of Mass vs. Displacement for the front spring. The spring constant
for this spring is approximately 280 g/cm.
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3
Mass (g)
Back Spring Mass vs. Displacement
1100
1000
900
800
700
600
500
400
300
200
100
0
y = 337.36x - 13.121
R2 = 0.9993
0
0.5
1
1.5
2
2.5
3
Displacement (cm)
Figure 9 Plot of Mass vs. Displacement for the rear spring. The spring constant for
this spring is approximately 340 g/cm.
Plot of Down Force vs. Speed
Partial Body Data
Full Body Data
Control Data
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Down Force (N)
12
10
8
6
4
2
0
0
10
20
30
40
50
60
Air Speed (mph)
Figure 10 Plot of Down Force vs. Air Speed for control data, full body data, and
partial body data ranging from 0 mph to 65 mph.
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70
Plot of Drag Force vs. Speed
Partial Body Data
Full Body Data
Control Data
10.00
9.00
8.00
Drag Force (N)
7.00
6.00
5.00
4.00
3.00
2.00
1.00
0.00
0
10
20
30
40
50
60
70
Air Speed (mph)
Figure 11 Plot of Drag Force vs. Air Speed for control data, full body data, and
partial body data ranging from 0 mph to 65 mph.
Plot of Down Force vs. Air Speed
18
16
Down Force (N)
14
12
10
8
6
Control
Full Body
Full Body + Rear Wing
Full Body + Underbody
Full Body + Diffuser
Full Body + Rear Wing + Underbody
Full Body + Rear Wing + Diffuser
Full Body + Underbody + Diffuser
Full Body + Rear Wing + Underbody + Diffuser
4
2
0
0
10
20
30
40
50
60
70
Air Speed (mph)
Figure 12 Plot of Down Force vs. Air Speed for control data, full body data, and
all possible component combination data ranging from 0 mph to 65 mph.
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Plot of Drag Force vs. Air Speed
12
Control
Full Body
Full Body + Rear Wing
Full Body + Underbody
Full Body + Diffuser
Full Body + Rear Wing + Underbody
Full Body + Rear Wing + Diffuser
Full Body + Underbody + Diffuser
Full Body + Rear Wing + Underbody + Diffuser
10
Drag Force (N)
8
6
4
2
0
0
10
20
30
40
Air Speed (mph)
50
60
70
Figure 13 Plot of Drag Force vs. Air Speed for control data, full body data, and all
possible component combination data ranging from 0 mph to 65 mph.
Plot of Down Force to Drag Force Ratio
1.40
1.20
Control
Full Body
Full Body + Rear Wing
Full Body + Underbody
Full Body + Diffuser
Full Body + Rear Wing + Underbody
Full Body + Rear Wing + Diffuser
Full Body + Underbody + Diffuser
Full Body + Rear Wing + Underbody + Diffuser
1.00
0.80
0.60
0.40
0.20
0.00
Ratio of Down Force to Drag Force at Maximum Speed
Figure 14 Plot of Drag Force to Drag Force Ratio for control data, full body data,
and all possible component combination data at an air speed of 65 mph.
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Distribution of Down Force Along Longitudinal Axis
75
Air Speed (mph)
60
45
30
Control
Full Body
Full Body + Rear Wing
Full Body + Underbody
Full Body + Diffuser
Full Body + Rear Wing + Underbody
Full Body + Rear Wing + Diffuser
Full Body + Underbody + Diffuser
Full Body + Rear Wing + Underbody + Diffuser
15
0
-4
-2
0
2
4
Distance From Center of Gravity (cm)
6
Figure 15 Distribution of Down Force Along Longitudinal Axis of RC car for
control data, full body data, and all possible component combination data at air
speeds of 0 mph, 30 mph, and 65 mph.
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Tables
Table 1 Estimated costs for project.
Product
Modeling Foam 24”×18”×2”
Fiberglass Cloth 1 square yard
Fiberglass Resin
and Hardener 6 fluid ounces
Acrylic Sheet 12”×12”×0.1”
Foam Glue Gun
Foam Glue Sticks 10 pack
CA+ Glue 20 grams
Spray Adhesive
Electric Carving
Knife
Provider
Unit Cost
Quantity
Total Cost
McMaster-Carr
$26.14
5
$130.70
Tower Hobbies
$5.49
4
$21.96
Tower Hobbies
$13.99
2
$27.98
McMaster-Carr
$3.86
1
$3.86
Tower Hobbies
$12.79
1
$12.79
Tower Hobbies
$7.59
1
$7.59
Tower Hobbies
$7.99
1
$7.99
Elmer’s
$4.99
1
$4.99
Black and Decker
$9.99
1
$9.99
Total Cost
$227.85
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