The Effect of Car Design on the Amount of Drag Created
Photo of Brad at Mid Columbia Science Fair

Researched by Brad K.
2005-06


PURPOSE

The purpose of this experiment was to determine the effect of front and back designs on the aerodynamic drag of a vehicle.

I became interested in this idea because I loved cars, specifically fast cars.  I knew that certain designs were more aerodynamic than others, and that less drag would allow for more speed.

The information gained from this experiment could help car designers choose what shape to base the design of a vehicle on.  A car with less drag could be faster and also more fuel-efficient.


HYPOTHESIS

My first hypothesis was that a square front and back design would be the least aerodynamic, and would create the most drag.

My second hypothesis was that a hemispherical front and a conical back design would be the most aerodynamic, and would create the least amount of drag.

I based my hypotheses on Landon Arnett’s 7th grade study in 2003.  He concluded, “My first hypothesis was that the hemispherical nose would be the most aerodynamic.  My second hypothesis was that the conical tail would be the most aerodynamic.  The results indicated that both hypotheses should be accepted.”


EXPERIMENT DESIGN

The constants in this study were:
•    The weight of the Styrofoam designs
•    The wind setting (medium)
•    The type and number of blowers
•    The wind tunnel
•    The lightweight spring scale    
•    The car base

The manipulated variable was the design of each of the cars.

The responding variable was the amount of drag created.

To measure the responding variable, I used a lightweight spring scale.


MATERIALS

QUANTITY ITEM DESCRIPTION
3-5
 Styrofoam Blocks
1
 Car Base
2
 Leaf Blowers
1
 Pocket Knife
1
 Wind Tunnel
1
 Lightweight Spring Scale
1
 Set of Earplugs
 

PROCEDURES
 
1)    Carve the designs out of Styrofoam
     a)    Design 1:  
     b)    Design 2:  
     c)    Design 3:  
     d)    Design 4:  
     e)    Design 5:  
2)    Set Up
     a)    Set up the wind tunnel
     b)    Put the blowers in the right positions
     c)    Put the blowers to the right wind setting
3)    Test the Designs
     a)    Place the car base inside the wind tunnel with one of the designs on it
     b)    Attach the lightweight spring scale to the car base, and to the wall of the wind tunnel
     c)    Put the earplugs on
     d) Start the blowers
     e)    Look at the lightweight spring scale and record the amount of drag created every 5 seconds 20 times per test. (5 tests per car, switch design after every test)
     f)    Stop the blowers
4)    Repeat step 3 five times
5)    Switch the design in the car base with one of the other designs.
6)    Repeat steps 3 and 4 until all 5 designs have been tested 5 times.
7)    Average the results for each design.
 
 
RESULTS

The original purpose of this experiment was to determine the effect of frontal, back, and windshield designs on the aerodynamic drag of a vehicle.

The results of the experiment were that Car Design 1 produced an average of 0.078 newtons of drag, Car Design 2 produced an average of 0.0515 newtons of drag, Car Design 3 produced an average of 0.0797 newtons of drag, Car Design 4 produced an average of 0.0701 newtons of drag, and Car Design 5 produced an average of 0.0342 newtons of drag.



See the table and graph below.
 

CONCLUSION

My first hypothesis was that a square front and back design (Design 2) would be the least aerodynamic, and would create the most drag.

My second hypothesis was that a hemispherical front and a hemispherical (Design 3) back design would be the most aerodynamic, and would create the least amount of drag.

The results indicate that my first hypothesis should be rejected, because Design 3 (hemispherical convex front, with hemispherical concave back) created the most drag.

The results indicate that hypothesis 2 should be rejected, because Design 1 (triangular front, flat back) created the least amount of drag.

After thinking about the results of this experiment, I wonder if I made more designs with more detail, would it affect the results?

If I were to conduct this project again I would use wood for my designs instead of Styrofoam to (hopefully) get better results.
 


RESEARCH REPORT

Human Need

Humans need cars because they need transportation.  Aerodynamically designed cars have less drag, and therefore are faster and more fuel-efficient.  A more fuel-efficient car uses less gas, which would cost less in the long run, and would produce less pollution.

Aerodynamics

There are four basic aerodynamic forces; they are thrust, weight, lift, and drag.  They all result from a transfer of force from a fluid to the surface of any solid object.  The force from the fluid creates a pressure and a shear stress on the surface.

Shear stress is a stress state where the shape of an object changes, usually because of friction.  Shear stress is measured in force per unit area, but shear stress is applied along the surface rather than perpendicular to the surface.  Shear stress occurs in a fluid because of the fluid’s thickness, there is internal friction that resists motion.  Friction occurs in fluid whenever a layer of fluid slides over another layer.  Because of the friction, layers resist sliding.

Total aerodynamic force is moved from fluid to the surface as a result of pressure and shear stress acting over the whole surface.  Lift and drag are both parts of total aerodynamic force.  Lift is the part that is perpendicular to the direction of the motion of the object.

Drag

Drag is the force that resists the forward motion of a solid object in fluid.  The object’s shape affects whether it produces a little drag or a lot of drag.  Objects that are shaped to produce a smaller amount of drag are called streamlined or aerodynamically clean.

Two types of drag – friction drag and form drag – act on all objects that move.  The third type of drag, induced drag, only acts on objects that fly.  The fourth type of drag is wave drag, which results when an object goes faster than the speed of sound.  In the metric system, a common unit of force measurement is the newton.

Friction drag

Friction drag is a component of drag because of shear stress.  Friction has the most powerful effect in the boundary layer, a layer of fluid next to the surface.

The amount of friction drag depends on whether the fluid flow is laminar or turbulent.  In laminar flow (a regime characterized by high momentum diffusion) fluid molecules move in neat paths essentially in the direction of the surface.  Turbulent flow occurs at higher speeds.  In turbulent flow, the fluid’s speed and direction of flow vary randomly from the average.

Form Drag

Form drag is a part of drag because of pressure. The amount of form drag on any given object depends on the objects form or shape.  If the object isn’t streamlined the drag force is mainly form drag.  If the object is streamlined drag force is mainly friction drag.

In form drag, the flowing fluid separates from the object.  The pressure next to the back surface of the object therefore decreases.  The decrease makes the pressure on the front surface larger than the force on the rear surface.  The net result is that the force pushes against the front of the object.  Car designers can reduce form drag by streamlining an object used for transportation, like a bike or a car.

Thrust

Thrust is the force that moves an object through the air.  Thrust is used to overcome drag and weight.  The engine of a car creates thrust by a kind of a propulsion system.

Thrust is a mechanical force, so the propulsion system must have physical contact with a working fluid to make thrust.  Thrust is most often generated through a reaction with accelerating a mass of gas.  Because thrust is a force, it is vector quantity, having a magnitude and a direction.  The engine does work on the gas, and accelerates the gas to the back of the engine; the thrust is generated in the other direction of the accelerated gas.  The magnitude of the thrust depends on how much gas is accelerated on the difference in velocity of the gas through the engine.

The physics involved in the generation of thrust is first taught in middle school, and studied in detail in high school and college.  To accelerate gas expanding energy is a necessity.

Human Need

The above explains how aerodynamics and drag work in slight detail, if that information had not been discovered, the only source of transportation would be to walk or run to get somewhere.

 

BIBLIOGRAPHY

Benson, Tom.  “What is Thrust?” http://www.grc.nasa.gov/WWW/K-12/airplane/thrust1.html

Geocities.  “Aerodynamics.”  http://geocities.com/MotorCity/76/0/aerodyn.htm

Pirie, Brian-Young, Benjamin  “Aerodynamics.”  http://insightcentral.net/encycloedia/enaero.html 2003

Plotkin, Allen.   "Aerodynamics." World Book Encyclopedia. 2005 World Book online.

Sharpe, Diane.  “Illustrated Science Encyclopedia Volume 1.”  U.S.A.  Raintree Steck-Vaughn publishers 1997 30-32

Williams, Frank & Briatore, Flavio.  “Renault Formula Motor Racing Book.”  United States D.K. Publishing Inc.  1996 6, 7, 9, 18, 37

WiseGEEK  “What are car aerodynamics.”  http://wisegeek.com/what-are-car-aerodynamics.htm 2005
 


ACKNOWLEDGEMENTS

I would like to thank the following people for helping make my project possible:
•    My dad for buying me supplies, and helping me cut the designs.
•    My mom for letting me use her craft table to cut the designs.
•    Mrs. Viernes for helping me with my graph and table.
•    Mr. Newkirk for letting me use supplies that I could not do my project without, such as the wind tunnel.

Top of page

Menu of 2005-2006 Science Projects

Back to the Selah Homepage