The Science of Aerodynamics

Drag is the force that opposes your car's motion through the air. Minimizing drag is crucial for maximizing your Pinewood Derby car's speed.

There are two main types of drag affecting your car:

• Form drag: Caused by the shape of your car pushing through the air.
• Skin friction drag: Caused by air molecules interacting with the surface of your car.

To reduce form drag:

• Create a streamlined shape with a pointed front and gradually sloping body.
• Avoid flat surfaces perpendicular to the direction of travel.
• Minimize the frontal area of your car.

To reduce skin friction drag, ensure your car's surface is as smooth as possible.

Streamlined shapes are designed to minimize air resistance. In Pinewood Derby, certain shapes have proven to be particularly effective:

• Wedge: Simple and effective, with a pointed front and sloping body.
• Teardrop: Rounded front with a tapering tail, inspired by nature's most aerodynamic shape.
• Bullet: Rounded nose with a cylindrical body, reducing turbulence.
• Supersonic: Inspired by supersonic aircraft, with a pointed nose and swept-back features.

Key principles for streamlined shapes:

• Gradual curves: Avoid sharp angles that can create turbulence.
• Smooth transitions: Ensure smooth transitions between different surfaces of your car.
• Minimize frontal area: The smaller the front profile, the less air your car needs to push aside.

Remember, the most aerodynamic shape may vary depending on your track's specific characteristics, so testing is crucial.

A smooth surface is crucial for reducing air resistance. Even tiny imperfections can create turbulence and slow your car down.

Follow these steps to achieve a super-smooth surface:

1. Start with coarse-grit sandpaper (around 80-grit) to remove major imperfections.
2. Progress through medium grits (120, 220) to smooth out sanding marks.
3. Finish with fine-grit sandpaper (400 or higher) for an ultra-smooth surface.
4. Apply thin layers of paint, sanding lightly between coats with very fine-grit paper.
5. Finish with automotive polish or plastic polish for a mirror-like surface.

Areas to pay special attention to:

• The nose of the car, where air first makes contact.
• Any curves or transitions between surfaces.
• The underside of the car, which is often neglected.

Remember, smoother isn't always faster beyond a certain point. Test your car's performance at different stages of polishing to find the optimal balance.

While professional wind tunnels are out of reach for most Pinewood Derby racers, you can create a simple DIY wind tunnel to test and refine your car's aerodynamics.

Here's how to create a basic wind tunnel:

1. Use a large cardboard box or clear plastic container as your tunnel.
2. Cut openings at both ends for air to flow through.
3. Place a fan at one end to create airflow.
4. Use smoke or lightweight streamers to visualize air movement.
5. Place your car inside and observe how air moves around it.

What to look for in your wind tunnel tests:

• Smooth airflow over the car's surface.
• Minimal turbulence behind the car.
• No areas where air seems to "stick" or create vortices.

Use your observations to make iterative improvements to your car's design. Even small changes can make a big difference in aerodynamic performance.