- Difficulty
- Beginner
- Build time
- 40-65 min
- Estimated cost
- $0-$7
- Age range
- 10-16
- Workspace
- A clear table about 90 cm wide
The finish line
What you will build
The car travels 2 metres from a fixed low-speed fan position, stays within a 50 cm lane, and completes three timed runs.
Learning goals
- Identify how moving air from a fixed household fan produces forward rolling motion.
- Construct and explain a airflow-to-linear vehicle travel system.
- Measure how sail area changes performance.
- Diagnose losses caused by axle friction and wheel slip.
Before you build
Materials, tools, and safety
Reuse-material cost: $0-$3 with reused materials. Supervision: Adult help recommended for sharp or heated tools.
Tools
- Ruler
- Pencil
- Scissors
- Masking tape
Low-cost swaps
- Use clean recycled packaging whenever it has similar stiffness.
- Substitute paper clips, binder clips, or twist ties for specialty fasteners.
- Use a large sheet of cardboard to create a hand-powered air pulse instead of an electric fan.
Project-specific safety
- Wear eye protection when stretched elastic, magnets, or spinning parts are present.
- Test at floor or tabletop height and keep the path clear of people.
- Use only a guarded fan with adult supervision, keep fingers and loose materials away from blades, and never modify the fan.
Orient the build
Place the build so moving air from a fixed household fan is on your left and forward rolling motion is on your right. Call the side facing you the front, the far side the back, the tabletop the bottom, and the opposite face the top.
Build it
Step-by-step instructions
Step 1
Build the chassis
Mark two square axle lines and reinforce the mast area.
Keep the base light and flat.
Step 2
Install bearings
Tape straws along the axle lines with equal overhang.
Sight through both to confirm parallel alignment.
Step 3
Add wheels
Center wheels on covered axles and leave side clearance.
Roll by hand through the test lane.
Builder checkpoint: After add wheels, the first subassembly should stay aligned when handled gently.
Step 4
Mount the mast
Brace a vertical mast at the chassis centerline with triangular gussets.
Push gently from each side to test stiffness.
Watch for: If this stage binds or drifts, inspect side force before adding more parts.
Step 5
Make three sails
Cut equal-shape sails with areas of about 100, 200, and 300 cm².
Add reinforced clips at top and bottom.
Step 6
Set the wind course
Place the guarded fan 1 metre behind the start line on low speed.
Tape a 2-metre lane and keep its position fixed.
Builder checkpoint: After set the wind course, operate the build slowly and confirm that forward rolling motion begins without binding.
Step 7
Run baseline trials
Attach the medium sail square to the airflow and release from a block.
Measure time and drift for three runs.
Step 8
Compare sail variables
Test sail area or angle one at a time.
Keep fan, car mass, lane, and release unchanged.
Builder checkpoint: At the final checkpoint, The car travels 2 metres from a fixed low-speed fan position, stays within a 50 cm lane, and completes three timed runs.
See the engineering
Why it works
- Input
- moving air from a fixed household fan
- Output
- forward rolling motion
- Motion
- airflow-to-linear vehicle travel
- Energy losses
- axle friction, wheel slip, sail leakage, side force
Why this works
Sail force
Air changes momentum as it meets the sail and applies a force to the car. A centered sail sends more of that force forward instead of turning the chassis.
Look for: Tilt the sail ten degrees and compare forward speed with sideways drift.
Where the energy goes
Efficiency and losses
The ideal model leaves out axle friction, wheel slip, sail leakage, side force. These effects turn some input energy into heat, sound, vibration, or unwanted motion, so measured performance will be lower than an ideal calculation.
Look for: Run the build slowly and locate the first place where axle friction becomes visible or audible.
Math bite
Calculate average speed
Formula: speed = distance / time
- Distance = 2.0 m
- Time = 4.0 s
Substitute: speed = 2.0 / 4.0 = 0.50 m/s
Result: The car averages half a metre per second.
A larger sail may increase force but also side drift and mast bending.
Fan airflow is not perfectly uniform.
Make it behave
Test, troubleshoot, and tune
Controlled test
Start here: Roll the car by hand through the lane before turning on the fan.
Success looks like: The car completes 2 metres and stays within the marked lane in three trials.
Measure: Travel time, drift, and mast angle.
Change: sail area
Keep constant: car, fan setting, fan distance, floor, lane, and release
- 100 cm²
- 200 cm²
- 300 cm²
| Symptom | Likely cause | Confirm it | Fix |
|---|---|---|---|
| The car veers | Sail or axles are not centered | Test rolling with the fan off | Square axles and center the sail |
| It barely moves | Rolling friction is high or sail too small | Spin each axle and feel airflow | Realign bearings and increase area |
| The mast bends | Sail force exceeds brace stiffness | Watch the mast during start | Add triangular gussets or reduce sail |
| Wheels slip sideways | Side force is high | View from above | Turn the sail square to airflow |
Choose your tradeoff
Low rolling resistance and straight alignment come first. More sail captures more air but can bend the mast and shift the center of pressure sideways.
Keep experimenting
Try another version
One-sail course
Tune only axle alignment.
Angle study
Compare 0, 10, and 20 degree sail angles.
Cargo delivery
Carry equal washers and score speed plus payload.
Build together
Classroom and access options
Classroom version
Teams can compare sail area while keeping car, fan setting, fan distance, floor, lane, and release. Assign builder, tester, recorder, and explainer roles; have each team predict the result before collecting three trials.
Access adaptations
- Offer pre-cut parts and tactile or high-contrast measurement marks.
- Split roles so one builder can hold, another assemble, and another measure.
- Use large clip-on sails and a release block so builders can change angle without fine knots.
Reflect on the design
- How did sail area change the measured result?
- Where did axle friction affect the build most strongly?
- What evidence shows that sail force explains the motion?
- Which change would improve forward rolling motion without creating a new problem?
Glossary
- Sail force
- Air changes momentum as it meets the sail and applies a force to the car.
- Input
- The action or energy supplied to a system; here it is moving air from a fixed household fan.
- Output
- The useful response produced by a system; here it is forward rolling motion.
- Efficiency
- The fraction of input energy that becomes useful output instead of friction, sound, heat, or unwanted motion.
Build your dreams
One build can start the next.
Share what you learned, change one variable, and help another builder understand what worked.
Explore more guidesSources and build notes
A familiar household engineering activity implemented with original instructions and controlled tests.
- Classroom engineering basis: A common educational challenge implemented with original dimensions, tests, diagrams, and instructions.
Written and edited by BrickLabClips. Published 2026-07-22; updated 2026-07-22.