- Difficulty
- Intermediate
- Build time
- 75-110 min
- Estimated cost
- $0-$8
- Age range
- 11-17
- Workspace
- A clear table about 90 cm wide
The finish line
What you will build
The rotor starts reliably in front of a household fan, turns without striking the guard, and lifts a 5-gram paper clip chain through 20 cm.
Learning goals
- Identify how moving air from a household fan produces rotor and winding-drum rotation.
- Construct and explain a airflow-to-rotary motion system.
- Measure how the blade pitch angle changes performance.
- Diagnose losses caused by bearing friction and blade flex.
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
- Low-temperature glue gun or tape
- Protractor
- Household fan operated by an adult
Low-cost swaps
- Use clean shipping-box cardboard instead of buying sheets.
- Replace hot glue with strong tape and folded tabs.
- Use index-card blades taped into a reused bottle-cap hub.
Project-specific safety
- An adult should handle craft knives and make difficult starter cuts.
- Let hot glue cool before pressing a joint or testing moving parts.
- Use only a household fan behind a full guard; never test outdoors in storms or near vehicle traffic.
Orient the build
Place the build so moving air from a household fan is on your left and rotor and winding-drum rotation 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 guarded tower
Brace two bearing walls and surround the rotor path with a wide cardboard ring.
Leave at least 3 cm tip clearance.
Step 2
Laminate the hub
Glue two circles with alternating corrugation and mark six equal radial lines.
Pierce one centered axle hole.
Step 3
Make matched blades
Cut six identical rectangles and score equal root tabs.
Weigh or compare them in pairs for balance.
Builder checkpoint: After make matched blades, the first subassembly should stay aligned when handled gently.
Step 4
Set the first pitch
Tape every blade at 20 degrees using a folded angle jig.
Keep leading edges oriented consistently.
Watch for: If this stage binds or drifts, inspect hub imbalance before adding more parts.
Step 5
Install axle and drum
Center the hub between straw bearings and add a small drum outside the airflow.
Limit side motion with paper collars.
Step 6
Balance the rotor
Let it settle repeatedly and trim or tape the heavy side.
Confirm every blade clears the guard.
Builder checkpoint: After balance the rotor, operate the build slowly and confirm that rotor and winding-drum rotation begins without binding.
Step 7
Run unloaded trials
Place the fan 1 metre away on low and time startup for three trials.
Keep fan position taped on the floor.
Step 8
Run lift trials
Attach a 5-gram chain and measure time to lift 20 cm.
Repeat at 10, 20, and 30 degree blade pitch.
Builder checkpoint: At the final checkpoint, The rotor starts reliably in front of a household fan, turns without striking the guard, and lifts a 5-gram paper clip chain through 20 cm.
See the engineering
Why it works
- Input
- moving air from a household fan
- Output
- rotor and winding-drum rotation
- Motion
- airflow-to-rotary motion
- Energy losses
- bearing friction, blade flex, air turbulence, hub imbalance
Why this works
Aerodynamic torque
Air exerts forces on angled blade surfaces. The tangential component acting at a radius produces torque, while the rotor accelerates until driving torque balances drag and load.
Look for: Use the same fan setting and distance while changing only blade pitch, then compare startup time.
Where the energy goes
Efficiency and losses
The ideal model leaves out bearing friction, blade flex, air turbulence, hub imbalance. 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 bearing friction becomes visible or audible.
Math bite
Calculate tip travel
Formula: tip distance per turn = 2πr
- Rotor radius r = 0.14 m
- π ≈ 3.14
Substitute: distance = 2 × 3.14 × 0.14 = 0.879 m
Result: A blade tip travels about 0.88 metres per revolution.
Tip speed equals this distance times rotations per second.
Slip in hand counting and turbulent airflow affect measurements.
Make it behave
Test, troubleshoot, and tune
Controlled test
Start here: Spin the rotor by hand with the fan off to confirm guard clearance.
Success looks like: The fan starts the rotor on low and the turbine lifts 5 grams through 20 cm.
Measure: Startup time, rotations in ten seconds, and lift time.
Change: the blade pitch angle
Keep constant: fan setting, distance, blade count, hub, load, and lift height
- 10 degrees
- 20 degrees
- 30 degrees
| Symptom | Likely cause | Confirm it | Fix |
|---|---|---|---|
| The rotor will not start | Blade pitch is low or bearings rub | Hand-spin with fan off | Free bearings and increase pitch slightly |
| The turbine shakes | Blade mass or angle is unequal | Let the hub settle and inspect roots | Match blades and rebalance |
| Blades fold backward | Cardboard is too thin or root tabs short | Run on low and watch deflection | Laminate roots or reduce area |
| The load stalls immediately | Drum radius or mass is too large | Test unloaded first | Use a smaller drum or lighter chain |
Choose your tradeoff
Balance and bearing friction often matter more than extra blade area. Increase pitch or load one step at a time, keeping fan conditions fixed for a fair comparison.
Keep experimenting
Try another version
Pinwheel rotor
Test four folded paper blades with no load.
Blade-count study
Compare three and six blades at equal total area.
Power estimate
Use lifted mass, height, and time to estimate useful output power.
Build together
Classroom and access options
Classroom version
Teams can compare the blade pitch angle while keeping fan setting, distance, blade count, hub, load, and lift height. Assign builder, tester, recorder, and explainer roles; have each team predict the result before collecting three trials.
Access adaptations
- Pre-cut repeated pieces and mark fold lines with high-contrast ink.
- Use large tabs, binder clips, and tape for easier one-handed assembly.
- Use large high-contrast blades and an audible paper rotation counter.
Reflect on the design
- How did the blade pitch angle change the measured result?
- Where did bearing friction affect the build most strongly?
- What evidence shows that aerodynamic torque explains the motion?
- Which change would improve rotor and winding-drum rotation without creating a new problem?
Glossary
- Aerodynamic torque
- Air exerts forces on angled blade surfaces.
- Input
- The action or energy supplied to a system; here it is moving air from a household fan.
- Output
- The useful response produced by a system; here it is rotor and winding-drum rotation.
- 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
An original BrickLabClips cardboard machine with dimensionally specified construction.
- Cardboard design verification: Dimensions, fold allowances, repeated-motion joints, and likely load paths received an editorial geometry review.
Written and edited by BrickLabClips. Published 2026-07-22; updated 2026-07-22.