Wind Turbine Blade Experiment: Count, Pitch and Power

Most kids’ turbine builds stop at “it spins”. This one gives you a number in watts. Wind a thread onto the axle, hang 20 g from it, start the fan, and time the lift: a 20 g load raised 30 cm is 0.059 joules of work, so a six-second lift is about 10 milliwatts. Now change one thing. Two blades or six. Steep pitch or shallow. Re-measure. That is a wind turbine blade experiment rather than a craft project, and the result is frequently the opposite of what a kid predicts, because the arrangement that spins fastest with nothing attached is not the arrangement that lifts the most.

Key Takeaways

• You’ll build one rotor with removable blades, then measure its power output in watts while changing blade count and pitch angle.
• The physics is that a blade converts wind into torque only when it meets the air at a useful angle, and both too-steep and too-shallow blades waste it.
• The quantitative core: power equals mass times gravity times height, divided by time. Two numbers and a stopwatch give you milliwatts.
• Best for ages 9 to 15. An adult sets up and supervises the fan throughout.
• What usually goes wrong: friction in the bearing swamps the measurement. If the rotor won’t lift anything, the straw bearing is the first suspect, not the blades.

What You’ll Need for the Wind Turbine Blade Experiment

Item	How much	Where to get it	If you don’t have it
Stiff card or manila folder	2 sheets	Stationery drawer	Cereal box card, or plastic from a milk jug
Corrugated cardboard disc, 10 cm across	1	Any box	A plastic jar lid
Bamboo skewer	1	Supermarket	A pencil, or a wooden dowel 4 mm across
Drinking straws	2	Kitchen	A short length of plastic tube
Tall cardboard box or a 1.5 L bottle weighted with sand	1	Recycling	A stack of books taped together
Masking tape	One roll	Hardware drawer	Any tape, but masking is easiest to reposition
Thread or fishing line	1 m	Sewing kit	Dental floss
Load: 20 g of coins in a small bag	1	Pocket	A 20 g fishing weight, or 20 washers weighed first
Box fan with an intact guard	1	Household	A hairdryer on cold, held by an adult at a fixed distance
Printed protractor	1	Print or trace	Fold a square of paper twice to get 45° and 22.5° references
Phone with a stopwatch	1	Pocket	Any stopwatch, or count seconds aloud consistently

Time: prep 25 min, experiment 35 min, cleanup 5 min Cost: nothing if you own a fan Mess level: low, aside from card offcuts

Do the Experiment

Step 1 — Build the hub and the bearing

Push the skewer through the exact centre of the cardboard disc and tape it so it cannot slip. Tape two 4 cm lengths of straw to the side of the box, 8 cm apart and perfectly in line, then thread the skewer through both. Spin it. A good bearing keeps turning for three or four seconds after you let go. If it stops instantly, the straws are misaligned.

Step 2 — Cut eight identical blades with fold tabs

Each blade is a 10 cm by 3 cm rectangle of card with an extra 2 cm tab at one end. Score and fold the tab. The fold angle is your pitch, and because every blade is identical, the only variable that changes between runs is the one you intend to change.

Step 3 — Mount three blades at 25 degrees

Mark three evenly spaced positions on the disc using a paper strip wrapped round the rim and folded into thirds. Tape each blade’s tab down so the blade sits 25 degrees off the plane of the disc, checked with the protractor. All three must lean the same way or the rotor fights itself.

Step 4 — Set up the measurement

Tie the thread to the skewer, wind five turns on, and tie the 20 g load to the free end so it hangs 30 cm below the axle. Place the fan 50 cm from the rotor, on its lowest setting, and mark the fan’s position on the floor with tape. Every later run uses that same mark.

Step 5 — Run the blade count sweep

With pitch fixed at 25 degrees, time the lift for 2, 3, 4, 6 and 8 blades, three runs each. If the rotor cannot lift the load at all, write “stall” rather than skipping it, because a stall is a result. Compute power as 0.059 joules divided by the lift time in seconds.

Step 6 — Run the pitch sweep, then plot

Fix the count at the winner from step 5 and repeat at 10, 25, 45 and 70 degrees. Plot power against blade count and power against pitch on the same sheet. Two curves, each with a peak.

What you should see: Two blades at 25 degrees often stall outright under a 20 g load, which surprises most kids because two blades spin fastest with nothing attached. Four to eight blades usually lift it, with lift times somewhere between 4 and 15 seconds, giving 4 to 15 milliwatts. The pitch curve typically peaks between 25 and 45 degrees, with 10 degrees barely producing torque and 70 degrees producing a slow, shuddering turn. Expect run-to-run scatter of 20% or more, which is why three runs per setting matters.

What the Wind Turbine Blade Experiment Measures

Setting	What you measure	Typical result	Why
2 blades, 25°	Lift time or stall	Often stalls	Low solidity means little starting torque
4 blades, 25°	Lift time	5–12 s	More blade area working at once
8 blades, 25°	Lift time	4–12 s, sometimes worse than 6	Blades begin to interfere with each other’s airflow
3 blades, 10°	Lift time or stall	Usually stalls	Angle of attack too small to turn wind into torque
3 blades, 70°	Lift time	Slow and shaky	Air separates behind the blade, mostly drag

Record all of it, including the stalls. The stalls are where the explanation lives.

What’s Actually Going On

A turbine blade does one job: it deflects moving air, and by Newton’s third law the air pushes back. The useful part of that push is the component in the direction the blade travels, which is what creates torque on the shaft. Everything about blade count and pitch comes down to maximising that component without wrecking the airflow.

Pitch sets the angle at which the blade meets the oncoming air, and there’s a reason both extremes fail. At 10 degrees the blade is nearly edge-on to the wind, so it barely deflects anything and produces almost no torque. At 70 degrees it deflects plenty, but the airflow separates from the back of the blade, leaving a turbulent wake. That separated flow is the same phenomenon that makes a real aircraft wing stall, described in NASA Glenn’s aeronautics guide as part of why drag rises so steeply with angle. Somewhere in between, the blade deflects a lot of air while the flow stays attached, and that’s your peak.

Blade count is really a proxy for solidity, which is the fraction of the rotor’s swept circle that is covered in blade. More solidity means more blade area meeting the wind at once, which means more torque at low rotational speed. That matters enormously here because your rotor is doing a high-torque job at low speed: hauling a weight up a string. Duquette and Visser’s 2003 study in the Journal of Solar Energy Engineering modelled exactly this trade and reported that “increasing the solidity beyond what is traditionally used for electric generating wind turbines led to increased power coefficients at lower tip speed ratios”, and that “an increase in the blade number at a given solidity also increased the maximum Cp for all cases examined”. Their list of side benefits included lower noise and a lower cut-in wind speed, which is the speed at which a turbine starts producing at all. A 2024 experimental study in Energy Reports revisited the same two variables on small horizontal-axis turbines.

So why do real wind farms use three blades? Not because three is aerodynamically optimal in isolation. Three is a compromise: it is close enough to the aerodynamic optimum at the high tip-speed ratios a generator wants, it costs less than six blades, it balances smoothly, it is quieter, and it produces less fatigue loading on the hub. Those reasons are economic and mechanical as much as aerodynamic, which is a useful thing for a child to hear about engineering in general.

There is also a hard ceiling nobody can design around. A rotor cannot extract all the kinetic energy from the wind, because to do that the air would have to stop dead behind the rotor, and then nothing could flow through. The theoretical maximum fraction works out to about 59%, the Betz limit, derived in standard references such as Manwell, McGowan and Rogers’ Wind Energy Explained. Modern utility turbines reach the high 40s. Your cardboard rotor will manage a fraction of a percent, and that gap is honest information rather than a failure.

Worth saying plainly: flat card blades are drag devices, not airfoils. Real blades are twisted, with a changing cross-section along their length, because the relative wind angle changes from root to tip. Your rotor is a fair test of count and pitch, and a poor model of a real blade’s shape.

How to Explain It to Your Kid

Ages 5–8: catching the wind

Hold one blade in the breeze from the fan and let them feel the push change as they rotate it. Flat to the wind, it shoves hard. Edge-on, nothing. Then ask them to guess which way to tilt it to make the rotor turn, and let them try. The physical memory of the push changing with angle is the whole lesson at this age.

Ages 9–12: measure power, not speed

Introduce the formula as a recipe: mass in kilograms, times 9.8, times height in metres, divided by seconds. That gives watts. Have them calculate the power of a six-second lift and a twelve-second lift and notice that halving the time doubles the power. Then let them discover that the fastest-spinning rotor is not the strongest one, which is the single most useful surprise in the activity.

Ages 13+: solidity and tip-speed ratio

Define solidity as total blade area divided by swept area, and have them compute it for each configuration. Then introduce tip-speed ratio: blade tip speed divided by wind speed. Measure the tip speed by counting revolutions in ten seconds from a phone video and multiplying by the circumference. Now the two curves they plotted have axes a real engineer would recognise, and the high-solidity result makes sense rather than looking like a fluke.

The question to ask: “If we gave this rotor a much lighter load, say 2 g, would the winning number of blades change? Which way, and why?”

Why Teaching This Early Pays Off

The transferable lesson is that a system’s best configuration depends on the job it’s doing. Two blades win a free-spinning race and lose a weightlifting contest, with no change to the physics, only to the task. Kids tend to look for the single best answer, and this is a cheap, physical demonstration that “best” needs a question attached to it.

It also pre-loads some specific school content. Power as work over time becomes a measured quantity rather than a definition to recall. Angles acquire a job. And the idea of a theoretical maximum efficiency shows up again in thermodynamics, in electrical engineering and in any sensible conversation about energy policy.

One honest note about the experience: this activity can feel frustrating in the middle, when the rotor stalls and nothing appears to be working. Deslauriers and colleagues found in 2019 that students in actively taught classes felt they were learning less while objectively learning more than those in polished lectures. Expect the same here. The stalls are data, and the moment your child starts treating a stall as information rather than failure is the moment the activity has earned its afternoon. For a gentler first build, the home wind turbine project gets something spinning before you start measuring.

Before You Start: What Parents Should Know

Best age	9–15. Younger children can do the blade-in-the-breeze part and the timing, with an adult running the fan
Supervision	Adult present the entire time the fan is on. The adult positions the fan, switches it on and off, and keeps the cord out of the walkway
Real hazards	Spinning card edges sting at the rim, so hands stay off a turning rotor. Use a fan with an undamaged guard and never remove the guard. Tie back long hair. The skewer point is sharp, so cap it with a cork or a blob of tape. Safety glasses if you switch to stiff plastic blades
Don’t	Don’t use mains electricity for anything other than the fan itself, and don’t open, modify or rewire the fan. Don’t substitute a desk fan without a guard

When It Doesn’t Work

• The rotor spins but lifts nothing. Almost always bearing friction. Realign the two straws so the skewer passes through both without touching the sides, and shorten the straws to 2 cm each to cut contact area.
• The thread unwinds instead of winding up. You wound it the wrong way for the rotor’s direction. Reverse the wind, or flip all the blades to the other lean.
• Every run gives a wildly different time. The fan is on a high setting with turbulent output, or its position shifted. Use the lowest setting, tape the fan’s position to the floor, and average three runs.
• The rotor wobbles and the skewer whips. The hub hole is off-centre or the blades aren’t evenly spaced. Re-drill the centre using the paper-strip method, and check spacing with the strip rather than by eye.
• Blades bend backwards under load. Card is too soft. Fold a 5 mm lip along the long edge of each blade, which stiffens it exactly the way a tube stiffens a paper column.

Take It Further

1. Blade length instead of count. Keep three blades and cut them to 6, 8 and 10 cm, measuring power each time. Swept area grows with the square of radius, so the effect is bigger than most kids predict.
2. Change the load, change the winner. Repeat the count sweep with 5 g and with 50 g. The optimum number of blades should shift upward as the load gets heavier, which is the tip-speed-ratio story made visible.
3. Measure the wind. Build a swinging-card anemometer, or hold a strip of paper at the rotor and measure the deflection angle. Once you have an approximate wind speed, you can estimate how much power is in the wind crossing your rotor’s circle and compute your own efficiency. Compare it with the figures in the US Department of Energy’s small wind guidebook.

What to Watch For Over the Next 3 Months

• Week 4: Counting blades on real turbines from the car window, and noticing that water pumps on farms have many blades while power turbines have three.
• Month 2: Using the word power correctly, as a rate rather than a quantity. Phrases like “it did the same work but faster” are the giveaway.
• Month 3: Designing a test rather than a thing. If your child proposes holding something constant to isolate one variable, in any context at all, that transferred.

Frequently Asked Questions

Why does my two-blade rotor spin fast but lift nothing?

Because speed and torque are different things. Two blades cover little of the swept circle, so at low rotational speed there is not much blade area pushing. With no load it accelerates until the torque it makes balances friction, which can be quite fast. Add a weight and it never gets started.

Do we need a box fan, or will a hairdryer do?

A hairdryer on its cold setting works, but hold it in a fixed clamp or have an adult keep it at a marked distance, because small changes in distance change the result more than the blades do. Never use the hot setting near cardboard.

How many blades is actually best?

For this task, usually four to six. For a utility turbine driving a generator at high rotational speed, three. The honest answer is that the question is incomplete without saying what the rotor is being asked to do, which is exactly what the two curves demonstrate.

Why is the measured power so tiny?

A fan’s wind is slow, your rotor is small, flat card is an inefficient blade shape, and the bearing eats a share. Milliwatts are the correct order of magnitude. The absolute number does not matter here; the comparison between configurations does. If you want visible output instead, the wind turbine build guide covers hooking a rotor to a small motor.

---

About the author

Ricky Flores is the founder of HiWave Makers and an electrical engineer with 15+ years of experience building consumer technology at Apple, Samsung, and Texas Instruments. He writes about how kids learn to build, think, and create in a tech-saturated world. Read more at hiwavemakers.com.

---

Sources

1. Duquette, M. M., & Visser, K. D. (2003). “Numerical Implications of Solidity and Blade Number on Rotor Performance of Horizontal-Axis Wind Turbines.” Journal of Solar Energy Engineering, 125(4), 425–432. https://doi.org/10.1115/1.1629751
2. Kulak, M., Lipian, M., Olasek, K., Pereira, M., & Ravelet, F. (2024). “Rotor solidity and blade pitch influence on horizontal axis small wind turbine performance: Experimental study.” Energy Reports, 12, 4568–4577. https://doi.org/10.1016/j.egyr.2024.10.036
3. Manwell, J. F., McGowan, J. G., & Rogers, A. L. (2009). Wind Energy Explained: Theory, Design and Application (2nd ed.). Wiley. https://doi.org/10.1002/9781119994367
4. U.S. Department of Energy. How Do Wind Turbines Work? Office of Energy Efficiency and Renewable Energy. https://www.energy.gov/eere/wind/how-do-wind-turbines-work
5. U.S. Department of Energy. Small Wind Guidebook. WINDExchange. https://windexchange.energy.gov/small-wind-guidebook
6. Deslauriers, L., McCarty, L. S., Miller, K., Callaghan, K., & Kestin, G. (2019). “Measuring actual learning versus feeling of learning in response to being actively engaged in the classroom.” PNAS, 116(39), 19251–19257. https://doi.org/10.1073/pnas.1821936116