Build a Windmill That Lifts a Weight

Understanding Build a Windmill That Lifts a Weight

A rotor works best when its blades have a useful shape and angle. A flat card facing straight into the air feels a push, but much of that push does not help it turn. Each blade needs to guide air sideways as it passes.

This produces a turning effect around the shaft, called torque. If the blades are angled too steeply, air becomes disturbed behind them and the rotor can slow down.

If they are nearly edge on, they catch very little air. Equal blade angles and equal blade masses matter because an unbalanced rotor shakes, wastes energy, and may rub against its supports.

The lifting system creates a tradeoff between speed and force. The tension in the string pulls back on the axle. That pull produces torque in the opposite direction from the rotor.

A narrow axle gives the string a smaller turning radius. This means the rotor can lift a heavier load more easily, but the string moves upward only a short distance during each turn. A wider axle raises the load farther per turn, but needs more turning force.

Friction can prevent lifting even when the blades are spinning. Common friction points include the axle supports, string rubbing on the frame, and a string winding unevenly over itself.

The height of the load gives a clear way to measure the useful result of the project. The work done in lifting equals the weight force times the vertical distance raised. The weight force depends on mass times gravitational field strength.

A heavier object gains more gravitational potential energy at the same height. An object raised twice as high gains twice as much energy.

Students should measure height from the starting position to the final position, not the length of string used. The rotor receives much more energy from the moving air than reaches the load, since some energy becomes sound, heat from friction, and motion of the spinning parts.

A fair test changes one feature at a time. Keep the fan setting, fan distance, load mass, blade material, and starting string position the same while testing blade angle or blade area. Measure the time needed to raise the load through a marked height.

Repeat each trial several times because small changes in airflow can affect the result. Record cases where the rotor spins but cannot start lifting, since this shows that its torque is too low to overcome the load and friction.

Keep fingers, loose hair, and clothing away from the spinning rotor. A light load and a strong frame make the results easier to observe without damaging the model.