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A motor and propeller mounted on one end of a pivoted beam, balanced against stones on the other.

🎒 What you need

a small DC motor salvaged from a broken toy, fan or drone, plus batteries and wire
a light stiff beam, a pivot, string, and small stones or coins as calibrated weights
a propeller cut from a plastic bottle or stiff paper, plus eye protection and an adult supervising

✋ Do this

SAFETY, and this one is not optional: propellers cause real injuries. Use paper or thin plastic blades, never metal. Protect your eyes, clamp the rig down, keep everyone to the side and never in the plane of the disc, and disconnect the battery before touching anything.
Calibrate your weights first. Find something of known mass and compare, or at minimum use identical coins so your unit is consistent. Uncalibrated results are just opinions.
Mount the motor at one end of the beam blowing downwards, pivot in the middle, weight pan at the other end. Measure both arm lengths — if they differ, your reading is scaled by that ratio.
Run the motor and add weights until the beam balances level. Record the thrust in your chosen units.
Vary one thing at a time: blade twist, blade length, battery voltage. Build a table with at least six rows. Longer blades and more twist generally give more thrust but draw more current and can stall the motor — watch for the motor bogging down.
Now compute the ratio that matters. Estimate the mass of a hypothetical aircraft using four of your motors, and work out total thrust divided by weight. Anything at or below 1 cannot leave the ground; controllable flight generally wants roughly 2 or more.

💡 Why it works

A thrust-to-weight ratio of exactly 1 means the aircraft can hover only at full throttle, with nothing left for correcting a gust — which means it is uncontrollable. Real designs keep spare thrust in hand, and that headroom is what lets the flight controller make the constant tiny corrections you practised in module 4. Your table of blade twist versus thrust is a genuine propeller characterisation, done the same way in a lab with a load cell instead of stones.

🔥 Challenge

Find the point of diminishing returns: keep increasing blade area until thrust stops rising and the motor starts labouring. Record the current draw if you can borrow a meter, and explain what is happening.

📖 New words

thrust-to-weight ratiototal thrust divided by total weight, which must exceed 1 to fly

characterisationmeasuring how a component performs across a range of conditions

stallwhen a motor or blade is loaded past the point where it can perform

⚖️

Thrust Engineer

Tap when you have finished this module.