rAs Kids↻ ↺ / ↺ ↻
🎒 What you need
- a bicycle wheel if you can borrow one, or a heavy stone on a long string
- a chair that spins, or a smooth patch of ground you can pivot on
- a friend to observe and record
✋ Do this
- Stand on a smooth spot holding a stone on a string. Swing it around your head one way. Feel yourself being twisted the other way.
- Swap direction. Feel the twist reverse. That is Newton's third law working on rotation, and you can feel it in your feet.
- If you have a bicycle wheel and a spinning chair: hold the spinning wheel and tilt it. You will turn. Have your friend film it, because nobody believes it the first time.
- Now draw a quadcopter and put clockwise arrows on all four propellers. Ask: which way does the body twist? Write your answer.
- Redraw it with diagonally opposite pairs spinning opposite ways. The twists now cancel. Point at the cancelling pairs.
- Last question, and it is the good one: if all four cancel exactly, how does a drone yaw deliberately? Work it out before reading the next paragraph.
💡 Why it works
The answer is that it unbalances the cancellation on purpose: speed up one diagonal pair and slow the other, keeping total thrust the same, and the leftover reaction torque rotates the aircraft. This is also why a single-rotor helicopter needs a tail rotor — without it, the fuselage would simply spin the other way. And it is why propellers are sold in matched clockwise and counter-clockwise pairs; fitting the wrong one is a guaranteed crash.
🔥 Challenge
Explain to someone who has never flown anything why a helicopter has a small tail rotor, using only your stone-on-a-string demonstration. If they get it, you understand it.
📖 New words
torquea twisting force
reactionthe equal and opposite force that always accompanies a force
counter-rotatingpairs of propellers spinning in opposite directions to cancel torque
🌀
Torque Tamer
Tap when you have finished this module.