Why helicopters cannot go much faster

In the picture

1. 1Advancing blade: tip speed plus forward speed, so it meets very fast air
2. 2Retreating blade: tip speed minus forward speed, so it meets very slow air
3. 3Zone of near-zero airflow on the retreating side, where the blade makes almost no lift
4. 4Direction of flight: the faster the helicopter goes, the bigger the gap between the two blades

Examples

• A cyclist riding into the wind feels a strong headwind, while one riding with the wind feels almost none.
• A spinning merry-go-round carried across a field: one edge moves faster than the ground and the other edge barely moves.

Quiz: Why helicopters cannot go much faster (Age 16)

1. 1

   When a helicopter flies forward, what happens to the airspeed of the blade that moves in the same direction as the flight?

   • It goes down, because the forward speed is taken away from the blade’s speed
   • It stays the same, because the rotor always spins at the same speed
   • It goes up, because the forward speed adds to the blade’s own speed
2. 2

   Why does the blade moving against the flight make almost no lift at very high forward speed?

   • The air around it becomes too thin to lift anything
   • The forward speed cancels most of its speed, so it meets almost no airflow
   • The engine cannot spin that blade as fast as the other one
3. 3

   What ultimately limits how fast a helicopter can fly, at around 400 km/h?

   • Lift becomes badly unbalanced between the two sides of the rotor
   • The body of the helicopter cannot cut through the air any faster
   • The engine runs out of power to push the helicopter forward any faster

The answers, each with its explanation, are in the app. This lesson has left the free week, so its quiz is part of the subscription.