The Helicopter Speed Limit - Helicopter Physics Series - #7 - Smarter Every Day 51

Helicopters have a physical speed limit governed by the laws of physics, not legal regulations.

Destin welcomes viewers to Smarter Every Day, a show dedicated to science education and exploration.

This episode explains the dissymmetry of rotor flight, a fundamental principle affecting helicopter performance.

A nighttime helicopter flight demonstration with light painting effects to visually illustrate the physics concepts.

The light painting reveals different radius of curvature on each side of the helicopter's rotor path.

The advancing blade moves faster relative to the air when the helicopter flies forward, while the retreating blade moves slower.

A rotor blade rig on a stick is used to demonstrate the effects of unsymmetric blade travel.

• One side shows a tight radius of curvature • The other side shows a much larger radius of curvature

The advancing blade travels forward with the helicopter, adding the helicopter's forward speed to the blade's rotational speed, creating higher relative air velocity.

If the advancing blade's air speed exceeds the sound barrier, shock waves form and damage the rotor.

The retreating blade travels opposite to the helicopter's forward movement, resulting in lower relative air velocity compared to the advancing side.

• Advancing blade: helicopter speed + blade speed = higher velocity • Retreating blade: blade speed - helicopter speed = lower velocity

Dissymmetry of Lift occurs when the advancing and retreating blades generate different amounts of lift due to their different air speeds.

To counteract the lift imbalance, the retreating blade is given more pitch to produce additional lift and maintain stable flight.

If the helicopter flies too fast, the retreating blade's pitch becomes excessive, causing it to lose lift in a condition called retreating blade stall.

Retreating blade stall is self-correcting due to gyroscopic precession, which pitches the helicopter down instead of rolling it, automatically slowing the aircraft.

Viewers should now understand enough about helicopter physics to make educated guesses about helicopter performance characteristics.

The episode challenges viewers to identify which of three US Army helicopters is the fastest based on the physics principles explained.

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The Chinook helicopter is the fastest in the US Army inventory.