The Physics of Flight Speed: How Pilots Know Their Airspeed

-Karthik Gurumurthy

Ever wondered how pilots know exactly how fast they’re flying? It’s not like they can stick their hand out the window with a speedometer!

The cockpit has this crucial instrument called an airspeed indicator (ASI) that shows how fast the plane is moving through the air. It has two needles – one for hundreds of knots and another for tens of knots. But how does it actually work?

The magic happens thanks to a clever device called a Pitot tube, invented by a guy named Henry Pitot back in the 1700s. This thin cylindrical tube sticks out from the wing, pointing forward into the airflow. As the plane speeds up, more air gets forced into this open-ended tube, increasing the air pressure inside.

But there’s a second tube involved too – this one’s sealed at the front end and has small holes along its sides. This “static tube” measures the normal atmospheric pressure outside the plane without being affected by the forward motion.

Both tubes connect to an expandable diaphragm inside the airspeed indicator. Through some smart mechanical engineering, the difference between these two pressure readings (the dynamic pressure from the Pitot tube versus the static atmospheric pressure) causes the diaphragm to expand or contract accordingly.

The indicator needles are attached to this diaphragm, so when the plane accelerates, the increasing pressure difference makes the diaphragm expand, moving the needles to show a higher speed. When the plane slows down, the diaphragm contracts and the needles show a lower speed.

t’s basically a sophisticated pressure differential system that translates air pressure variations into a speed reading that pilots can instantly understand at a glance. Pretty ingenious when you think about it!