The Atmosphere — Page 32, Lesson 39

Right, let’s pick this up with the airspeed indicator. We’ve been talking about dynamic pressure as the thing that drives every aerodynamic force on the aircraft. Now I want to show you how we actually measure it and present it to the pilot. The key idea is that all aerodynamic forces are determined by dynamic pressure. So it’s essential to have a way of measuring dynamic pressure and presenting that information to the pilot. That’s exactly what the airspeed indicator does. Here’s how it works. We start with a sealed tube, open at the forward end. It’s located where it will collect air when the aircraft is moving — that’s the pitot tube. When the aircraft moves forward, air is rammed into this open end. The pressure inside that tube is dynamic plus static. In this context, we call it “Pitot” pressure, simply because the air is inside the pitot tube. But pitot pressure alone isn’t enough. We need to remove the static pressure from it, because we only want the dynamic part. So we use a vent — a hole in a surface that is parallel to the airflow. That vent senses static pressure. Now here’s the clever bit. Look at the diagram: if the pressure from the pitot tube is fed to one side of a diaphragm mounted in a sealed case, and static pressure is fed to the other side, the two static pressures cancel each other out. What’s left is only the dynamic pressure. So the diaphragm movement is influenced only by changes in dynamic pressure. That movement of the diaphragm moves a pointer over a scale, so the flight crew can observe changes in dynamic pressure. But there’s a catch. The instrument is calibrated at ISA sea level density. So it will only give a true indication of the speed of the aircraft through the air when the air density is 1.225 kilograms per cubic metre — that’s the standard sea level density. Now, is that a problem? No, because the pilot needs an indication of dynamic pressure, and that’s exactly what the instrument provides. The instrument is made in such a way that it indicates the square root of the dynamic pressure, in nautical miles per hour — that’s knots — or statute miles per hour, mph. So if this indicated airspeed is doubled, the speed of the aircraft through the air will also be doubled. That’s the key relationship: the airspeed indicator is essentially a pressure gauge. The needle indicates changes in dynamic pressure. The pitot tube feeds pitot pressure — static plus dynamic — to one side of the diaphragm, and the static vent feeds static pressure to the other side. The difference is what moves the needle. So, to sum up: pitot pressure minus static pressure gives you dynamic pressure, and the airspeed indicator displays the square root of that dynamic pressure as indicated airspeed in knots or mph. That’s the whole principle of the ASI.