The Airspeed Indicator (ASI) — Page 39, Lesson 47

Let’s start with the principle of operation, because everything about the airspeed indicator hangs off this one idea. When an aircraft sits on the ground in perfectly still air, the only pressure acting on it is the atmospheric pressure around it. We call that static pressure, and I’ll label it S. Now, the moment the aircraft moves through the air, something new appears. The leading edges of the aircraft — the nose, the wing leading edges — are ramming into the air. That impact creates an additional pressure on those surfaces. We call that the dynamic pressure. So in flight, the total pressure felt on a leading edge is the sum of the dynamic pressure plus the static pressure. That total is called the pitot pressure, and I’ll label it P. So the relationship is simply: Pitot = Dynamic + Static. That’s the fundamental equation of the whole instrument. Now, how do we sense these pressures? We use two sensors. The pitot head senses the pitot pressure — that’s the total pressure, dynamic plus static. The static/vent senses the static pressure alone. Both of these pressures are piped into the airspeed indicator, which is really just a differential pressure gauge. A differential pressure gauge measures the difference between two pressures. Here, it subtracts static from pitot, and what’s left is the dynamic pressure. Why does dynamic pressure tell us airspeed? Because of the equation: Dynamic Pressure = ½ ρV². Let me unpack that. The Greek letter ρ is rho, the density of the surrounding air. V is the true airspeed, which we abbreviate TAS. So dynamic pressure equals half the air density times the true airspeed squared. In other words, if you know the dynamic pressure and the air density, you can work out the speed. The ASI does exactly that — it measures dynamic pressure and displays the result on a scale, usually calibrated in knots. And one knot is one nautical mile per hour. Now, here’s the subtlety. Dynamic pressure cannot be measured directly. You can’t put a probe in the airflow and read dynamic pressure off it. You can only measure the difference between pitot and static pressure. So the whole construction of the instrument is built around subtracting static from pitot to isolate that dynamic pressure. Let me walk you through the construction, because it’s elegant. The static pressure is fed into a hermetically-sealed instrument case — that means the case is completely airtight. Inside that case sits a thin metal capsule, and the pitot pressure is piped directly into that capsule. The capsule is capable of expansion and contraction — it’s like a little bellows that can flex. Here’s the clever part. The static pressure is present on both the inside and the outside of the metal walls of the capsule. Wait — the inside of the capsule has pitot pressure, not static. Let me re-read that carefully. The static pressure is present on both the inside and the outside of the metal walls of the capsule, and so it cancels. So the pressure differential between the inside and outside of the capsule is (Dynamic + Static) − Static, which is just Dynamic. The static pressure on the outside of the capsule pushes inward, and the static component inside the capsule pushes outward, so they cancel each other out. What’s left acting on the capsule walls is purely the dynamic pressure. So the expansion or contraction of the capsule is directly proportional to changes in dynamic pressure, which in turn are produced by changes in airspeed. Faster airspeed means more dynamic pressure, which means the capsule expands more. Finally, those capsule movements are transmitted through a temperature-compensated magnifying linkage to the pointer on the face of the ASI. The linkage magnifies the small capsule movements so the pointer moves a useful amount across the dial. And it’s temperature-compensated because temperature changes could cause the metal parts to expand or contract and give false readings — the compensation cancels that effect out. So the whole chain is: pitot head senses pitot pressure, static vent senses static pressure, the differential gauge subtracts static from pitot to get dynamic pressure, the capsule expands proportionally to dynamic pressure, the linkage magnifies that movement, and the pointer shows you airspeed in knots. One thing to note — the equation uses true airspeed, V, but the instrument is calibrated to display in knots. That’s a distinction we’ll build on later when we talk about the difference between indicated airspeed and true airspeed. For now, just hold onto the core idea: the ASI is a differential pressure gauge that measures dynamic pressure, and dynamic pressure is ½ ρV².