The Atmosphere — Page 32, Lesson 38

I want to walk you through the very foundation of this subject: the atmosphere, and why a pilot has to understand it. We're starting with something called dynamic pressure, and I want you to hold onto this idea because it drives everything else in Principles of Flight. Here's the problem a pilot faces. You need to know how much dynamic pressure is available to you in flight. But you can't measure dynamic pressure on its own. Why not? Because static pressure is always present, all around you, pressing on everything. So any instrument you have is always seeing static pressure mixed in with whatever dynamic pressure is there. So we define a combined quantity. The sum of static and dynamic pressure, in this context, is called 'Total' pressure. And you'll also hear this same sum referred to as Stagnation pressure, or Pitot pressure. Those three names — Total, Stagnation, Pitot — all mean the same thing in this context: static plus dynamic. Let me write that relationship out for you. Total Pressure equals Static Pressure plus Dynamic Pressure. And we can rearrange that equation to solve for what we actually want. Total Pressure minus Static Pressure equals Dynamic Pressure. That's the key takeaway — subtract the static part away, and what's left is the dynamic pressure you're after. Now, I cannot overemphasize how significant dynamic pressure is to understanding Principles of Flight. It's the whole ballgame. And here's the crucial part: dynamic pressure depends on two things — the density of the air, and the speed of the aircraft through the air. So if you want to understand dynamic pressure, you have to fully appreciate what affects air density. That's what the rest of this lesson is about. There are three factors that change air density, and I want you to know each one precisely. First, temperature. Increasing temperature decreases air density. Warm air is less dense. And changes in air density due to temperature are significant during all phases of flight — not just some, but all of them. Second, static pressure. Decreasing static pressure decreases air density. Lower pressure, less dense air. And again, changes in air density due to static pressure are significant during all phases of flight. Third, humidity. Increasing humidity decreases air density. Now, why is that? Because the density of water vapour is about 5/8 that of dry air. So when you add water vapour into the air, you're replacing some of that denser dry air with lighter water vapour, and the overall density drops. Humidity is most significant during take-off and landing — that's when it really matters to you as a pilot. Now, there's one more relationship that ties these together. What happens as you climb, as altitude increases? Air density decreases. Why? Because the effect of decreasing static pressure is more dominant than the effect of decreasing temperature. Both are changing as you go up — pressure drops, temperature drops — but the pressure effect wins out, and the net result is that density goes down with altitude. So let me pull it all together. Dynamic pressure is what you need, and it equals total pressure minus static pressure. Dynamic pressure depends on air density and aircraft speed. And air density is governed by temperature, static pressure, and humidity — each with its own direction of effect, and each with its own significance in different phases of flight. That's the foundation you're building on for everything that comes next in this course.