The Atmosphere — Page 25, Lesson 32

I want to walk you through the very start of Principles of Flight, and we begin with the medium everything happens in — the atmosphere. Think of it as the ocean the aircraft swims through. It's the properties of that atmosphere, changed by the shape of the wing, that generate the lift force we need. And the single most important property is air density — the "thickness" of the air. Here's the key fact I want you to hold onto: if air density decreases, the mass of air flowing over the aircraft in a given time will decrease. Now, mass flow is measured in kilograms per second — kg/s. We don't usually dwell on it in this subject, but keeping that idea in the back of your mind helps a lot. A given mass flow will generate the required lift force. So if density drops, the mass flow drops, and lift would drop with it. To maintain that required lift force when density decreases, the speed of the aircraft through the air must be increased. The increased speed of airflow over the wing keeps the mass flow — and therefore the lift force — at its required value. So density down, speed up. That's the fundamental trade. Now let's look at the physical properties of air, because air has substance. It has mass — not much compared to other matter, but a significant amount. A mass of moving air carries considerable kinetic energy. For example, at 100 knots, the kinetic energy of air can inflict severe damage to man-made structures. So never think of air as nothing. Air is a compressible fluid — it can flow and change its shape when subjected to even minute pressure differences. And here's the direction rule: air will flow in the direction of the lower pressure. Its viscosity is so low that very small forces can move the molecules in relation to each other. Viscosity is essentially the internal friction of a fluid — how much it resists layers sliding past each other. Air has almost none. Now, consider the portion of atmosphere where most aircraft operate — up to 40,000 feet. As altitude increases, the characteristics of air undergo a gradual transition from those at sea level. Because air is compressible, the lower layers contain much the greater part of the whole mass of the atmosphere. So most of the air is down low. Pressure falls steadily with increasing altitude. Temperature, however, falls steadily only up to about 36,000 feet — above that it remains constant through the stratosphere. So pressure keeps dropping, but temperature stops dropping at that level. Next, static pressure. The unit is newtons per square metre, N/m², and the symbol is lower-case 'p'. Static pressure is the result of the weight of the atmosphere pressing down on the air beneath. It exerts the same force per square metre on all surfaces of an aeroplane — so it pushes equally on the top, bottom, and sides. The lower the altitude, the greater the force per square metre. It's called static pressure because of the air's stationary, or static, presence. And an aircraft always has static pressure acting upon it — you can never escape it. Now, the units. Newtons per square metre is the SI unit for pressure. 1 N/m² is called a pascal, and it's quite a small unit. In aviation we use the hectopascal, hPa. 'Hecto' means 100, so 1 hectopascal is 100 pascals. And here's the handy equivalence: 1 hectopascal is the same as 1 millibar. So when you hear a QNH of 1013 hPa, that's the same as 1013 millibars — same pressure, two names. So to tie it together: the atmosphere is compressible, most of its mass sits in the lower layers, pressure falls steadily with altitude, temperature falls only to about 36,000 feet then holds steady, and static pressure — measured in hPa — is always acting on the aircraft. And remember the density–speed relationship, because that's the engine behind lift generation.