Overview and Definitions — Page 18, Lesson 21

Right, let's get into the heart of this. We're starting the formal definitions that underpin the whole of Principles of Flight, and I want to walk you through them properly because these aren't just words in a glossary—they're the precise tools we use to describe and analyse everything an aircraft does. Let's begin with the Pitot Tube. This is a tube with an open end facing upstream—that means the open end points directly into the oncoming airflow. Now, here's the key physical fact: at speeds less than about four tenths of the speed of sound, the pressure inside that tube is equal to the total pressure. And for practical purposes at this stage of your training, we regard total pressure as equal to pitot pressure. So the pitot tube is our instrument for capturing the total pressure of the airstream, which is the sum of the static pressure and the dynamic pressure from the air's motion. Next, a Pod. This is simply a nacelle—an engine housing—that is supported externally from a fuselage or a wing. So instead of being buried inside the structure, it hangs out on a pylon. Then we have Propagate. This is a verb meaning to pass on, to transmit, or to spread from one to another. You'll hear this most often when we talk about sound waves or pressure waves propagating through the air. Now, one of the most important concepts you'll use constantly: Relative Airflow, also called Relative Wind or Free Stream Flow. This is the direction of airflow produced by the aircraft moving through the air. The critical point is that the relative airflow flows in a direction parallel and opposite to the direction of flight. So if the aircraft flies forward, the relative airflow comes at it from the front. This means the actual flight path of the aircraft determines the direction of the relative airflow—not the aircraft's attitude, but its true path through the air. The term also refers to the air in a region where pressure, temperature, and relative velocity are unaffected by the passage of the aircraft through it. That's the "free stream" condition—the undisturbed air far ahead of the aircraft. Let's look at Scale. If we consider a 1/10th scale model, all the linear dimensions are 1/10th of the real aircraft. But here's the trap: the areas are 1/100th—because area is length squared—and if the model is constructed of the same materials, the mass is 1/1000th of the real aircraft, because mass scales with volume, which is length cubed. So the model is to scale in some respects, but not others. This is a crucial limitation when we test models in wind tunnels. A Schematic is a diagrammatic outline or synopsis—an image of the thing, representing something by a diagram. It's a simplified, symbolic drawing rather than a realistic picture. Separation is a term you'll hear a lot. It's the detachment of the airflow from a surface with which it has been in contact. When the boundary layer can no longer follow the surface, it breaks away—that's separation, and it's central to stalling. Now, a Shock Wave. This is a narrow region crossing the streamlines, through which there occur abrupt increases in pressure, density, and temperature, and an abrupt decrease in velocity. The key detail is that the normal component of velocity relative to the shock wave is supersonic upstream and subsonic downstream. So the air hits the shock at supersonic speed, and leaves it at subsonic speed, with those abrupt property changes across that thin region. Side-slip is the motion of an aircraft, relative to the relative airflow, which has a component of velocity along the lateral axis. The lateral axis is the axis running wingtip to wingtip. So in a sideslip, the aircraft has a sideways velocity component relative to the air it's moving through. A Slat is an auxiliary, cambered aerofoil positioned forward of the main aerofoil so as to form a slot. It's a high-lift device—a small curved surface ahead of the wing's leading edge, and the gap between them is the slot. A Spar is a principal spanwise structural member of a wing, tailplane, fin, or control surface. "Spanwise" means running along the span—from root to tip. It's the main load-bearing beam of the structure. Now, Speed. In most formulae we use metres per second, m/s. But in aviation we commonly measure aircraft speed in nautical miles per hour, or knots, abbreviated kt. And here are the conversion facts you need: there are 6080 feet in 1 nautical mile, and 3.28 feet in 1 metre. Next, the Speed of Sound, denoted by the symbol a. Sound is pressure waves which propagate spherically through the atmosphere from their source. The crucial fact is that the speed of propagation varies ONLY with the temperature of the air. The lower the temperature, the lower the speed of propagation. On a 'standard' day at sea level, the speed of sound is approximately 340 m/s, which is 660 knots TAS—true airspeed. So remember: temperature is the only variable that changes the speed of sound. Stability is the quality whereby any disturbance of steady motion tends to decrease. In other words, if something pushes the aircraft off its steady path, stability is the tendency for that disturbance to die away and return toward the original condition. A Stagnation Point is a point where streamlines are divided by a body and where the fluid speed is zero, relative to the surface. Think of the air hitting the leading edge of a wing—at one specific point, the flow comes to a complete stop. That's the stagnation point, and it's where the pressure is highest. A Static Vent is a small aperture in a plate fixed to form part of the fuselage, and it's located appropriately for measuring the ambient static pressure. This is the counterpart to the pitot tube—it samples the undisturbed static pressure of the surrounding air. Finally, the Throat is a section of minimum area in a duct. This is critical in convergent-divergent ducts, where the flow accelerates through the throat—the narrowest point. Now, let me tie a few of these together, because they work as a system. The pitot tube gives you total pressure, the static vent gives you static pressure, and the difference between them is the dynamic pressure, which is what drives your airspeed indicator. That's the schematic you'll see in the instrument. And the stagnation point is exactly where that total pressure is captured at the pitot tube's open end. That figure shows the schematic of the airspeed indicator—the pitot tube feeding total pressure, the static vent feeding static pressure, and the diaphragm inside that senses the difference. Keep that picture in mind as we build on these definitions.