Overview and Definitions — Page 11, Lesson 15

Let’s start with the very foundation of this chapter: the definitions that govern how we talk about motion and force in aviation. First, acceleration. Its unit is metres per second per second, written as m/s². I want you to think of it as the rate of change of velocity — how quickly your speed is increasing or decreasing. The key relationship here is Newton’s Second Law, expressed as a formula: Acceleration = Force ÷ Mass. So, for the same mass, a bigger force gives a greater acceleration. And for the same force, a larger mass gives a slower acceleration. That’s why a heavy aircraft needs more thrust to accelerate at the same rate as a light one. Now, a specific example to lock it in: a force of 1 newton acting on a mass of 1 kg will produce an acceleration of 1 m/s². That’s the definition of the newton itself — it’s the force needed to accelerate one kilogram at one metre per second squared. Next, momentum. Its unit is mass × velocity, written as kg-m/s. Momentum is the quantity of motion possessed by a body — the tendency of a body to continue in motion after being placed in motion. Here’s the worked example: a body of 10 kg mass moving at 2 m/s has 20 kg-m/s of momentum. And note the contrast: at the same velocity, a large mass has more momentum than a small mass. That’s why a fully loaded aircraft is harder to stop than an empty one at the same speed. Then we have Newton’s Third Law: every action has an equal and opposite reaction. In practical terms, if a force accelerates a mass in one direction, the body supplying that force will be subject to the same force in the opposite direction. That’s the principle behind thrust — the engine pushes air backwards, and the aircraft moves forwards. Now let’s move into the glossary, because this chapter is built on precise definitions. I’ll go through them in order. Aerofoil — a body so shaped as to produce aerodynamic reaction normal to the direction of its motion through the air without excessive drag. So the lift force acts at right angles to the airflow, and the shape is designed to do this efficiently. Aft — to the rear, back, or tail of the aircraft. Simple positional term. Air brake — any device primarily used to increase drag of an aircraft at will. So it’s a pilot-controlled drag device. Ambient — surrounding, or pertaining to the immediate environment. Ambient temperature, ambient pressure — the conditions around you. Amplitude — largeness, abundance, width, range, extent of repetitive movement, from extreme to extreme. So for an oscillation, amplitude is the full swing from one extreme to the other. Attitude — the nose-up or nose-down orientation of an aircraft relative to the horizon. This is about pitch, not direction of travel. Boundary Layer — the thin layer of air adjacent to a surface, in which the viscous forces are dominant. That’s the layer where friction with the surface matters most. Buffeting — an irregular oscillation of any part of an aircraft, produced and maintained directly by an eddying flow. So turbulent airflow causes the structure to vibrate irregularly. Cantilever wing — a wing whose only attachment to the fuselage is by fittings at the wing root; it has no external struts or bracing. The attachments are faired-in to preserve the streamline shape. So the wing is self-supporting, with no wires or struts outside. Control Lock, also called Gust lock — a mechanical device designed to safeguard, by positive lock, the control surfaces and flying control system against damage in high winds or gusts when the aircraft is parked. It physically locks the controls. Control Reversal — this one has two distinct meanings, and you need both. At high speed: the displacement of a control surface producing a moment on the aircraft in a reverse sense because of excessive structural distortion. At low speed: the displacement of an aileron increasing the angle of attack of one wing to or beyond the critical angle, causing a roll in the direction opposite to that required. So in both cases, the aircraft responds opposite to what you commanded. Convergent — tend towards or meet in one point or value. Convergent airflow, convergent duct — things coming together. Critical Mach Number, abbreviated MCRIT — the free stream Mach number at which the peak velocity on the surface of a body first becomes equal to the local speed of sound. That’s the speed at which the first point on the aircraft reaches Mach 1 locally, even though the aircraft itself is still below Mach 1. That’s where the excerpt cuts off — we’ve covered acceleration, momentum, Newton’s Third Law, and the glossary up through Critical Mach Number. When you’re ready, we’ll continue with the next definitions.