Let's begin with the first entry — Page 11, Lesson 17

We're starting a new section now — the glossary of definitions for the whole Principles of Flight course. This is the foundation. Every term we use for the rest of the syllabus gets defined right here, so I want to walk you through them properly. Let's begin with the first entry. We have the free stream Mach number — that's the Mach number of the undisturbed air relative to the aircraft — at which the peak velocity on the surface of a body first becomes equal to the local speed of sound. That's a critical aerodynamic boundary. When the fastest-moving air over a surface just reaches Mach 1.0 locally, that's the condition being described. We'll build on this concept later when we talk about critical Mach number and drag rise. Next, Damping — to slow down the rate; to diminish the amplitude of vibrations or cycles. Think of it as the mechanism that reduces the size of oscillations over time. Geometric Dihedral — the angle between the horizontal datum of an aeroplane and the plane of a wing or horizontal stabilizer semi-span. So if you look at a wing from the front, the upward angle of the wing from the horizontal reference line is the geometric dihedral. It's measured on the semi-span — half the wing. Divergent — inclined or turned apart. And Divergence — a disturbance which increases continually with time. So a divergent condition is one that gets worse, not better. Eddy — an element of air having intense vorticity. That's a small swirling mass of air with strong rotational motion. Effective Angle of Attack (αe) — the angle between the chord line and the mean direction of a non-uniform disturbed airstream. So when the airflow is disturbed and non-uniform, we use the mean direction of that flow to define the effective angle of attack, rather than the free stream direction. Now, Equilibrium — a condition that exists when the sum of all moments acting on a body is zero AND the sum of all forces acting on a body is zero. Both conditions must hold simultaneously. No net force, no net moment. Fairing — a secondary structure added to any part to reduce its drag. It's an aerodynamic smoothing cover. Feel — the sensations of force and displacement experienced by the pilot from the aerodynamic forces on the control surfaces. That's the tactile feedback you get through the controls. Fence — a projection from the surface of the wing and extending chordwise to modify the wing surface pressure distribution. So it's a vertical plate on the wing, running from leading edge to trailing edge, that changes how pressure is distributed over the wing. Fillet — a fairing at the junction of two surfaces to improve the airflow. So where two surfaces meet, like a wing and fuselage, the fillet smooths that junction. Flight Path — the path of the Centre of Gravity (CG) of an aircraft. So the aircraft's trajectory is tracked by its centre of gravity. Fluid — a substance, either gaseous or liquid, that will conform to the shape of the container that holds it. Air is a fluid, and so is water. Free Stream Velocity — the velocity of the undisturbed air relative to the aircraft. That's the airflow far ahead of the aircraft, before it's been disturbed by the aircraft's presence. Gradient (Pressure) — rate of change in pressure with distance. So how quickly pressure changes as you move through space. Gust — a rapid variation, with time or distance, in the speed or direction of air. Gust Lock — see control lock. That's a cross-reference; we'll cover control lock elsewhere. Instability — the quality whereby any disturbance from steady motion tends to increase. So an unstable aircraft amplifies disturbances rather than resisting them. Laminar Flow — flow in which there is no mixing between adjacent layers. The air moves in smooth, parallel layers without turbulence between them. Now, Load Factor — the ratio of the weight of an aircraft to the load imposed by lift. The correct symbol for load factor is (n), but it's colloquially known as (g). So the formula is: Load Factor = Lift / Weight So if lift equals weight, load factor is 1. If lift is twice the weight, load factor is 2. Mach Number (M) — the ratio of the True Airspeed to the speed of sound under prevailing atmospheric conditions. The formula is: M = TAS / Local Speed of Sound (a) So Mach number is True Airspeed divided by the local speed of sound. Magnitude — largeness; size; importance. Now, Moment (N-m) — the moment of a force about a point is the product of the force and the distance through which it acts. The distance in the moment is merely a leverage and no movement is involved, so moments cannot be measured in joules. That's an important distinction — even though the units are newton-metres, which look like energy units, a moment is not energy because there's no movement involved. It's a turning effect, a torque. Nacelle — a streamlined structure on a wing for housing engines (usually). So the engine housing pod on the wing. Normal — perpendicular; at 90°. Oscillation — swinging to and fro like a pendulum; a vibration; variation between certain limits; fluctuation. And finally, Parallel — lines which run in the same direction and which will never meet or cross. That's the full set of definitions for this opening section. These terms will recur throughout the entire course, so it's worth being comfortable with each one.