Overview and Definitions — Page 11, Lesson 10

Let’s start with the big picture, because everything in this chapter hangs off one idea: an aeroplane in motion has four main forces acting on it — WEIGHT, LIFT, THRUST and DRAG. I want you to hold those four in your head as the skeleton of the whole subject. To generate lift, the aeroplane must be propelled forward through the air by a force called THRUST, and that thrust comes from the engine or engines. So thrust is the force that pushes the aeroplane forward. The moment the aeroplane begins to move, the air resists that forward motion with a force called DRAG. So drag is the resistance of the air opposing the motion. And these four forces are all closely interrelated — they form a chain. The greater the weight, the greater the lift requirement. The greater the lift, the greater the drag. The greater the drag, the greater the thrust required. And so on. So you can see they’re not independent; they feed into each other. Now, a couple of important realities before we go deeper. Air has properties that change with altitude, and knowing those variables and their effect on the aeroplane is a prerequisite for fully understanding the principles of flight. Also, the structural and aerodynamic design of an aeroplane is a masterpiece of compromise — an improvement in one area frequently leads to a loss of efficiency in another. And one more crucial point: an aeroplane does not ‘grip’ the air the way a car grips the road. An aeroplane is often not pointing in the same direction in which it is moving. Keep that in mind — it will matter a lot later. Now let’s get into the general definitions, because these are the exact terms you’ll use for the rest of your career. First, MASS. Its unit is the kilogram, abbreviated kg. The definition is: ‘the quantity of matter in a body.’ A body here means a substance — any substance, a gas, a liquid, or a solid. The mass of a body is a measure of how difficult it is to start or stop. So the larger the mass, the greater the force required to start or stop it in the same distance. And mass has a big influence on the time and/or distance required to change the direction of a body. Next, FORCE. Its unit is the newton, abbreviated N. The definition is: ‘a push or a pull’ — that which causes or tends to cause a change in motion of a body. And here’s the connection: there are four forces acting on an aircraft in flight, pushing or pulling in different directions — those are our weight, lift, thrust and drag. Next, WEIGHT. Its unit is also the newton. The definition is: ‘the force due to gravity.’ And here’s the formula: F = m × g. In that formula, m is the mass of the object, and g is the acceleration due to the gravity constant, which has the value 9.81 metres per second squared. So a 1 kg mass ‘weighs’ 9.81 newtons. Let me give you the worked example from the text: if the mass of a B737 is 60 000 kg, and F = m × g, then we need 60 000 kg × 9.81 m/s², which gives 588 600 N of lift force. So that’s the lift force that has to be generated just to hold that weight up. Next, CENTRE OF GRAVITY, often abbreviated CG. This is the point through which the weight of an aircraft acts. Two key facts: an aircraft in flight rotates around its CG. And the CG of an aircraft must remain within certain forward and aft limits, for reasons of both stability and control. So that’s a limitation you’ll always respect. Finally, WORK. Its unit is the joule, abbreviated J. A force is said to do work on a body when it moves the body in the direction in which the force is acting. The amount of work done on a body is the product of the force applied to the body and the distance moved by that force in the direction in which it is acting. So Work = Force × Distance, where distance is through which the force is applied. And here’s the critical qualifier: if a force is exerted and no movement takes place, no work has been done. The example given: if a force of 10 newtons moves a body 2 metres — and that’s where the excerpt cuts off, but you can see the pattern: 10 newtons times 2 metres gives you 20 joules of work. So to tie it together: mass is the quantity of matter, force is a push or pull, weight is the force due to gravity calculated as m × g, the centre of gravity is where that weight acts and around which the aircraft rotates, and work is force times distance moved in the direction of the force. These definitions are the foundation — every later chapter builds on them.