<< Page 1 Page 2: Page 3 >>   The Four Forces We’re going to start with the very basics here. If you have ever read anything about airplanes, even most kids’ books, you know that there are four forces that rule an airplane: Weight, Thrust, Lift and Drag, in no particular order. Weight is, well, weight. Because of gravity, the stuff that makes up an airplane (like anything else) is attracted towards the center of the earth. So weight always points straight down. You can’t ever really change weight (other than by making your plane lighter). Thrust happens because the engine of the plane pushes air and exhaust gases backwards, so air and the exhaust gases exert a forwards force on the engine (Newton’s Third Law). Sometimes, some airplanes’ engines (in some helicopters and some military aircraft) can push the air and exhaust gases in directions other than backwards, so this pushes the plane in directions other than forwards. This is true in very few, rare, unusual airplanes, as we’ll see, and even in those, thrust pushes forward nearly always. Lift is the upwards force generated by the wing (and sometimes by the fuselage, tail, and other features, in smaller amounts). It occurs because the wings divert air downwards when air is blown by them (Newton’s 3rd again). We will see why this happens. Lift is always directed upwards or just-off-upwards: for example, when an airplane makes a banked turn, some lift is generated towards the inside of the turn, pulling the plane inwards as well as up, so it turns. Drag is, simply, air resistance. It is the force the air exerts on an airplane (or anything else moving through the air, or water, or any fluid) trying to slow it down. Drag always points opposite to the direction of flight. As an airplane flies through the air, it drags some of the air forward with it, so that air pulls the airplane back (you guessed it – Newton’s 3rd again). This is in fact caused by different forces and different effects, always changing in many ways. Some of it is quite non-intuitive. For example, we will see how drag sometimes is higher at low speeds than at higher flight speeds. As you probably have gathered from this, drag balances thrust and lift balances weight. An imbalance of weight and lift would cause the airplane to climb or drop, and an imbalance in drag and thrust would cause the airplane to accelerate or slow down.