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Theory of Flight

The theory of flight or aerodynamics is an amazing topic, people have for years looked up at something flying through the air and wondered, "how does it fly?" The following information is designed to make it as clear as possible for anyone to understand theory of flight, it can be a simple subject to understand, or for those that really want too, a deep and complex one, depending of course on how deep you are willing to go.

I am not sure how deep we will get, let's just take it a piece at a time and see how we go. I am writing this from a position of 30 years of interest and working life in aviation, and theory of flight is one of the first topics I tackled when I was still in school. Hopefully you will learn something here, I will endeavour to make it as interesting yet as simple as possible.

Get Something for Nothing?

I have always stood by the saying that with anything mechanical, you do not get something for nothing. It is the law of conservation of energy, energy is only ever transferred from one object to another in its various forms. One of the most common every day item to observe this law is a light bulb, the electrical energy is converted into light and heat, yes, heat too, some energy is lost to heat. It is the same for an internal combustion engine, only around 30% of energy put into the engine ends up driving the wheels, the rest is lost in heat, out the exhaust pipe, and the radiator, even the gearbox and differential generates heat as the power from the engine is passed through it. So it is with an aircraft, forgetting for now the engine side of an aircraft, if you want an aircraft to fly, you need to move it through the air, we will get to how the wings work soon, but first, forward motion is essential to get things moving. Most aeroplanes have one or more engines somewhere, be it on the wings, fuselage or the tail, pointing forwards or backwards, an aeroplane needs something to move it forward. This energy required to move an aircraft forward is almost always in the form of thrust.

Forces at Work

There are four forces at work on any aircraft, thrust, drag, lift and weight, and these forces are opposing each other, trying to make the aircraft sit still or maintain its current position or state. When something is sitting still or at a constant state, in regard to the four forces, it is in equilibrium. To be in equilibrium means that all four forces are equal, thrust equals drag, and lift equals weight. To alter one of these, the equilibrium must be changed or upset, so to move forward thrust must increase, as it does, so too will drag, until they are once again equal. An aircraft will maintain its state until moved, this includes directional changes, the aircraft may be flying through the air at five thousand feet straight and level, it is in equilibrium, by decreasing the thrust, the aircraft will slow down until drag is equal, the other affect here is that lift will be decreased, so other factors must change to maintain height, more on that later, but for now to keep it simple, if lift decreases but weight has stayed the same the aircraft will descend until such point where lift once again equals weight.

There are many other factors at work here, the above is only a brief and simple explanation of the forces of flight, the very foundation required to continue. The next step - how is lift generated?

Bernoulli's Theory

Bernoulli's theory is the very basis for aerodynamics and the creation of lift, there are many ways to explain it, however one I remember from my early years in school seems the easy way- "When air (fluid) travels through a convergent divergent duct, the sum of all pressures is equal" - that's the simple way I remember it. The theary can be expanded as follows; there are two types of air pressure, static and pitot (or ram/velocity), static pressure is what we feel, or don't feel all around us, it is also referred to as barometric pressure and is measured in millibars, inches of mercury or hectopascals depending where you live. We don't really feel it as air is pressing on us from every angle equally, however pitot or ram air pressure is best described as the force you feel when you stick your hand out the window of a car while driving.

How does that fit with Bernoulli? As air travels through a duct that is converging, that is, getting narrower, it speeds up, as it speeds up the ram air pressure increases and the static air pressure decreases, the sum of both remains equal. As the duct opens out again, the air will slow down, ram air pressure decreases and static pressure once again increases - all the way, the sum of static and ram air pressure is always equal or constant. This gives us the basic theory needed to know how a wing works to create lift for subsonic flight, for supersonic flight it is a little different, remember here that the angle of attack is also very important.