By Jordan Bullock CFI, Boeing 737 Pilot
The age old question every examiner is destined to ask: how does an airplane fly? Outside of pure magic, the answer can be a bit involved. Planes are designed to soar through the sky through a complex combination of power and aerodynamics. With advancing technology throughout the aviation industry new designs are constantly being introduced. Wing designs change, but the aerodynamic principles serving them don’t: Thrust, Lift, Drag and Weight.
Where and how an airplane get’s its power varies as well. Propellers were the first power plant installed in an aircraft, and still serves its purpose today. Though, now we have turbo props, jet props and jet engines. With ever changing technology and advancements, the four forces which move a plane through the air remain the same. As I have stated above, the four principles of flight are thrust, lift, drag and weight. As we discuss these four principles, think of an airplane from a two dimensional side view, this will help you understand how all four forces work together to achieve flight!
Let’s start with the simplest one: thrust. We need something to pull, or push, the airplane through the air. Most pilots learn on a simple single engine airplane (like a Cessna 172) powered by a piston engine with a propeller attached to the nose. As the propeller spins, it pushes air around the body, or fuselage, of the aircraft.
You can also think of the propeller as being the pulling force. In modern passenger-carrying Jet aircraft, the thrust is generated through high performance jet engines which essentially suck air in, compress it, and then ignite it to produce thrust out the back of the engine and push the jet into the air. In fighter jets, this thrust is located on the tail-end of the aircraft.
Thrust must overcome drag in order to achieve flight.
So now we have our thrust which, for lack of better terms, pushes us down a runway. How do we get in the air? Well, we rely on lift. Lift is achieved through the design of the wings. Similar to a bird's wings, the wings on an airplane are designed the same way and with the same mission: force the high pressure to the underside of the wing.
If you know nothing about air pressure, you should know one thing. Low pressure will always try to get to high pressure. It’s true in weather and it’s true when talking about lift. The shape of the airfoil on a wing produces an effect that creates high pressure air below the wing. Conversely, their is faster moving lower pressure air above the wing. Thus, the wing is essentially "sucked" upwards. The low pressure above the wing pulls in the high pressure below the wing. This is how lift is achieved.
Lift must overcome weight in order to achieve flight.
Drag is the force which will act opposite towards the forward motion of the object. This is probably the easiest to grasp and simplest principle of flight to discuss. Think of your airplane flying in the sky. Easy right? Now attach a giant metal square box to the front of the wing, what happens to the airflow that is supposed to hit the wing and separate, sending the high pressure beneath and the low pressure above? The airflow is halted, and fails to produce lift. Drag is created by air friction against the body of the plane itself.
Drag must be overcome by thrust in order to achieve flight.
Weight is the force caused by gravity which acts counter to lift. Let’s assume our airplane is in the sky cruising along. If we pack our single engine prop plane up to the brim with luggage and people, we now weigh more. This will result in us needing more and more lift to stay in the sky.
Understanding the four principles of flight and how they correspond with one another is a vital step to becoming a pilot. As you go through training, building a foundational understanding of aerodynamics will serve you well, and it starts with understanding these four basic principles.
By Jordan Bullock CFI, Boeing 737 Pilot
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