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Four forces keep an airplane in the sky. They are lift, weight, thrust and drag.

Lift pushes the airplane up. The way air moves around the wings gives the airplane lift. The shape of the wings helps with lift, too.

Weight is the force that pulls the airplane toward Earth. Airplanes are built so that their weight is spread from front to back. This keeps the airplane balanced.

Don't forget the pilot! Image Credit: NASA Thrust is the force that moves the airplane forward. Engines give thrust to airplanes. Sometimes an engine turns a propeller. Sometimes it is a jet engine. It doesn't matter as long as air keeps going over the wings.

Drag slows the airplane. You can feel drag when you walk against a strong wind. Airplanes are designed to let air pass around them with less drag.

An airplane flies when all four forces work together. But, most airplanes need one more thing: They need a pilot to fly them!

Two primary principles contribute to the creation of lift, which is what makes flight possible. Those two principles are Bernoulli's Principle and Newton's Third Law. Let's break it down and look at each principle individually.

Bernoulli's Principle By definition, Bernoulli's Principle states:

For an inviscid flow, an increase in the speed of the fluid occurs simultaneously with a decrease in Pressure. From a practical standpoint, this basically means that as a fluid (air, water, etc) moves faster, it's internal pressure decreases.

But how does this help an airplane create lift?

Well, let's think about this. Picture an airplane's wing - but cut in half so we can see the shape of it (referred to as an airfoil). The top of the wing is more curved than that of the bottom of the wing. The reasoning behind this is that the increased curvature on top of the wing will take advantage of something called magnus effect.

Magnus effect? What the heck is that? Well - before we continue let's define magnus effect in a nutshell. I'll do this through an example. Close your eyes and envision a baseball game. How does the pitcher get the ball to move in a desired direction? He or she can curve the direction of the ball's flight left, right, down, or even up if desired. But how?

Well, magnus effect states that a rotating ball or cylinder moving through a fluid (air, water, etc) will create faster moving fluid in the direction of rotation, thus lowering pressure and "pulling" the ball or cylinder in that direction. This force is not created when the object is stationary, which is why a baseball pitcher puts a "spin" on the ball when he or she wants a curveball.

Phew - okay, so back to our discussion on the wing. So we know the top of the wing is more curved than the bottom. But how does that have anything to do with magnus effect? Basically, the shaping of the wing "fools" the air around it into thinking it is a long rotation cylinder, and forces the air to travel faster over the top of the wing than that of the bottom. And according to Bernoulli's Principle, faster moving air = lower pressure. If we have lower pressure on top of the wing than we do on the bottom of the wing, we now have an inequality of pressures acting on the wing. There is more pressure pushing up on the bottom of the wing than there is on the top pushing down, which means we now have a total net force pushing UP. And voila.. we have LIFT.

With real airplanes and airplane models, Bernoulli's Principle (related to the curvature of the wing) and the magnus effect have very little to do with "flying". If what the author says were true - real airplanes could not fly upside down. But if you have ever seen an air show -- you know real, powered planes regularly fly upside down in air shows.

Almost all of the lift created on the underside of a wing is created as (A) the underside of the wing is blasted by the air rushing past the wing -- because the airplane engine is pulling the airplane very fast through the air and (B) the plane's geometrical configuration is holding the wing at an angle such that the front edge of the wing is a little higher in the direction of flight than the back edge of the wing. A & B, together mean the air pressure on the bottom of the wing is higher than the pressure on the top of the wing. Thus the wing is forced upward. You can get the same effect by holding your flat hand out the window of a moving car and tilting the front edge of your hand up or down.

Large gliders, in general, can't fly upside down because they do not have engines. They get lift from surrounding air currents which force the plane upward by either (a) rising warm air -- called "thermals" or (b) air that is being directed upward by a hill. Gliders can take off at one height and land at a higher elevation if they are near thermals and air moving over hills or mountains.

Clouds fly because they are lighter than the air around them.

Newton's 3rd Law This one is much less complicated than Bernoulli's Principle.

Newton's 3rd Law is defined as: To every action there is always an equal and opposite reaction.

Airplane wings are fixed onto the airframe of the plane at a slight angle. It may not be easy to see for the untrained eye, but upon a close examination of an wing's attachment point to the body of a plane, one will see a slight angle. This angle creates a deflection of air downward. As air hits the underside of the wing, even in straight-and-level cruise flight, it is forced downward. And according to Newton's 3rd Law, the forcing of the air downward causes an equal and opposite upward force on the wing, thus contributing to the creation of lift.

Now, one thing worth mentioning about Newton's 3rd Law is that its contribution to lift is highly debated within the industry. Some say it only has a tiny effect, while others argue it has barely any effect.

I will say this much; without Newton's 3rd Law the creation of lift would be MUCH more difficult. It contributes greatly to the creation of lift. To make an example out of this, picture an aerobatic airplane in knife-edge flight (flying on its side). Bernoulli's Principle doesn't properly work in this configuration because the wing is at an unusual attitude. So I ask you this: how does the plane stay aloft? Newton's 3rd Law is the only explanation for such a flight scenario. As air is hitting the airframe and control surfaces of the plane, it is being forced downward at a great rate, creating an opposing force - LIFT - in the opposite direction.

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