Flying was once a dream of our ancestors and we are living their dreams at present. Don’t you think flying is one of the biggest achievements of human beings? In a sense, we defied the rules of nature that didn’t give us wings to fly. But we have made airplanes now that can fly faster than the birds nature created. This very thing makes us humans, we are the ones who can go beyond the boundaries set by nature. This article is about aerodynamics forces and how an aircraft flies . Do trust me, it is so simple that you don’t necessarily have to be an aerospace aficionado to understand this.

You can simply understand how an aircraft flies simply by understanding the aerodynamics forces. Aerodynamics is the study of the properties of moving air and the interaction between the air and solid bodies moving through it. Simply, Aerodynamics is the way objects move through air and the rules of aerodynamics explain how an aircraft is able to fly. There are four major aerodynamics forces in an aircraft in a steady horizontal flight. They are Lift, Drag, Thrust, and Weight.

Aerodynamics forces in an aircraft

Lift Force

As you can see in the figure above, lift is the aerodynamics force acting in the upward direction. This upward forces balances the weight of the body and if it is greater than the weight the aircraft moves higher. So, how is lift generated? Lift is generated by aircraft wings.

Shape of Wing airfoil

See the shape of wing airfoil. This shape is what makes it possible for aircraft to fly. The aircraft wing is curved on top and flatter on bottom.

Usually it is believed that when air rushes over the curved upper wing surface, it has to travel further than the air that passes underneath, so it has to go faster (to cover more distance in the same time). According to a principle of aerodynamics called Bernoulli’s law, fast-moving air is at lower pressure than slow-moving air, so the pressure above the wing is lower than the pressure below, and this creates the lift that powers the plane upward. This is called equal transit time theory. Experimentally, it is found that two air molecules passing through upper and lower surface don’t necessarily meet at the trailing edge. In fact, the air molecule through the upper section reaches the trailing edge earlier. So, this explanation is wrong.

So what’s the real explanation? As a curved airfoil wing flies through the sky, it deflects air and alters the air pressure above and below it.

Why does this happen? As air flows over the curved upper surface, its natural inclination is to move in a straight line, but the curve of the wing pulls it around and back down. This creates curved streamline. Because the air is changing direction there must exist a centripetal force acting normal to the direction of motion. This force can only be generated by pressure differences (all other forces are ignored), which implies that the pressure on one side of the particle is greater than that on the other.

flow of a particle in a curved streamline

In other words, if a streamline is curved, there must be a pressure gradient across the streamline, with the pressure increasing in the direction away from the centre of curvature. As a result, there is low pressure at the upper section of airfoil and hence the airflow is faster.

In essence, An airfoil splits apart the incoming air, lowers the pressure of the upper air stream, and accelerates both air streams downward. As the air accelerates downward, the wing (and the plane) move upward. The more an airfoil diverts the path of the oncoming air, the more lift it generates. Learn more about how lift is generated here.

Lift of an aircraft is calculated by using the formula;

The lift coefficient mentioned in the formula depends upon the type of airfoil used and the angle of attack. Also click here to find out how an airfoil is manufactured

Drag Force

In every motion, there is certainly presence of some resistive force. Hence, drag is the resistive force that opposes the motion of aircraft. The shape of an object also affects the amount of drag. For example, round surfaces usually have less drag than flat ones and narrow surfaces usually have less drag than wide ones. The more air that hits a surface, the more the drag the air produces.

Whenever we talk about maintaining aerodynamic shape of an object we are focusing on minimizing the drag force. So, an aircraft is designed in the way that produces minimum drag and maximum lift.

Notice that the formula to calculate lift and drag are similar except the coefficients. Like lift coefficient, drag coefficient also depends upon wing shape and angle of attack. Lift also induces certain amount of drag so there exist a relation between lift and drag coefficient.

lift drag polar



Thrust

Thrust is the force produced by the engine that pushes the airplane forward. The airplane moves in forward direction when thrust is higher than the drag force which is in opposite direction. So, how is thrust produced?

Turbojet engine

For this, let’s talk about the functon of engines. All the engines whether propeller or jet, take in air through the inlet, compress the air through compresser, burns with fuel in combustion chamber generating energy in turbine. Then the high pressure air exits through the nozzle with high velocity. I am sure you do know Newton’s third law of motion. When high speed air exits from the engine, it pushes the plane in forward direction.

Have you ever wondered why engines nowadays are bigger? Look at the formula above and we can find that thrust increases if we increase the mass flow rate. Which is why, engines are now bigger for passing high amount of air through inlet.

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Weight

Weight is the simplest of all forces but it is the most crucial one. The main law of any aircraft structural engineer is:”thou shall design as light as possible”. Let me tell you a wonderful fact. Even a slight weight reduction in design can harvest further reductions. This effect is called snowball effect.

For example; if you reduce weight of an aircraft then you require less amount of lift force to fly. Lower lift required means you can decrease the wing size. Again, smaller engines result in less drag so less thrust is required which can be achieved by smaller engines. In the end, smaller engines and wing size lead to further reduction in aircraft weight.

Hence, learning about all four aerodynamics forces helps to know how an aircraft flies. Feel free to give your thoughts in the comment.

Also check out our post about Aerodynamics forces on a Car here https://www.geniuserc.com/basics-of-car-aerodynamics-drag-and-downforce/

You can visit this wikipedia link to learn more in detail about aerodynamics https://en.wikipedia.org/wiki/Aerodynamics