How do planes fly?
Air travel is one of humanity’s greatest inventions but how exactly are such giant balls of metal able to fly? The answer is based on a branch of physics called aerodynamics. Aerodynamics is defined as the study of the interaction between an object (usually a solid) and air. There are 4 main forces of aerodynamics that allow planes to fly:
- Thrust: Force provided by the engines of an aircraft.
- Drag: Force that acts in the opposite direction of motion (thrust). Drag is caused by friction between the aircraft and air particles.
- Weight: The force that is caused by gravity. Proportional to the mass of the object.
- Lift: The force that allows planes to fly in the air. This force is mainly created by the wings of an aircraft
The fourth force listed above (lift), is the key component in describing how a plane is able to take off.
An airplane wing is shaped in an airfoil shape which makes it easier for the air passing above the wing to move faster and air moving below the wing to move slower. From a theory called Bernoulli’s principle (equation shown on the right), higher velocity (v2) results in less pressure (P2), holding all other variables constant. Pressure is proportional to force, which is given by the equation [P = F/A], where P = pressure, F = force, and A = area. Holding A constant, when pressure is raised, then the force must be greater as well. The air that is moving above the wing will exert a downward force, and the air moving below the wing will exert an upward force. But since the air moving below the wing is slower, the pressure is greater and therefore the force is greater. The upward force is able to counteract the downward force caused by the fast-moving air above the wings, which leads to a net upward force. This net force is defined as lift. The lift increases as the plane moves faster, eventually reaching a point where the upward force caused by the lift is able to overcome the downward force (weight) due to the aircraft. This is how an airplane is able to take off. The lift increases with speed since the lift equation [ L = C*(ρV*2)/2*A] (which is derived from Bernoulli’s principle) where L = lift, C = lift coefficient, ρ = density, V = velocity of the plane, A = surface area, shows that when the velocity is increased, the lift increases as well when holding all other variables constant. The lift coefficient is dependent on a variety of factors including: shape, surface roughness…etc.
Aerodynamics in cars
Even though aerodynamics is commonly used to describe how planes are able to fly, they are also used to design cars with improved fuel efficiency and top speeds. In car designing, the drag force is an important component. The equation of drag is given by [D = C*(ρV*2)/2*A], where D = drag force, and C = drag coefficient (the rest of the variables are defined in the previous section). The drag coefficient here plays an important role. The lower the drag coefficient, the lower the drag force, which is better. A cube for example, has a terrible drag coefficient, since air piles up in front of the brick creating a region of high pressure and the absence of air right behind the brick creates a region of low pressure. This pressure difference results in a huge drag force in the opposite direction of the brick’s motion. Therefore, it is reasonable to conclude that modern cars tend to have a more airfoil-like shape, because an object with the shape of an airfoil has less drag coefficient, allowing modern cars to go much faster than older cars with the same amount of power. In the previous section however, we’ve learned that an object with an airfoil shape leads to positive lift when travelling at high speeds. In a car, we do not want that since positive lift in a vehicle can lead to not only shaky rides but also catastrophic accidents. Cars therefore, often have splitters on the front and spoilers on the back of the car.
Splitters are small extensions added to the bumper of a car and they generate a region of high pressure in front of the car. This forces high pressure gas to flow above the car and low pressure gas to flow below the car. When this happens we get a negative lift force. A spoiler is a tail-like accessory that is attached to the trunk of a car and it disrupts the flow of the air moving above the car as shown on the figure in the right, making a region of high pressure gas and therefore, resulting in a net negative lift.
Sources
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[4] (Donut Media). (2018). “Wings and Spoilers; Lift and Drag | How It Works”. https://www.youtube.com/watch?v=AXjiThF1LXU&t=527s&ab_channel=DonutMedia Last Accessed: 2 August 2020.
[5] (2018). “What is A Car Lip Splitter and What Does It Do? | All-Fit Automotive”. All-Fit Automotive. https://allfitautomotive.com/blog/what-is-a-car-lip-splitter-and-what-does-it-do-allfit-automotive/ Last Accessed: 2 August 2020.
[6] Benson, Tom. (2018). “Modern Lift Equation”. National Aeronautics and Space Administration. https://wright.nasa.gov/airplane/lifteq.html Last Accessed: 2 August 2020.
[7] Hall, Nancy. (2015). “The Drag Equation”. National Aeronautics and Space Administration. https://www.grc.nasa.gov/www/k-12/airplane/drageq.html
[8] “Aerodynamics_Spoiler | Build Your Own Race Car!” Build Your Own Race Car!. https://www.buildyourownracecar.com/race-car-aerodynamics-basics-and-design/aerodynamics_spoiler/ Last Accessed: 2 August 2020.
[9] Lucas, Jim. (2014). “What Is Aerodynamics?” Live Science. https://www.livescience.com/47930-what-is-aerodynamics.html#:~:text=Aerodynamics%20is%20the%20study%20of,over%20and%20around%20solid%20bodies. Last Accessed: 2 August 2020.
[10] George, Patrick E.. “How Aerodynamics Work”. HowStuffWorks. https://auto.howstuffworks.com/fuel-efficiency/fuel-economy/aerodynamics.htm Last Accessed: 2 August 2020.
(Thumbnail Image Source) [11] Free-photos. (2015). “Plane Trip Journey – Free photo on Pixabay”. https://pixabay.com/photos/plane-trip-journey-explore-841441/ Last Accessed: 16 September 2020.
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