Basic Aerodynamics

You may be wondering why this page exists. However, don't forget that one of the fluids that we hold dear to survival and regular transportation is air. The following topic lets us know about the basics of aerodynamics:

We know that airplanes fly through the air at high speeds kilometers above the ground. Thus, it's obvious that some upwards force needs to be counteracting the plane's weight acting downwards. This force is known as the aerodynamic lift on the plane.

​

So, why does lift occur? It's all about Newton's Third Law.

​

The explanation has to do with the idea that the airfoil of a plane's wing is curved such that fluid, namely air, will be pushed downward by it. For context, the airfoil of a plane is the shape of the cross-section of its wing, which is ideally curved. The airfoil pushing the air downwards causes the air to push the foil(and thus, the rest of the plane) upwards. This upwards force from Newton's Third Law is known as lift. You may be wondering what happens to the air flowing on top of the wing. Shouldn't that push down on the wing? No, thanks to the Coanda Effect. The Coanda effect is the tendency of a fluid to stick to a surface. An example would be if you took an empty jar and poured water over it. The water wouldn't just slosh off the surface of the jar; it would flow around the jar's surface and then continue on its path. The Coanda effect allows the air on top of the foil to stick to the curved edge of the foil itself so that once it moves past the airfoil, it is forced downwards along with air on the bottom. Thus, in the end, the air on the top also generates lift because it is forced downwards along with the air on the bottom by the end of its path around the wing.

This explains how generating lift works but you may also want to know a few ways to generate more lift. There are two main ways to do this: creating greater curvature on the wing or increasing the angle of attack.

​

If we curve the top of the wing more, the air at the top will be deflected(forced) downwards much more intensely which will cause more downwards force from the wing on the air, thus generating more lift. Since this case, you may be wondering why commercial airliners don't just curve their wings extremely convex. The reason is that if you curve the foil too much, the structural support of the wing becomes much weaker which will cause the plane to not work properly. Thus, you have to optimize the curvature of the airfoil such that you can generate more lift but also support your plane.

​

The angle of attack is described as the angle the airfoil is oriented at relative to the oncoming airflow. If this doesn't make sense, imagine a view of the airfoil from its side. If we take oncoming airflow to be perfectly horizontal, we can draw a horizontal line near the airfoil representing the airflow. Now, let's draw a chord going through the airfoil angled such that it goes perfectly through the center. The angle between this chord and the horizontal line representing the idealized airflow is the angle of attack. Of course, airflow isn't perfectly horizontal but it's a good approximation to make in this case If you increase the angle of attack of the airfoil, you'll increase the curvature of the air on the top of the airfoil which, as stated before, increases the downward deflection of air and thus generates more lift. However, if you increase the angle of attack too much, the airflow on the top won't properly "stick" to the surface of the airfoil and it will diverge from the wing, causing the plane to stall due to not being able to create sufficient lift while slowing down heavily due to drag. Thus, there's an optimal angle of attack you want the plane to be at, known as the critical angle of attack. Interestingly enough, the angle of attack is the key reason why airplanes can still fly fine upside down as you can orient an upside down plane such that it'll generate significant lift.

​

The equation for the lift acting on the wing is given as:

Drag is the force created by the movement of a solid object, usually a plane, through the air. You can think of it as aerodynamic friction, caused by the sliding motion between an object and air itself. Also, in many physics classes, the air resistance that is often ignored can be synonymous with aerodynamic drag.

​

The equation for aerodynamic drag is given as:

where A is the area of reference, C is the dimensionless drag coefficient which is determined by the Mach Number, the Reynolds Number, and other factors. Lower case rho is the density of the fluid, usually air which is 1.20 kg/m3 at 20 degrees Celsius (293 degrees Kelvins). Lastly, v is the velocity of the object. This proportionality between drag and velocity is why you feel air resistance when you put your hand out the window of a moving vehicle but you likely wouldn't feel that same resistance if you put your hand out on a leisurely walk. Since the drag coefficient is based on the Reynolds Number of the oncoming airflow, this explains why you'll feel lots of air resistance if you decide to take a walk in a stormy setting. Storms are characterized by extremely turbulent gusts and airflow so the Reynolds Numbers of stormy air will be extremely high, causing the drag coefficient and thus the drag force to be very high.

​

Interestingly enough, the equation for drag is very similar to the equation for lift. The reason for this is simple. The force of lift is caused by the same force that creates drag. Both are created by the resistive force of air on a given solid object. Lift is the reaction force that is perpendicular to the airflow while drag is the parallel component, although this gets more complicated when factors like the angle of attack and thrust become involved.