The problem about looking stuff up online is the absence of an algorithm that quantifies the amount of bullshit exposed in websites. Therefore, when someone tries to understand what really goes into making aircraft fly, usually one out of two things happen:
a) you get incredibly overwhelmed with controversial information, or
b) you simply accept that there is thrust, drag, lift and weight and that that’s everything you know for sure.
I am saying this because it has happened to me.
But after some experience, analysis and deeper research, here is a reasonably simple explanation for the basics behind wing aerodynamics.
On a macroscopic level, the afore mentioned forces are indeed what makes an airplane fly. Considering basic physics:
- Thrust > Drag: you accelerate
- Lift > Weight: you rise
- Drag >Thrust: you slow down
- Weight > Lift: you pray
This is as simple as it gets.
Thrust is generated by engine propulsion. Drag due to air resistance. Weight thanks to gravity. But how do you generate lift?
Actually, the magic happens on the wings and to understand the big picture, first we need to look into the microscopic level.
Some Fluid Dynamics theory
They say when there’s a will, there’s a way. In fluid dynamics, when there’s a pressure drop, there’s also a way. Fluids usually move from places with higher pressure to regions under lower pressure. This is basically what makes a fluid move inside a pipe! The flow suffers continuous pressure loss due to wall friction, therefore P1 > P2 and thus a net force is created that keeps the flow moving forward:
This phenomenon is the Holy Grail of several engineering applications involving fluid flow. Keep it in mind if you want to understand flows: fluid has a tendency to move from higher to lower pressure!
Visualize yourself as being a passive air particle looking at an airplane.
The plane is moving forward in high speed due to propulsion and it continuously flies into stationary air.
Meet the Airfoil, a section view of an airplane wing:
1. It’s all about Pressure
When the air particles encounter the front of the wing, the air will split in two directions: upwards and downwards of the leading edge.
Here comes the first revelation: lift is generated in part because the average pressure contour at the bottom of the wing is higher than the pressure at the top.
And here is why:
When the air encounters the wing, the bottom surface will bend the air, pushing it forwards and downwards, increasing the pressure beneath the airfoil (red and orange arrows in the picture).
The same thing happens, momentarily, at the top surface: an impact wave forms when the air hits the leading edge of the wing, increasing pressure at the tip of the airfoil (pink and yellow arrows).
This is exactly the same effect as sea waves that from at the tip of a forward moving boat.
But as soon as the air travels further ‘downhill’ of the wing, it encounters the “hidden” area which isn’t exposed to the dynamic incoming air, due to the high angle of attack (AoA):
This area is usually thinner than the leading edge, creating a kind of “gap” for the air to “fill”, and this sudden change of direction and increased space for the incoming flow causes the pressure to drop.
At this point, the air at this “hidden” surface will accelerate in consequence of the pressure drop it suffered, therefore the streamlines of the upper surface of the wing reach the leading edge first in comparison to the streamlines of the downside:
This can be associated with the Bernoulli principle! The Bernoulli principle states that there is an inverse relation between pressure and velocity in fluid flow. The faster the flow is moving, the less pressure the fluid particles will exert.
2. Lift Generation: it’s Newton baby
The mentioned pressure gradients end up being good for one, solid reason: every action creates an opposite reaction, a.k.a. Newton’s 3rd Law (karma).
- if pressure gradients generate a net pressure force due to the fluid’s tendency to move from high to low pressure, the airfoil suffers an upward net force due to the average pressure difference between top and bottom surface.
- the wing bends the air downwards thus the solid wing is exerting a downwards and forwards directed force on the fluid. According to Newton’s 3rd law, the fluid will therefore apply an upwards and backwards directed force on the wing, the resultant force:
where the upwards component is called Lift and the backwards component, Induced Drag.
Now to fully understand why this is possible, it is important to consider the reason air actually remains attached to the wing’s surface. Ever thought about that?
3. Air follows the wing shape and… sticks to it?!
Say hi to Pascal’s law.
The air around an airplane is at a characteristic pressure value for a given altitude. Let’s call it simply atmospheric pressure.
Pascal’s law states that pressure applied to a fluid particle is transmitted equally in all directions, meaning that the entire wing surface is constantly being pressurized by the atmospheric air around it.
Air is naturally viscous, meaning that it is sticky and wants to glue to every surface is finds, creating something called a boundary layer.
If air wasn’t viscous, there would be no way to fly whatsoever because it would simply ignore any surface instead of acting upon it.
That’s why plane crashes can happen when ice forms on plane wings: it would dettach the boundary layer from the surface and create aerodynamic stall.
- atmospheric pressure acting on the wings + natural air viscosity = mistery solved: flow keeps attached to the surface.
You can also correlate this phenomena with Newton’s 1st law: a body moving in a certain direction will continue to move in its trajectory unless an external force acts upon it.
Funny how things connect to each other.