Lift Force - Aerodynamic Force Resolution
Updated: Jul 21, 2021
What is the theory exactly behind lift force?
Table of Contents
There are many theories on how lift force is seen or its concept. Let's discuss all the theories and understand which exactly is right!
It is surely a debatable topic. If so, let's debate, it would be fun, right? Before that let's get into few basics as usual.
When there is relative velocity between fluid and solid molecules, Aerodynamic force acts on the body and acts at a point called Center of Pressure.
Factors that affect Aerodynamic Force:
Shape and size of the object: As the geometry and the roughness of the object change, the aerodynamic force associated with it also changes.
Velocity and direction of flow: In the above formula itself, one can see as velocity increases, the aerodynamic force also increases.
Mass, viscosity, and compressibility of air: If these change, the aerodynamic force also changes.
Aerodynamic Forces are resolved into two components, namely Lift and Drag force.
Let's get into Lift Force as of now...
Theory I: Longer Path/Equal Transit Theory by Bernoulli's Principle
Lift force can be explained easily by Bernoulli's Principle.
Let us consider the figure above. The body over which the fluid flows is called an airfoil. There are two streamlines that are blue and red. The blue streamline flows over the airfoil, and the red streamline flows under the airfoil. When you compare both the paths in the figure, the fluid molecule traveling along the blue streamline travels a longer distance and vise versa. Let both the molecules start to flow along with blue and red at the start point, at the same time.
By Bernoulli's Principle, the mass flow rate of fluid before and after flowing over the airfoil should be the same (Law of conservation of mass). This means, for example, the mass flow rate before flowing over the body is 10 cu.m/s then it should remain the same after reaching the other end of the airfoil. Hence, the velocity of the fluid molecule flowing over the blue streamline is higher, because the distance is longer. Therefore, a low-pressure region is generated (By Bernoulli's Principle). Similarly, to maintain the mass flow rate, the velocity of the fluid molecule flowing over the red streamline is lower (as the distance is shorter). Therefore, a high-pressure region is generated.
We know that fluid always flows from the high-pressure region to low pressure region. Or you can imagine it as the lower pressure creates a suction effect and the fluid flows to the low-pressure zone. Going back to the figure, the lower pressure region is created above the body and the high-pressure zone is created below the body. Hence, the fluid moves in an upward region, and lift is created.
What is wrong with this theory then?
Bernoulli's principle clearly states that it can be applied only between a single streamline. But we compared the mass flow rate, between two different streamlines. Which is one of the reasons why it does not explain lift force. Also, one more observation made by the scientists is that, if the mass flow rate should have been the same when the two streamlines meet, the blue streamline molecule flows so fast that the red streamline molecule fails to catch up at the other end. Additionally, even if the blue streamline travels a shorter distance than the read streamline, the blue streamline molecules flow with faster velocity. You can refer to the diagram below for your reference.
Theory II: Venturi Theory
As the name of the theory suggests, the principle of venturimeter is used here.
This theory states that the upper body of the airfoil acts as a venturi nozzle. Similar to the nozzle, the area of the upper body of the airfoil reduces. Hence, the velocity increases and pressure reduces. And similar to Bernoulli's Theory of lift, the air will flow from high to low pressure, and lift is created.
What is wrong with this theory then?
The first statement itself is wrong here. This theory simply assumes that the upper body of the airfoil acts as a venturi nozzle. Also, it just considers the upper body of the airfoil. One cannot apply the same theory to the lower body.
Theory III: Skipping Stone Theory by Newton's Third Law with Coanda effect
Newton's Third Law: For every action, there is an equal and opposite reaction.
Coanda effect: It is the tendency of a jet of fluid to curve towards a surface.
This theory states that, as the fluid molecules that hit the lower body of the airfoil, an opposite reaction is produced. Due to this reaction force, the lift is generated. To support this explanation this law is clubbed with Coanda Effect.
The Coanda effect can be explained by considering the upper surface of the airfoil. When we take a closer look at the upper airfoil, one can observe that the fluid follows the curve of the airfoil, instead of traveling in a straight line. The reason is explained by the Coanda effect, as the surface near the airfoil has lower pressure than that of fluid. Hence, the fluid sticks to the upper part of the airfoil.
So is this theory right then?
Talking about Newton's Third Law of Motion, it is not right. Because the upper body of the airfoil too can push the body in the opposite direction as a reaction. But, it is observed that it is true in some cases. When the velocity is high and density (of the fluid) is low.
Coming to Coanda Effect, it is a very debatable topic. Some agree others don't. Also, some people mistake the Coanda effect for Surface tension.
The theory why lift is produced is very complicated and cannot be explained by everyday physics. It can be explained with help of Euler's Equation (Navier-Stokes Equation).
Lift is produced due to relative velocity between fluid and the solid to be lifted.
Bernoulli's Principle was proved to be wrong as it was applied across two streamlines to explain lift.
Venturi Theory is not right as airfoil was compared with venturimeter, but airfoil is not venturimeter.
Newton's Third Law with Coanda Effect is neither rejected completely nor accepted.
Understanding Lift Force on the surface level is very difficult. It is a very controversial topic and I am sure you guys have a lot to say or ask. Do comment on your thoughts!