If you have studied aerodynamics or fluid dynamics, you can easily explain what is lift. However, it might not be easy to correctly explain how lift is created. The question about how lift is produced may look simple and easy but the popular explanations found on internet are ambiguous and misleading. This post is about some popular misleading theories, how they fail and some correct explanations.
Theories of lift generation are often based either on Bernoulli’s principle or Newton’s laws of motion. Both Bernoulli’s principle and Newton’s laws of motion are without any doubt flawless, however, misleading interpretations of these laws are made to explain generation of lift. Some popular misleading theories are as follows;
Equal Transit Time Theory
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.
Though the Bernoulli’s principle is correctly applied in this theory, the overall assumption that the fluid particle meet at the trailing edge is wrong. Therefore, this theory fails to successfully explain lift generation.
Skipping Stone Theory
Skipping stone theory is based on Newton’s third law of motion which states, “For every action, there is an equal and opposite reaction”. The theory is based on the idea that lift is the reaction force to air molecules striking the bottom of the airfoil as it moves through the air.
Again, the problem doesn’t lie in the Newton’s law of motion. This hypothesis is concerned with only the interaction of the lower surface of the moving object or airfoil and the air. It assumes that all of the flow turning and hence the lift is produced by the lower surface. But in reality, upper surface also contributes in the lift generation. According to this theory, two airfoils with different upper surface but same lower surface should produce equal lift which is never true.
Furthermore, this theory doesn’t explain how angle of attack affects lift as there will be negative lift at negative angle of attack even though the flow is turned at the lower surface. Although this theory appears to be correct sometimes, it fails to correctly explain lift generation.
This theory suggests that air flowing over the top of a cambered wing surface is compressed, as in a Venturi tube, and therefore moves more quickly than air that is not compressed. And then it applies Bernoulli’s principle to show low pressure in the upper surface that is responsible for lift generation. According to this airfoil theory, the top of the airfoil is curved, which constricts the flow. Since the area is decreased, the velocity over the top of the foil is increased. Then from Bernoulli’s equation, higher velocity produces a lower pressure on the upper surface. The low pressure over the upper surface of the airfoil produces the lift.
This appears to be convincing in the way it uses Bernoulli’s equation. However, this theory deals with only the pressure and velocity along the upper surface of the airfoil. It neglects the shape of the lower surface. If this theory were correct, we could have any shape we want for the lower surface, and the lift would be the same. The flow can still be constricted in the airfoil at negative angle of attack but still it generates negative lift. Hence, Venturi theory is also an incorrect explanation.
Better Explanation of lift generation
Both the Newton’s laws of motion and Bernoulli’s principle are the foundations for calculating any forces. The fundamental problem is that neither theory fully explains real- world observations. Bernoulli’s principle tells that the faster air on top of the wing experiences reduced pressure which is correct but does not explain why it’s correct. It also does not explain inverted flight. That’s where Newton’s second and third laws come into play. Taken together, Newton’s laws describe how we can fly reversed and how angle of attack works. However, this is still not convincing as seen above in skipping stone theory.
When both these theories are combined we can get the following explanation;
“Applying Bernoulli’s Principle of Pressure, the increase in the speed of the air across the top of an airfoil produces a drop in pressure. This lowered pressure is a component of total lift. The pressure difference between the upper and lower surface of a wing alone does not account for the total lift force produced.
“The downward backward flow from the top surface of an airfoil creates a downwash. This downwash meets the flow from the bottom of the airfoil at the trailing edge. Applying Newton’s third law, the reaction of this downward backward flow results in an upward forward force on the airfoil.” To be honest, the combined explanation is still ambiguous.
Wouldn’t it be better if we don’t have to use both Newton’s laws and Bernoulli’s principle to explain lift? There is indeed a simpler explanation of lift generation that uses neither of the above principles.
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.
The relation between pressure gradient and the local radius of curvature is as follows;
If a streamline is straight, R → ∞ and dp/dn = 0. Therefore, there is no pressure gradient across straight streamlines. 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 higher pressure at the lower section. This concept holds true for inverted flight as well as different angle of attacks. Do follow this link to learn how this theory holds true– http://www3.eng.cam.ac.uk/outreach/Project-resources/Wind-turbine/howwingswork.pdf
I hope this post clears some popular misconceptions. Also feel free to comment down other popular misconceptions and also correct explanations if there are any.
Also Check out this post- http://geniuserc.com/dalemberts-paradox-and-its-resolution-explained/