Debunking Common Aerodynamic Myths for Beginners
And also for those aero folks who have been given incorrect information
Before we begin, here are a few terms that you need to know if you are new to aeronautics. Those already aware of the basics can skip ahead!
Airfoil: The cross-sectional shape of an airplane’s wings or of a helicopter’s rotor blades is an airfoil. It is a 2D shape, while the wing is a 3D object. For example, a rectangle is a two-dimensional shape, which, when extruded, gives the three-dimensional cuboid. A large number of airfoil sections placed side by side gives the wing.
Angle of attack: Simply put, it is the angle between the relative wind and the airfoil’s chord. It is mathematically denoted by the Greek letter alpha (α).
Bernoulli’s principle: Precisely, Bernoulli’s principle is a statement of conservation of energy. For purposes in this story, we shall consider only the conclusion of Bernoulli’s principle: the increase in fluid velocity in a region is accompanied by a decrease in pressure at that region, and vice versa.
With those definitions in place, we are ready to move ahead.
It is a well-known fact that aircraft remain airborne due to the lift generated by definite parts such as wings, rotor blades, or propeller blades. All these devices have one common characteristic: they all have a streamlined cross-section. Every aeronautical engineering course teaches the generation of lift as a part of introductory aerodynamics. This simple concept is explained using Bernoulli’s equation and/or Newton’s laws of motion, the latter is also taught in high school physics classes. Things get worse when wrong ideas are added to the broth and the same is taught to learners.
The most prevalent explanation for the generation of lift that I have come across in countless sources is along the following lines:
The airflow upstream splits into two parts as it approaches the airfoil: one part flows over the airfoil, and the other part flows under the airfoil. The streamlined shape of an airfoil is such that the upper camber (curvature) of an airfoil is longer than the lower camber. Therefore, the air molecules over the top of the airfoil must travel with a greater speed than those below the airfoil to meet up at the trailing edge. Therefore, according to Bernoulli’s principle, the pressure atop the airfoil is lower than that beneath, hence pushing the airfoil upwards.
This explanation, which is unfortunately also taught in many universities, is only partially correct. Bernoulli’s theorem in fact states and proves, through simple conservation of energy equation, that the increase in flow velocity at any point is accompanied by a decrease in pressure at that point. However, the part which says that two air particles at the leading edge that flow past the airfoil must meet at the trailing edge is absolutely wrong. This falsity that has come to mix with the generation of lift is the ‘Equal transit time fallacy’.
The flow over the top of a lifting airfoil does travel faster than the flow beneath the airfoil, but not because they travel a longer distance and have to catch up with the air particles flowing beneath. There simply is no need for the flow over the upper surface and that under the lower surface to rejoin at the trailing edge. In fact, the flow is much faster than the speed required to have the molecules meet up at the trailing edge. This is beautifully demonstrated in this wind-tunnel experiment. If the idea that air travels faster on the upper surface because it has a longer distance to travel was true, then a kite must not be able to stay aloft, nor a sail must be able to generate enough push to steer a boat because the lengths of the top and bottom surface of the kite and sail are equal.
In truth, the streamlined shape of an object does not decide whether or not it can generate lift. Any object, from a flat plate to a cylinder, that can cause a pressure difference between its upper and lower surface can generate lift. Airfoils are only shapes that are designed to optimise lift-to-drag ratios, such that they can be used in practical flight applications.
Another inaccurate theory for lift generation is popularly known as the ‘Skipping stone theory’, as it is analogous to the way a flat stone skips across water when thrown at an angle. It is based on the impact of air on the airfoil. The theory says that a lift force is produced as air molecules strike the bottom surface of the airfoil, thereby generating a lift force. However, this theory does not take into account the influence of the upper surface. Hence it is incomplete and incorrect. There is also a third, popular and faulty explanation for lift generation which analogates flow over an airfoil to the flow through a venturi tube. In short, it states that flow is squeezed as it flows over the cambered part of the airfoil, causing it to accelerate (similar to how water speeds up when the opening of a hose pipe is restricted). Then, as per Bernoulli’s principle, a low-pressure region is created over the airfoil surface. This proposition fails to explain lift generation over a flat plate, which does not have a camber to restrict the flow of air. It also fails to consider the relationship between the lower surface and lift.
Now let us debunk these common myths once and for all. Lift is generated as a consequence of the pressure distribution around the airfoil. Without going into mathematics and equations, I would agree with the following explanation for lift generation:
Air that approaches the airfoil splits into two parts (one flowing over the airfoil and the other flowing beneath), forming a stagnation point A on the airfoil, whose location depends upon its orientation. It is at this point that the flow gets divided.
There is also another stagnation point B formed at the trailing edge of the airfoil, but I will be mentioning this in a while. The mass of air over the airfoil remains in contact with the airfoil due to the Coandă effect. As air moves away from stagnation point A, it makes a curved turn to flow past the airfoil while being attached to it. This results in an Euler’s centrifugal force exerted on the mass of the air, away from the turn radius. It decreases the local pressure (in comparison with the freestream pressure) and consequently increases the local flow velocity, which is in accordance with Bernoulli’s principle. This pressure difference generates lift force.
As air flows over the airfoil, it takes a curved turn along the boundary (known as downwash), due to the aforementioned Coandă effect. A change in the momentum of air is caused due to its deflection by both upper and lower surfaces. According to Newton’s second law of motion, a force is produced when the momentum of a moving body changes and the magnitude of this force is equal to the rate of change of momentum of the body. That means, according to this law, the airfoil exerts some force on the air in turning its path. Therefore, air also exerts an equal force in opposition (according to Newton’s third law of motion), which is the lift.
Although the flow velocity over the airfoil’s upper surface is greater than that below the lower surface, these two different masses of air leave the trailing edge smoothly and a stagnation point is formed at the trailing edge. This phenomenon is described by the Kutta condition. Realistically, the different flow velocities along the upper and lower surfaces of the airfoil, the different pressure distributions on the upper and lower surfaces of the airfoil, and the downwash, all take place simultaneously and are all different components of a single phenomenon.
A topic of debate that has split up many aeronauticians into two groups, is an argument on which theory on the lift is correct: Newton’s or Bernoulli’s. It must be highlighted here that neither Bernoulli nor Newton attempted to explain the generation of lift. Their theories are simply used to explain the generation of lift. Bernoulli’s theory explains the generation of lift in terms of pressure and flow velocity, while Newton’s laws explain the same phenomenon in terms of fluid momentum.
Simplifying 2*6+9÷3–8 and (2⁶.ln(e)/16 — sqrt (81)) yields the same answer 7. Neither of them is wrong. One represents the number 7 purely by simple arithmetic operations while the other does the same using exponents and logarithmic functions. Similarly, both Bernoulli’s principle and Newton’s laws offer intuitively correct explanations, only from different angles. So it is not a case of Newton vs. Bernoulli; rather Newton and Bernoulli vs. misleading claims and explanations that falsely propagate their theories.
To conclude:
- Lift generation over an airfoil is due to air pressure distribution between the upper surface (with low pressure, high velocity) and lower surface (with high pressure, low velocity).
- Airflow over the upper surface of the airfoil is much faster than beneath the lower surface. So adjacent air molecules upstream that split up into two parts do not meet each other downstream.
- Both surfaces of the airfoil contribute towards lift generation, not any one alone.
- Bernoulli’s principle and Newton’s laws of motion are not contradicting. They take different perspectives on the same phenomenon.
- Explanation of generation of lift is not just limited to airfoils or streamlined cross-sections but to any object with any general cross-section.
I hope this piece of writing helped clear up some common misconceptions. Would love to know about any other popular but flawed beliefs in aeronautics from your end!
