there is a net change of momentum in the vertical plane between the leading and trailing edges of the airfoil, and by necessity of Newton’s third law, this creates a lift force. Hence, in accordance with the speed-pressure relationship of Bernoulli’s principle, this constriction of the streamtubes means that we have a net pressure differential that generates a lift force.Īsymmetric airfoil with pronounced camber Another explanation that is often cited for explaining lift is that the airfoil pushes air downwards, i.e. Similarly, to conserve the mass flow rate in the lower streamtube as it widens, the fluid must slow down. To conserve the mass flow rate in the upper streamline as it becomes narrower, the fluid must flow faster. no fluid enters from the outside, and no fluid particles are created or destroyed. As two adjacent streamlines form the boundaries of the streamtubes, the mass flow rate through each streamtube must be conserved, i.e. The definition of a streamline is the line a fluid particle would traverse as it flows through space, and thus, by definition, fluid can never cross a streamline. The flow between two adjacent streamlines is often called a streamtube, and the upper and lower streamtubes are highlighted in shades of blue in the figure below. If we start from a phenomenological observation of streamlines around an airfoil, as depicted schematically below, we see can see that the streamlines are bunched together towards the top surface of the leading edge, and spread apart towards the bottom surface of the leading edge. In order to generalise the above theory, while maintaining the mathematical relationship between speed and pressure given by Bernoulli’s principle, we can relax the initial assumption of equal transit time. Hence, this theory of equal transit times is often referred to as the “Equal Transit Time Fallacy”. does not explain all phenomena.Īs this video shows, the air over the top surface does indeed flow faster than on the bottom surface, but the flows certainly do not reach the trailing edge at the same time. It fails to explain why aircraft can fly upside down, i.e.the underlying assumptions are certainly not as simple as possible. There is no physical law that requires equal transit times, i.e. This explanation has a number of fallacies: along a streamline an increasing pressure gradient causes the flow speed to decrease and vice versa, is then invoked to deduce that the speed differential creates a pressure differential between the top and bottom surfaces, which invariably pushes the wing up. For this to occur, the airflow over the longer top surface must be travelling faster than the air flowing over the bottom surface. This theory assumes that the individual packets of air flowing across the top and bottom surfaces must reach the trailing edge of the airfoil at the same time. The first explanation of lift that I came across as a middle school student was the theory of “Equal Transit Times”. Keeping this definition in mind, let’s investigate some popular theories about how aircraft create lift. The diversity of phenomena the theory can explain, including phenomena that other theories could not explain.The simplicity of its assumptions, ideally as few as possible.In science, the simplicity of a theory is a hallmark of its elegance. According to Einstein (or Louis Zukofsky or Roger Sessions or William of Ockham… I give up, who knows), “everything should be made as simple as possible, but not simpler.” Hence, the strength of a theory is related to: In fact, the theory that requires the simplest assumptions and least abstraction is typically considered to be the most useful. This is not necessarily because more advanced theories are harder to understand or require a lot maths. Most importantly, the equal transit time theory, explained further below, is taught in many school textbooks and therefore instils faulty intuitions about lift very early on. Just throughout my time in school and university, I have been confronted with several different explanations of how wings create lift. Given its importance to flight, it is surprising how many different and oftentimes wrong explanations are being perpetuated online and in textbooks. How airplanes fly is one of the most fundamental questions in aerospace engineering.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |