![]() ![]() In order to take the curve, there should be more pressure at the top of the particle than at the bottom. But after the curve, why doesn’t it move straight as shown ? Fig:6: After the curve, particle does not move straightĮxamine this curved motion more closely. The particle approaches the airfoil and takes a curve as shown in Fig:6. Now, we will investigate the science behind lift generation by applying the laws of physics correctly. Fig:5 Newton's second law of motion applied Coanda effect It is just Newton's second law of motion applied along a fluid streamline (Fig:5) Some people applied it incorrectly and caused confusion. And this surface – the same argument indicates that this should produce an incredible amount of lift, as shown in Fig:4.įig:4 According to the equal time argument, both these surfaces should produce a lift force: the second surface should obviously produce an incredible amount of lift forceīernoulli's equation is completely right. ![]() According to the equal time argument, this surface should also produce a lift force. Bernoulli's equation should be applied strictly along a streamline, this is illustrated in Fig:3 Fig:3 Bernoulli's equationĮven after pointing out these mistakes, if you still support this widespread myth, just take a look at this shape. The second mistake is that you cannot apply Bernoulli's equation between 2 streamlines. The 2 particles can leave for a completely different journey and may not meet in their lifetime! That is a completely absurd argument.There is no law in physics to support it. The first mistake pertains to how 2 particles starting from the same location reach the trailing edge at the same time. The equal time argument is a beautiful way to explain lift, but it’s completely wrong. This argument more specifically is known as the “equal time argument” Fig:2 According to the Bernoulli's principle, there is more pressure at the bottom and less pressure at the top surface The difference in the pressure generates lift. This means that according to Bernoulli's principle, there is more pressure at the bottom and less pressure at the top surface(Fig:2). Since both particles should reach the trailing edge at the same time, the upper surface particles should have more velocity than the lower surface particles. This means the particles on the upper surface should travel a greater distance than the particles on the lower surface. We will also include an interesting brain teaser at the end of the article.įig:1 The airfoil and lift force production Bernoulli's principle.įirst, let's see what is the argument that uses Bernoulli's principle.įrom the shape of the airfoil it is clear that the upper surface is more curved than the lower surface. Skeptics include NASA scientists and Professor Holger Babinsky of the University of Cambridge, who, in his popular YouTube video, proved both experimentally and theoretically that the equal time argument is incorrect.Īnyway, we will approach this problem rationally and use computational fluid dynamics and experimentation to support our findings. ![]() Some textbooks point to Bernoulli’s principle, but many people reject this claim. But what is the source of this lift? Is Bernoulli's principle or Newton's third law responsible for it ? Or both the effects? What is lift force?Īn airfoil produces a lift force when fluid flows over it, this is illustrated in Fig:1. In this aricle will unveil the physics behind the simple shape that revolutionized the engineering world. Wind Turbines, gas turbines and hydraulic machines, all work on the principles of airfoil. The maximum phase lag occurs at $k \approx 0.25$.įor most rigid-body motions in commercial aircraft, the reduced frequency is low and the quasi-steady assumption is valid.How does an Airfoil generate Lift? July 5, 2019Īirfoil technology helped human beings to fly. As $k$ increases, there is increasing attenuation in lift, up to 50%. As $k \to 0$, there is no attenuation in lift and no phase lag this is the quasi-steady aerodynamics. $$C_l=\text$, denotes how many airfoil chord lengths per flow distance traveled in one motion period ( $\omega$ is the angular frequency of the cyclic motion), and is a measure of how much the cyclical motion affects the flow on the airfoil. Using thin airfoil theory in incompressible flow, the lift coefficient for an airfoil undergoing cyclical pitching and/or heaving can be expressed as (Ref. Shed vortices from unsteady flow over an airfoil result in decrease and phase lag in lift. ![]()
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