It only takes a minute to sign up. Connect and share knowledge within a single location that is structured and easy to search. I have read on many sites that for lift to occur, the pressure below a wing must be higher than the pressure above it.
This happens by somehow moving air faster over the top of the air foil so the pressure above the wing decreases. What I can't understand is: Why does the air above the wing have to move faster?
Because on many sites it is stated that there is a misunderstanding that air moving upwards has to flow faster to reach the end at the same time with the air moving downwards. So why else would the air upwards want to move faster? Why don't they both move at the same speed?
It is true that the air on the "top" suction side is faster than on the "bottom" pressure side. The interesting thing is that the air on the suction side is so fast, it overtakes the air on the pressure side see illustration from Wikipedia :. The key to understand the velocity distribution around an air foil is not to look at it independent from the pressure distribution and curvature of the airfoil.
Let's start by describing the flow from the stagnation point this is the point at the very left in the figure where the free air flow hits the airfoil first leading edge and move along the suction side to the end of the air foil trailing edge. At the stagnation point we have a very high static pressure imagine holding your hand out of the car window when driving really fast. Around the stagnation point the static air pressure is a lot higher than the air around it.
From there on the flow is first accelerated to the point of maximum profile thickness and then decelerated until it reaches the trailing edge. The flow around and air foil is governed by a set of equations i. These are the Navier Stokes Equations. Without going into detail the equations are not linear. This means we end up with a chicken-and-egg problem. It means one cannot really answer your question in a step-by-step way. The answer to your question is actually which is not really satisfactory : The flow field which develops around the airfoil is the only possible state this equation system allows for conserving momentum and energy.
Let's look at an example:. The flow which is approaching the cylinder left to right is converging in front of the stagnation point. The distance between the streamlines is getting smaller and the flow is moving around the cylinder. This happens because the high static pressure at the stagnation point pushes the air away from the cylinder and thereby compressing the flow.
When the flow has reached the top or bottom of the cylinder, the streamlines diverge. The distance between the stream lines increases because they flow towards the centre line.
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Anyway, we will approach this problem rationally and use computational fluid dynamics and experimentation to support our findings. We will also include an interesting brain teaser at the end of the article. From the shape of the airfoil it is clear that the upper surface is more curved than the lower surface. This means the particles on the upper surface should travel a greater distance than the particles on the lower surface.
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 that according to Bernoulli's principle, there is more pressure at the bottom and less pressure at the top surface Fig The difference in the pressure generates lift. The first mistake pertains to how 2 particles starting from the same location reach the trailing edge at the same time.
That is a completely absurd argument. There is no law in physics to support it. The 2 particles can leave for a completely different journey and may not meet in their lifetime! The second mistake is that you cannot apply Bernoulli's equation between 2 streamlines. Bernoulli's equation should be applied strictly along a streamline, this is illustrated in Fig Even after pointing out these mistakes, if you still support this widespread myth, just take a look at this shape.
According to the equal time argument, this surface should also produce a lift force. And this surface — the same argument indicates that this should produce an incredible amount of lift, as shown in Fig Bernoulli's equation is completely right. It is just Newton's second law of motion applied along a fluid streamline Fig:5 Some people applied it incorrectly and caused confusion.
Since the velocity of the fluid below the wing is slower than the velocity of the fluid above the wing, to satisfy Equation 3, the pressure below the wing must be higher than the pressure above the wing. In a qualitative look at Euler's Equations, the movement of the fluid flow around the curved upper surface of the wing may be likened to that of a car going around a bend. Similarly, as the fluid particle follows the cambered upper surface of the wing, there must be a force acting on that little particle to allow the particle to make that turn.
This force comes from a pressure gradient above the wing surface. Starting at the surface of the wing and moving up and away from the surface, the pressure increases with increasing distance until the pressure reaches the ambient pressure. Thus, a pressure gradient is created, where the higher pressures further along from the radius of curvature push inwards towards the center of curvature where the pressure is lower, thus providing the accelerating force on the fluid particle.
Thus due to the curved, cambered surface of the wing, there exists a pressure gradient above the wing, where the pressure is lower right above the surface. Assuming a flat bottom, the pressure below the wing will be close to the ambient pressure, and will thus push upwards, creating the lift needed by the airplane.
At angles of attack below around ten to fifteen degrees, the lift increases with an increasing angle. However, if the angle of attack is too large, stalling takes place. Stalling occurs when the lift decreases, sometimes very suddenly. The phenomena responsible for stalling is flow separation see Figure 9. Flow separation is the situation where the fluid flow no longer follows the contour of the wing surface. Fluid particles flowing along the top of the wing surface experience a change in pressure, moving from the ambient pressure in front of the wing, to a lower pressure over the surface of the wing, then back up to the ambient pressure behind the wing.
The region where fluid must flow from low to high pressure adverse pressure gradient is responsible for flow separation. If the pressure gradient is too high, the pressure forces overcome the fluid's inertial forces, and the flow departs from the wing contour. Since the pressure gradient increases with an increasing angle of attack, the angle of attack should not exceed the maximum value to keep the flow following the contour. If this angle is exceeded, however, the force keeping the plane in the air will decrease, and may even disappear altogether.
Viscosity can be described as the "thickness," or, for a moving fluid, the internal friction of the fluid.
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