QUESTION: Why DOES the air go faster over the top of a surface inclined w/ a positive angle of attack? In addition, I think I understand "lift is a force generated by turning a moving fluid" (I can draw a sensible vector diagram, particularly if I know the airflow over the top of the wing is faster) (www.lerc.nasa.gov/www/FMD/airplane/lift1.html), but can you explain (in terms a 5th grader could understand) how this is related to Bernoulli's law? ANSWER from Andrew Hahn on April 23, 1998: This is a great question and it brings out one of my pet peeves. In order to make the concept simple, some common and quite wrong explanations have been offered historically. Wrong explanation #1: An airfoil is flat on the bottom and curved on top so air has to flow faster over the top to meet the air from under the airfoil at the trailing edge. Faster air has lower pressure and so the airfoil lifts. There are a lot of problems with this like; the air particles don't actually meet at the trailing edge, flat plates fly, airplanes fly upside down, etc. I could go on, but I think you get it. Wrong explanation #2: Believe it or not, my industrial design professor in grad school was teaching another wrong explanation (to be fair, it is outside of his bailiwick). He said that if a curved plate were in a channel, then the flow area above is restricted while the flow area below expands and the net effect is lower pressure above and higher pressure below. The main problem with this is that it doesn't really model our airplane because the airflow isn't really constrained. This too leads to inconsistencies. The most obvious is, if you place a flat plate at an angle of attack, then the flow area above the leading edge increases (decreasing speed => increasing pressure) while the flow area below the leading edge decreases (increasing speed => decreasing pressure). Since the geometry is symetrical, there is no net lift but there is a pitching moment. In reality, a flat plate at an angle of attack has lift but no moment. So, the result is exactly backwards. The common problem with these explanations is that they are modeled on variations of the venturi, which is the classic device used to demonstrate the Bernoulli principle. Unfortunately, airfoils and venturi's change the airflow velocity through two totally different mechanisims. The venturi changes area to speed up or slow down the flow while the wing induces circulation. So, what is circulation? Circulation is a component of flow that goes in a circular motion, just like a tornado. Early researchers found that they could represent the complex circulatory flow around an airfoil with a simple vortex (tornado) that runs along the span of the wing (We do this sort of thing all the time. For example, we represent the complex pressure distribution on the surface of the wing, both top and bottom, with Lift, Drag and Moment vectors). When this horizontal vortex is put in a horizontal flow, lift is generated. This concept made calculation of the lift on a wing possible by hand and was a huge breakthrough in understanding. As an undergrad, I used to think that this was just a convenient mathematical concoction that gave the right answers, but that this didn't really occur. How wrong I was. So many effects are explained by circulation that it can't be just convenient. What's so special about the wing (airfoil) shape that generates this circulatory flow? The sharp trailing edge. I'm going to leave out some of the more obscure arguments and just relate the most common explanation of what happens at the trailing edge. This is a little hard to do without pictures, but I'll try to explain this as best I can. All fluids have viscosity, or a resistance to deforming. Some fluids resist deforming a lot (like molasses) while some fluids deform easily (like air). Still, air does have some viscosity and this makes air very reluctant to turn sharp corners. So now, take a flat plate (your hand will do in a pinch) and put it in a horizontal flow (out the car window), level (no angle of attack). The air hits at the front (leading edge) and leaves at the back (trailing edge). Now, rotate your wrist so that the leading edge goes up and the trailing edge goes down (increase angle of attack). You will notice that there is a lift and drag force generated. This occurs because the air has viscosity. If there were no viscosity, the air would hit somewhere on the lower side of your hand near the leading edge and it would split. Half of the flow would travel foreward, turn around the leading edge and then flow backwards along the top of the plate. Like a reflection, the other half of the flow goes backwards along the bottom of the plate until it gets to the trailing edge. It would then turn around, and flow forward along the top until it met the first half of the flow. Where they meet, they would leave your hand. With this invicid flow, there is no lift. But remember, this doesn't really happen. The difference is that the second half of the flow never negotiates the turn around the trailing edge because it is sharp and the velocity required to do this is theoretically infinite. Viscous fluids can't do infinite and so the lower half just gives up and leaves at the trailing edge. It is this difference between what the flow wants to do and what it can do that sets up the circulatory flow. This circulatory flow modifies the local air velocities over the whole wing, but the general effect is to speed up the air over the top and retard the air underneath. (This still works for thicker airfoils, but the picture is a little more complicated) Bernoulli's principle says that faster air has lower pressure, so the air above the wing sees a low pressure. Since air always flows from high to low pressure, this starts air moving downward. Newton said that Force = Mass x Acceleration, so even though air has just a little mass, we can generate a usable force (lift) if we accelerate a lot of air downward a little. Well, that was a mouthful. Let's recap. The wing shape forces the air to leave the trailing edge smoothly which sets up a circulatory flow pattern which causes the air velocity to increase over the top of the wing which lowers the pressure which turns the flow downward which causes lift. There, that wasn't so bad was it.