Happy New Year from Team UV! Hope you all enjoyed the past year and are looking forward to some new and beautiful things to come in 2015! Being the first post of the year, I’ve decided to start it off right with something uplifting!
Airfoils. Who needs them, right? Well actually a lot of people do, especially if you want to fly. Looking at an airfoil, you might believe that lift is created because one side is curved and the other flat. This however would suggest that lift is only possible in that particular orientation but that is simply not true! If it were, then how could airshow pilots routinely fly upside down for extended periods of time? These are the questions we should be asking people!
To fly, it is not necessary to change the shape of the wing when inverted. Actually, any normal shaped wing can fly fine when inverted. It may look a bit weird and the inverted-stall may not be optimized but it does create lift by the same principles as the right-side up wing. Remember, lift is more a function of angle of attack than simply shape.
The chord line is a straight line drawn from the leading edge to the trailing edge of the airfoil. The mean camber line is a curved line drawn from the leading edge to the trailing edge that always stays halfway between the top and bottom of the airfoil. The amount of camber an airfoil has is determined by the greatest distance between the mean cambered line and the chord line. If the mean chamber line and the chord line are the same, then you have a symmetric airfoil. A general rule of thumb is that symmetric airfoils are preferred to highly cambered airfoils in small angles of attack, and the opposite is true for higher angles of attack. This is shown in the figure below where the airfoil designated 631-012 is symmetric and 631-412 is the slightly cambered version. Besides one being slightly cambered, the airfoils are pretty much identical. Beyond 12 degrees relative angle of attack you can see that the cambered airfoil has a big advantage over its symmetric counterpart. The cambered airfoil does not stall until a higher angle of attack around 18 degrees and as a consequence its maximum coefficient of lift is much greater. An intuitive explanation would be simply that the slight camber allows for the leading edge to slice through the air easier. Taking this to another extreme, a reverse cambered airfoil would not be a good idea because it would stall even earlier than then symmetric airfoil (which is why you don’t see these). A reason for having a large camber would be in landing or lifting off situations and usually extending the flaps of the wings is sufficient in increasing the camber.
Again it is important to reiterate that lift is more a function of the angle of attack rather than simply the shape of the airfoil. If the coefficient of lift was only dependent on the camber of the wing (which doesn’t change often during flight) then the plane would only be flyable at a special airspeed designed for that particular cambered wing. In reality however pilots have to constantly change the angle of attack to maintain steady lift. For more on lift, airfoils, and flight please visit: http://www.av8n.com/how/htm/airfoils.html. Have a happy new year!