Here is a picture from UKPPG Specialistsm (http://www.ukppg.com/dangerofvortexs.htm):
Tuesday, April 14, 2009
Why planes always pull up right before they crash (in the movies)
Actually it's very simple. You just have to sit through several fluid dynamics classes to have someone point it out. So planes fly because of the vacuum above their wings. There is lower pressure above the wing because the air has to travel farther to meet the air that went on the bottom of the wing and went strait. Air that moves faster has less pressure. Wow, I just summarized three fluid dynamics classes in a paragraph. So when you get to the end of the wing at the tip you have low pressure on top and high pressure below and fluids like to move from areas of high pressure to areas of low pressure so the air below the wing moves up to the area above the wing in a circular manner (that's not as easy to explain) but it explains why planes like the F4 and many commercial airplanes have the vertical wing tips. You lose a lot of lift to the wing tip vortexes. Anyway, when you are very close to the ground there is not enough air below the wings to vortex around so there are no wing tip vortexes and you get a lot more lift. A lot more lift = the ability to go up.