Bernoulli 's Principles

Often, I see birds flying. Then, sometime later, I see a plane flying. Birds are light weight, have hollow bones and have to flap or use updrafts to fly. Planes, are huge metal things that have engines and literally, weigh tons. So how does a plane fly? Bernoulli was a scientist who observed that with any fluid, an increase in velocity equaled a decrease in pressure. It applies in actual fluids, like water, and with gas, like what we inhale and exhale. The airplanes wings are shaped so air has to cycle around it, creating low pressure above the wing and high pressure beneath it. That helps the plane to fly. There are always four forces acting on a plane. The planes weight, thrust, drag, and lift. The thrust has to be greater than the drag for it to go forward. For it to go upwards as well, it has to have greater lift than its weight. One of pilots worst enemies is gravity since it always wants to pull down the plane from 5000 feet up. Anyways, planes couldn't fly if they had  a square shaped body. Why? The air wouldn't be able to circulate around it. Bernoulli said that the increase in velocity, for planes its mostly speed instead of direction, meant a decrease in pressure. http://www.centennialofflight.gov/essay/Dictionary/bernoulli/DI9.htm