飞机之所以能飞起来，是因为它产生向后的力，使飞机前进。飞机的机翼是上部分稍弯曲、呈弧形，下面稍平。所以上面的气流流动快，根据努伯利原理，机翼上面的压强小于下面的，飞机就受到向上的力了。而不是发动机向下喷气。
If so, an aircraft won't be able to fly upside down, which fighter aircrafts both jet and propeller alikes do all the time.
Here is a link about more accurate explaination.
http://www.grc.nasa.gov/WWW/K-12/airplane/right2.htmland here is the typical wrong model
http://www.grc.nasa.gov/WWW/K-12/airplane/wrong1.html