Expert: Paul Soderman - 10/26/2007

Question
I understand that when a plane is flying, lift is created by the air rushing over the top of the wing at a faster rate than the air going over the bottom (at least I think that's right).  
I always wondered how or why say fighter jets can fly upside down.  I would think the theory of how lift is created would bring them straight down.
Thanks,
Jim

Answer
Jim
You are correct - lift is created by high speed airflow over the wing upper surface, which creates lower pressure than the lower speed airflow under the wing.  That lift is controlled by a number of things including airfoil shape and angle of attack.  Airfoil shape, bulbous on top plus camber, forces the airflow to accelerate over the upper surface.  But angle of attack is an even stronger effect - nose up for positive lift, nose down for negative lift (now the air has to accelerate over the lower surface).  

So, a fighter jet that flew upside down and still kept the same angle of attack would generate lift that would drive it straight down as you said.  But if the pilot induced a negative angle of attack, the lift would be skyward or toward the lower side of the wing.  You can see the same thing yourself by sticking your hand out the car side window while someone else drives. Palm up, you get positive lift, palm down you get negative lift.  You are now qualified to fly an aircraft upside down.
Paul