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How airplane still flies with air at altitude pressing down on it

Asked by: 2609 views , , ,
Aerodynamics

I'm currently training for my FI rating and was doing a demo class with cfi's yesterday. As I was discussing the theories of lift (did an introduction with air), one instructor asked how an airplane still flies (ex. At 2000') when air directly above it (troposphere to 2000') is pressing down on the airplane. 

I said it was because our airplane is also exerting an equal and opposite force against the air (3rd law) pressing downward on it, but they were unsatisfied. I wanted to say not all air above the airplane is pressing down on it (only the air being affected) but it might contradict my statement where i said "the atmosphere is being held together by the gravity".I feel the answer is extremely simple but I got really lost.

Any help? Thanks

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2 Answers



  1. Kris Kortokrax on Oct 29, 2016

    I don’t know if your cohorts are just uneducated or are trying to play devil’s advocate to throw you off.

    Static air pressure acts uniformly all around the aircraft. There is not a lack of pressure on the underside of the airplane. If their theory were viable, when you blow up a balloon, the top side would be flat, with 14.7 pounds of pressure acting only on the top side of the balloon.

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  2. MasterFlight on Oct 30, 2016

    Agree with Kris, they may have been trying to throw you off and did. Or they may have been raising a common point of confusion students have. Or they may be struggling with the concepts themselves.

    The weight of the atmosphere above compresses the air and creates the ambient *static* air pressure. As a quick aside, the atmosphere above is not limited to the troposphere, but would also include the stratosphere, mesosphere, thermosphere, and exosphere. Best just to stick with all-inclusive “atmosphere” to keep it simple.

    Kris is also correct that this static pressure acts uniformly around the entire aircraft, thus there is an equal amount of upward static pressure to downward static pressure. They cancel each other out and have no effect on flight.

    Lift comes from the *dynamic* air pressures created by an airfoil moving through the air. The net result of these dynamic pressures is that air is accelerated downward and to a lesser degree forward. The equal and opposite reaction to this acceleration is therefore upward (lift) and backward (induced drag).

    To better visualize how an airfoil accelerates air, consider a propeller. This is an airfoil oriented vertically (ignoring aircraft pitch and engine mounting for the moment). As the propeller spins, the “downward” air off the bottom of the airfoil is blown aft to the tail of the aircraft, and the “forward” air creates the slipstream the spirals around the airframe. The resulting equal and opposite reactions are thrust and torque respectively.

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