Air Pressure
Although air pressure is a simple concept, the way it is often presented in flight is very confusing. Air pressure is just the force placed on an object by the striking of air molecules. Air can only put pressure on an object. The lower pressure on top of an airplane wing does not “suck” the wing up. It reduces the force on top of the wing so the pressure on the bottom can lift the wing. When a can is collapsed by drawing the air out, it is being crushed from the outside. Though we are not really aware of it, the air pressure at sea level is 1 ton/square foot. A cubic yard (3ft x 3ft x 3ft) weighs 2 pounds. That’s the same weight as a quart of milk.
There are three pressures associated with flowing air. The first we will discuss is static pressure, Pstatic. This is what is meant when people refer to “air pressure”. It is the pressure one would measure in a building or riding in a hot-air balloon.
The second is dynamic pressure, Pdynamic. This is not technically a pressure but has the detentions of pressure. It is due to the motions of the air. It cannot be measured directly. When you stand in a strong wind the side the wind is striking is the sum of the Pstatic and the Pdynamic. Your downwind side feels only the Pstatic pressure. So, the force that is trying to push you over is due to the Pdynamic.
The total pressure, Ptotal, is the sum of the Pstatic and the Pdynamic. It is the Ptotal on your windward side in the strong wing. These three forces are used to measure the “indicated airspeed” of an airplane with a Pitot tube.
The Pitot Tube
The airplane’s instrumentation measures the indicated airspeed by exposing two ports to the airflow. The first is the static port in the side of the fuselage. See Figure 1. The static port measures the Pstatic independent of the speed of the airplane. This port is also used to measure the altitude and to provide air pressure to other instruments. The static port is usually on the side of the airplane fuselage near the front, though it is also occasionally placed on the side of the Pitot tube itself.
The Pitot tube faces directly into the air flow as shown in Figure 2. This measures the Ptotal. As shown in Figure 1 both the static port and the Pitot tube are connected to a differential pressure gauge. That means the Pstatic is subtracted from the Ptotal yielding the Pdynamic which is related to the speed of the air. The differential pressure gauge measures the difference between these two ports and is calibrated in speed. This is the airspeed indicator. The “indicated airspeed” must be corrected for air density and temperature to give “true airspeed”.