Total Pressure, Static Pressure and Dynamic Pressure
Today one of the major aviation organizations published an article about being nice to your aircraft pitot tube. In the article the author stated that the pitot tube measures dynamic pressure. That statement is totally false and, actually, physically impossible. At the end of the article there is a reference to the Pilot Handbook of Aeronautical Knowledge (PHAK, FAA-H-8303-25C), Chapter 8, Flight Instruments. The first paragraph in that reference, titled Impact Pressure Chamber and Lines, explains clearly that the pitot tube measures the combination of both the static pressure and the dynamic pressure, often referred to as Total or Ram pressure. Pt = Ps + Pd
It is impossible to measure just the dynamic pressure because static pressure is always present around us. It is the ambient atmospheric pressure that we use to indicate our altitude.
Dynamic pressure, on the other hand, is the air pressure felt due to the movement of the air. It is represented by the equation: Pd = 1/2 ρ V2 where the Greek letter Rho, ρ is the air density and V is the air velocity, the velocity that the aircraft is moving through the air mass.. This shows that the dynamic pressure is related to the aircraft velocity.
So how do you separate the dynamic pressure from the total pressure? Well, simplifying things, inside the airspeed indicator is a diaphragm with a flexible divider in the middle dividing it into two sealed chambers. One side of the diaphragm is connected to the line from the pitot tube and, therefore, receives total pressure. This moves the divider an amount relative to the total pressure. The other chamber of the diaphragm is connected to a line from the static port. The static pressure them moves the divider back in the other direction in relation to the value of the static pressure.
The result is that the flexible divider is moved an amount that is relative only to the dynamic pressure. In effect, the instrument has ‘subtracted’ the static pressure from the total pressure.
The rest of the mechanism within the airspeed indicator can then provide a display that is relative to the airspeed. [If the pitot tube measured dynamic pressure directly, there would be no need for the airspeed indicator to be connected to the aircraft static pressure port.]
All this was figured out by a French hydraulic engineer, Henri Pitot (PEE-toh) in around 1732. Smart guy. Even the fancy glass panels with their digital sensors use the same basic method to determine airspeed.
I’m not going to get into what happens when this port is plugged or that one is plugged. You can read about those from other sources. I just wanted to make sure that you got the correct answer – the pitot tube, whether in a glass panel or a round-dial airplane, measures Total Pressure, a combination of both static and dynamic pressure. The airspeed indicating system subtracts the static pressure from the total pressure to arrive at a value for dynamic pressure which relates to airspeed.
As an aside, if you listened to any of the manned space launches, or a movie like Hidden Figures, at some point in the launch the astronauts are advised they have reached ‘max q’. In engineering, the dynamic pressure is referred to by the letter q. That advisory is saying that the vehicle has reached the point in the launch sequence where it is feeling the maximum dynamic pressure. If you look at the equation for q you can see that as the vehicle accelerates to escape velocity in the launch the value of V will initially be the overriding factor in determining the value of q. But as the vehicle gets higher and higher in the atmosphere, the decreasing density of the air will reach a point where it is decreasing faster than the velocity is increasing. At that point the air density term will override the velocity term and q will start to decrease. The vehicle will have reached ‘max q.’