Air speed measurement

Bernoulli’s equation gives rise to a simple method of measuring air speed. You can see that the dynamic pressure is related to the density and the speed, (dynamic pressure = ypV2), so if we could measure the dynamic pressure and the density we could determine the speed. Fortunately there is a very simple way of measuring the dynamic pressure at least.

If we point a tube directly into the flow of air, and connect the other end to a pressure-measuring device, then that device will read the stagnation pressure. The reason for this is that the tube is full of air and its exit is blocked, so no air can flow down the tube; the oncoming air therefore is brought to rest rela­tive to the tube as it meets the open end of the tube. This type of tube is called a pitot tube and provides a means of measuring stagnation pressure. A dif­ferent result is obtained if we make a hole in the side of a wind tunnel or in the fuselage of an aircraft, and connect this via a tube to a pressure-measuring device. The hole will not impede the flow of air, so the pressure measured will be the local static pressure. A hole used for this purpose is called a static vent or tapping. Since static pressure plus dynamic pressure equals stagnation pressure as shown above, it follows that