Pitot and Static Sources — Page 18, Lesson 19

Let's start with the static head, because that's the foundation of everything we're about to do. A static head is a tube with its forward end sealed, but with holes or slots cut into the sides. Those slots don't face into the airflow — they're perpendicular to it — so in theory they sense only the static pressure, the ambient pressure of the air around the aircraft. In practice, there's a suction effect as air flows past, so the sensed static pressure comes out slightly lower than true static when the aircraft is moving. That's a real error we have to live with and calibrate for. This static pressure is what supplies the static line to the pressure instruments — the altimeter, the vertical speed indicator, and the airspeed indicator. Now, the pitot source is the other half. A pressure sensing system with separate pitot and static heads is shown in Figure 2.1. But we can also combine them into a single unit called a pressure head, where the static tube surrounds the pitot tube, with separate pressure lines still leading to the instruments. Figure 2.2 shows that combined arrangement. So whether they're separate or combined, the key point is that pitot and static are two distinct pressures, each with its own line. Let me be precise about what each one carries. The pitot pressure is the total pressure — the ram air pressure that builds up when the moving aircraft rams air into the open end of the pitot tube. The static pressure is the undisturbed ambient pressure. The difference between them is dynamic pressure, and that's what the airspeed indicator actually measures. That's the core of the whole system. Now, the requirements of a pitot tube. It must be positioned outside the boundary layer — that's the thin layer of slow-moving air clinging to the aircraft's surface. If the pitot sat inside that layer, it would sense disturbed air and give wrong readings. So it's usually a head on a strut if mounted on the side of the fuselage, or a tube placed on the nose, ahead of the fuselage. And the opening must be designed to be parallel to the airflow in the normal flight attitude — meaning in level cruising flight, the pitot opening faces straight into the relative wind. There are two ways the pitot pressure can be handled. One way: the air is brought to rest in the pitot probe against a stagnation wall — that's simply a pressure-measuring flat surface — and then transmitted up the pitot pipelines to the ASI and the Machmeter. The other way: the pitot pressure is passed directly up into the pitot pipelines to the instruments, which is more usual in elementary aircraft. Either way, the pressure reaches the instruments. Now, why is this so critical? Measurement of dynamic pressure is essential to safe flight. At too low a speed, the aircraft will stall. At too high a speed, it will be overstressed. So the ASI — the airspeed indicator — is critical to flight safety, and it cannot be allowed to block because of ice. That's why an electric anti-icing heater coil is usually incorporated. Any errors due to the heating effect can be reduced by design and calibration. But here's the clever part: if water is drained, ice should not be able to form. So drain holes are provided. Those drain holes will, of course, cause some loss of pressure, but that loss can be suitably calibrated and allowed for in the design. So the system is a balance — you accept small pressure losses from the drain holes, and you accept small errors from the heater, but you calibrate them out so the ASI stays accurate and, above all, unblocked. Let me tie it together. We have two pressures: static from the static head, pitot from the pitot tube. They can be separate or combined in a pressure head. The pitot tube must sit outside the boundary layer, facing parallel to the airflow. The pitot pressure can be brought to rest against a stagnation wall, or passed directly up the line. Dynamic pressure — pitot minus static — is what drives the ASI, and it's essential because it tells us whether we're about to stall or overstress. And because ice could block that critical line, we have an anti-icing heater and drain holes, with the resulting small errors calibrated out in design.