Pitot and Static Sources — Page 18, Lesson 22

Let's start with the static source, because that's the foundation of everything we're about to do. The static source — whether it's a simple hole in the fuselage or part of a combined probe — must have its opening at right angles to the airflow. That's the key requirement. Why? Because we want it to sense only static pressure, with no component of dynamic pressure. Think of it this way: static pressure is the pressure of the air around you, the ambient pressure. Dynamic pressure is the pressure caused by the aircraft's motion, the ram effect of the air hitting you. If the opening is angled into the airflow, it'll pick up some of that dynamic pressure, and our reading will be wrong. So the opening must be perpendicular to the airflow to isolate static pressure. Now, some static sensors — especially those in a combined pitot/static probe — may be fitted with electric heating. That's to prevent ice from blocking the opening, which would be catastrophic for your instruments. Let me introduce the combined pitot/static pressure head. This is a single probe that houses both the pitot opening, facing forward into the airflow, and the static openings, positioned at right angles to the airflow. Now, here's where it gets interesting. We have a problem called Position Error. I want you to remember this term, because it's also called 'pressure' error. Here's what happens: if the aircraft has forward motion, the static pressure sensed will be slightly lower than if the aircraft is stationary. That's due to suction — the airflow rushing past the opening creates a slight vacuum effect. As the speed increases, due to turbulent airflow in the region of the pitot/static heads, this error becomes greater. And turbulence will also affect the pitot reading, not just the static. Let me be precise about this. Position error depends mainly on three things: the positioning of the pressure head, the airspeed, and the aircraft attitude. And there's a subtle but important point about turbulence. The turbulence produced in the airstream by the pressure head itself affects the value of static pressure sensed, rather than the pitot pressure. Why? Because the turbulence is downstream of the pitot opening. The pitot opening faces forward, so it's upstream — the turbulence generated behind it doesn't disturb the ram air entering the pitot. But the static openings are on the side, so they're affected by that downstream turbulence. Now, at large angles of attack — which are usually associated with lower airspeeds — the pressure head is inclined at an angle to the airstream, so position error is usually greater. And here's a practical detail: flight manuals may list different values of position error for different flap settings. So when you're flying with flaps down, the position error might be different than with flaps up, and the manual gives you those values. Because of all this, the Static Vent was introduced. This is a key development. Instead of using a static head combined with the pitot, they separated the two. The static vent became the source of static pressure, and pitot pressure is then sensed by a simple pitot head. A static vent separated from the pitot head is shown in Figure 2.4. Now, why does this help? Because there's usually some place on the airframe — usually on the side of the fuselage — where true, or nearly true, static pressure is obtained over the whole speed range of the aircraft. A flat metal plate is fitted at this position. That flat plate is the static vent. By mounting it flush on the fuselage, away from the turbulent wake of the pitot head, we get a much cleaner static pressure reading. So let me tie this together. We have two separate systems now: the pitot head, facing forward, sensing ram air pressure, and the static vent, flush on the fuselage side, sensing ambient static pressure. The position error is reduced because the static vent isn't in the turbulent wake of the pitot head. But position error isn't eliminated entirely — it still depends on the positioning of the pressure head, the airspeed, and the aircraft attitude, and it's still greater at large angles of attack. That's the core of it. The static source must be at right angles to the airflow, the combined probe has heating, position error is the error caused by turbulence and suction, and the static vent was introduced to separate the static source from the pitot head to reduce that error.