Powered Flying Controls — Page 186, Lesson 247

I want to walk you through artificial feel units, because this is where we solve a real problem that appears the moment we go fully powered. Think about what happens when hydraulic actuators move the control surfaces. The hydraulic pressure does the work, so the pilot's muscles are no longer in the loop. That sounds convenient, but it creates a serious danger: the pilot loses all sense of how much control surface movement is actually happening. Without any feel, you have no idea whether you've moved the elevator a little or a lot, and you're in danger of over-controlling the aircraft. So the very thing that makes the controls effortless also makes them dangerous. The solution is an artificial feel unit. These are fitted to the system specifically to give the pilot control feel that is proportional to two things: the speed of the aircraft, and the amount of control surface movement made. So the harder you push, and the faster you're flying, the more resistance you feel. That restores the feedback loop the pilot needs. Now, there's an important distinction here between two types of powered control systems. A fully powered flying control unit is irreversible. That means the control surface does not feed back to the cockpit — the pilot gets no natural feel from the surface itself. Because it's irreversible, it requires an artificial feel system. By contrast, a power assisted flying control unit is reversible. That means it allows feedback to the cockpit controls, so the pilot does get natural feel, and therefore it does not require an artificial feel system. So remember the pairing: irreversible means artificial feel is required; reversible means it's not. Now let's look at how these units are actually built. They vary from a simple spring box to a 'Q' pot operating system. Let me start with the spring box. The idea is that movement of the control column in either direction will compress one or other of the springs. So if you push the column forward, one spring compresses; if you pull it back, the other spring compresses. That compression gives you resistance proportional to how far you move the column — that's the "amount of control surface movement" part of the feel. But the spring box only gives you feel proportional to movement. It doesn't give you feel proportional to speed. That's where the 'Q' pot comes in. Let me walk you through the simple 'Q' pot unit. It contains a simple piston which is connected through a double linkage to the control column. The clever part is that whichever way the control column moves, the piston will be pulled forward — the double linkage ensures the piston always moves the same direction regardless of which way you push the column. Now here's the key: the piston is pulled forward against pitot pressure, which is admitted to the forward side of the pot. The rear side of the pot is open to static. So the front of the piston sees pitot pressure, and the back of the piston sees static pressure. The pressure on the front side of the piston therefore measures dynamic pressure. And that's exactly what makes the control feel proportional to aircraft speed — because dynamic pressure is a function of speed. Let me make sure you understand the relationship, because it's the heart of this unit. The equation is: Pitot pressure minus Static pressure equals Dynamic pressure. In symbols, we write it as P plus one-half rho V squared, minus P, equals one-half rho V squared. Let me unpack that. P is the static pressure. The pitot pressure is the static pressure plus the dynamic pressure, which is one-half rho V squared — where rho is air density and V is the true airspeed. So when you subtract the static pressure P from the pitot pressure, the P's cancel, and you're left with one-half rho V squared, which is the dynamic pressure. That's the 'Q' in 'Q' pot — Q is the standard symbol for dynamic pressure. So the piston is pulled forward against this dynamic pressure, and the resistance the pilot feels grows as dynamic pressure grows, which means it grows as speed grows. That's how the 'Q' pot gives you feel proportional to aircraft speed, while the spring box gives you feel proportional to control movement. Together, they cover both requirements of the artificial feel unit. One more thing to note: this is a simple 'Q' pot. In practice, real aircraft use more sophisticated versions, but the principle is exactly what I've described — a piston sensing dynamic pressure to generate speed-proportional feel. And remember the context: this whole system exists because a fully powered, irreversible control system removes natural feel, and without artificial feel you'd be over-controlling the aircraft.