Powered Flying Controls — Page 186, Lesson 248

Let’s pick this up right where the mechanism gets interesting. We’re inside the artificial feel unit now, and I want you to picture the ‘Q’ pot — that’s the pitot-pressure-operated feel device — working together with a hydraulic spool valve selector. Here’s the setup. The ‘Q’ pot itself is a chamber divided by a diaphragm. Pitot pressure — the ram air pressure from forward flight — is fed into the upper chamber of the ‘Q’ pot section. That pressure pushes down on the diaphragm. The faster the aircraft flies, the greater the pressure on top of that diaphragm. So the diaphragm’s position is a direct measure of airspeed. Now, that diaphragm is mechanically connected to a spool-type selector valve. When the diaphragm moves down, it opens the pressure port and partially closes off the return port. So hydraulic fluid is admitted to the unit, passes to the forward side of the piston, and also flows through a narrow channel to the underside of the spool valve. That channel is there to dampen the spool’s downward movement — it smooths out the response so the valve doesn’t slam. Here’s the cause-and-effect chain I want you to hold onto. Faster flight means higher pitot pressure, which pushes the diaphragm down harder, which opens the pressure port wider. A wider pressure port means more hydraulic pressure builds in front of the piston. And that increased pressure in front of the piston is what increases the resistance to further control movement. So the feel you get at the controls grows with airspeed — that’s the whole point of the artificial feel unit. Now, one critical design detail: the return port is never fully closed. If it ever closed completely, you’d trap fluid and form a hydraulic lock in the system — that would jam the controls. So the valve is deliberately arranged so the return path always stays slightly open. Also note this: large control movement produces a similar effect on control feel as high-speed flight does. In other words, a big, fast input from the pilot also raises the resistance you feel. So the unit gives you both speed-related feel and input-rate-related feel. One more thing before we move on — the architecture. Figure 9.7 shows the two principal units in any fully powered flying control system, and there’s a note I want you to remember: the artificial feel unit is connected in parallel to the pilot’s control column. Parallel, not in series. That means the feel unit senses and resists the pilot’s input without being in the direct load path of the control run — it’s a separate branch that generates the artificial feel. So, to tie it together: the ‘Q’ pot converts pitot pressure into a diaphragm displacement, the spool valve converts that displacement into hydraulic pressure, and that pressure becomes the artificial resistance the pilot feels. The return port stays cracked open to prevent hydraulic lock, and the whole feel unit sits in parallel with the control column.