Powered Flying Controls — Page 196, Lesson 249

Let's pick up with the artificial feel system. This is the part of the powered flying controls that gives the pilot a sense of resistance — a "feel" — when moving the control column, and it's critical because at high speed you need more resistance to prevent overstressing the airframe. The system uses both spring feel and hydraulic feel. Spring feel units may be adequate at low speeds, but at higher speeds, greater resistance to cockpit control movement is needed. Why? Because at high speed, the aerodynamic forces on the control surfaces are enormous, and without artificial resistance, a pilot could easily yank the controls hard enough to overstress the aircraft structure. So the artificial feel system is there to protect the structure. Look at the double cam on the aft elevator control quadrant. That cam illustrates a key tendency of the artificial feel system: it wants to put the control column back into the neutral position. If the pilot moves the control column, he must compress the spring and overcome the force exerted on the hydraulic piston. So the pilot is literally fighting both a spring and a hydraulic force just to move the controls. Now, where does the hydraulic feel come from? It comes from the feel computer. Here's the mechanism: pitot pressure is delivered to the top side of the airspeed diaphragm, and static pressure is fed to the other side of the diaphragm. The difference between pitot and static pressure is dynamic pressure, which is proportional to airspeed. The diaphragm then exerts a downward force on two sets of springs — one set on top of the stabilizer position cam, the other above the metering valve. And that downward force is proportional to aircraft speed. So the faster you fly, the greater the downward force. Now let's talk about the metering valve, because it's the heart of the hydraulic feel. The metered pressure forces exerted against the internal horizontal surfaces of the metering valve balance each other and tend to hold it in the neutral position. So the valve sits in equilibrium. If the metered pressure exerted against the relief valve at the top of the metering valve is enough to balance the downward force exerted on it by the diaphragm and the spring, then the pressure inlet port remains closed. In other words, when the forces balance, no hydraulic pressure flows in. When airspeed increases, the downward force on the metering valve increases and overcomes the metered pressure force. That moves the metering valve down, opening its interior to the hydraulic pressure line. Hydraulic fluid flows in until the metered pressure builds up enough to balance the downward force on the metering valve again. So the metering valve continually modulates — it keeps adjusting — to compensate for metered pressure bleeding to return. It's a constant balancing act. Now, what happens when the pilot actually moves the control column? He has to force the hydraulic feel piston up into the cylinder, and in doing so he overcomes the hydraulic force acting on the piston. That force the pilot exerts is transferred to the relief valve, which opens slightly against the pitot pressure acting downwards on it, and allows hydraulic fluid to bleed to return. So the pilot's input is resisted by hydraulic pressure, and the relief valve vents that pressure to let the control move. So the whole system works together: the feel computer senses airspeed via pitot and static pressure, the metering valve regulates hydraulic pressure to match that airspeed, and the pilot's control movement is resisted by that hydraulic force, with the relief valve bleeding pressure to allow movement. The result is a control feel that increases with speed — protecting the structure from overstress. That's the artificial feel system.