I've heard many people say that a digital autopilot is better than its analog equivalent. How do the two differ in design and the way they control the plane?
1 Answer
Early autopilots would only control one degree of freedom. Starting with the yaw damper of the Me-262, they were a simple feedback loop involving a gyro, a single control surface and some damping circuitry. Example: A roll gyro was coupled to the ailerons and kept the wings level. Commanding a bank angle would simply shift the zero point of the gyro to a different angle, and some damping made sure that the movement was slow and did not overshoot.
This, however, would neglect any cross coupling. Banking flight needs more thrust to keep altitude, but the roll autopilot would not "know" of engines or throttles, just ailerons. The same was true for analogue pitch and altitude hold systems which had a simple feedback loop with the throttle and the elevator, but no other controls.
Modern digital autopilots, starting with the one in the X-31, control all axes in parallel and use a multitude of sensors and feedback loops to control the whole aircraft in one go. This is much easier to achieve with digital control, and gives smoother responses since cross-coupling effects can be anticipated and corrected before they manifest themselves.
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$\begingroup$ The Henschel 129 had the first yaw damper, I think. $\endgroup$– user7241May 24, 2015 at 14:14
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$\begingroup$ @jjack: This is new to me, and considering that it flew mostly at low level, had a decent fuselage length and only a top speed of 400 km/h, I wonder why it should have needed one. $\endgroup$ May 24, 2015 at 20:05
"Digital is NEVER better."
That is a very incorrect statement. Digital signals are generally much more error/interference-resistant than analog ones and, for that reason alone, are very often better than analog signals. Digital signals are very often designed such that there will be no data/precision loss at all until such time as the entire signal is unrecoverable, which usually results in much better quality reception than an equivalent analog signal, even when the analog signal is using more bandwidth. See NTSC vs. ATSC or FM vs. digital radio, for example. $\endgroup$