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?

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    $\begingroup$ A lot of people think that anything newer is automatically better... $\endgroup$ – jwenting May 10 '15 at 4:42
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    $\begingroup$ Digital is NEVER better. Digital is just different. Because digital systems are usually newer than analog ones, they just often tends to be better. Look at audio files: MP3 has yet to match the quality of a vinyl, but yet it has many advantages over it. And many computers rely on analog devices as source of informations (sensors, ...) $\endgroup$ – Antzi May 10 '15 at 5:58
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    $\begingroup$ @antzi, I'd have to say that is completely wrong. Digital is more flexible than an analog system because you can do far more with code than you can with discrete elements of a circuit. Your audio example describes the difference between a continuous signal and a discretely sampled signal. That has nothing to do with the quality of digital vs analog. $\endgroup$ – chris12892 May 10 '15 at 18:15
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    $\begingroup$ @antzi: Good luck making an analog GPS unit. $\endgroup$ – Nathan Tuggy May 10 '15 at 21:40
  • $\begingroup$ @chris12892 nonsense. Digital systems have many problems analog problems avoid, especially in the discrete nature they are by force required to adopt. Analog systems are usually continuous, meaning changes over time are more correctly defined. $\endgroup$ – jwenting May 11 '15 at 11:19

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.

  • $\begingroup$ The Henschel 129 had the first yaw damper, I think. $\endgroup$ – user7241 May 24 '15 at 14:14
  • $\begingroup$ @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. $\endgroup$ – Peter Kämpf May 24 '15 at 20:05

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