David Megginson <[EMAIL PROTECTED]> said: > Curt and I have been having a discussion offline about algorithms for > NAV mode on a simple autopilot (like those typically found in a light > Cessna or Piper). The current autopilot does not have a working NAV > mode -- it was just a quick kludge, with ability to correct for a > crosswind. > > What algorithms are commonly used to get and keep the CDI centered in > a simple AP? It should be easy enough to start with the rate and > direction of CDI deviation from center. >
The technique that I had in mind is probably one that would work for most aircraft. Flying to the needle causes some issues especially with airliner sized aircraft. The current code adjusts the heading by taking the needle deflection and multiplying it by the distance to the navaid. In order to get the 747 on the radial at all I had add a tighter clamp to the heading output of the current code once it got inside the cone, because at 10 or 15 miles out the adjustments were way too high. At 15 miles X 2 degrees deviation it was commanding a 30 degree turn! The momentum of the 747 caused it to stay on 2 degrees (or go even higher) until there was a steep roll angle going that would shoot it way over the other side of the radial. Basically what I had in mind was an adjustment of the target heading based on the resulting devation values. I don't have the actual formula written down, but it would involve doubling the devations and subtracting them from the target heading and including an integral accumulated error adjustment similar to the PI method that Curt used for altitude/gs. For example if the needle indicates off radial by 0.3 degrees, the adjustment to the heading would be -0.6 degrees adjusted with a small fraction of accumulated error so that 0 could be acheived. Pardon me if I have the sign reversed on one of the values in the example, but the idea is to fly on a course that attempts to cross back over the radial, using the accumulated error to drive the size of the errors and resulting course adjustments to 0. Very strong crosswind gusts possibly could blow it too far off course, but I'm not sure that wouldn't be normal behavior for most AP systems. Note that this will only operate while the signal is intercepted. Something else (arm type modes) will have to do the actual interception and trigger the system into the NAV mode at which point the above method would engage. Best, Jim _______________________________________________ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
