Worth Repeating
A recent article, Practical Autopilot Theory, written for the May 1997 Issue of Avionics News by Gary Picou bears quoting."... Rate information is a critical item within this loop. (ed. The Inner Loop) The rate of change in aircraft attitude and direction, as sensed by the input, be it gyro, CDI or VDI, or acceleration. Sensing rate is essential to smooth control. Rate control prevents the airplane from exceeding the design limitation, and makes for a smooth ride. It prevents over and under correction. Rate is predicated on the autopilot knowing that the attitude is changing. This requires either accelerometers or a rate sensing circuitry in the system.
To process the rate information properly, the autopilot and its rate sensor must be matched and calibrated. If the system needs to roll at 10 degrees per second to smoothly exit a turn, but the gyro is sending out a maximum of seven degrees per second, the performance will be sloppy. The critical point for technicians in this area are the gain adjustments. Too much gain (i.e., 10 degrees looks like 15), and the autopilot will under bank and under shoot the pitch commands. This causes hunting as the autopilot tries to complete a maneuver without a large enough 'cut' at the attitude or heading. Too little gain (10 degrees of actual attitude appears to be seven at the autopilot) and the airplane may still be banked or not at the proper attitude as the maneuver is completed. This causes overshooting of the desired path, wing rock, pitch porpoise, etc.
Rate is so useful and important that S-TEC autopilots have done away with absolute attitude (ed. thus derived rate) altogether in all axes, in favor of reliable accelerometers and rate gyros. This simplifies the autopilot system, and has led to the most successful general aviation autopilots to date. Although they operate on slightly different principles, the airframe concepts still apply. ..."
Gyros
"... Gyros provide the master source of aircraft attitude. In a rate/acceleration based system, like S-TEC, the turn coordinator gyro gives us a critical bit of information. The difference is, the rate gyro is less prone to gyroscopic errors like precision. In a position-based system, you need to understand some gyro theory to recognize some problems.Precession is the greatest factor. Gyroscopic precession says that any force exerted on a spinning body will result in the resultant deflection ninety degrees from the application, along the rotational axis. So, if you tip a gyro rotor to the left, the rotor will try to dip down (although down and left are irrelevant under gyroscopic stability). No gyros are precession free, because they are captured in gimbals in the airplane. As you bank, the gyro wants to induce roll and vice versa. Autopilot designers have engineered these tendencies to a minimum, if the system is functioning normally, and the gyro precession is within tolerance. Otherwise, pitch porpoise induced by a roll, or wing rock that begins with a pitch maneuver can be traced to the attitude gyro. ..."
Excerpts reprinted with permission from Avionics Magazine, May 1997.
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