Hello,
But to go back to the C172 example, does anyone actually understand how this issue is handled in a cessna autopilot or any of the others that might be installed in a 172? Will it stall the aircraft? Or...hmmm...trying to remember... does the 172 even have an AP for the pitch axis?
I can't give an exact matching case, but here is some info on a S-TEC System Twenty/System Thirty autopilot in a Piper Warrior II, a plane in the C172 class.
The Twenty is a 1-axis AP, controlling the ailerons. Feedback to the controller is from the turn coordinator (with the AP built into the same case). There are 4 operating modes. One is a stabilizer, with a panel knob to control the pitch. With the knob centered, you have a wing leveler. Turning the knob left or right gives a controlled left or right bank, up to 90% of a standard rate turn.
Mode 2 is heading hold, where the input is from the heading bug of the DG. It adjusts the bank to keep the bug centered. Modes 3 and 4 get input from the needle on the CDI, tracking to/from a VOR or following a localizer on an approach. Mode 3 is lower gain for a smoother ride when the VOR signal is a bit erratic. Mode 4 holds more actively as you would want on an approach.
The model Thirty adds pitch control in the form of altitude hold. It uses an absolute pressure transducer for altitude sensing, and an accelerometer is mentioned but not discussed. You first stabilize at the desired altitude, and switch the pitch control on, which then maintains altitude. If the elevator servo runs out of capability, two lights signal the pilot to adjust the pitch trim up or down to get back into control range. No attempt is made to help on climb or descent, that would likely put it out of price range for these smaller single engine planes.
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Bill Earnest [EMAIL PROTECTED] Linux Powered Allentown, PA, USA
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