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AUTOPILOT AWRY
Hi James,
Thanks for your feedback. I think we are talking about two different generations of Piper / Mitchell autopilots. Mine is an Altimatic II, installed in a 1964 Piper Aztec. It obviously is original equipment, factory installed. All the electronics are composed of germanium transistors, so that also dates it. My info all comes directly from the Piper Service Manual entitled "Autocontrol I & II, Altimatic I & II Service Manual". The Piper part number is 753 798 with a first publish date of June 1961. It contains permanent revisions numbered PR710920 and PR721020, dated September 20, 1971 and October 20, 1972, respectively. There are also pages with various dates between these dates. The 10Mhz frequency information is contained in the theory of operation section and it describes in good detail the construction and operation of the roll vane in the AI. It shows it as being composed of an Iron Ferrite piece on one side and of an Aluminum piece on the other. As the aircraft banks to the Iron side, it says the inductance of the circuit is increased and when it banks to the Aluminum side, the inductance of the circuit decreases. It also details a similar pickup in the AI for pitch and a variable capacitor in the DG, variable capacitors in the each servo's "follow-up" circuit and variable capacitors in the amplifier and control console. It goes step by step on how a change in bank and a change in pitch and altitude changes each reactive component and how all of the these reactive components work together to maintain a given resonant frequency. Maybe the 10MHz figure is just an example - I have not measured the frequency. However, the roll and pitch servos in my 1964 Aztec definitely have the feedback or "follow-up" capacitors. I've seen them with my own eyes and checked their timing marks to see if they are lined up properly. There are RF type variable capacitors in the control console and in the console amplifier to adjust the circuit reactance (level the wings and level the pitch). There is also an RF type variable cap in the altitude bellows assembly to detect a change in pressure altitude. The trim-turn and pitch controls on my control console are also variable caps, along with an altitude / pitch offset adjustment. The amp has two cans which look like IF transformers with paper labels on them that state "Do not adjust discriminator". The whole thing is connected together with coax cables and the service manual is clear about using the proper length and type of cables otherwise the resonant frequency will be shifted. The service manual shows a drawing of these cable as being composed of a hollow tube that contains a curly wire on the inside and a braided shield on the outside. The whole thing is covered with an outer jacket of rubber or plastic material. It reminds me of the radio antenna coax used in automobiles in the 1960s. When I went looking for an instrument shop to overall my AI, I ran into several that stated they would not work on "those old RF type gyros". All of this taken together makes me believe that the system does in fact use an RF signal as the method to detect changes in pitch and bank and to detect the position of the servos. From the limited information I can find, it appears that the later Mitchell / Piper versions switched to the pickup system that you describe. Maybe this was the Autocontrol IIB / Altimatic IIB and later versions? I don't know. I wish I had a schematic of my console amplifier. My service manual is lacking in this area. It treats the amplifier as a black box. It does provide a schematic of the radio coupler, but no theory of operation information, or other useful info like the excitation frequency it supplies to the DG, voltage levels, etc. Any information you'd care to share along those lines would be greatly appreciated. Just FYI, the earliest Autocontrol and Altimatic systems used what the service manual refers to as a "0" heading directional gyro. This device did not allow a specific heading to be selected, but rather just indicated a change in heading which was used to keep the wings level. Also, these early models had amplifier modules attached to the back of the AI and to the back of the DG. There was no central "console" amplifier. When I first got the service manual and saw this, I knew these didn't exist in my airplane and thought there were missing components. However, as I dug in more, I finally figured out that these were consolidated in a later version and that the DG was changed to a resolver type wired to the radio coupler rather than the older RF type wired to the autopilot amplifier. What is the excitation frequency of the radio coupler to the DG in the later Mitchell / Piper autopilots? Maybe it is the same in mine. FYI, my coupler has a 4-pin Amphelnol connector for the radio input and a 9-pin Amphenol connector for connection to the DG and autopilot amplifier. In my case, the coupler could not be eliminated from the system by simply moving connectors around. I'd be happy to learn all you're willing to teach me about these things. If you have any schematics or theory of operations on the early Autocontrol / Altimatic I/II amplifiers and/or on the early radio couplers, I'd really appreciate getting a copy. Ronnie "jmk" wrote in message ups.com... Hmmm... I have to admit, that's quite a bit different than a standard Autocontrol (aka Century) for a normal Piper system - and I would ask if you are sure of your information? The standard "wing level" indication from the AH is indeed off a pickup coil, but it's about 5 KHz, no RF. And there is no frequency shift - the inside of the Century AH/AI just has two coils and a metal plate behind them (looks like an iron butterfly). As the thing shifts from one side to the other you get a shift in amplitude, either in phase or out of phase (180 degrees) with respect to the ROLL EXCITATION signal. [Or the HDG EXCITATION signal, if it goes through the nav coupler.] With the AH centered in roll, the amplitutde is zero. There is no frequency shift regardless. It would be great if there was some sensor back from the ailerons, but there isn't - so it has no way of knowing their position. Having said all the above, your "list of the usual suspects" is the same one I would come up with. #3 is the one that surprises most folks, but if you have a nav coupler then it uses HDG EXCITATION to drive the ROLL coils, so lose of HDG signal affects just operating in the WINGS LEVEL mode. You can move the ROLL cable that goes from the computer to the NAV Coupler to go to the AI instead and restore use of ROLL EXCITATION for a test. |
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