Google has recently scanned and posted all of Popular Science's back issues (and they go WAY back), and there are some gems in there from the early days of autopilots. One of them is this, which describes an autopilot the legendary Maynard Hill (yes, the guy who flew a hobby UAV across the Atlantic in 2003) built that uses minute electrostatic differences with altitude to create an analog autopilot.
Here are some other pictures from the article, but you should really read the whole thing. Anyone know what went wrong with this approach and why he abandoned it?
Comments
I think the reason why this project was not taken any further was that the electrostatic sensors were made of radio active material banned in Germany at the time and probably throughout the world by now! I also believe the autopilot was confused by hills and terrain.
This concept is still valid if suitable sensors could be investigated and designed. With all the knowledgeable folks here I bet someone could come up with a solution!