Maynard Hill developed a wing leveler for some of his altitude
record setting model planes (about 30,000 ft) in the mid to
late 70's . He used a system that measured the earth's
electrostatic potential.

At one time he planned to patent the sensor I am not sure that
it ever happened. The implementation was very simple. He
essentially measured the the potential difference between
the wing tips. The tricky part was that we had to ionize the
air at each wing tip to bleed off accumulated charge. At that
time he used a small radioactive source from a record lint remover.
(My age is showing) There are other (safer) ways to do this
Find a library with old Model Airplane News or RCModeller can
probably point in the right direction.

Walter Banks

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