I read about a similar device recently in a radio control aeroplane magazine
(RCM+E  [UK] if I remember correctly though I haven't got that edition
anymore to check). It was based upon AVR uCs AFAIR. Seemed mostly to rely on
accelerometers rather than the ultrasound approach, though I like the idea.
A couple of issues though....
Is the accuracy of ultrasound enough though? A small inaccuracy could result
in a big movement. You'ld presumably also need a "rudder" stabilising
circuit/device (eg a gyro) to stop the tail wandering. Wouldn't the US
system get rather confused when taking off from grass? I would expect the
downdraft would make the grass wave all over the place resulting in a
terribly noisy signal? Perhaps not for a simple pulse reflection
system.......
In addition, if the heli did go off the horizontal at any height (ie due to
a gust of wind) won't the signal get reflected away from the heli completely
bogging up the system? Out of control egg whisk time! Ouch....
Interesting project though :)
Ben

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