Peter Dohm wrote:
"brian whatcott" wrote in message
...
I expect, like me, you thought you knew what heading hold gyros were all
about? If you haven't been around model helicopter enthusiasts lately, you
have no idea! Try googling 'heading hold gyro'.
It turns out, that radio control modelers stick a heading hold gyro on
their model helicopter and hook it to a fast servo that modulates the tail
rotor pitch. The device takes over when a rotate ("pirouette') command
stops, and stops the tail dead on that heading - fast, and holds it
against wind-drift and weather cock effects.
The HH gyro runs $40 to $150 and a fast (digital) servo might add another
$40 on it.

Think of the possibilities for a heading stabilize function in a
homebuilt!
A HH gyro driving a big servo, controlling a servo tab on the rudder.
Something similar could be arranged for pitch hold (a sort of super cheap
altitude hold/augment?)

Brian W

One of the avionics makers, Bendix IIRC, had a system available about 25 or
so years ago that used an air stream across a pair of resistance elements,
or two pairs of resistance elements for a two axis autopilot. Basically,
each pair of elements was an uninsulated resistor on each side of the
airstream for heading hold and top and bottom for pitch hold--not really the
same thing as altitude hold but possibly usefull in smooth VFR conditions.
AFAIK, the system worked because the resistance of each element (for most
materials) tends to increase with rising temperature and decrease with
falling temperature--so that the system can work by comparing the
resistances with little regard for the actual resistance values. My best
recollection is that the device could also maintain a standard rate turn.

I only vaguely recall that there may have been an article in one of the
homebuilding magazines--possibly in the early 1980s.

Peter



I remember this project well, it was the brainchild of a research
engineer at one of the NASA labs - NASA Ames, if I remember.
I was so intrigued, I called him up to talk about it.
A few people scratch built the device, the "gyros" were four thermistors
on a circle, with a little tube blowing air into the center of the
circle. Rotation in either axis shielded one from the air jet, and
ventilated the other, of each pair. Thermistors generate a little heat,
and their resistance varies dramatically with temperature. Each pair fed
an operational amplifier as differential inputs.
A weakness of the design was the little motor and fan involved.
These days, a two axis (piezo) gyro is more stable, more sensitive and
more reliable, not to mention using much less power. And you can buy it
off the shelf!

Regards

Brian W