"brian whatcott" wrote in message
...
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

Some very good points there, especially regarding the advances in some of
the components that are now available, which also remind me of a problem
that could be either general or local in nature.

Most of the sources that I used to know about have dissappeared from the
local business landscape--so I not not sure were one would buy solid state
gyros, abolute pressure sensors, mass flow sensors, thermistors, or any of
the resistors and SSI and LSI components that used to be so common.

Peter