"Peter Dohm" wrote in message
...
"RogerN" wrote in message
m...

"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

The heading hold gyros are rate gyros and use a microcontroller integrate
the error. They hold heading real well but drift over time. Most of the
time we can trim out the drift well enough to not be a problem but
remember most model helicopter flights last no more than 15 minutes and
if a constant heading was held for an entire 5 minutes it would be a long
and boring time for the pilot.

But gyros in model helicopters do make a world of difference. When I
started trying to learn to fly model helicopters (1981) gyros weren't
very common. I tried for years and was never able to hover out a tank of
fuel before a crash. After purchasing a simple mechanical rate gyro, I
flew some 70 full tanks of fuel before crashing, and that crash was a
result of getting too far away and losing orientation (the heli turned
black in the bright sky!).

On my larger model helicopters I have gyro's that once sold for nearly
$400 and a servo that sells for $130, I bought most of my stuff used and
sometimes crashed. By that time I had so much experience rebuilding
crashes that I would buy heli's needing work for a fraction of the new
cost. I have my own home machine shop and make most of the shafts simply
by cutting drill rod to length and cross drilling the holes for the
bolts, a $20 main shaft costs me about $2 + 10 minutes.

RogerN


As a matter of fact, I really allowed myself to get way off track early on
in this thread--along with nearly everyone else.

Basically IIRC the rate gyro systems are traditionally called wing
levelers, while a real heading gyro is just that--and frequentle slaved to
a flux gate--so that an autopilot on heading hold will really continue to
follow the same magnetic heading. The rate gyro allows the autopilot to
intersept a new course or heading without commanding an acrobatic maneuver
to accomplish it.

Peter

The problem with sensing magnetic heading in model helicopters is that they
want them to hold the heading relative to the model. For example if you
were heading north and did a loop you would be heading south at the top of
the loop, that would be interesting if it tried to auto correct magnetic
heading during a loop or other aerobatics.

But for stabilizing an aircraft that wasn't performing aerobatics perhaps a
rate gyro, magnetic heading, and perhaps an accelerometer might be able to
do the job.

RogerN