Model Forum / Radio Controlled / Helicopters / July 2004

Hi Tippy,

You're right in one thing, HH gyro or not, you still have to learn to "fly"
the helicopter.  Compared to the old mechanicals that a lot of us learned
on, HH gyros are a God send but you still have to fly the machine.  Hovering
was not a problem but, in fact, it took me a while to get used to flying
around with a HH gyro.  The tail commands are different and it took a little
getting used to.  It wasn't a big deal, just different.

As for the "rate" part, I usually use the term "rate, command," this is how
I generally describe it to people who ask.

When flying the gyro in HH mode, it's trying to do just that, hold heading.
The gyro doesn't really "know" which way it's pointed.  What it can do,
however, is measure how much the model has turned and how fast.  It can then
apply a tail command to counter that movement and bring the machine back to
where it was.  All of this is predicated on the tail rotor having enough
authority to do what is needed.

Back when we used mechanical gyros, we had to set the ATV's or travel
adjustments in the transmitter so the tail rotor servo linkages didn't bind
at the extremes.  Actually, we'd deliberately set the system up to bind a
"little" at the extremes on the bench.  Once the helicopter was in flight,
the gyro would naturally take a little bit of that back, because it couldn't
tell if the turn was commanded by the pilot or not.

With HH gyros, the "gyro" controls the tail rotor servo, "not" the pilot.
That's why you have to do a setup procedure with the gyro itself to set the
servo ATV's or travel adjustments so that the tail rotor pitch controls
don't bind at the extremes.  This is "very" important especially with the
new digital servos.  If the servo binds at the extremes, it would draw a LOT
of current off the battery, trying to force the linkage to a point that it's
not capable of reaching.  That's bad for the battery "and" the servo.

What about the "rate, command" part I mentioned above?  The HH gyro will try
to rotate the helicopter at a given rate in degrees/second based on how much
tail command the pilot is giving from the transmitter.  How fast the
helicopter will rotate is dependant on how much tail rotor authority the
model has, what the ATV's or travel adjustment settings are in the
transmitter, and how far the pilot is holding the stick over.  Assuming that
tail rotor authority is a non issue, and it usually is in most of the models
available these days, the higher the ATV's or travel adjustment settings
are, the faster the model will spin for a given stick position being held by
the pilot.  If the tail control is too fast for the pilot, all he/she has to
do is turn the ATV's or travel adjustments down on the tail rotor (rudder)
channel in the transmitter.  The gyro doesn't care if the ATV's or travel
adjustments are set to 100% and the pilot is holding the stick 50% over, or
the ATV's or travel adjustments are set at 50% and the pilot is holding
stick "all" the way over.  It's all the same to the gyro.  The nice part of
all of this is that the pilot can hold the stick at a certain point and the
gyro will try to provide a given yaw rate regardless of what the wind is
doing.  With the old mechanical gyros, the pilot had to gradually (sometimes
not so gradually) increase the tail command as the tail rotor pushes into
the wind.  Then, when the tail is pointing straight to the wind and crosses
over to the "downwind" side, the pilot has to immediately reduce and
sometimes reverse the tail rotor command to maintain the same yaw rate.  It
was difficult to perform a constant rotation rate pirouette in the wind.
Now, with HH gyros, the gyro does all the work.  It will apply whatever tail
command is needed to maintain the "commanded" yaw rate regardless of whether
or not the tail is pushing into the wind or being pushed by the wind.  If
that means taking the tail rotor to full control right, then full left,
it'll do it, even though the pilot is only holding 1/4 stick over.  That's
what I meant when I said that the gyro controls the tail rotor servo and not
the pilot.

One way to see this rate command phenomenon is to set the tail rotor ATV's
or travel adjustments to a certain level, say 80%.  Take the helicopter up
(I'm assuming you're capable of doing this safely, or letting someone who is
try this!) and give a full tail rotor command from the transmitter and note
how fast it spins.  Next, land and reset the tail rotor ATV's or travel
adjustments to 40% and perform the same test.  You'll see that it spins
significantly slower than at the 80% level.  That's what they mean by a
"rate," or as I like to say, "rate, command" gyro.  It's really cool as far
as I'm concerned.

Hope this makes sense.
Fly Safe,
Steve R.

Then surely it's incorrectly set up?  A standard rate gyro should enable you
to hover the machine hands off, no tail yaw and certainly no spinning,
shouldn't it?

I accept that if you fly the machine sideways etc it WILL then yaw, and it
might even weather cock, but hovering into wind it should exhibit no motion
at the tail that requires permanent rudder stick to be applied.

It should pirrouette well of course when you end-stop the stick!

Thats my understanding anyway. Its how mine work for me.

D