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- Myths and Facts about setting up Gyros correctly
I have seen a lot of different
the forums from time to time talking about how to do this, and have seen
some VERY ILL INFORMED answers that I hope people haven't followed.
The most common myth is that if
you have to initialize the helicopter at power up with the gyro in HH
(Heading Hold) mode, you don't have to bother about setting it up in
Rate Mode - UTTER RUBBISH IN ALMOST ALL CASES.
Let's talk about what a gyro does
first so we can understand why you do things in a certain order to get a
well set up tail gyro...
Firstly, you may imagine that you
just stick a gyro on the aircraft, and it goes off and works out where
the centre point is and then holds the tail in a given position for you
- TOTALLY WRONG AGAIN.
If you put your helicopter,
without a gyro on it, on the ground, or better yet on a Lazy Susan
(turntable), tighten it down so it cannot come off, and then increase
the throttle to around 50 % it will almost certainly spin madly.
Why is this ?
Because the rules of Physics tell
us that "for every action there is always an equal and opposite
This rule applies to everything,
even during space walks in zero gravity - check a film and watch how
space walkers are moved around by their power tools !
So back with our helicopter, let's
assume the rotors turn clockwise, as many do. The obvious reaction
to the motor pushing those rotor blades around clockwise is for the
helicopter itself to turn anti clockwise - WITH ME SO FAR ?
To stop it doing that you can
apply right rudder to try to hold it, but as soon as you change the
power setting, our rule of Physics comes in again, and you will need a
new rudder stick setting to hold the tail steady.
So you can see that there is NO
ONE SINGLE RUDDER POSITION THAT IS CORRECT FOR ALL LEVELS OF THROTTLE. -
Now that's a bit of a problem isn't it. ?
Yes, which is where the gyro comes
in to the picture.
To use the gyro, we have to
give it a starting point to work from, and the accepted position is
the position where the tail holds reasonably steady at 50% throttle
setting, as this has proved to be the best median setting.
So how do we achieve that then you
may ask ?
Simple, the first thing to do is to DISCONNECT THE GYRO
completely, and connect the tail servo directly to the rudder output of
At no time do you touch the rudder
stick, which should be centered, and the rudder trim must be centred
Set the rudder servo horn to be
absolutely perpendicular at 50% throttle. You should have already done
this during the mechanical setup I just know you have all done by now
? If not WHY NOT....
Now you spool the helicopter up
slowly, and adjust the servo position, or the length of the tail push
rod or whatever you need to do to adjust the tail rotor position until
it pretty well holds steady at 50% throttle. DON'T TOUCH THE
RUDDER STICK OR TRIMMER. If you look at the tail you will see the slider
hub will NO LONGER BE IN THE MIDDLE OF ITS TRAVEL. That is because
it needs to be offset against the rotor blade rotation to hold the tail
OK, so now what, the tail is
holding at 50% throttle ? Again easy, reduce throttle to 25% and
watch what happens - Oh damn, the tail is not being held any longer is
NO - THAT'S WHY WE
However, what we have
done is to establish the correct centre point for the gyro to start it's
Reconnect the gyro
into the tail control circuit now, and put it into RATE MODE, NOT NOT
NOT HH mode.
Repeat the exercise,
and you will almost certainly find that you will need to make a further
small adjustment to the rudder position to get it to hold to 50%
throttle with the gyro working.
bringing the throttle back down to 25% and see the difference. The tail
will not be held solidly in place, but the gyro will resist it's attempt
to rotate against the rotor blades. That is what is meant by RATE
MODE, It simply dampens the RATE of movement the tail servo will make,
BUT IT DOES NOT HOLD THE TAIL IN THE SAME ORIGINAL DIRECTION .
Another test is to
return to 50% throttle, and then carefully try to make the helicopter
rotate on the turntable.
out for the main rotor blades AND the tail rotor blades,
they can give you a nasty cut at 50% throttle.
You should be able to
feel that resistance against your hand, but the helicopter will let you
turn it around to another heading, and it will then stay in that
So far, so good, what
we have done so far is to let the gyro establish it's natural centre
position which holds the tail steady at 50% throttle.
Now you can switch it
from Rate to HH mode. Most gyros will require a small a mount of
retrimming when switching between Rate and HH modes, that is perfectly
normal in most cases - the really expensive 3D gyros may not need this.
You can use the
rudder trimmer to make these small adjustments in flight.
Now can you
initialize your helicopter in whatever gyro mode the documentation
recommends, and please make sure you get it right, as some use Rate
mode, and other HH and there is no pattern to it.
HH Mode is not magic,
the important thing to note is that what it does is to pick up settings
from the RATE MODE settings, and then applies electronic
"smarts" to hold the heading all the time until you tell it to
change with the rudder stick.
Try moving the
helicopter around in HH mode and you will feel the difference
immediately, as the HH mode will resist you quite heavily, and will
always try to return to the original position.
If you do not set
your gyro up as discussed above, you will never get a stable helicopter,
unless you just happen to be really lucky.
Hovering with your HH gyro working correctly.