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Last month we had a chat on yaw rate
gyros, what are they and what types are available, this month
let's take a look at the inexpensive Telebee heading hold gyro
Most heading hold gyro starts out at around
$200. The Telebee, at less than $100 (call HeliProz for the
best deals on this gyro!) is the most inexpensive heading hold
gyro on the market. First, let's go over what is a heading hold
gyro. A heading hold gyro uses the same piezo element as a yaw
rate gyro to sense a "rate" change. Except, a heading
hold gyro has an extra circuitry that processes the "rate"
signal to obtain "angular position" information. Yaw
"rate" means how fast the helicopter is changing its
heading. Rate is measured in degrees-per-second. Angular position
is a measure of which direction the helicopter is facing. Angular
position change is measured in degree of deviation from the original
heading. A heading hold gyro tries to maintain the heading of
a helicopter and not its yaw rate. The heading hold gyro feeds
the output signal as a command to tell the tail rotor servo to
move and correct any heading deviation.
A heading hold gyro's job is to make the aircraft
lock onto a heading that the pilot desires. For example, if
the pilot wants to fly sideway at 30 mph, then the heading hold
gyro will not allow the helicopter nose to turn when the helicopter
moves sideway at 30 mph. If the pilot want to fly backward,
then the heading hold gyro will lock the helicopter nose so it
will not weathervane. This ability allows beginner 3-D pilots
to practice 3-D flying with greater ease.
We often hear all kinds of buzz words used
in the magazines. Let's get the technical jargon correct. When
a rate gyro is used, it makes the yaw control a "rate command"
system because the pilot stick deflection determines the yaw
rate. When a heading hold gyro is added, it makes the yaw control
a "position command" system because the pilot stick
deflection determines the "angular position."
The difference between an expensive heading
hold gyro and a less expensive heading hold gyro is how well
the gyro can lock on the heading for a "long time."
Since the heading information is generated mathematically from
the rate signal coming out of the rate sensor by doing a time
integration, integration error can build up over time. Modern
control feedback engineers have designed "lagged rate"
and "washout" feedback schemes that will minimize this
error build up. For example, the Futaba GY-501 has very little
error buildup even after 20 minutes of flight and has a good
circuit to counter act drift due to temperature change. If you
were to hover a helicopter with the Futaba GY-501 in the heading
hold mode for 20 minutes, you will find the GY-501 does a pretty
good job at holding that nose steady and will require no tail
rotor trim change.
Between the beginning and the end of a flight,
the Telebee may require a clicks of tail rotor trim to remove
the drift. But it is not a real concern, the Telebee can hold
the heading very well in dynamic situations, such as during maneuvers.
For example, in backward flight, loops, flips, or sideward flight.
The Telebee does a fine job at these maneuvers.
In the beginning of a flight I adjusted the
tail rotor trim on the ground so the tail rotor servo arm and
pushrod will not move in the heading hold mode. Near the end
of the flight, the model's heading may want to drift slightly
to the left or to the right. Simply add a few clicks of tail
rotor trim to eliminate it. I can live with that for sport 3-D
flying.
The amazing thing is once you have taken off,
you will not likely to notice the drift until you land. The
reason is the helicopter is constantly moving, even when you
are in stationary hover, the gyro is constantly working. The
error never has a chance to build up to make the helicopter yaw
to one side because the washout circuit constantly removes the
error as long as there are dynamic signals coming in. The drift
is most noticeable when the helicopter is on the bench. Don't
worry too much, just minimize the drift as much as you can with
the rudder trim and then go fly.
Here is my cookbook on how to set up the Telebee
heading hold gyro. The Telebee can operate in either the Normal
rate sensing gyro mode or the heading hold mode. Which mode
it is in depends on the toggle switch position on your transmitter.
For example, I was using the Telebee with my older Futaba 8UH
radio. I connect the gyro connector to channel 8 on my Futaba
8UH. When the toggle switch on channel 8 is up, my Telebee is
in the heading hold mode. When the toggle switch is down, the
gyro is in the Normal rate sensing mode. The gain is set by
the ATV (also called Travel Adjust, or end points by other manufacturers)
feature of channel 8 (if you plug the gyro gain pig tail into
channel 8). Increasing the ATV or end point value will increase
the gyro sensitivity. You need to adjust two different end points.
One end point is for the toggle switch in the up position and
another end point value for the toggle switch in the down position.
For example, in the normal rate gyro mode, my ATV (end point)
is set to about 80% and in the heading mode, the ATV is 70%.
If the gain/sensitivity value is too high,
then the helicopter tail will oscillate and wag like a dog.
You want to set the gyro sensitivity to as high a value as you
can before the tail starts to wag. When the gyro is set at a
very high gain, the helicopter pirouette rate may become very
slow. Now you need to increase the rudder channel ATV to achieve
the yaw rate that you desire. I set the ATV or endpoints on
the rudder channel to about 90% to 110% for both direction.
If you find the model spins faster to one direction than the
other, then increase the rudder ATV for the direction that pirouettes
slower. I have found that with the Telebee on the Raptor, I
need to set the left rudder ATV at 90% and 110% for right rudder
ATV. I think this differential will vary with different Telebee
units and different servos and different helicopters.
Typically, the tail rotor pushrod ball link
should be mounted on a steel ball that is about 15 mm from the
center of the servo arm. A longer servo arm increases the effectiveness
of the gyro. I usually try to use as short a servo arm as possible
that will still give me the gyro holding power that I need.
This way the freeplay in the servo gear train will be minimized.
When my helicopter is properly set up, I can give a quick yaw
command to spin the helicopter nose 90 degrees to the left or
right and upon releasing the tail rotor control stick, the helicopter
nose will come to an instant stop; with no ringing or oscillation
or overshoot. If it oscillates, then the gyro gain is too high.
If the helicopter overshoots, then the gain is too low.
On many modern radios, for example, the Airtronics
RD-6000, JR-652 and JR-8103, and the 8UH Super the idle up switch
can be used to automatically change the gyro setting. On these
radios, I connect the gyro gain pig tail to channel 5 and use
"Reverse" feature in the transmitter to make sure that
the gyro operates in the normal rate sensing mode when throttle
is in the Normal mode, and the gyro operates in the heading hold
mode when idle-up is turned on. This is just a personal choice.
The reason is I take off in the Normal throttle. I only use
the heading hold capability in idle-up for aerobatics.
Another advantage of this idle-up switching
setup is radios like the RD-6000, JR-8103 and Futaba 8UH Super
use digital trims. They can remember different trims for different
flight modes. Therefore, the tail rotor trim can be used to
make the helicopter hover straight when you are in the Normal
throttle model, and normal gyro mode. Then, in idle-up, the
tail rotor trim is used to remove any drift in the heading hold
operation.
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Author's Telebee gyro in the
Ergo 60 helicopter.
Author's Telebee tested in
the Raptor 30.
If you have an older helicopter radio, such
as the original Futaba 7UH or the original 8UH, or JR 622, then
you do not have separate trims for each flight mode. Then, you
have to use either tail rotor trim, or subtrim in the transmitter
to eliminate drift for the heading hold mode, then physically
adjust the tail rotor pushrod length so the helicopter does not
yaw in hover in the Normal gyro sensing mode. This is not as
convenient because most piezo crystals are temperature sensitive
and it will require the pilot to make a few clicks of fine tuning
for the tail rotor trim or subtrim to eliminate the drift. This
means in the non-heading hold mode, the tail rotor trim will
be screwed up slightly. The alternative solution is never use
the normal rate sensing mode and always leave heading hold on.
One distinct advantage of heading hold gyros
is they do not require setting up any tail rotor to main rotor
revolution mix. This task is daunting for beginners because
beginners do not know how to climb or descent vertically, yet.
Always leave the tail rotor revolution mixing off when you are
in the heading hold mode.
It is highly recommended to use a high speed
tail rotor servo so the gyro feedback command can be applied
quickly to the tail rotor to instantaneously stabilize the tail.
The best high speed tail rotor servos on the market now have
a transit speed of around 0.1 second for 60 degrees of servo
arm rotation. The Futaba 9205, 9402, JR 8700G, , and the JR
digital 8417, and the Airtronics 95257, 95157, and the Hitec
HS-925, and the digital 5925 all meet this spec, and they employ
high quality coreless motors. But these servos cost around $100
or more each. They are worthwhile investment for the $200 to
$350 heading hold gyros. Since the Telebee is a very affordable
gyro that's most likely selected because one is on a budget,
then I recommend using alternative lower priced high speed servos
for the Telebee. The Airtronics 94742 and the Hitec HS-525 or
HS-525MG (metal gear) are excellent choice. These second tier
servos have a transit speed of around 0.16 second for 60 degrees
travel. The 94742 has a 3-pole motor, the HS-525 and HS-525MG
use 5-pole motors. The 94742 by itself is $40, and the HS-525
is around $29 and the metal gear HS-525MG is around $39. I use
the HS-525MG on cyclic and collective for 30-size helicopters
and the HS-525 for the tail. Heliproz has a very good combo
package for the Telebee and the 94742 servo. Now, you can try
heading hold at a very affordable cost. (Note - JR also has
a very good non-digital hi-speed servo, the 4735)
If you are building a high end $1000 helicopter,
like the Xcell 60 SE graphite, you should still go for the expensive
gyros and servos. But for the new wave of inexpensive 30-size
and inexpensive 60-size helicopters, the affordable Telebee will
do the job. Also note, the Telebee gyro can also be used without
connecting the remote gain control pig tail. Now the gyro sensitivity
is controlled by the trim pot on the gyro body.
Is there any reason to use a non-heading hold
gyro? A helicopter using a heading hold gyro will not weathervane.
For example, in a 540-degree stall turn, if you ended the maneuver
with the tail facing toward the ground, then a heading hold gyro
will hold that attitude while the helicopter rushes toward the
ground. On the other hand, with a non-heading hold gyro, the
helicopter will weathervane itself so the nose will face toward
the ground. Also in a turn, one needs to use more tail rotor
when flying with a heading hold gyro because the nose will not
swing around due to weathervane. For these reasons, some pilots
and beginners may prefer non-heading hold gyros. The heading
hold is only a feature of the electronic circuitry, the quality
of the piezo sensor itself is very important. In general, the
really inexpensive non-heading hold gyros also have a very inexpensive
gyro sensor. If you want a non-heading hold gyro, then the Thunder
Tiger TG-8000 is a good choice because it is about $160 and comes
with a very high quality sensor unit that's on par with the $300
JR and Futaba gyros. In hover, the TG-8000 will lock on as good
as any heading hold gyro and it has almost no drift. Both Telebee
(heading hold) and TG-8000 (non-heading hold) offer good value.
Here is a quick tip on how to set up a TG-8000.
Connect the gain control pig tail to channel 5. Set ATV for
Channel 5 to +0 and 0 for the two end points. The toggle
switch position for channel 5 will now have no effect. Adjust
the gyro sensitivity by using the Subtrim for channel 5. A more
negative value Subtrim reduces the gyro sensitivity and a larger
value increases the sensitivity. The TG-8000 is very sensitive
and do not be alarmed if the tail always wags very, very slightly
in hover. Another note is when using a high quality, high speed
servo such as the JR 8700G, 2700G, or the digital 8435, or the
Futaba 9205, these servos will always buzz non-stop. I don't
like it when these expensive servos buzz, but that's normal.
Photo captions
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