|Gyros, or gyroscopes to give them
their full title, are used in everything from autopilots in
aircraft, the navigation system on the space shuttle down to our
own sR/C helicopters.
So what’s a gyro? Well,
In simple terms it’s a device that can sense and measure how
quickly an object turns.
In helicopters, gyros are
used to dampen the tail movements or in the case of the heading
hold gyro, to keep the tail in a constant position.
However, the modern RC
helicopter gyro is actually an accelerometer. Accelerometers
produce a signal as they’re rotated about an axis just like a
traditional gyro, so the more the acceleration the stronger the
signal it will generate.
However we will still call
these accelerometers gyros as they work in very much the same
On your RC helicopter, the
gyro works by measuring yaw or rotational acceleration around
the mainshaft. It then mixes that with the pilots rudder
commands and adds dampening to the helicopters yaw axis
rotation. eg: If a gust of wind makes your helicopter turn
counter clockwise in the yaw axis, the gyro senses this and
moves the tail rotor in the opposite direction to slow or dampen
There are three main
types of gyros:
1. The original
gyro for R/C Helicopters was the mechanical rate gyro that had
an electric motor that span a small disc or flywheel that was
able to pivot on one axis and had springs to return it to
centre. When the gyro was moved about the axis that it is
sensing, the spinning disc tilted and this tilt was picked up
electronically by a potentiometer.
The faster the gyro is
rotated, the greater the deflection was, and based on the
deflection the correct signal to counter the movement could be
fed into a servo.
These mechanical gyros are
very rarely used to day in R/C Helicopters due to their physical
size and weight.
2. The current
gyros are mostly piezoelectric gyros which use a rapidly
vibrating crystal. As this vibrates any applied rotational force
will cause disturbances in it’s wobble which create a small,
but measurable electric current proportional to the rate at
which the gyro is rotated.
Piezo electric gyros are
much more sensitive than a mechanical gyro and because there are
no moving parts, and they are a lot smaller and lighter, so they
are really ideal for modern R/C Helicopters.
The one small disadvantage
with piezoelectric gyros systems and that they are very
sensitive to temperature, so going from hot to cold or vice
versa will cause them to act erratically. Most have built in
temperature protection circuits, but I would suggest that you do
not rely on these, so if you’re going to take a gyro from the
warmth of your car and fly in cold weather, give it 10 or 15
minutes to adjust before flying.
3. Finally the most
modern type of gyro which is called the MEMS or Micro
Electric-Mechanical System gyro.
MEMS are molecule sized
machines that are fabricated on top of a piece of silicon, along
with the electronics to interface to them. They vibrate at a
high rate just like the piezoelectric gyro, and as the gyro
rotates so does the signal generated by the gyro.
the different types of gyros, there are two primary ways that
gyro’s operate, rate and heading hold mode.
Mode Vs. Heading Hold Gyros
Rate gyro’s are often
used in scale RC helicopters because they allow more realistic
flying, because YOU, the pilot, needs to control the tail at all
times, while heading hold gyros are used because they make some
types of flying such as hovering a lot easier.
Despite peoples beliefs,
rate gyros do not provide any form of heading hold functionality
at all. So once the helicopter has been turned the gyro cannot
return the helicopter to the original heading, or even really
keep the helicopter facing in the same direction.
Rate gyros WILL HELP to
control your RC helicopter’s tail to RESIST rotation in the
direction they measure. In other words, it “dampens” the
The amount of thrust
provided by the tail can be adjusted by using the revo mixing
function on your radio transmitter, although very few helicopter
pilots use this today. Revo mixing will allow you to set
the tail rotor thrust to match the throttle curve so that it
exactly counters the main rotor’s thrust, but frankly this is
a complex thing to set up and probably best left alone.
Equally, you CANNOT USE REVO MIX if you are going to use your
gyro in HH mode.
Heading Hold Gyros
Heading hold or heading
lock gyros are really a simple extension of rate gyros.
In a heading hold gyro, a
built-in processor keeps track of and remembers how far the
helicopter has turned from its set position. Based on the
deflection from that set position the gyro will control the
rudder servo and try to return the helicopter back to the
So as you increase the
throttle or head speed of your heli, the heading hold gyro will
generally counter the main rotors thrust automatically and keep
your helicopter's tail in its original position.
Heading hold gyros are
very popular and pretty much standard among RC helicopter pilots
because they will hold the tail in the same position no matter
what you’re doing unless of course you input a rudder command.
This applies even if you’re doing 3D aerobatics or flying in a
With a heading hold gyro,
the rudder signal from your transmitter no longer directly
controls the tail – it simply tells the gyro how many degrees
to turn per second. It will also reset the gyros stored position
to the new position you move your heli to.
NB - Revo mixing on your
radio must be disabled when using heading hold
If you’re going to fly
scale and want the more realistic characteristics often
associated with scale RC helicopter flight, you’ll probably
want to use a heading hold gyro, preferably a piezoelectric or