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   INFORMATION - All you ever wanted to know about how gyros work
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 whats a gyro? Well, In simple terms its 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 theyre 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 way.

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 the rotation.

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 its 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 youre 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.

Besides the different types of gyros, there are two primary ways that gyros operate, rate and heading hold mode.

Rate Mode Vs. Heading Hold Gyros

Rate Gyros

Rate gyros 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 helicopters tail to RESIST rotation in the direction they measure. In other words, it dampens the tail movement.

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 rotors 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 original position.

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 youre doing unless of course you input a rudder command. This applies even if youre doing 3D aerobatics or flying in a strong wind.

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 gyros.

If youre going to fly scale and want the more realistic characteristics often associated with scale RC helicopter flight, youll probably want to use a heading hold gyro, preferably a piezoelectric or MEMS.

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