RC Helicopter Gyroscopes Explained

Gyros or gyroscopes are used in everything from full computer mice to the navigation system on the space shuttle to RC helicopters.

So what’s a gyro? In layman’s terms, it’s a device that can sense and measure rotation or how quickly an object turns.

In helicopters, gyros are typically 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 isn’t really a gyroscope at all – it’s an accelerometer. Accelerometers produce a signal as they’re rotated about an axis just like a traditional gyro and the more it accelerates, the stronger the signal is.

For the purposes of this article and real life, we’ll still refer to accelerometers as gyros because they function similarly and the end result is much the same.

On your RC helicopter, gyros work by measuring yaw or rotational acceleration, then mixing that with the pilots’ rudder commands to add dampening to your helicopters yaw axis rotation. For example, if a gust of wind makes your helicopter turn counterclockwise 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 first is the mechanical rate gyro uses an electric motor to spin a small disc or flywheel that can pivot on one axis and has springs to return it to center. When the gyro was moved about the axis that it’s sensitive to, the spinning disc tilts and this tilt is picked up electronically by a potentiometer.

The faster the gyro is rotated, the greater the deflection is and based on the deflection, the corrective signal can be fed into a servo.

2. The second type of gyro is the piezoelectric gyro which uses a quickly vibrating crystal. As the crystal vibrates, an applied rotational force will cause disturbances in its wobble which creates a small, but measurable electric current proportional to the rate at which the gyro is rotated.

The piezo element is similar to that used in a gaslighter system like those found on a barbeque.

Piezoelectric gyros are much more sensitive than a mechanical gyro and because there are no moving parts, they are a lot smaller.

A disadvantage of piezoelectric gyros systems is that they’re very temperature sensitive and going from hot to cold or vice versa will cause them to act erratically. Most have built-in temperature protection circuits, but they’re not perfect, so if you’re going to take a gyro from warm your car and fly in cold weather, give it 10 or 15 minutes to adjust before flying.

3. And the third and most modern type of gyro is 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 the As the gyro rotates, so does the displacement of the mass and the signal generated by the gyro.

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

Which gyro?

The gyro is one of those little magic contraptions that is invaluable. In the early days of helicopters, they weren’t used (probably cause they hadn’t been invented) and the pilots would compensate for tail movements manually. Talk about difficult! I don’t think I would have even bothered getting into helis if gyro’s weren’t around.

A good gyro is worth it’s weight in gold. What does it do? Well simply put, it makes flying easier cause it looks after the tail.

Gyro’s come in two flavours. Heading hold ability and non-heading hold.

When you’re looking to buy a gyro, you’ve got to take into account a number of things. Some of these are:

• How much it costs. Obviously important in any purchasing decision.
• Local knowledge. While these systems may look complicated (and they are!), they’re not too hard to use. However, if you know someone who already uses the gyro you are thinking of buying and can help you set it up, it’s much easier.
• Reputation What’s the general opinion of this gyro?

Non-Heading Hold

A non-hold gyro allows the tail of the helicopter to ‘weathervane’ behind the helicopter. This means that the tail follows the helicopter in the direction it’s heading. This is good for smooth fast forward flight where the tail just follows the rest of the helicopter. However when hovering, it means you have to keep an eye on the tail and make any corrections when it starts drifting.

Heading Hold

A heading hold gyro (when in heading hold) will keep the helicopter pointed in the direction the pilot last pointed it no matter what’s going on. The wind can blow all it wants and the helicopter will stay pointing in your chosen direction. This is really good for hovering cause it allows you to concentrate on everything else that’s going on, comfortable that the gyro is looking after the tail.

It’s also really handy in fast backward flight. In normal mode (ie non-heading hold mode), the tail will tend to wander when flying backwards. However, in Heading Hold mode it won’t.

I personally, find it really handy when flying inverted. I’ve got my hands full with the rest of the controls and it’s nice to not have to worry about compensating for wind, weathervaning etc when you’re upside down.

Most heading hold gyros will do both heading hold and non-heading hold. Most also have the ability to be remotely adjusted from the transmitter (in my opinion, an invaluable function).

Gyro’s come in all sizes for all budgets. There are many ‘non-name brand’ gyros that are cheap, but once you get advanced you’ll probably find yourself looking to a more able system.

Rate Mode Vs. Heading Hold Gyros

Rate Gyros

Rate gyro’s are often used in scale RC helicopters because they lend themselves to a more realistic flying experience while heading hold gyros are used by almost everyone else because they make flying easier.

Rate gyros only sense the turn rate or angular acceleration of your helicopter, not the absolute orientation of the helicopter and do not provide a heading hold capability. For example, once the helicopter has been turned, it cannot return the helicopter to the original orientation, nor keep the helicopter facing a constant direction.

Rate gyros will simply control your RC helicopters tail servo so as to resist rotation in the direction they measure. In other words, it “dampens” the tail movement.

Because a rate gyro “slips” when trying to counteract the main rotor’s thrust, it can’t effectively counteract the main rotor’s thrust on its own.

The amount of thrust provided by the tail is set by the revo mixing function on your radio transmitter.

Revo mixing allows you to set the tail rotor thrust to match the throttle curve so that it exactly counters the main rotor’s thrust. There’s no formula for setting the values – they must be set by experience and trial and error.

Heading Hold Gyros

Heading hold or heading lock gyros are a conceptually simple extension of rate gyros.

In a heading hold gyro, a built microprocessor that keeps track of and remembers how far the helicopter has turned from its set position. Based on the deflection from the set position, the gyro will control the rudder servo such that the gyro returns the helicopter to the set position.

Therefore, as you increase the throttle or head speed of your heli, the holding hold gyro will counter the main rotors thrust automatically keeping your heli’s tail in its original position.

Heading hold gyros are very popular and pretty much standard among RC helicopter pilots for that very reason – they’ll hold your tail in a constant position no matter what you’re doing as long as you don’t input a rudder command, even if you’re 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. Revo mixing on your radio must be disabled when using heading hold gyros.

In conclusion, unless you’re going scale, and are looking for the more real characteristics often associated with scale RC helicopter flight, you’ll want to purchase a heading hold gyro, preferably of the piezoelectric or MEMS variety unless.

Final Words

I originally got started with a CSM 180 which is a non-heading hold gyro. It wasn’t too bad a gyro for the price. But any adjustments had to be made on the gyro itself on the helicopter, which became a real pain in the a$$. It did pretty well though, I learned to hover and I started to do nose in with it etc. Then I got a great deal on a GY501 gyro which does both heading hold and non-heading hold. Wow what a difference! It really accelerated my learning. When I was doing nose in I just stuck it in heading hold and concentrated on the other controls. Once I got confident with nose in I was able to do it in normal mode also.

In my opinion, the market leaders are CSM, JR and Futaba. You won’t go wrong with either of these choices. Currently (June 2001) the best bargain is the Futaba GY401 with servo combo.