Understanding the RC Helicopter Gyro

I’ve been digging deeply into RC helicopter gyros and have some great information to share with you. I’ll attempt to demystify gyro gain settings and explain the differences between the various gyro types. But first, I’ve got to tell you a funny story.

The other day we were out with the kids and my wife wanted to stop by the mall to buy some clothes (uggh). I don’t know about you but I hate going to the mall… especially when I get to keep the kids entertained while my wife does whatever it is that shopping wives do. While this was going on I bumped into an interesting kiosk (you know, those stands in mall hallways where people try to sell you ridiculously¬† priced cell phone cases or a carved egg with an elephant in it). Any way, at this particular kiosk, there was a guy selling micro helicopters. Cool! naturally I had to go check it out.

The guy came over and started telling me about the helicopters. He explained how great these helicopters are because they have this great new feature called a gyro. Then he proceeded to point to the flybar and explain that this gyro is what gives the helicopter all its stability. Huh?

In case you’re wondering what the flybar really is, it’s an arm with weights (or paddles) on either end that attaches to the axle of the main rotor. The flybar produces a gyroscopic effect that stabilizes the helicopter. However, this is not what people are referring to when they talk about gyros. I’ll talk more about flybars some other time.

What is the purpose of a Gyro”
I’ll tell you what a gyro is in a moment, but first let me explain its purpose. The gyro is there to keep the helicopter stable. Helicopters have a tenancy to spin around when you don’t want them to. This can be caused by a number of things including torque from the rotating blades or wind if you’re flying outside. It would be extremely difficult to fly a helicopter with out some method to compensate for these outside forces. Enter the gyro.

What is a Gyro?
A gyro is not really a gyro at all. Confused? Lets look at the definition of gyro.

gyro – rotating mechanism in the form of a universally mounted spinning wheel that offers resistance to turns in any direction

The animation above is a true gyro (gyroscope). Apparently, back in the day rc helicopters were equipped with mechanical gyros. This in itself would dampen any unwanted helicopter rotation but I understand that these early helicopters also used a sensor of some type generate an electrical signal from the gyro to feed to the tail rotor. This would cause the tail rotor to counteract those unwanted movements.

Okay, that’s enough of a history lesson. Fast forward to today and mechanical gyros are no longer used. These days, digital accelerometers are used instead. The same things they put in Nintendo Wii remotes and iPhones to detect motion. Accelerometers usually contain piezoelectric sensors that use tiny crystals (inside of a microchip). The crystals are stressed and vibrate when acceleration forces are applied which causes a voltage to be generated. That voltage is used to measure the forces. Accelerometers have no moving parts, they are inexpensive, tiny, lightweight, and far more sensitive than their mechanical counterparts. For whatever reason, despite the switch to accelerometers, these devices are still referred to as gyros.

Today’s gyros work by converting the accelerometers output into a signal that is fed to the tail motor, which in turn counteracts, or at least suppress unwanted rotational movement. This is handled in two ways that divide gyros into two categories. “Yaw Rate Gyros” and “Heading hold Gyros”.

Yaw Rate Gyros
Yaw rate gyros monitor the angular acceleration (how fast the heli is turning) and when it starts to move too fast, it sends a signal to the tail motor to slow down or spin it in the opposite direction. The yaw rate gyro won’t stop outside forces from affecting the helicopter, but they will slow down the helicopters reaction to those forces which should give you time to react and maintain control. One thing to consider about yaw rate gyros is that they react the same regardless of what is causing the helicopter to rotate. That means when you are intentionally spinning the helicopter, the gyro will slow it down. There will be a reduction in performance and responsiveness when using a rate hold gyro.

Heading Hold Gyros
Heading hold gyros work a little differently than yaw rate gyros. This type of gyro holds the heading as the name implies. The heading hold gyros can actually sense the heading of the helicopter and will control the tail motor to maintain that heading (unless the operator changes the heading via the transmitter). This is really cool because the gyro electronics actually have full control of the tail rotor at all times. It monitors the accelerometer and the transmitter signals simultaneously to ensure that the helicopter stays in the desired orientation. For example if you are flying straight and a breeze picks up that tries to spin the helicopter, the gyro will kick in and engage the tail rotor as hard as it needs to in order to keep the tail in the same orientation.¬† While this is happening, if you provide stick inputs to rotate the helicopter opposite the wind direction, the gyro will tell the rotor to apply even more power (it’s already spinning to compensate for the wind) as needed to rotate the helicopter an amount corresponding to the stick inputs provided. Conversely, if you provide stick inputs¬† to rotate the helicopter the same direction that the wind is pushing, the gyro will reduce the tail rotor compensation just enough to allow the wind to move the helicopter to where you want it to go. Pretty cool huh? Most decent micro helicopters these days are equipped with heading hold gyros (or programmable gyros that can be programmed as either heading hold or yaw rate).

Gyro Gain adjustment
I’ve read a TON of conflicting information about what gain is and how to set it. Luckily, after 10+ years in electronics engineering, I know enough about what gain means in that context to make sense (I hope) of what people on the internet are describing in terms of rc helicopter gain settings.

Gain refers to the amplification of a signal. In this case, the signal is the output from the accelerometer which gets amplified prior to reaching some sort of logic (likely a micro-controller, at least in the case of heading hold gyros). Before I get into the effects of changing the gain, it’s important to understand another concept, threshold.

The idea behind threshold is that some amount of signal has to be present before the gyro will do anything. Without threshold, the gyro would constantly be moving to correct for every microscopic movement that the accelerometer detects. This would probably cause the helicopter to rapidly twitch back and forth. Definitely not what we want. So, lets assume that the threshold is set to a voltage that corresponds to a relatively small amount of accelerometer detection.

There is one other concept to understand before I get back to discussing gain. Once the threshold has been reached, and the gyro starts reacting, the higher the voltage, the more of a correction signal will be sent to the tail motor.

Okay, back to gain. The accelerometer detection is proportional to the voltage output, but, as the gain is increased, the voltage output will become higher for the same corresponding accelerometer detection. This will do two things…

  1. It will cause the threshold to trip earlier
  2. Once the threshold has been tripped, it will cause the gyro to apply a larger correction signal to the tail rotor.

In practical terms, that means that by increasing the gain you cause the gyro to kick in sooner and to react more aggressively to rotational forces. If you reduce the gain, the gyro will be slower to kick in and will have a more subdued response to rotational forces.

This video is an excellent demonstration of a heading hold gyro vs a yaw rate gyro. Special thanks to Kendoo74 who uploaded the video.

I’ll be the first to admit that I don’t have a lot of experience
with RC helicopter gyros, but I’m confident that this is a good
description of them based on tons of research that I’ve done combined
with my knowledge of electronics. After all, that’s what this site is
all about; Learning as much as I can about micro helicopters and sharing
what I learn about their features, mods, repairs and news with you the
readers.



One Response to “Understanding the RC Helicopter Gyro”

  1. Andre
    November 24, 2012 at 7:41 am #

    I have been flying rc heli for the past few months and enjoying it tremendously however the little crashes can be frustrating at times but I really enjoy the rebuild and getting to understand the mechanics.

    Your writing and video have been great and has clarified the issue I has regarding gyros.

    Have fun with your heli

    Cheers

    Andre
    Cape Town

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