So what exactly is a gyro and why do I need it? A gyro is a device that senses angular movement and generates feedback to the control system in order to stabilize it. We use a gyro in our helicopter to stabilize the rotation around the main rotor shaft (yaw axis). The gyro helps reduce the pilot workload associated with keeping the helicopter pointing in a given direction by counteracting any unintentional yaw caused from the forces exerted on it by wind and torque. A sensitivity (gain) adjustment on the gyro determines the amount of effort put into corrections. When the gain is adjusted too low, the gyro will not effectively stabilize. When adjusted too high, the gyro will overcorrect, resulting in oscillation (wagging).
What about the different modes of operation?
We know the gyro will sense and correct, but how it does this varies depending on its mode of operation. The two common modes are "rate" and "heading hold."
• Rate
A rate gyro senses angular velocity (see sidebar) and counteracts it with input to the rudder servo. It detects when the helicopter rotates and will reasonably attempt to offset it. I say reasonably because a rate gyro has no sense of heading; it simply provides correction based on velocity and timing. It has no way of determining if it actually moved back to the original position.
Unfortunately, rate gyros exhibit an unwanted side effect: They may actually resist (or dampen) rudder commands initiated by the pilot.
• Heading Hold
As its name implies, a heading hold gyro (a.k.a. Angular Velocity Control System or tail lock) will provide the correction necessary to maintain a heading. Any deviation to the helicopter's heading not initiated by pilot command causes the gyro to apply correction until it's returned back to its original heading.
Unlike a rate gyro, the heading hold gyro does not resist pilot command.
What about the features?
In addition to the gain adjustment, a gyro may feature additional functionality and adjustments. The majority of gyros offered today contain most if not all of them.
Where and how should I mount it?
Some believe that the gyro must be as close to the main rotor shaft as possible. This is simply not true, as I explained in the angular velocity sidebar. There are, however, some other things to consider when choosing a mounting location.
Protection — The gyro should be protected in the event of a crash. It's for this reason that I don't advise mounting it on top of the tail boom mount. It can be easily whacked by the flybar or the flybar paddles. Good candidates for location would be under the tail boom mount, inside the frame, or up front.
Accessibility - You'll most likely need to make adjustments to controls on the gyro itself. If you mount it inside the canopy, you'll have to remove the canopy every time you need access to the gyro.
Interference - In electric helicopters, you'll want to keep it away from sources of electrical interference, such as the motor and ESC.
Secure - Mount it to a clean, solid surface with one layer of double-sided vibration damping tape. As an additional precaution, it can be further secured with a Velcro strap or a nylon cable-tie.
Bench setup
Initial Transmitter Settings
Dial the following settings in for the rudder channel:
• Servo Reverse: Check for proper direction.
The helicopter's nose must turn in the same direction as you move the rudder stick. Tail rotor pitch should increase when the rudder stick is moved to the right. Set servo reverse accordingly.
• Trim: 0 (centered)
This ensures that you start with a good mechanical setup.
• Sub-Trim: 0
Once again, this ensures that you begin with a good mechanical setup.
• Adjustable Travel Volume/End Point Adjustment:
Maximum (in both directions)
Normally, this would control the amount of servo travel. However, because of the gyro's design, this controls the pirouette rate—not the travel limit. We'll adjust travel later on mechanically, and if available, with the gyro's limit adjustment.
• Exponential: +30%
This compensates for the non-linear pushrod movement that occurs due to the circular path of the servo arm (see sidebar on pushrod geometry). It will bring the pushrod movement closer to linear with respect to rudder stick movement. Some advanced pilots prefer the feel of no exponential on the tail.
• Dual-Rate: 100%
We may adjust this later. If the gyro's electronic travel limit is used to reduce servo travel, sometimes you'll notice that the servo will reach its limit without full rudder stick deflection. Dual rate can be used to bring the servo's limit and the rudder stick's full deflection in sync with each other.
• Revo Mixing: Inhibited
This will be adjusted later if you're using a rate gyro or intend to operate a dual mode gyro in rate mode.
• Gyro Mode & Gain/Sensitivity: 70%
If you have a dual mode gyro, one side of neutral puts the gyro in rate mode, and the other side puts it in heading hold mode. The further away from neutral you are, the higher the gain in its respective mode. If you use a two-position auxiliary channel (such as the gear channel), you'll make the adjustments with the channel's ATV/EPA feature.
Initial Gyro Settings
Where available, dial in the following at the gyro:
• Gain/Sensitivity — 70%
This is a good starting point. Some radios have a gyro menu that set the values differently. Follow your radio's manual to set up the gyro sensitivity appropriately.
• Limit — 75%
If your gyro provides limiting, you want to make sure it won't produce excessive travel and cause binding prior to adjusting it.
• Delay - 0 (no delay)
No delay is required initially.
• Digital Servo or High/Low Frame Rate - Set accordingly
Mechanical Setup
Here comes the meat of the setup procedures. I cannot overstress the importance of proper mechanical setup. In fact, gyro performance depends on it. There are five things we're looking to achieve:
1. Smooth operation of the pushrod and tail rotor pitch change mechanism
2.Formation of pushrod at a 90-degree angle with the rudder servo output arm at neutral
3.Tail rotor pitch control at its center of travel with rudder servo at neutral
4. Maximum tail rotor pitch travel with nominal rudder servo travel of approximately 80 degrees
5.Correction provided by the gyro in
the proper direction
Begin by detaching the pushrod from the servo arm. The pushrod and pitch change mechanism must move freely in order for the servo and gyro to efficiently and accurately do their job. Move the pushrod by hand and make sure there is no binding in it or in the tail rotor pitch change mechanism throughout its entire range of movement. It should be silky smooth.
Turn on your transmitter. If you have a dual mode gyro, make sure to set it in rate mode. Now turn on your receiver. With the rudder stick at neutral (or centered), the rudder servo output arm should be such that it forms a 90-degree angle to the tail rotor pushrod. It may be necessary to remove the arm from the servo and reposition it on the output shaft. Get it as close to 90 degrees as possible without using any trim or sub-trim.
To mechanically set the pushrod's travel range, we'll need to measure the distance the pushrod travels when moving the tail rotor pitch control mechanism from stop to stop. Now we'll need to do a little math (see sidebar on pushrod geometry). This is how you'll determine how far out on the servo's output arm to attach the pushrod.
With the rudder stick centered, adjust the length of the pushrod so that the tail rotor pitch change mechanism is at its center of travel when the servo end of the pushrod is held over the attachment point. Attach the pushrod to the servo's output arm.
Now set the servo's travel limit. This is done via the limit adjustment on the gyro, not from the transmitter. Move the rudder stick and observe the movement of the tail rotor. Increase the limit until the pitch change mechanism is just short of hitting the stops with full rudder stick deflection. Check it in both directions. Once the limit is set, you may notice that the servo reaches its limit before the rudder stick reaches maximum deflection. The rudder dual-rate can be used to correct this. Decrease the rates until the servo reaches its travel limit, just as the rudder stick reaches full deflection.
[sidebar]NOTE: If you don't have a limit adjustment on your gyro, measure the full servo deflection. Use that angle in your calculation.
Finally
To complete the bench setup, you'll need to verify that the gyro provides correction in the proper direction. Observe the direction in which the pushrod moves when you move the rudder stick to the left. While looking at the helicopter from behind, bump the tail to the left. The pushrod should move in the same direction as it did when you moved the rudder stick to the left. If it didn't, switch the reverse switch on the gyro.
transmitter's servo reverse on the rudder channel if you reversed he direction of gyro correction. Always check for proper operation.
On to flying &
the final adjustments
The remainder of the setup will need to be done at the flying site.
Final Trim - If you have a dual mode gyro, verify that it's in rate mode. If you don't have a rate mode gyro, move on to the gain adjustment. Hover the helicopter with the nose pointing into the wind and trim as necessary using the transmitter's rudder trim lever. If you added trim, land the helicopter and mark the position of the tail rotor pitch arm with the rudder stick at neutral. Re-center the rudder trim lever and adjust the pushrod length so that the tail rotor pitch arm is back at your mark. Operate the control and check for binding. You may need to redo the limit and dual-rate adjustments.
Gain Adjustment — If you don't experience a tail wag in a hover, keep increasing the gain until you do. Then, decrease it just enough to stop the wagging. If you have a dual mode gyro, you'll need to do this individually for both modes of operation.
Delay — While hovering, initiate some relatively hard rudder input. If the tail wags when you stop, you can increase the delay adjustment until it doesn't do this anymore.
Revolution Mixing — Although this has nothing to do with the gyro, per se, you can use revo mixing to help keep the helicopter straight with changes in collective/power. There are individual adjustments for "up" (correcting for increase in power with positive pitch), "down" (correcting for decrease in power with positive pitch), and "negative pitch" (correcting for increase in power with negative pitch).
Revo mixing should only be enabled while operating in rate mode.
Angular Velocity
The definition of angular velocity is "the rate of turning of a body about an axis expressed in angle turned per unit." The axis of concern is the yaw axis, which is the helicopter's rotation around its main rotor shaft.
Visualize the main rotor shaft as a point and the helicopter's frame as extending out from that point. You'll notice that if you rotate that line around the point, any position on that line will traverse the same angle.
It's for this reason that regardless of where you mount the gyro in the helicopter, its ability to sense angular velocity will be the same. It's only important that the gyro's sensing axis is perpendicular to the main rotor shaft.
Pushrod GeometryThe adjustable travel volume (ATV) or end point adjustment (EPA) on the radio allows you to specify the travel limits on each side of neutral, typically from 0% to 150%. This translates to a servo rotation of 0 to 60 degrees from center. Using this information, 100% servo travel would produce a servo rotation of 40 degrees each way and a total travel of 80 degrees.
The pushrod's travel is not linear with respect to degrees of rotation. As it moves further away from neutral, the pushrod moves less for each degree of servo rotation. Therefore, it's desirable to get the control deflection required with the travel limits at 100%.
Looking at a servo wheel (or arm) with a pushrod attached, you can use basic right triangle trigonometry to calculate how far out from the center or rotation to attach the pushrod in order to get the control deflection you require.
Your ATV/EPA values on the tail affect the maximum pirouette rate, the actual ATV/EPA value for your tail must be adjusted on the gyro.
The formula we use is Sin( ) = Opposite/Hypotenuse. The opposite side of the triangle is half the distance we want the pushrod to travel. The hypotenuse is the distance out from the center of rotation that we need to attach the pushrod, which is what we're looking to determine. Solving for the hypotenuse, we can use the formula Hypotenuse = Opposite/Sin( ).
As an example, let's say your total pushrod travel needs to be 20mm. That would be 10mm travel from neutral on each side, the opposite measurement in our formula. In our case, the angle we use is the maximum servo rotation of 40 degrees, and Sin(40) = 0.64. If we plug in the numbers Hypotenuse = 10mm/0.64, or 15.6mm, this is the distance out from center that we must attach the pushrod to the servo arm.
Some final adviceA heading hold gyro wants to maintain a heading. It doesn't know that you carried it to another spot. If you move the helicopter and happen to put it down facing a different direction, the gyro will provide correction and return the helicopter to its original heading as soon as it lifts off. This could be quite unnerving.
If you need to move the helicopter after you turn it on in heading hold mode, simply do one of the following:
• Leave the gyro in heading hold mode and move the rudder stick around while carrying the helicopter and keep moving it until just after you put it down.
• Switch the gyro to rate mode, move the helicopter, then return the gyro to heading hold mode.
The flight characteristics of the two modes of operation are different. With a rate gyro, the tail of the helicopter tends to follow the helicopter through a turn. A helicopter with a heading hold gyro, on the other hand, must be steered through a turn.
Happy flying ...
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