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Knowledge base Quark Common problems

Question #20
The helicopter tail drifts.
Answer

Spartan gyros offer exceptional tolerance to the vibration levels that are commonly found on model helicopters. However, there are many factors that can manifest as drift varying from transmitter configuration, gyro mounting, very rich engine or helicopter mechanical faults.

In order for heading hold gyros to achieve drift free operation they require precise knowledge of the rudder stick neutral as well as high quality - low vibration and noise - measurements from the gyro sensor. As such the causes of drift fall into two main categories.

Radio system configuration related
When the rudder stick is operated the gyro's Status light flashes twice to indicate that the gyro detects a command to yaw the helicopter.  If the Status light flashes even when the rudder rudder stick is centered the cause of drift is falls in this category.

When heading hold  gyros initialise they capture and memorise the rudder stick neutral position as seen at the receiver output. Therefore this measurement also includes any rudder trim and sub-trim which is set on the radio. During operation any deviation from the stick neutral position is interpreted as a command to yaw the helicopter. Even minute changes like 1 or 2 rudder trim clicks can be seen as a yaw request and therefore manifest as a continuous slow rotation, thus drift.

  • Some radios offer different trim setting on each flight mode (Normal, Idle-1/2, Hold). Ensure that the trim setting for all modes is identical. If the gyro's Status light starts to flash as you change between flight modes it is an indication that some adjustment on the radio is making the rudder channel change.
  • Never use trim to cancel drift in AVCS mode. Trim will only hide the real cause of drift. Also, at the next power on the gyro it will memorise the new rudder neutral including the trim and thus your adjustment will no longer have any effect.
  • Check that your transmitter rudder stick centring has good repeatability. Operate the rudder stick both direction and then allow it to return to center. Check that the gyros Status light stops flashing and stays continuously on. If the stick centring is not very accurate increase the gyro's deadband adjustment slightly so the gyro makes allowance for it.
  • Always power on in AVCS (heading hold) so the gyro can capture the rudder position and update it internal rudder neutral calibration. Normal mode may require tail rotor mixing set on the radio which affects the rudder signal the gyro see. As such the gyro will not update the rudder neutral in Normal mode; it will use the last calibration instead.
  • Check that the Status light is permanently lit at the end of gyro calibration. If you are using a Spektrum radio rebind your receiver with the rudder stick at neutral. The gyro may see and initialise on the default rudder position (bind position) if the link with the transmitter has not been established before the gyro has finished calibrating. Rebinding your receiver ensures that the real rudder neutral from the transmitter is the same as the default bind neutral.

Mechanical, installation and vibration related
The second group of drift sources includes problems that ultimately affect the gyro sensor; most commonly from mechanical vibration. Anticlockwise drift if often the result of vibration generated from the main rotor; however there can be exceptions. Therefore it is best to investigate the main rotor first but we should not rule out other parts of the helicopter.

Due to their magnitude and frequency not all vibrations are visible to the human eye but in certain cases they can still affect the gyro operation. On a model helicopter vibration is typically produced from main rotor, tail rotor, tail drive shaft and engine and can be the result of faulty, misbalanced, bent, badly engineered mechanical components, blades or ball bearing.

  • The correct operation, performance and stability of your gyro can be greatly affected by the way it is fixed on the aircraft. It is essential that the gyro is mounted on a flat surface accurately perpendicular to the main shaft. When possible choose a rigid mounting location close to the main shaft. Avoid mounting the gyro on weak structures such as the electronics tray at the front of the helicopter; these locations are often subject to substantially more vibration.
  • The supplied mounting kit contains a stainless steel plate and adhesive foam pads which have been selected by Spartan RC to give the correct mass and shore hardness for effective vibration damping. Using other types of adhesive tapes will affect the balance and the performance of your gyro. Harder tapes will pass more vibration to the gyro while softer tapes may allow the gyro to wobble thus incorrectly sensing motion as a result of gyroscopic precession.
  • In high vibration environments the use the supplied steel plate and two layers of the 3mm foam tape for mounting the gyro.
  • Do not fix the cable to the helicopter for the first 5cm (2 inches) from the gyro end to reduce transmission of vibrations through the cable.
  • Do not fit cable braid over the gyro wire. It stiffens the wire and allows vibrations to be transmitted through it.
  • Avoid mounting the gyro in direct proximity to other electronic equipment that produce high levels of electrical and magnetic noise. This includes in particular high current digital servos and the ESC. Also ensure that the gyro and its wire harness are kept away from ESC and high current wires to the battery and motor.
  • Check you engine is not excessively rich. If it is a new engine you may need to tolerate the drift until the engine is broken in and running smoothly.
  • Check the blade tracking and most importantly balance . Blades must have matching CoG as well as equal weights.
  • If you have a spare set of main and tail blades it worth trying them on the heli to rule out the possibility of blade misbalance. 
  • If your helicopter offers more than one gyro mounting location try moving the gyro there.
  • Change the routing of the gyro wire harness.
  • Unclip the tail pitch control rod from the servo arm and check that it can move freely with minimal amount of force in both directions. Ensure that there is not excessive backlash in the tail pitch control mechanics.
  • Check your servo runs smooth and does not have excessive backlash or a worn potentiometer which often results in a dead-band.

Unfortunately in some cases a persistent drift may continue to exist despite following all the best practises for building a new helicopter kit and installing the gyro. Tracking down the faulty part can be difficult, time consuming and will require having the helicopter in front of you. As such, Spartan RC can not help in this process but we will be happy to test your gyro free of charge as part of our free repair service.

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