From RC Helicopter Wiki
Helicopter tail setup seems to be often thought of as a bit of a black art. In truth, it is generally not difficult, although there are multiple conflicting variations with different advantages and disadvantages, and a wrong step can lead to instantly crashing your model in an uncontrollable pirouette the first time you spool up.
The descriptions on this page assume a typical model helicopter with a clockwise-rotating main rotor.
Basic mechanical setup
First we must ensure that the tail rotor is turning the correct way, and the blades are on the correct way. Virtually all helicopters are intended to have the tail blades come 'up' towards the front of the helicopter, so that the tail blades travel up through the main rotor downwash where the downwash is strongest. If the tail rotor rotates in the opposite direction, then the chances are that your tail belt is twisted the wrong way; correct this, being careful to ensure that it only has total of half a twist along it entire length.
If possible, you will probably want the pitch control to be on the 'leading edge' of the blades, as this will probably give you the largest useful tail pitch range, and also means that the tail pushrod will be under tension (rather than compression) as the pitch (and therefore the load on the pushrod) increases. Not all tail mechanics give you a choice of leading edge or trailing edge control.
Next check the tail blades. They should be rotating 'thick edge' first.
Rate gyro electronic setup
The simplest setup, although not most effective, is a rate mode gyro with the gyro gain set by a small screw adjustment on the gyro itself. The gyro is connected between the receiver and the tail servo; the gain is set approximately mid-way, and is then temporarily mounted on the helicopter in the correct orientation for sensing yaw (if the manual doesn't tell you which way this is, then by turning the gyro back and forth with your fingers in different orientations, one will be found where the servo follows the turning movement well).
The transmitter's rudder stick is operated to check that the tail blades operate in the correct direction---pushing the stick right should try to push the nose right by increasing the blade pitch to blow more air to the right (and so create thrust left, or clockwise, relative to the mast); and pushing the stick left would blow less air right and may even pass through the zero degrees point and move to blow air left. If it does the opposite, then the rudder channel on the transmitter will need to be reversed.
Rest a finger on the tail servo arm, or otherwise monitor the tail servo, and operate the controller to feel or see how the left and right tail inputs move the servo. Turn the helicopter sharply through about 90 degrees counterclockwise, and compare the movement of the servo with that you felt a moment ago: it should match the right rudder input, and similarly turning the helicopter sharply clockwise should match the left rudder input. If it does not, then the gyro sense will need changing, either by the reverse switch, or by turning the gyro upside down.
Rate gyro mechanical setup
By trial and error we will now set the appropriate tail pitch mechanically at the hover point. Set the linkages to give a few degrees pitch to blow air right, and carefully attempt a hover---often it is useful to use training gear as the balls will allow the helicopter to turn, letting you recognize a wildly wrong pitch setup much sooner). Use trim to correct any constant tail input needed to hold the helicopter's heading; land, note the amount of pitch required by the trim, and reset the trim to center and apply the same amount of pitch by adjusting the linkage rods. By repeating this sequence in calm air, it should be possible to reach the point where an out of ground effect hover can be sustained with minimal trim or rudder input.
If the helicopter's tail wags from side to side then likely the gyro gain is too high.
For a small helicopter, the neutral tail pitch can be set approximately by tying the helicopter to a turntable or lazy suzan, and spooling up and noting the helicopter's response as above.
Typical rate mode gyros will try to counter any turning of the helicopter, be it from changes in torque reaction, wind, or changes in tail pitch caused by pilot commands. An old trick was to use a programmable mix from rudder to gain, to reduce the the sensitivity and hence the resistance when rudder inputs were made, but keep the helicopter stable otherwise.
As a simple rate gyro tries to counter even inputs from the pilot, the travel of the servo seen on the ground (and limited by the servo end points set on the transmitter) will likely bear little relation to that seen in the air.
Heading hold mode gyro setup
HH electronic setup
Nearly all heading hold gyros can have their gain controlled remotely; see controlling gyro gain.
There are a few minor differences between rate and heading hold gyro electronic setup. In heading hold mode, when the helicopter is on the ground with the rotors stationary, the tail servo will not center as normal if a rudder input is made and the stick released. This is expected: the gyro is still trying to turn the helicopter in reaction to your last command (you can use this to your advantage when checking the gyro sense: turn the helicopter clockwise and check the pitch has reduced; turn the helicopter anticlockwise and check the pitch has increased.)
A consequence of this is that if your transmitter trim or subtrim is not zero, the the servo may creep to one side, as your gyro's concept of servo centre pulse length differs from your transmitter's. The first step must be to use the subtrim or trim to stop this drifting as much as possible. It may be necessary to power cycle the helicopter a number of times while tuning this to end up with a consistent setting; as the gyro warms up, its concept of centre pulse may appear to change. Other than in this (now hopefully rare) case, trim and subtrim should be strictly avoided when using heading hold gyros (some very cheap HH gyros do require the occasional small amount of trim to correct them, normally due to temperature drift in the sensor). Normally, good gyros will use whatever signal it receives when it starts up to set the center pulse value, neatly sidestepping this issue.
The gyro is connected identically as a rate gyro is, and you need to check the rudder control direction and gyro sense is correct in the same way.
The rudder channel end points bear no relationship to how far the servo may be moved, and are used soley to limit the pirouette rate.
HH gyro mechanical setup
This is the main area of controversy, as different gyros operate differently and operate under different constraints. There are three main parts to the puzzle:
- Neutral pitch
- The pitch of the tail when the gyro is first switched on, and it moves the tail servo to the center position may need to be set close to the OGE hover pitch mechanically as with a rate mode gyro, or it may be allowed to be any value.
- Servo travel
- As the rudder channel travel now bears no relation to how far the servo can be driven, most HH gyros allow the limit to be set on the gyro itself. Sometimes this range is adjustable independently either side of servo center, and sometimes only a single value can be set, changing the range equally on both sides of center.
- Rate mode trim
- If the tail has not been configured to have the correct pitch to hold heading in a hover at servo center, then switching into rate mode will cause the helicopter to start to yaw. This could be fixed with trim, but then when the pilot switches back to heading hold mode, the trim will appear as a yaw command, and the helicopter will turn until the pilot removes the trim. Some gyros have the facility to 'automatically trim' the tail for rate mode (usually triggered by repeatedly toggling the gain control switch while hovering), allowing the pilot to switch modes freely if desired.
Some of these constraints necessarily work against each other. For example, if a gyro requires the neutral pitch to be set to for a rate mode hover, but only has a single travel control, then it is possible that the travel will have to be unnecessarily reduced on one side to avoid binding the servo at the other extremity; or risk binding on that side to allow full range on the other.
A good example of different approaches to gyro design are the Logictech 2100T and the Futaba GY401. Both are heading hold gyros with remote gain control. The 2100T requires the neutral tail pitch to be set up for a rate mode hover, and can set the servo travel independently on either side to compensate for any asymmetry of travel. The GY401 on the other hand has a single servo travel control but doesn't require the neutral pitch to be set to any particular value, so the neutral position is set to simply give maximum servo throw; to work well in rate mode, the gyro features an automatic rate mode trim.
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