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TAIL DRIFT/WAG

If you are experiencing unusual tail drift in HH mode after setting gain (usually nose left) it is more than likely a gyro mounting vibration issue.
Firstly make sure you have full mechanical expo/trim to fly a straight tail in rate mode with no radio (electronic trim=0). This is achieved with about 8-10 degrees of tail blade pitch in the direction of torque compensation for the main rotor. 

You will need to test various mounting methods for the best result.

1) Fit the gyro to heli in the following manner; one 3mm foam pad, metal plate, one 3mm foam pad, then adhere gyro.

2) Fit the gyro to heli in the following manner; one 3mm foam pad, metal plate, one 3mm foam pad, adhere gyro then strap gyro down tight to the heli with a nylon cable tie.

3) Fit with one 3mm foam pad, metal plate, one 1mm double sided tape then adhere gyro with no cable tie.

4) Try different manufacturer of double sided tape the results from brand to brand are surprising.

5) Reposition gyro to a different location there may be a change in resonance frequency. Pictured below right is an unorthodox mounting position on elevator servo which produced better performance than on the plastic electronics tray on the GFZ. Try what works best.

If this fails swap & test servo then gyro. Make sure you have selected the correct pulse band width, for the type of servo you are using within your Gyro settings.



Tail Wag or tail twitch

Tail wagging & tail twitch (side to side) is usually an indicator of an error in the mechanical or electrical systems of the tail control. There are many links in this chain & they all need to be isolated & checked. This is a quick guide;
 

Check for binding & slop of all bearings, bushings & linkages in the system including one way bearing & main shaft bearings, clean, lubricate & make sure all have free effortless movement. 

Check all drive gears & belts within the system for damage, wear or missing teeth including the main gear, tail servo gears & pinion wear. Make sure proper tensioning & meshing of all components. 

Check correct gain settings within the radio & also mechanical gain at the servo (may be to great move up one hole), check correct gyro mounting method & position making sure that the first 50mm of gyro lead is free & not touching the frames. 

Tail servo's can wear overtime or be manufactured with excessive slop causing this issue. Hold the tail servo horn & check for excessive free play or slop.

Check gyro & servo compatibility & selection. If persists change out gyro & test, then tail servo if no effect. Worn radio pots can cause this effect so if 100% sure all checks have been carried out swap TX bind & test, then RX. With more modern Flybarless gyro's, some have the ability to monitor performance & interaction through software & the worn radio pot issue can be picked up quite early.

What I recommend is that you do one check at a time then test so the source of the problem is recorded for future monitoring & reference. It also helps establish a maintenance program for a particular component. I have become religious about information as it helps in design modification or if a component is failing at an early life cycle.

Tail Bobbing

Tail bobbing (up & down movement) is a different motion to tail wag (side to side) or drift (slow circular movement to one side only). Bobbing is usually an indicator of low head speed or an imbalance within the head. There could be many factors causing bobbing including; 

  • Low head speed.
  • Blade grips too tight, imbalanced blades or poor blade tracking.
  • Worn or damaged head dampeners.
  • Slightly bent feathering or main shafts.
  • A loose or bent component within the head & swash structure including any ball links.


 

Tail Blow Out 

In a hover & basic flight the helicopter performs as expected but in a rapid pitch pump the tail of the Heli will not hold heading & spin opposite to the main blade direction i.e. CCW if main blades spin CW (newtons 3rd law for ever action there is an opposite & equal reaction).

Main causes,

  • Loose tail belt or tail drive gear on a shaft
  • Poor quality or undersized tail blades.
  • Not enough anti torque mechanical gain.
  • Radio mixing setting.



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