Hi, I need help analyzing the vibrations on my helicopter (TREX 700 GASSER). I did a long test flight where I tried various rpms and noticed that the vibrations were lower at 1900 rpm, but there were moments where the global vibrations reached 500. Mechanically, the helicopter is fine, and the blades are tracking correctly. Do you have any idea what might be causing the vibrations?
Also, during hovering, the helicopter tends to tail down; I can't achieve a steady, stable hover.I've attached the log and current configuration. Thank you
It is completely normal for vibrations to be much higher on a combustion engine model than on an electric motor model. In Gasser models, vibrations are even higher than in Nitro models.
This is due to explosions in the combustion chamber, which cause strong vibrations on the three axes, and movements on opposing axes of the piston, connecting rod, shaft, fan, clutch, starter shaft, and resonance from the exhaust/silencer, which cannot be eliminated.
It is for this very reason that in the instructions for assembling and positioning our flight control units on the model (panel 2 of the configuration wizard) we state that for models with combustion engines it is MANDATORY to use double-sided adhesive tape to attach the control units to the model that is softer than the tape we supply for models with electric motors, so that the double-sided adhesive tape filters the vibrations that are transferred from the model to the flight control units.
We also recommend the double-sided adhesive tape models that have given the best results during our tests and those of our beta testers, because some users mistakenly think that using two or three layers of double-sided adhesive tape for electric models can be useful, when this is not the case.
Vibrations can also be transmitted from the model to the flight control unit through the cables, which for this reason should never be kept taut but loose.
(In this regard, it is important on models with strong vibrations to fix the plastic connectors to the upper edge of the flight controller casing using hot glue, which is easily removable if necessary, in order to avoid false contacts or the connectors coming loose over time).
Obviously, the flight control unit must be completely free above and around and must not touch any mechanical part of the model, but must be held in place on the model solely and exclusively by double-sided adhesive tape.
The practice of passing cables through and “sticking” them between the edge of the flight control unit case and the model must also be strictly avoided, as it is as if the flight control unit were resting on the frame.
Apart from the above, please note that high vibrations do not cause problems for the gyroscopes that control the model's attitude, but can cause problems only when the accelerometers are activated for the Auto Level function with or without automatic collective pitch control (Rescue).
From the flight log file sent to us, we can see that the vibrations are not exceptional (they are within the grey area), although, as expected, the vibrations of greater amplitude occur at the engine rotation speed, while at the rotation speed of the tail rotor and main rotor, the vibrations are significantly lower.
From the configuration file sent to us, we noticed that in panel 12 of the Wizard, the value of the “Fail Safe Idle Value” parameter has been set to the maximum (+20% compared to Throttle 0% /Throttle Cut). This could be correct due to its mechanical control configuration between the servo and carburettor. However, we would like to point out that this value must be set so that in the event of a fail-safe or deliberate autorotation activated by the Throttle Hold switch, the engine runs at idle speed and cannot shut down in flight but can restart as soon as the radio signal becomes stable or the Throttle Hold is deactivated.
However, it must not be too high a value to the point of triggering the clutch. Otherwise, in the unfortunate event of a crash, what is known as “chicken dance” would occur and the model, now on the ground but with the blades still rotating, would 'grind' and “crumble” over the long time it takes for the fuel to run out, since in such a condition it would be very dangerous to approach the model, which continues to jump, spin and rotate on itself in an attempt to shut down the engine.
From the configuration file, we see that no checkbox for activating Auto Level is active.
Therefore, if the model tends to move backwards when hovering, it can only be due to:
- Incorrect balancing of the model (when lifting the model with the fingers of both hands placed under the blade mounting bolts perpendicular to the model, the model should be horizontal and the main shaft should be perpendicular to the ground).
- An inclination of the swashplate that is not parallel to the ground but tilted backwards. Tilt forward by changing the length of the links between the servos and the swashplate.