helicopter vibration

[camerart]

Hi M,
As I have a more visual way of seeing things, I think of ideas such as adding a xenon tube that is triggered by the accelerometer. This sometimes gives a different way of approaching things.
Camerart.

 

[dknguyen]

Hi M,
As I have a more visual way of seeing things, I think of ideas such as adding a xenon tube that is set off by the accelerometer. This sometimes gives a different way of approaching things.
cmaerart.

Xenon tubes? Not LEDs?

 

[alec_t]

An optical sensor, plus a reflective mark on the rotor shaft, would give you a phase reference.

 

[mark sheldon]

This is something I have been looking at due to the fact there is no permanent attachment or potential for damage to the model. Please excuse my ignorance/naivety but how would the phase data of the helicopter and then the shaker unit be displayed enabling me to ensure they were 180 degrees opposite? Thank you for your time over this

 

[alec_t]

You could add a functionally similar sensor to the shaker to get its phase information, then use a phase-locked loop to bring the shaker and rotor frequencies into alignment. The 180 phase shift, if needed, could be provided by a simple logic inverter.

 

[camerart]

This is something I have been looking at due to the fact there is no permanent attachment or potential for damage to the model. Please excuse my ignorance/naivety but how would the phase data of the helicopter and then the shaker unit be displayed enabling me to ensure they were 180 degrees opposite? Thank you for your time over this

Hi M,
I note that A has given you one idea.

I would suggest using a timer circuit with one or more knobs that allow the o° and perhaps one or more flash trigger points for comparison. You would also need a readout to indicate where in the corcle the frashes were triggering. All in a darkened room.

Also if cameras were left with their shutters open you would get images of vibrations. (I've never done it )
C.

 

[JonSea]

The vibration produced by the rotor and by the exciter will indeed be 180° out of phase to cancel out.

However, the relative phases you measure will not simply be 180° degrees apart because of the relative measurement points and differences in signal processing time. Consider the case of using an optical tach and trying to cancel out radial imbalance caused by a mass imbalance. If you move the location of the optical tach or the reflective tape, the measured phase will change. Nothing about the imbalance has changed, only the measured phase angle.

Suggest you search for articles on "in place balancing with phase." This techique always starts with a trial weight to determine measured phase angle vs. actual phase angle.

 

[alec_t]

trying to cancel out radial imbalance caused by a mass imbalance.

I understand the TS to be trying to cancel vertical aerodynamically-induced vibration, not mass imbalance-induced vibration? Whatever, appropriate positioning of optical sensors seems a good starting point.

 

[JonSea]

The point is still the same. The phase sensor, whatever is used, will be in an arbitrary point compared to the phase measured on the shaker.

Also, since the helicoptor rotor is overhung (e.g., there is not a bearing on both sides of it), a mass imbalance will produce a strong or even predomimant 1x vibration in the axial (vertical) direction.