helicopter vibration

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mark sheldon

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Good morning,
i am currently attempting a university module looking at reducing vibration in a helicopter.
i have access to a "model" puma helicopter that is used to record and change track and balance of the Main and tail rotor blades, i am looking at attaching a tachometer to calculate the phase of the "helicopter" and then i need to find the phase of my shaker (a dc motor) to enable me to set them 180 degrees opposite with the outcome being a reduction in vibration. my question is what component would be best to use to provide me with the phase measurement of the helicopter and the shaker and would it be possible to show this data in real time on a smartphone/laptop/tablet etc? Thank you in advance for your time over this, best regards
mark
 
Positional feedback in machinery is usually done with either an encoder or a resolver, an encoder generates pulses as the encoder rotates, and a pulse every rev, and a resolver generates a sine & cosine wave as it rotates, a 3 phase version of the latter very popular in aircraft is the synchro.
If your motor is always 180 degrees out then you could just connect the rotor & counter shaft with a timing belt.
 
Interesting project.
Given that there are large time-varying out-of-balance forces acting on the main rotor shaft as a result of cyclic pitch changes, presumably the shaker will need to provide a complementary cyclically-varying force of similar magnitude. How will the DC motor achieve this?
 

Mass imbalance of the blades isn't likely changed significantly with pitch changes of the blades.
 
True. I was thinking rather that the cyclically-varying vertical lift forces on the blades would impart varying horizontal torque on the main shaft and hence cause lateral vibration.
 
I am looking at reducing vibration whilst the helicopter is on the deck for this project. I will be looking at reducing the r1 vibration in the vertical axis through the airframe as opposed to r4 vibration in the lateral. The reason for the dc motor is that I need to produce a “shaker” which will be used to counter the vibration from the helicopter, by using the dc motor 180 degrees out of phase to the helicopter then I should be able to reduce the helicopter vibration proving my theory works albeit on a smaller scale
 
It’s not a continuation just a different approach. I had to admit defeat with that concept as I was unable to produce a prototype due to my lack of knowledge when it came to coding/sketching Arduino circuits
 
mark sheldon said:
It’s not a continuation just a different approach. I had to admit defeat with that concept as I was unable to produce a prototype due to my lack of knowledge when it came to coding/sketching Arduino circuits
Click to expand...
Is an Arduino even fast enough to do that? I've never used one but I had a coworker who tried tried using one for something similar (reading an encoder and doing something at several points within each rotation motor and according to him it was so slow so as to be unusable, even for a slow spinning large motor running off the 60Hz mains.
 
But, is your DC motor's vibration a good model? With a turbine-powered aircraft, I suspect aerodynamic "vibration" (for example the propellers in a multi-engine airplane) outweighs mechanical vibration per se from the engine, yet your DC motor only provides the latter.
 
If you want to cancel only the 1x force in the vertical direction, you should use a pair of synchonized counter rotating eccentric disks. They will only generate a vertical force The sideways forces from each disk cancel because they are in opposite directions.

Do you have some reason to believe the 1x force in the vertical direction is the primary concern?
 
My original idea was to transfer HUMS data via bluetooth instead of using a PCMCIA card this is where the Arduino board was to be used. As I said the stumbling block was the coding and my lack of knowledge. I have decided to continue with the HUMS but instead of data transfer i was looking at vibration specifically and the possibility of reducing the vibration generated by the helicopter itself through the vertical axis. By proving i could reduce vibration on a small scale then this would reduce aircrew fatigue, increase component life and improve airworthiness
 
2R,4R etc, (dependant on blades) is the vibration that companies are constantly trying to reduce. For the purpose of my project, i have an accelerometer fitted in the vertical axis showing vibration. The problem I have is that in the real world of helicopter vibration the Phase would be measured by a magnetic pickoff connected to the rotating component that I would be interested in (the Main Rotor Head) Once I knew the phase of the component I would then need to match the shaker unit 180 degrees out of phase. To achieve this on a real system the shaker would also need some sort of pickoff (or feed from the MRH pickoff) This would essentially synchronise the two rotating components so the phase could be set opposite. This is what I am unsure whether or not I will be able to achieve and replicate the synchronization on the prototype???
 
To match the phase (well, be 180° out of phase), you would probably turn the shaker with a servo motor matched to rotor speed, so you could match the phases. The phase difference between the rotor signal and signal from the shaker will have to be determined experimentally - the relative positions of the sensors, signal processing time and other factors will control the exact relationship between the signals.

Note that with the shaker I have described and also with a motor with a mass imbalance, the resulting force is a fuction of speed, so either method would only work over a narrow range of rotor speeds. For the shaker I described, the force can be adjusted by moving the weights in or out; in the shaker I have used, this is accomplished by bolting the imbalance weights at the necessary radius.

An electrodynamic shaker would be the usual method of controlling both the frequency and generated force, but to counter rotor imbalance, an excessively large unit would prpbably be required.
 
Am I misunderstanding this? If you work out the imbalance and use a motor to correct it, how is that different to adding a weight to the "other" side of the rotor to the out of balance weight?

Mike.
 
Pommie said:
Am I misunderstanding this? If you work out the imbalance and use a motor to correct it, how is that different to adding a weight to the "other" side of the rotor to the out of balance weight?

Mike.
Click to expand...

Adding mass to the light side, or removing mass from the heavy side...

I'm not advocating the approach, just adding information to help with the stated idea.

Irrelevant true story about balancing a ceiling fan (not yours truly) :

An engineer was tired of his wobbly ceiling fan so he decided to balance it. He grabbed a handy weight and taped it to one on the blades. Hevhad made a good guess and the fan was much improved but still not perfect. To adjust the weight, he removed the cap of the red Sharpie he had taped to the blade and gave it another try.

Nice and smooth at slow speed. Click. Good at medium. Click. Perfect at high speed. Our engineer was enjoying the breeze, patting himself on the back for a job well done. Then he noticed a light red line just below the ceiling starting to appear around the room, growing darker as he watched!
 
The model I am using is very expensive military equipment. It is used primarily to detect track and vibration data of the main rotor and tail rotor. Although I will achieve data for this vibration through the vertical axis from the test equipment fitted I need to prove that I can reduce this vibration. To do this I need to measure/calculate the phase of MRH & then the "shaker" to ensure they are 180 degrees out of phase. I am unable to drill into the model so am looking at alternate ways of collecting this data
 
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