Vibrations in statically balanced main rotor blades

[dimachorny]

Dear friends,

My helicopter with a teetering rotor and two composite blades is not stable enough and has very bad autorotation characteristics.
To improve these properties I decided to add mass to the blades by injecting epoxy to the tips. Total addition of the weight was 28%, but in the terms of moment of inertia this added 60% to each blade.
After injection the difference of the mass is about 0.01%, and 0.04% difference in moment of inertia between two blades. Also the span wise CG is almost the same (0.05% variance).
There is a relatively big change in the chord wise CG: 2.74% toward the trailing edge. There is also a small change in the airfoil profile in the range between 75%-95% of the blade radius. The variance is relative to the original blade and to the each other (there is a mismatch in thickness of about 2% in the skin of the upper blade surface).

On the ground there is a good tracking of the blades, but when hovering, the helicopter starts to vibrate, I think laterally, nevertheless perfectly balanced blades.
Does anybody have the idea what can cause these vibrations?

Thanks a lot.

 

[Sparweb]

From your title...

Vibrations in statically balanced main rotor blades

...obviously comes "due to dynamic imbalance."

This is more complex. I fear the process of injecting the epoxy was not controlled as well as you believe it was. It may have distributed itself in different ways in the two blade tips. The unequal distribution of mass, particularly in the chordwise direction, is a red flag. It sounds like the two operating blades are copies of an original blade - used to make the mold? Apart from balance, have you investigated tracking? That causes lateral vibrations, too.

...mismatch in thickness of about 2% in the skin of the upper blade surface...

Ummm not sure which you mean. Is that mismatch total or relative?
Do you have one blade with 10%C thickness and another 12%C thickness, relative to the chord, or is one blade just 2% thicker than the other (the difference between 10 mm and 10.2 mm)?
I hope it's not the former...



STF

 

[dimachorny]

Hi SparWeb,

Thank you for the response.
The blades are the original: I've injected the epoxy into the original blades.
I compared the chord wise CG by hanging the blades one next to other on the same axle and observed that they are aligned.
Moment of inertia I measured by swinging the blades like pendulum while measuring the time period.

have you investigated tracking?

As I mentioned, on the ground the blades tracked each other very good

Is that mismatch total or relative?

Fortunately it's not the former but this doesn't help me. It is one blade 2% thicker than the other.

 

[dwallace1971]

What sort of damper system do you have on the rotor head?

"On the human scale, the laws of Newtonian Physics are non-negotiable"

 

[dimachorny]

The rotor head is a teetering one with elastomer dumpers, no underslung, precone of 2.25 deg. The blade moves freely about lead-leg hinge.

 

[Sparweb]

You're sure the vibration comes from the blades? Perhaps you have a vibration analysis that can tell if the oscillations come at 1/RPM, 2/RPM, or other frequencies?
Sorry, nothing jumps out at me yet, so all I can offer you is the "Socratic Method" of asking more annoying questions.
Is this a certified type, a gyroplane, or something experimental?
Are you certain that the materials that were "injected" into the blade remain in place (don't move around). How did you come up with that strategy, and what did you do to assure the process would be uniform in the two blades?

STF

 

[dimachorny]

You're sure the vibration comes from the blades?

First we've checked the helicopter with unmodified blades and didn't discover any vibrations, after that we replaced them with the modified set and than the vibrations started. I didn't measure frequencies.
The helicopter is a light unmanned experimental one.

Are you certain that the materials that were "injected" into the blade remain in place (don't move around). How did you come up with that strategy, and what did you do to assure the process would be uniform in the two blades?

The material is an epoxy that cured at least 72 hours. I don't believe it may move after that. Regarding the uniform distribution of the material in the blades, I believe it is uniform according to the measurements of the weight, CG, moment of inertia.
Is displacement of the chord wise CG 2.74% backward significant and may cause the flutter?
May difference of 2% in the sickness between two blades in the area between 75% to 95% of the radius may cause vibrations and if yes (lets say difference in drug and lifting forces), why trim and teetering motion can't compensate this, perhaps in forward flight there is the same phenomena of difference in these forces.