Reducing gyro vibrations

[Jukka]

We used a different method to reduce gyro vibrations. Hope you find this interesting:
http://www.tervis.fidisk.fi/vibrations.html

Jukka

[Vance]

Thank you for putting you creative process to work on this.

It seems a simple, elegant way to test a hypothesis on teeter height.

The way that hub bar is designed it seems very rigid in bending.

It seems to me that the blades have to manage any change in coning angle by bending.

Do you prefer a rigid hub bar to a more flexible one?

I love your gyroplane performance calculator and use it often to answer “what if I changed this?” questions.

The gyroplane I fly climbs at 1,100 feet per minute and I am building one that should climb faster. I would love to see the curves above 1,000 feet per minute climb.

I would also find it useful to be able to compare different reductions in drag and the effect they have on performance.

Thank you for continuing to be an inspiration of creative thought in our little corner of the world, Vance

[ccluck]

Your solution is elegant. How did you calculate the rotor CG ?

[Passin' Thru]

I really like it!

[C. Beaty]

In the case of a seesaw rotor, mass above and below the teeter bolt is forced to rotate in a 2/rev circle during forward flight. Equalizing mass above and below the teeter bolt minimizes but does not eliminate 2/rev vibration.

The better solution is adjustable teeter height as used in Dominator rotorheads.

[hillberg]

The weight acts like a tuning hammer. I removed the stick shake in the Rotormouse by added chord area on the root of the blades.

[twistair]

That's very interesting and useful thread, thanks to Jukka for rising it.
I'm currently trying to guess two things in this direction:
1) how to find by practice if certain rotor's CG is above or below teeter?
2) what amount of misplacement in teeter height can be considered as, say, "suitable" (1 mm, 5 mm 10 mm etc)?
It's known that, say, lateral misplacement of rotor between towers (or simply out of axis of rotation) should be less than 0.1 mm and if it is more than vibs come really high. So the question is what amount of misplacement in undersling can be considered suitable?
Last weeks I often fly other guy's tandem gyro with DW28 rotor. When he asked me first time to check it, 2-per-rev stick shake was really violent. Teeter bolt stood in it's usual position for 28ers - in the upper hole. I moved teeter one hole down (in a standard RFD rotorhead) and the 2-per-rev went way lower but I still feel that it should be decreased. As I can guess this may be achieved - only by making another teeter block? Probably the idea of fine tuning for undersling by adding weights is really simplier?

[msmfi]

Originally Posted by Jukka

We used a different method to reduce gyro vibrations. Hope you find this interesting:
http://www.tervis.fidisk.fi/vibrations.html

Jukka

Welcome on bord Sir !!!
It is an honour to have You here.

[Mike G]

Jukka
In this thread http:
//www.rotaryforum.com/forum/showthread.php?t=29961
we discussed the 2 per rev vibration I was measuring during vertical descents in a Magni M16. My aim was to balance the rotor and after a lot of input from this forum I came to the conclusion that my rotor was reasonably well balanced and that the 2 per rev was either:
1) unequal blade pitch.
2) in plane resonance as promoted by Chuck Beaty
3) In correct teeter height.

1) The Magni blades are (as you know because it's your design) not adjustable in pitch, so I'm going to try to fit reflectors at each blade tip to simply see if my rotor is somehow incorrect.
2) I'm still working on a method to check the in plane resonant frequency of my rotor.
3) I couldn't believe that Magni would have got this calculation wrong however you seem to demonstrate the contrary.

If you have the time to read through my thread about my vibration measurements I would appreciate any comments.

Mike G

[Jukka]

Hi!

I have tried to answer some of the posts here but the RWF does not work or works very, very poorly in my iMac/OSX10.6 with the newest versions of Safari and Firefox browsers.
Please, send me email if you want to ask any specific questions. (jtki@icon.fi)

[gyrogreg]

I have had good results identifying rotor shake with the following methods:

Note that a tight cyclic to rotor head linkage is very helpful to eliminate loose shake in the stick that does not represent actual rotor head vibrations.

1 per rev rotor shake: This is due to two sources: Mass dynamic imbalance, and aerodynamic imbalance – unequal lift on each blade – often referred to as tracking mismatch. Aerodynamic imbalance is normally adjusted by shims under the teeter block of the rotor hub bar.

NOTE: Perfectly tracked blade tips does not guarantee aerodynamic balance. Symmetry of blade airfoil shape at each station from root to tip and symmetrical flexibility of each blade are factors.

2 per rev shake: Can be due to several sources – primarily, mis-matched teeter undersling height relative to coned rotor vertical CG. Other sources that should be eliminated are side-to-side slop on the teeter bolt – allowing the rotor to “flop” from fore to aft each rev. Also, friction in the teeter bearing should be minimized. 2 per rev shake is caused by forward movement requiring teeter action.

Identify which shake you have. You may have both. Eliminate 2-per rev shake by flying in a vertical or slow airspeed decent. This eliminates most of the 2-per rev shake and leaves only 1-per rev shake.

Minimize 1-per rev shake by mass balance adjustments with chord sideways adjustments and span wise adjustments with tip weights. Note: that adding weight to the tips – span wise balance - also can change the tracking of the blade tips because it can tip the cone of the rotor toward the heavier blade. So, if the aerodynamic center of the rotor is not concentric with the rotor CG, chord wise adjustments are only a compromise and 1 per shake may not be able to be well reduced. If tracking adjustments are available, it is possible to move the aerodynamic center of the rotor to be concentric with the mass CG, but this can be a tedious trial and error effort.

Just to mention, Magni rotors are held very well aerodynamically and mass symmetrical from the production processes. That means the mass and aerodynamic centers are well aligned. In this case adjusting span wise balance with tip weights fine tunes both the span mass dynamic balance and the tracking. So, span dynamic balance can be expected to be best when the tracking is best. There are no tracking adjustments on the Magni rotor, so blade tracking is accomplished by adjusting tip weights. A dynamic prop balancer works well on such precise rotors to mass balance the rotor.

2 per rev shake: This shakes the stick twice as fast as the 1 per rev shake observed at slow airspeed or in a vertical decent. 2-per rev shake increases with airspeed because this requires more teeter action. As mentioned, slop in the teeter bolt and/or friction in the teeter bearings can create two per rev shake when moving forward. Minimize these issues first. Then teeter undersling mismatch to the rotor coned CG may be the remaining 2-per rev shake issue.

I find a fairly simple way to see which way the undersling needs to go is as follows in flight. (Don’t do this if you are flying a pitch unstable gyro – the gyro must return to trimmed airspeed upon a pulse pitch input to the cyclic.)

From normal straight and level cruise, pulse the cyclic slightly aft to initiate a slight nose-raising, increased G load. Immediately, let go of the stick and observe the stick vibration. Do the same with the cyclic in the forward direction to induce a slightly decreased G load transient. If the stick shake is the same in both directions for a short period during the transient, your undersling is about right. If the stick shake gets better in one direction, and worse in the other direction transient, the undersling is not matched to the rotor coned CG. The reason this tells you something is that the rotor coning angle changes during these transients and the rotor CG moves vertically relative to the teeter pivot. When the gyro is experiencing a lesser load on the rotor during a nose lowering transient, the coned rotor CG is lowering and may be either a better or worse match to the teeter undersling position. When the gyro is experiencing a higher load on the rotor during the nose raising transient, the rotor cones a bit more and the CG raises relative to the teeter bolt. If a raising nose transient increases the stick shake momentarily, the rotor CG is moving further above the teeter bolt indicating the undersling should be higher. Visa versa is true if the 2-per rev shake gets better upon this positive G-load transient.

As Chuck said, you can’t completely eliminate 2-per rev shake. The rotor coning angle will change during wind and maneuver transients, and cyclic input momentarily moves the rotor CG off center from the spindle axis. But, if the amount of 2-per rev shake induced in the above test is the same in each transient direction, the undersling height is probably about right.

In summary, nothing beats a quality rotor that is symmetrical between blades both aerodynamically and in mass distribution. Nothing beats a good dynamic balancer for fine tuning mass balance.

- Thanks, Greg

[troed@aon.at]

The Godfather of modern European gyroplanes himself in this humble forum.

It´s an honour to have You here.

I carefully read Your article on how You eliminated rotor/rudder vibes on Your Magni M24 Orion.

Actually I fly the same and it is from the first serial-series back in 2008. I flew one M16, one M22 and now the M24 and with none I never ever had any stick-shake, the rotors smooth as silk the stick rock-steady at every speed.

BUT: especially in thermal conditions during summer I experience some cabin-shake on the M24 when the rrpm drops from 368 to 350 with a maximum at 354. It never happens when constantly above 370 rrpm (which is always with PAX). The cabin shake lasts about 2 to max. 4 seconds and it´s not really strong but I can feel it. When I leave the rudder pedals all alone this cabin-shake is lesser in this rrpm-range

What would be Your guess about this ? Could it be the tail-flutter You mentioned inducing this slight cabin shake and could it be dampened by Your weight-solution in the rudder section ?

[dragonflyerthom]

Very interesting read. I have never seen balance bars on two bladed gyros but they are used quite frequently on helis. I'm wondering if this could be used on an unpowered system.

[choppergabor]

Fascinating findings. Thank you so much

[Arnie Madsen]

Originally Posted by Jukka

We used a different method to reduce gyro vibrations. Hope you find this interesting:
http://www.tervis.fidisk.fi/vibrations.html

Jukka

Yes , I did find it interesting.
Arnie