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!:first:

[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]

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

[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 :)

[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 ?

[Arnie Madsen]

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

[500e]

How is everyone starting the balance procedure ?
I note there is a lot of discussion regarding tip weight, what about root weights?
Vibration is such a destructive force that anything that can be done to reduce eliminate is well worth the effort, this is without the pilot fatigue that it also induces.

[Redbaron]

My rotorhawks had no provisions for mass balance so just recently I drilled a hole just slightly under 5/16" about 1" deep into my lighter blades spar then tapped a 5/16" thread part way into the hole so I can fill the hole with lead shot and screw in a small piece of threaded rod.

wish I would of done it like this before I used epoxy. :sad:

[QUOTE=500e;448117]How is everyone starting the balance procedure ?
QUOTE]

[500e]

My rotorhawks had no provisions for mass balance so just recently I drilled a hole just slightly under 5/16" about 1" deep into my lighter blades spar then tapped a 5/16" thread part way into the hole so I can fill the hole with lead shot and screw in a small piece of threaded rod.

wish I would of done it like this before I used epoxy. :sad:
[QUOTE=500e;448117]How is everyone starting the balance procedure ?

QUOTE]
Thanks RB.
Where are you drilling hole? longitudinally We have found with Heli blades a lot out of balance root & tip, the consensus is centripetal forces allow the balance to be done at tip as this will require less weight, we have found that balancing both root & tip give much smother run up (We work on 2Grams max variation across all blades)+ smother flight, less wear as one would expect with less vibration.
Blades that are 300+ (12 Ozs) Grams out are not unknown the worst we have found was 382 grams variation between blades.
Drilling holes is a NO NO for us.

[Redbaron]

I'm probally the wrong person to ask but I drilled the tip of my spar. Like you said with the centrifugal forces it makes more sence to balance at the tips since you need less weight. My hawks were around 10 grams off balance which makes quite a stick stir.


[QUOTE=Redbaron;448129]My rotorhawks had no provisions for mass balance so just recently I drilled a hole just slightly under 5/16" about 1" deep into my lighter blades spar then tapped a 5/16" thread part way into the hole so I can fill the hole with lead shot and screw in a small piece of threaded rod.

wish I would of done it like this before I used epoxy. :sad:

Thanks RB.
Where are you drilling hole? longitudinally We have found with Heli blades a lot out of balance root & tip, the consensus is centripetal forces allow the balance to be done at tip as this will require less weight, we have found that balancing both root & tip give much smother run up (We work on 2Grams max variation across all blades)+ smother flight, less wear as one would expect with less vibration.
Blades that are 300+ (12 Ozs) Grams out are not unknown the worst we have found was 382 grams variation between blades.
Drilling holes is a NO NO for us.

[500e]

What I am getting at is do not ignore the root weight it does make a difference, Do root first then check tip.

[Mike G]

Jeff S
When you say your blades were "10 grams off balance" do you mean that one was 10 grams heavier than the other?
Mike G

[hillberg]

With Reeds "wedge"(A Canadian Home Rotors single place) we had inboard root & chorde weights,Tip weights & China Weights to get the thing to fly right.

[Mike G]

As you may have read in my thread "Magni FFT rotor vibration analysis", I've been playing with 2/rev vibrations for some time now. I was pretty convinced that there wasn't a problem of undersling because I thought that Magni couldn't get that wrong, could they?
Reading Jukka's thread I thought that maybe I was being naive and that there really was a problem of undersling.
So I took some rough measurements of the blade carrier and some old data I had for the rotor blade weight and C of G position, put it all on an Excel sheet and calculated the C of G of the combination of half the rotor carrier + bolts + a blade.
As explained by Gregg G, in the Magni blade carrier the blades are held by two horizontal bolts and these bolts appear to be set at a coning angle of about 3°. I didn't have any accurate way of measuring this because I haven't disassembled the rotor but cross checking the dimensions 3° seems to come out every time so I presume that is the pre set coning angle used by Magni.

Using the position of the combined blade and half blade carrier C of G the calculated required undersling as about 84 mm ( 3.3") whereas the actual undersling (assuming that the line of application of the axial force on the blade passes through the two bolts) is 56 mm (2.2"). I then added the Jukka correction mass into the calculation and found that a mass of 1.8 kg (4 lb) would pull the combined C of G inboard to get the required undersling to 56 mm. Considering that Jukka found that a mass of 2 kg worked on his rotor which has a longer blade carrier that's pretty convincing for me.

The attached drawing (half blade carrier dimensions.doc) shows what half of a Magni blade carrier looks like with the dimensions I used plus and the sketch (M16 undersling.doc) shows the trigonometry I used to do the calculations. Looking at the blade carrier drawing the two bolt holes for the blade look too far apart, if anybody has a Magni short blade carrier handy and can give me more accurate dimensions I'd be grateful, I don't think it'll change much but I'd like to be as accurate as possible.

The next thing I did was to try to measure the coning angle of the rotor in flight. I thought of all sorts of schemes with lasers and cameras and then realised the obvious. If I know the gyro+pilot+fuel weight, the combined blade and half blade carrier weight plus its C of G, by flying at different speeds and noting the rotor RPM it's simple to back calculate the actual coning angle. The results of this simple test are shown on the attached sheet (Magni M16 coning angles v speed) and you can see that for my gyro, with pilot only, the coning angle varies from about 3.2° at 40 mph to 2.75° at 100 mph. Of course the apparent "accuracy" is only thanks to Excel (you can calculate to as many decimal points as you wish) but it shows that Magni were in the right ball park of 3° coning angle but seem, to me, to have miscalculated the undersling.

Doing the above calculation I took the gyro weight as the total weight minus the blade weight does anyone have any thoughts on this?

As I said at the beginning I can't believe they got it wrong because there are Magni pilots who say they have zero vibration but based on Jukka's experience and my recent amateur trials there seems to be something wrong.
Any ideas anyone??

Mike G

[Redbaron]

10 grams in one blade tip for the least amount of stick shake. :ohwell:

Anyone wanna trade an old set of 23' riveted rotor hawks for sport copters? :lol:



Jeff S
When you say your blades were "10 grams off balance" do you mean that one was 10 grams heavier than the other?
Mike G

[ckurz7000]

...
As I said at the beginning I can't believe they got it wrong because there are Magni pilots who say they have zero vibration but based on Jukka's experience and my recent amateur trials there seems to be something wrong.
Any ideas anyone??

Mike G

Mike, I really like the scientific approach you are taking. Keep it up! More specifically to your question of undersling:

The amount of undersling you calculate using Excel is based on several assumptions, but most notably it assumes that the rotor blade is hinged right at the rotor axis. In reality the rotor blade is attached to a rigid rotor hub. And not even by a hinge, at that. So there is some bending moment in the blade and only over a certain distance will the rotor blade assume a more or less constant angle. The overall effect of this is that the undersling is smaller than calculated by the idealized formula.

If you want to infuse more reality into the undersling calculation you would have to treat the rotor blade as a cantilevered beam under the influence of centrifugal and lift forces. This is actually a nasty problem since both forces depend on radius (and hence cannot assumed to be point forces). Furthermore, the centrifugal force component acting to deflect the blade depends on the sine of the coning angle, which is what you want to calculate in the first place. All in all, you wind up with a 2nd order differential equation that you have to solve numerically.

Greetings, -- Chris.

[Mike G]

Chris
I wouldn't call it "scientific" you should visit the French gyro forum there are some discussions there about rotor geometry that are on a much higher scientific level.

I agree that I'm treating the blade fixture as a hinge but I think the problem of the cantilever beam you are explaining is covered in Jukka's other thread that deals more with the importance and relevance of having the cone angle pre set into the hub bar (carrier). I tried to illustrate this in that thread (attached "undersling difference.doc"). What this diagram tries to show is that the left hand blade is fixed at 90° to the axis of rotation (or more correctly the coning axis) and therefore has to bend to achieve any coning angle. Whereas the right hand blade that is hinged or fixed at the correct coning angle doesn't bend. Since at the end of the day the gyro weight has to be supported the only solution for the left hand blade is to turn faster. OK I know it's a simplification but I still think it's true.

My belief (I'll try to check with a mathematical demonstration later if I get the time) is that if the coning angle is pre set in the hub bar the blade has no bending moment at the hub bar when the rotor flies at the same coning angle, as such it's as if it was hinged.

Also I'm using the same basis to calculate the coning angle when flying and I get the same 3°. Don't forget this is only an attempt to ball park the numbers and as such I had hoped it would open the discussion and help understand the question of undersling and 2/rev vibration.

I don't know if I'd have had the courage to post my results if I hadn't seen some one as notable as Jukka come up with a similar experience.
Thanks for replying, I just might translate my post and let it get torn to pieces by the French forum.
Mike G

[C. Beaty]

In practice, the smoothest seesaw rotor results from undersling being a bit less than required for centering rotor mass.

This produces a 2/rev input that is out of phase with aerodynamic excitation.

[Redbaron]

Hi chuck, slightly off topic question but assuming the static balance of our blades is off should we not match the cg of each blade and then from there add weight at the cg spot?

This is how I've always balanced blades on my rc helis. If we just add weight at the tip wouldn't the balance change as the rpm changes? how do we adjust the blade cg assuming its off, paint, glue? Whats your thoughts on this chuck? thanks.

[Mike G]

Chuck
You've just answered my question from Jukka's other thread

"Is it because a 2 bladed teeter rotor is more sensitive to 2/rev vibration when the undersling is too low (ie rotor blade C of G above the teeter bolt) than when it is too great (ie rotor blade C of G teeter below teeter bolt)?"

Thanks.
Mike G

[Mike G]

Chuck
You'll have to explain your sketch to me, why does the masss above the teeter cause a forward acceleration?
Mike G

[C. Beaty]

Hi chuck, slightly off topic question but assuming the static balance of our blades is off should we not match the cg of each blade and then from there add weight at the cg spot?

This is how I've always balanced blades on my rc helis. If we just add weight at the tip wouldn't the balance change as the rpm changes? how do we adjust the blade cg assuming its off, paint, glue? Whats your thoughts on this chuck? thanks.It could, Red, because the mass of a rotor doesn’t lie in a single plane.

It’s like balancing wheels.

A bicycle wheel isn’t fussy because everything lies nearly in a single plane. Static balance is good enough.

An automobile wheel, because of its width, must be dynamically balanced.

A rotor, by rights should be dynamically balanced. But, because rotorblades are normally quite uniform, static balance is usually good enough.

[C. Beaty]

Chuck
You'll have to explain your sketch to me, why does the masss above the teeter cause a forward acceleration?
Mike GMass above (and below) the teeter bolt is being forced to rotate in a 2/rev circular motion. It would rather not and instead, fly outward.

It can make its presence know to the outside world only crosswise to the rotor.

This stuff is difficult to visualize but becomes quite clear if you make a welding rod model with removable weights.

[Mike G]

Chuck
I think I see what you mean.
Mike G

[Mike G]

Chuck
In your post 27 are you saying:
When the C of G of the rotor is just above the teeter axis (a little bit less undersling than required) there is a forward force each time the rotor is in the 9 -3 o'clock position and this tends to cancel the rearwards 2/rev force due to the difference in drag of the rotor in forward flight in the same 9 -3 o'clock position ?

Mike G

[ckurz7000]

Mike, yes this is true. When there is a bit to little undersling, the rotor CG will move back and forth twice per revolution in the following fashion:

1. With the blades fore-aft the rotor CG is a bit behind the teeterbolt.
2. As the blades turn, the rotor CG moves forward.
3. When the blades point left-right, the rotor CG is right above the teeter bolt.
4. as the blades turn, the rotor CG moves backward.

Since the CG of the rotor changes direction from going forward to going backward at the time the blades point left-right, it imparts a forward momentum to the mast at the same point the increased drag of the broadside blade imparts a backward momentum.

If you do it all correctly, those two impulses can work against each other resulting in reduced 2/rev vibrations.

-- Chris.

[KD>]

Hello All, new guy here. I realize that I am really late to the party as the last post was in November/2011. However, if vibration in a gyro is as serious as it is in other aerodynamic systems, I think this is a critical topic worthy of ongoing discussion (at least to a person new to gyros). Considering that point, I have several questions regarding the vibration issue, to wit:

1) Are there any data on how much, if any, sympathetic vibration from the control linkages/components add to the overall vibration and/or subsequent resonance of the rotor assembly?

2) Has there been any resonant coupling noted between the rotor and the propeller/engine?

3) Has anyone measured vibration levels with the hard linkages removed and replaced with either "fly-by-wire" or cable (Teleflex?) systems?

4) Does anyone have any experience with using Sorbothane gaskets, grommets, pads or other such vibration sinks in gyros?

I brought about half a square meter of Sorbothane with me to Australia from the USA. I had been experimenting with it to for ultra-lightweight, virtually solid-state, shock absorbers for landing gear. As such it may have a tad too low a durometer but it would still be worthwhile for testing (the lower the durometer the softer it is and the more rapidly it would wear out). If anyone here in Australia is interested in working with it, I may be able to part with a chunk of it (it is really expensive so it wouldn't be a huge piece...don't mean to be such a cheapskate :sorry: )

If you are in the USA or Europe, it would probably be easier and cheaper just to buy it there or request a sample.

Jukka, is there any chance you will be writing-up the results of your additional testing? If so, I would be very interested in reading it.

Anyway, thanks everyone for your time.

Regards,
KD