Reducing gyro vibrations

Mike G

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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

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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

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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
 

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Redbaron

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

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

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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

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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
 

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C. Beaty

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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.
 

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Redbaron

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span wise balance

span wise balance

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

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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

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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

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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

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

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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

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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>

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Vibration Questions?

Vibration Questions?

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
 

Mike G

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Did anybody keep a copy of the report by Jukka Tervamaki, I would like to read it again and it is no longer on his website.
Mike G
 
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