Tempest wobble

[Hardcarve1]

Hi guys I tried this in RR http://www.runryder.com/helicopter/t99909p1/ but I guess a lot of the MA guys are here know so I may as well come and join the party.

I have a FAI tempest 8.18 with YS91 and have set this up to the instructions and have installed the paddels with the max weights.
The head is very soft and I have read that if you have low head speed you can get the wobbles. I have the problem that if I put some head speed on it starts getting a slow wobble. If I bring the head speed down the wobble goes away, I noticed that it also wobbles in FF with high head speed. It's only a guess but I recon it starts the wobble at around 1600 head rpm. I tried tightening the blades with no luck and I also note that you should not lubricate the "O" ring dampers which would be my first guess as whats wrong.
My next guess would be to put stiffer dampers to stiffen the head and then maybe reduce fly bar weights.

Neg. delta = -3.3

How much shim 0.25mm

Blade type = MAH 700

How tight are the blades in the grips = 25mm of fly bar flex when tip of fly bar is held and tip of blad is moved.

50/50 or 50/60 orings = 50 on inside & 60 on outside.

I then did this.

OK I took the head apart to make sure I built it right. It was all assembled the correct way so I put 60/60 O rings and the thickest shim to stiffen the head up.
Did the first flights and the problem was still their. I did full powered climbs and FF full powered runs. In both occasions the wobble would start and get worse the longer I held full throttle.
I then loosen the main blad bolts to where they were just enougth to stop the blade moving in the grips. ( My experience was this was very loose) and then did all the test flights again. I found that this solved the problem for powered climbs and fast FF. I did find that if I used cyclic and continuously rocked the heli using elevator or roll the wobble would come back again.
When I assembled the kit it had fibreglass washers for the blade grips. I did not use these because the MAH blades were a tight fit in the grips, by not using these I wonder if this could cause the wobble problems.
My next plan is to see if the fibreglass washers will fit and I will use light paddels.
Has anyone got any other idea's.
Alex

[Dr.Ben]

No, I'm not stalking you <g>

I couldn't be sure from your latest description.............have you switched out to another set of blades yet?

If the blades are a snug fit in the grips without the shim washers. there should be no issue in leaving them out. The only way their exclusion should cause a problem is if the blades were a really loose fit, and you were distorting the grip to achieve tension on the blade.

Gordie, this is your favorite topic, I know. I've been through the usual stuff. What do you have to add? Same for you David.

Ben

[Shannon Davis]

I had the same wobble at 1550 and it went away at 1580. I thought it was the dampening was too stiff for this low head speed--not to soft :?

When I losened the blades in the grips the wobble got less-when at 1550. The tighter the blades the worse it wobbled.

I also run MAH and have to use the FG washers--they are a tight fit and a little tricky- but that may be your problem--you may be distorting the grip.

[DavidH]

Alex,
On the problem of oscilation ( wobble, mast bumping) there can be couple of causes.

Usually the tighter the dampening ( more shims, harder orings) the higher the rotor head has to be to prevent this from happening.
Softening the dampening ( less shims, softer orings) the rotor rpm can be lower without the wobble occuring.

Now the stiffness of blades also come into play in the above equation. The MAH blades are very stiff blades. You might try either raising your headspeed or softening the dampening. Or you can try another brand of blade with the same setup and see if they make a difference. This is not saying anything about the MAH blades, just do to there stiffness they may not be working best for how your running your setup.

My set up is .9 to 1 ratio, -3.3 mm delta, 50 and 60 duro oring on each side. The 60 is inboard on each side. One .25 mm shim on each side.
I can hover the model down to about 1450 rpm without it wobbling. But that depends on what blades I have on the model. I can swap blades and I am not able to hover the same model below 1500 rpm without it wobbling.
If your only going to fly the model for 3D style flying, then I would suggest to set the model up to run about 1700 rpm plus without it wobbling. About the only time the model would be below that rpm is on spool up and sometimes during auto rotations.

The correct pressure on the blades in the grips. You should be able to take the model and hold it on its side without the blades falling under there own weight. Then give it a slight shake and they should move. That usually works for me.

David

[Krister T]

I'm only flying F3c and really like to have low head speed at hovering.
My set up is the same as David H. have.

When I tried different pairs of rotorblades of the same model so did I also get different results.
One thing that I have found out is that the chord wise c.g. also is one of the things that helps get the mast bumping as having to tight pressure in the blade grips.
On the TZ91 II so was the lowest head speed 1490 before the mast bumping occurred. And as I wasn't satisfied whit that so did I rebalance them on the koll-rotor. With the blades rebalanced I can now hover down to 1380 where the mast bumps starts and still have a tight pressure in the blade grips.

To get a little from the subject so do I still have problems with the perfect matched rotorblades.
At 1950rpm:s from a vibration free helicopter so do I get more and more vibrations but I think that I could be the thrust brg:s that needs replacement. When I found it out so will I post it here.

Krister T

[Hardcarve1]

Thanks guys.
I took the fly bar assembly apart because I could feel just a small amount of play in the bearings. I put it back together still with a bit of play as the fly bar diameter is smaller than the bearings. I removed all the lead in the paddles, put the washers for the blade on and did the bolts up as to what DavidH described. I left the head setup with 60/60 O rings and thickest shim.
Went to the field and the wobble is completly gone thank god but the thing flys like a cut snake and I need to just tame it down a bit for my flying style. So I guess it was the washers, blade pressure or the heavy 55g paddles or maybe a combination.
Thanks for you help guys.
Alex