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Aerobatic FAI F3C Contest > trex 700 FAI, low head speed vibration problems
 
 
jhirahara
New Heliman
Location: Los Angeles, CA

My Posts This: Topic  Forum		I was curious if anyone has been able to resolve the vibration issues with running slow head speeds on a Trex 700 with flipped grips. The consensus seems to be the dampeners are too stiff. Any solid solutions? The head seems to stabilize at about 1700. I've gotten very low head speeds to work also, but it's finicky. I know I can try thinner shims, but I'm not convinced that will totally solve the issue.
I am running a 8:1 gear ratio from FAD-heli. I seems to work well.
I would love to hear any suggestions.
Thanks
10-29-2009 04:13 AM
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jhirahara
New Heliman
Location: Los Angeles, CA

My Posts This: Topic  Forum		anyone?
10-29-2009 07:30 AM
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MessyDan
Heliman
Location: Johannesburg, South Africa

My Posts This: Topic  Forum		Flipped my grips with great results! ok so she didnt hover like my sylphide but hovered and flew really nicely!

The trick is to add an additional spacer between the grip and the ball link... played with 8-9mm, used 9mm eventually...
Definetly have to use softer dampers for this to work
- Tried o-rings, but they did not fit well (this may work though if you get the right ones)
- Tried drilling holes in the stock dampers (dont know what i was thinking here, ended up getting a wobble since the damping was now non-linear)
- Ended up using some 75 soft sylphide dampers, trimmed a little to fit, along with the 1mm shims that are used with the stock 700 head.

Ran 720 blades, but these are a little long and did bounce in the wind (slightly) would suggest 700s or so...

I was happy with the 700, anything better would be overkill for my skill level (700 is going back to 3D and sylphide is coming out of the woodwork now though )
10-29-2009 08:01 AM
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jhirahara
New Heliman
Location: Los Angeles, CA

My Posts This: Topic  Forum		Thanks Messy!!

Yeah, I did the 8mm spacer mod. and it seems to work well. Even with a higher head speed the 700 hovers nice. I too am using 720 rotortechs. Sounds like slightly shorter may work better, I'm gonna stick with these for now. Sylphide parts are hard to come by and I've found difficulty finding the right part number. I'm going to try finding some o-rings.

thanks again for the info.
10-29-2009 05:25 PM
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F1 Rocket
Senior Heliman
Location: Melbourne, Florida USA

My Posts This: Topic  Forum		I made my dampeners using 2 O-rings with heat shrink tubing around them. The O-rings can be found at www.mcmaster.com. Take 2 of these, stack them together and shrink a piece of heat shrink tubing around them. Trim the tubing to the edges of the O-rings. One set will take the place of the stock dampener. Use shims to adjust the stiffness. These flew quite well on my 700 head before I went FBL with it.

Part Number: 9262K782 $7.06 per Pack of 100
Type
O-Ring

O-Ring Type
Standard

Cross Section Shape
Round

Width
3.5 mm

Inside Diameter
10 mm


Danny

Esprit Model
10-29-2009 06:11 PM
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jhirahara
New Heliman
Location: Los Angeles, CA

My Posts This: Topic  Forum		Thanks F1Rocket. Funny, I actually spec'd the same mmc o-ring today. I'm also going to try this one:

#9263K765 Metric Viton® Fluoroelastomer O-Ring 3.5 mm Width, 10 mm ID, packs of 10

It has a slightly softer durometer rating than the buna-n one. I'm a little concerned about the longevity of the shrink wrap, but it's definitely worth a try.

I've read your other threads and they have been very helpful by the way.
10-30-2009 03:27 AM
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MessyDan
Heliman
Location: Johannesburg, South Africa

My Posts This: Topic  Forum		Another thing is that the RT720SGs should be run around 1650 during hover and 2050 for flying (from what I have heard anyway, and what is on SGs website) So your 8:1 mod should allow 2000 rpm at least for flying

I also found that the blades need to be super loose on the 700 grips (this made all the difference), due to the grips having a small contact area with the blades they tend to bite (also sanded any rough edges)

My 720 needed a bit of balancing, although I dont think this is usual.
10-30-2009 09:35 AM
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Synthax
Senior Heliman
Location: Warsaw - Poland

My Posts This: Topic  Forum		I've solved this problem with special dympers and right RPM. About 1250-1280

http://www.youtube.com/watch?v=TQx13a_AUFI

One day it just became your lifestyle.
10-31-2009 11:14 AM
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MessyDan
Heliman
Location: Johannesburg, South Africa

My Posts This: Topic  Forum		wow thats slow! are the blades hapy there?
11-02-2009 07:54 PM
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Synthax
Senior Heliman
Location: Warsaw - Poland

My Posts This: Topic  Forum		Please do not miss the strobo affect with camera. But at this rpm it works very well to me.

One day it just became your lifestyle.
11-02-2009 11:40 PM
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jhirahara
New Heliman
Location: Los Angeles, CA

My Posts This: Topic  Forum		This past weekend I made my own dampeners from MMC part # #9263K765 Metric Viton® Fluoroelastomer O-Ring 3.5 mm Width, 10 mm ID. Used 2 o-rings, shrink wrapped them together, trimmed the edges so no shrink made contact with the o-ring to the washer and head, lubed it all up and bolted back together. Bolted the blades very loosely. The results were extremely positive.

No vibrations from spool up to hover. I had it about 1650 RPM. Hovering was solid. Forward flight felt very locked in. Huge improvement to stock 3D set up for FAI.

I'll have some pics by this weekend. Head mods seemed to work well. The Fad-heli gear set seemed to help hold the tail well. No low speed tail kicks.
11-10-2009 03:32 AM
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Aerobatic FAI F3C Contest > trex 700 FAI, low head speed vibration problems
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