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Author Topic: 700 nitro pro.. shaking on low throttle  (Read 211 times)

Offline Kez

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700 nitro pro.. shaking on low throttle
« on: June 09, 2018, 00:26:39 »
New toy - I'm told it has been set up and flown. So I bind my tx and set the paramaters, then head off to the field.

Started easy enough, although I think it needs a tune. However the thing thats unnerving me is when i increase throttle everything starts shaking.
I'm wondering, is something bent/unbalanced or is this normal. I mean, does it smooth out if i increase more? I was still on a fairly low throttle - nowhere near enough to take off.

Offline Two-Six

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Re: 700 nitro pro.. shaking on low throttle
« Reply #1 on: June 09, 2018, 11:09:41 »
All RC helicopters do wobble and shake quite a lot initially at spool-up because the blades haven't been pulled out straight enough by centrifugal force.  They don't find their "centers" until they are going at a good speed.  Perhaps your blade bolts are too tight.

A tip for how tight your rotor blade bolts should be.  Tighten them up enough so that they can hold the blade horizontal with the helicopter held sideways BUT will let go of the blade so it falls down vertically when you give the heli a reasonably hard bump. 

How hard a bump?  With small helicopters they should let go with an increasingly hard tap, with a 700 they need to be tighter, so a reasonably hard fist bump is probably about right....

You could try getting a blade ballancer but I am sure your blades will be balanced, no harm in checking.  You might also have a bent main shaft.  Test by removing and rolling on a very flat surface and check for wobbles.

So the heli was set up and flown...hummmmmm :hmm:

Well perhaps it was.  I would recommend you have a very close look at it all before flying it.  Even giving it a good strip down and re-build because you don't know if bolts are tight and thread locked and things are properly lubricated.  You can also have a good look for any damage.

Helicopters should be almost vibration free, so almost no detectable blurring/vibration of the skids or tail at all.  Usually any vibrations will be quite high frequency vibes.

Nighthawk Pro, Trex 450 L Dominator 6 cell *FLOWN*, Blade 450-3D, MCPX-BL, MCPX-V2, Hubsan X4, Seagull Boomerang IC .40 trainer, HK Bixler, AXN Clouds fly,, Spektrum DX7, Taranis, AccuRC

Offline Bad Raven

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Re: 700 nitro pro.. shaking on low throttle
« Reply #2 on: June 09, 2018, 12:55:03 »
RC helicopter blades like real world Heli's lead true alignment when retreating towards tail and lag when going forwards. It is common for blades to be over tightened and then then this does not happen efficiently. 

Also the blades are balanced and their CG is set during manufacture to be correct at flying head speed under that level of centrifugal force. (particularly true for flybarless blades as their CG differs).

Heli engines/clutches also contribute to the vibration at certain revs below their operating range.

While spooling up you may have mild vibration even when blades are correctly set/tensioned.

The trick is to get through this resonance period asap.

I have been around the heli flying arena for more than twenty years and currently have over 30, around 10 being engine powered, and have just got back from a 3D session at the field with a 230S, which though a great little Heli, isn't great while spooling up!

Offline Kez

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Re: 700 nitro pro.. shaking on low throttle
« Reply #3 on: June 09, 2018, 14:37:28 »

Thanks for the replies people.
I spoke to the previous owner and think I found the issue. Basically I wasn't revving it enough, and using a different fuel it needs a major retune. Then it should pick up better and smooth out.
Previously it was tuned for 40% nitro, but I can't seem to get hold of that now. I'm told we can't buy more than 25% without having a license to store it?

Offline Bad Raven

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Re: 700 nitro pro.. shaking on low throttle
« Reply #4 on: June 10, 2018, 06:30:46 »
If it genuinely was "tuned for 40%" (and that is rare and usually counter productive, making me a little suspicious ) then it will 99/100 have a head spacer/thicker gasket to lower the compression ratio.  Remove this! If it was done by modding other than that they needed their bumps feeling!!

I had years ago a couple of full on competition Heli OS Black head (gold body) engines. These had extremely high compression low nitro set up (as the rules withheld high nitro use), high on both both head and crankcase.

SO high that even a Dynatron starter on 24v had difficulty when the engine was cold in turning it over WITH THE PLUG OUT!!!!

I used to have to spin if over with the plug out and no fuel to friction warm the bore/piston, then fuel it and spin it over, still plug out, then finally replace the plug only half screwed in and get it firing to warm it up, then stop, refit plug properly, and restart. It was fine then for subsequent flights that session.

A right PITA, and done away with asap, though the POWER, OOOOOH the power!! (maniacal laugh)   ::)

Whip the head off, if you find a thick gasket/spacer, or more than one, remove it down to as thin as poss. If it hasn't then pass it on.

Thunder Tiger Redline engines are great, I standardised all my fleet on them after lots of trials.