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120 SR Blade (eFlite) 120 SR Helicopters Information and Help


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Old 03-12-2011, 09:38 AM   #1
BrandonB
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Default Fix for Toilet-Bowl Effect (TBE) and vibration

My two week old 120 sr started developing some low frequency shakes as well as some good TBE after numerous crashes. From my MSR, I remembered a few things to check:

1) make sure that linkage guides are not bent wide from crashes. they should be straight so that there is not much play such that the flybar won't rotate more than 5-10 degrees independently of the rotor (read my quick fix for this at bottom of this post).
2) make sure the blades aren't too tight: I've noticed that many people here recommend you tighten em up a bit, but I found the best advise is to tighten em so that they will stay in position when the heli is turned sideways, then carefully loosen each screw until the blade drops under its own weight.
3) assuming that there is no other obvious thing wrong like twisted tail boom or cracked blades, all we need to do now is adjust the trim: get it up and out of ground effect then trim the controller in the opposite direction of its first unintentional movement. Keep doing this until the heli doesn't drift!

Also, balancing the blades and flybar do help a little, but may not be worth it to everyone. There are other posts in this sub-forum which cover a couple methods to do this.

Disclamer: The steps listed below could melt your heli. Do so only if you have the spare parts already and just don't want to replace it if you don't have to. Only do this with adult supervision, away from any fire hazards, and take care to not flame treat any part which does not need it!!!

A quick fix for bent plastic linkage guides: Remove the linkages. Carefully squeeze the guide so that it's straight again. Using a cigarette lighter, quickly 'kiss' the flame to the part which is turned white from getting bent. It should only take one or two 'kisses' until you see the part turn all black again and no more white. If the part gets soft from the heat, make sure it stays straight until it cools. This will resist future re-bending but will make it fragile over repeated treatments. Also, don't heat up the blade grips while you do this, be smart. Don't flame treat any part which does not need it!!!
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Old 03-12-2011, 05:09 PM   #2
RoundWing
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The root cause of TBE is the flybar getting out of its normal 45 phase angle. The model drifts out of hover, the flybar corrects, but the correction is slightly out of phase so the correction isn't a linear pendulum motion. Causes include a bent or cracked flybar and slop/binding in anything that moves the flybar and rotors swash, links, guides, etc.

The rotors dish into a V shape under load by design. The dishing has the effect of dampening any imbalances in the rotor blades or flybar. The blades also need to pivot and flap up and down to compensate for dissimilar lift on the leading / trailing blades. Over-tightening the blades in the grips so they can't do all those things will exacerbate any imbalances and other factor causing TBE.
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Old 03-13-2011, 08:37 PM   #3
shadow102
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i found out my main shaft was split in half and that caused a ton of vibration and TBE. didnt realize it until i switched to an aluminum shaft just thought it was bad flying on my part
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Old 03-14-2011, 03:21 AM   #4
bulesz
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Just asking, why did you switched to alu shaft??? The carbon is really good I think, due that is cant bend!

?
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Old 03-14-2011, 03:55 PM   #5
jeffpack1957
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you should also add blade tracking.
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Old 03-15-2011, 12:38 PM   #6
BrandonB
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I don't know how important tracking is for this size heli. On my bigger birds I have pitch adjustments for each blade grip, but the mSR and the 120 SR obviously don't have that option.
Generally, good condition blades that are equal weight along with good rotor head/feathering shaft and grips (no bends, no white lines on the plastic) should track fine. If they don't then at least one of the blades is probably bent. I check for this by taking the blades off the grips, laying them over each other to see if they are out of shape. Then I do my best to make them the same with gentle bending. It worked with my mSR, and it worked once with my 120 SR. I'd like to know what other people do.
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Old 03-15-2011, 06:31 PM   #7
jeffpack1957
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I put new blades on mine, and blade tracking was off by 1/4in.

Fixed blade track, no more shakes.
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Old 03-16-2011, 08:37 AM   #8
jeffpack1957
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How to do blade tracking:

I use permanent felt markers, 1 red- 1 Blue and mark each blade tip one color.

Then, putting the SR on a level spot, I use a bright flashlight, and spin up the SR.

I hold my light in my mouth, one hand holding down the SR.

Shine light parallel to blade track, and you can see how close blade tracking is.

To adjust, I just use a heat gun to adjust the blade.

courtesy of John Salts book on the SR 120.




Thats how ya do it Brandon
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Old 04-10-2011, 08:14 AM   #9
Twinkles
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Hi, just got my 120 SR a few weeks ago and I noticed a wobble while hovering. Been looking at various posts and cannot find any answers. What should I look at as a root cause?
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Old 04-10-2011, 09:24 AM   #10
whirlEbyrd
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I would call it an oscillation. My MSR and 120SR will both occasionally do it for a second or two while hovering. With the slightest control input it stops. I suggest moving a control as the cure.
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120 SR Blade (eFlite) 120 SR Helicopters Information and Help

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