SR 120 Toilet Bowl Effect - Solved!

[crashmatt]

Hi guys,

I am pretty much brand new to RC, never mind helicopters. I have been loving my SR 120 since I got it about 3 weeks ago. After a few fairly major crashes (misjudged dives from 40+ ft), my chopper developed the infamous TBE, shaking quite badly whilst it hovered.

I examined my rotating components, tail unit was fine. The blades were messed up so I changed them - no effect. What I had noticed however when I changed them was that the blade grips were rather tight on the new blades.

On close inspection I found them to have been bent slightly inwards. As the blades are not completely free to move (and there is no way to have them perfectly aligned pointing 180.000 degrees from each other) this leads to dynamic out of balance of the sytsem, hence the shaking.

As a side note I would also advise all people to not over-tighten their blades as many people suggest. This can damage your grips (leading to a pronounced toilet bowl effect through the above mechanism) and also prevents your blades from dynamically balancing themselves out by flinging out to a perfect 180.000 degrees between each blades centre of gravity.

If this does not work it is likely to be your flybar having been slightly bent. You may not be able to see the kink, but when it spins up the weakened arm will not flex the same as the opposite arm (the plastic may have been stress hardened from previous crash) leading to the TBE also.

I hope that helps someone out.

Matt

[kushflyer]

TBE is inevitable on any 45 degree flybar head (like the 120 sr) and is only worsened by over tightening your grips. I'm not sure about your post title, it's literally impossible to rid the 120 sr of ALL TBE because of its flybar angle. You need a true collective pitch 90 degree flybar (or flybarless) system to fly without any TBE.

[Bloosee]

Blades should be just lose enough that they move freely. Too lose is better than too tight. TBE could also be caused by the hole in the flybar where the main shaft runs thru becoming enlarged. I've noticed after quite a bit of flying that the flybar starts to wear, becoming lose, creating a bit of slop, time to be replaced.

Make sure all the links are snapped on nipple out, if they are reversed they will be too tight. I like to occasionally spray a bit of silicone lube around the rotor head and links as well, keeps all the parts moving freely.

To me with the 120SR, TBE really isn't an issue unless you're hovering, and with this bird I rarely ever hover, I take off and go immediately into FFF.

Just my 2 cents....

[65L88]

Originally Posted by kushflyer

TBE is inevitable on any 45 degree flybar head (like the 120 sr) and is only worsened by over tightening your grips. I'm not sure about your post title, it's literally impossible to rid the 120 sr of ALL TBE because of its flybar angle. You need a true collective pitch 90 degree flybar (or flybarless) system to fly without any TBE.

I'm not sure I agree. One can split hairs and argue there is some remaining undetectable TBE in any case, but as far as eliminating TBE to the extent that it is visually undetectable, it's entirely possible. I have a 120 SR and an mSR that both hover absolutely solid, with the only drift attributable to wind or breeze. When TBE onsets, it's a cue that something is out of spec, and it's then time to think about replacing some parts or doing a rebuild.

[Pfloyd]

+1

[Dontfakelife]

TBE seems to come into play only when I put new parts in.. it drives me crazy! I had TBE on my previous set up but after a while it worked itself out (primarily with the flybar) but this weekend when torque finally broke one of the balls off and replace it the TBE came back in full force.I lubed everything up but it still it throwing my bird all over the sky!

[kushflyer]

Originally Posted by 65L88

I'm not sure I agree. One can split hairs and argue there is some remaining undetectable TBE in any case, but as far as eliminating TBE to the extent that it is visually undetectable, it's entirely possible. I have a 120 SR and an mSR that both hover absolutely solid, with the only drift attributable to wind or breeze. When TBE onsets, it's a cue that something is out of spec, and it's then time to think about replacing some parts or doing a rebuild.

Its easy to dial in a solid hover with no TBE via adjusting cyclic trim mechanically (giving servos equal throw) but there is no debating whether or not ANY 45 degree flybar has TBE issues when flying all out in fast forward flight. If you've flown a CP heli with a 90 degree flybar or flybarless system you know that you aren't dependent on momentum alone to keep the heli moving (as you are with a 120 sr). A 45 degree flybar will always have a tendency to want to move back to center. Its a fact.

[65L88]

I thought we were talking about TBE, not self centering, LOL... I think you're comparing apples and oranges. A well tuned heli (45 degree flybar) will go into a stable hover after any maneuver after centering the sticks (it will pendulum a bit, but I don't consider this to be TBE). And yes, I own three ccpm helis, including a fbl 500, so I'm well aware of their characteristics, not to mention spending a good part of my life working on, around, and flying in full size helis.

[kushflyer]

[65L88]

Originally Posted by kushflyer

Gee, how mature. Thanks!!

[RoundWing]

On a cause and effect level TBE occurs because the model drifts out of perfect hover and when the self-leveling pendulum effect of the 45° flybar kicks in it is slightly out of phase.

Instead of rocking straight back like a pendulum as it does when flying forward at speed and releasing and centering the cyclic the out of phase correction makes it drift in a circle. The CW rotation of the rotor and CW thrust of direction of the tail rotor make it TBE in a clockwise circle unless the flybar (which controls rotor blade cyclic pitch) is out of phase in the opposite direction.

There are a lot of different factors which can cause the flybar to get out of its designed 45° phase angle with the rotor such as bent /cracked flybar, loose / binding linkages. Anything which causes the flybar to change orientation incorrectly and change the cyclic pitch the rotor blades without any operator input.

Blade grip tension affects TBE because by design the rotor tips must be allowed to lead/lag and flap during rotation to keep the rotor stable. The grip tension (and design of the plastic parts) also control the amount of V shaped dishing of the rotor under load. The dishing has a stabilizing effect that will help minimize any inherent TBE. When the grips are tightened too much the rotor blades can't dish or flap properly adversely affecting handling.

Most of the dynamics of rotary flight were sorted out by Juan de la Cierva in the 1920s in his auto-gyros which preceded helicopters. The early copter designers like Sikorski, Hiller and Young (Bell designer) incorporated the flapping hinges and other solutions that de la Cierva devised. The Hiller paddle system and Young devised "Bell" 90° weighted stabilizer bar were first combined on CP RC models in the 1970s. The 45° flybar is a very recent innovation resulting from making models smaller and fixed pitch. The first reference I've found to it in a patent search is a 2009 application by Hirobo. The narrative for that patent application for an "acute angle" flybar design mentions that it was a result of trial and error to solve stability problems with the fixed pitch flexible rotor micros.

More on detail on TBE and development chronology on my web site: http://roundwing.nova.org

[kushflyer]

Originally Posted by 65L88

Gee, how mature. Thanks!!

just for you bud

[65L88]

Very good post, RoundWing. One thing I have found with the 120 SR and the mSR is, after a few crashes, the holes in the top of the main shaft that the "jesus bolt" or screw go through has a tendency to oval out, which will let the rotor head lag behind the rest of the rotor assembly. This out of phase condition can actually be enough in the extreme to make the heli virtually unflyable. I actually cracked or split the top of the mainshaft on my 120 SR through the top screw holes. When I took everything apart it looked normal, until I looked closely and saw the cracks, which let the shaft "spring" just enough under the combination of clockwise motor torque and counterclockwise rotor drag to let the head run out of phase. Once I superglued the cracks and reassembled, everything was fine again. Just something to watch out for if your heli suddenly quits flying like it should.

[toys2cars2toys]

A common mod on the msr was to modify the flybar to take a swashplate ball as the pivot, I never tried it myself though
I haven't seen it done on the 120SR, I wonder if it would help

Here's an image that I found

[65L88]

A novel idea, worth checking out!!

[CrashCanuck]

I am currently trying to get the pivot ball mod working in my 120. I have tried it in my MSR and it has reduced the pendulum effect quite noticeably. I shortened the flybar arms as well. It now flies, I feel, the way it should have from the factory. The 120 has taken a little longer, I am using the ball out of my original SR swashplate. The main shaft size is the same as the 120. I need to find some small O rings to hold the ball in place without getting in the way of the movement of the flybar. I tried an old pair of dampeners out of my SR but they are to chunky. Need to locate some thinner ones. I also shortened the arms on a flybar but made the mistake of removing too much material from the center hole of the flybar. This has to be able to just fit over the pivot ball. I have a little too much play so I think I will pick up another flybar and give it another go. Hopefully will find some O rings this weekend. Will post back when and if i get it flying right.

Later

[kenh]

Originally Posted by CrashCanuck

Hopefully will find some O rings this weekend. Will post back when and if i get it flying right.

Later

I don't have any in front of me right now, but I'm thinking the O-rings from the idle mixture screw of any motorcycle or ATV should be about the right size. Check with a MC shop to see if they are available. We keep a small bag of about 40 or 50 to use when rebuilding carbs.