Not really an elevator "Bounce"

[helihammer]

I have the BD3SX installed on a Pantera P6. This is a 600 class machine swinging 602 mm blades. This helicopter has the aileron servos connected directly to the swash and the elevator servo connected to the swash through only one 90 degree arm. The FBL conversion for this model costs nearly nothing to do due to the virtue of using stock parts, but the disadvantage is that the pitch range is limited to +/- 10 collective and +/- 9 cyclic. Anything beyond that and the swash binds on the main shaft and the servo travel is pretty much maxed out. This should be enough for the BD3SX though.

Now, on to what is happening. I don't fly 3D, I'm just looking for a nice easy hovering around machine to learn orientations and cruise around on.

Ever since the FBL unit has been installed on this helicopter, I have been plagued by a sharp elevator movement. That is to say in a hover, the aileron movements are smooth and predictable, but when given only a small amount of forward elevator, the helicopter tilts forward quite a bit in a hurry. To put it another way, when the heli is tilted ever so slightly backwards and heading towards me, a small elevator correction will make the tail lurch upward unusually fast.

I wouldn't characterize this as an elevator bounce. It's more like a lurching or rocking. This only seems to occur on forward elevator inputs.

All the settings are from the 600-700 default file from the sticky.
I have 30% expo turned on for Aileron and Elevator in the DX8.

Using Real Time Tuning, I have played with the following parameters:

Elevator Gain (slider normally at 16, same as aileron gain)
20 gave a cripser response, and I started to see some actual elevator bouncing upon hard stops. This also made the "rocking" motion somewhat smaller, but it was still there.
A setting somewhere around 12 made the heli feel much too unresponsive.

Elevator Filter (slider normally at 4)
This seemed to have no effect in reducing the rocking no matter what value I tried.

D-Portion for elevator (slider normally at 10)
This also seemed to have little effect.

I also played with agility and it affected the overall feel of everything, but I was unable to get rid of the rocking.

I'm running an 1800 RPM headspeed, so I would think that's safely above what may cause elevator wobbles. This really isn't a wobble, however. It's a rocking in one direction only.


I wonder if the mechanics of the helicopter are causing this or if it's something I'm missing.
Anyone have an ideas? Perhaps I should try increasing the gain and then change other parameters from there. I would think it futile to change things like the elevator filter if the gain was not set properly. Also, I have noticed that when tilting the swash forward at zero collective, the elevator servo has to move a lot whereas the aileron servos don't have to move nearly as much. Perhaps this could just be a design issue?

P.S. This model was originally flybarred. I can't remember if this problem existed then or not, but I'm going to err on the side of not.

[F1 Rocket]

Make sure the dampeners in the head are not worn. There should be no free play at all.

[rhodesengr]

I am going to take a WAG. maybe you feedback is wrong.

Carefully check your feedback directions. If the elevator feedback is not opposite your control direction, it might act as you describe.

You don't actually set feedback direction with the 3SX. But you have to choose the right mounting orientation and not change any TX reverses after you get things right on diagnose.

Anyway, easy enough to check. Tilt the heli forward and the swash should tilt backwards

[helihammer]

The dampers in the head are the hardest ones available for this model (88 durometer).
They are not worn, but if I wiggle the blades up and down, I get about half an inch of travel at the tips. I think this is largely due to a ganging of tolerances in the blade grips. The bearings in the grips can wiggle the slightest bit on the spindle shaft, and the grips themselves can wiggle a little bit on the bearings (I assume because of manufacturing tolerances).

The feedback directions are also correct.

[helihammer]

It seems like the elevator is kind of loose, so could I increase the overall gain and then reduce the aileron gain if necessary? Currently, the elevator gain is at 13 with an overall gain of 55. This means the actual elevator gain is 7.15. When I turned the elevator gain up to 20 with RTT while keeping the overall gain at 55, this meant an actual elevator gain of 11. It seemed to reduce the rocking movement, so maybe more elevator gain than I can get with a head gain of 55 is needed.

[helihammer]

Update:

I've since gone back to a flybar setup and the elevator issue is no more, so I can safely say that the problem only existed with the FBL setup. I'm thinking it was not enough elevator gain. If I decide to go back to FBL, I'll start there.

[jsbach#1]

I chased the same thing for an entire year with 3sx and goblins. Its only noticable when you give the sharp inputs, not during flying for me.

At the end I minimized it doing exactly what Rocket said, tightened up my dampers.

Now I have no bounce at all, flying a protos and a brain lol. BUT I had a crash yesterday I would NOT have had if I was still flying 3sx. It didn't rescue fast enough man.

[helihammer]

I agree, it is only noticeable when sharp inputs are given. I changed the dampers to softer ones and have since CA'd the bearings into the blade grips. Since doing this, the play at the blade tips has gone from half an inch to less than a quarter inch. Maybe this in combination with more elevator gain would solve the problem. I want to go back to FBL so I can use the 3SX to its full potential instead of just a tail gyro.

[helihammer]

As soon as the weather gets nice, I'm going to try out the FBL setup shown in this picture.



I'm using the newest default setup from the sticky as before so we shall see how it goes.

[helihammer]

Also, I just remembered I caught an elevator servo failure during a preflight, maybe that could have exacerbated the problem. The servo has been replaced with a new one of the same type.

[helihammer]

Well, it turns out more gain was needed. I turned the Head gain up to 70 and the elevator gain to 16 for a total elevator gain of 11.2. The bounce is mostly gone. Some more tweaking is needed as always, but I'm on the right track! I was able to turn the aileron gain down to 12 to keep the craft from wobbling too much.

I'm not sure why so much elevator gain is needed, but every machine is different.

[helihammer]

I tried reloading the defaults from the sticky with a head gain of 60. I also put 20% expo in the radio and this seemed to help greatly. This change is enough to make me want to keep the flybar in storage for now.