Wild oscillations

[GunPilot]

Hey Everyone,

I'm new to helifreak but not all that new to RC helicopters.

I recently converted my T-Rex 600E to flybarless with the SK360. It responds fine to left and right cyclic applications, but for and aft cyclic applications cause it to develop a very large, and scary oscillation! the manual suggests adjusting the hiller gain for slow oscillations, and Dampening gain for rapid oscillations. But it says nothing about LARGE oscillations. Has anyone else come accross this issue? if so how is it corrected?

Thanks!

[jvthert]

Large oscillations in FF mean you have to fiddle with your dampening gain. Oscillations in a hover after steering could say something about your bell gain. Does it react aggressively? If so you need to lower the bell gain a bit.
Last one is your hiller gain. Gradually increase it until the heli starts to hunt. Then dial it down a few % to make it stable again. That should fix the oscillations.

The fact that it's slow and large makes me think your gain is too low somewhere. It it only happens in flight and not in a hover it's definitely something with your dampening gain.

[GunPilot]

I've adjusted the dampening gain all the way up, and down to almost nothing and I still get the oscillation. I suppose I should have mentioned that this is occoring in hovering flight. I haven't transitioned into forward flight yet because I don't think that would be a sound decision with it responding the way it does in hovering flight. I'm still new to this forum, but I'm assuming FF stands for Forward Flight, am I correct?

[GunPilot]

I've adjusted the dampening gain all the way up, and all the way down to almost nothing and I still get the oscillation. I suppose I should have mentioned that this is occurring in hovering flight. I haven't transitioned into forward flight yet because I don't think that would be a sound decision with it responding the way it does in hovering flight. I haven't messed with the Bell gain yet, so I'll give that a shot. When I make the adjustment, should I only do it in the elevator axis or both? I'm still new to this forum, but I'm assuming FF stands for Forward Flight, am I correct?

[jvthert]

Yes that's correct. FF = fast forward. I don't know how good your flight skills are so unless you're confident you can control a oscillating heli you'd better stay away from forward flight.

Perhaps a no brainer but are you 100% sure the mechanical setup is correct and there are no vibration issues?
Second can you post your skookums setup. Screendumps are the easiest way but you can also include a setting file.

Having seen the profiles for the 500 and 450 I reckon your dampener gain should be in a 8 - 15 range. Depending on how stiff your blades and dampening is. Soft dampening equals higher gains and harder lower gains. Vibrations have a lot of influence on this setting.

If it oscillates only after you give stick input and go back to neutral lower your bell gain a bit. Most people have settings close to the stock settings. That means hiller gains in the 45 - 55% region and Bell gains in the 30 - 35% region.

[GunPilot]

I'm very confident in the mechanical setup, but I don't know what the vibration tolerance is for this system. What are some techniques to reduce the effects of vibration on the system other than the foam tape I used to secure it? My blades are well balanced and the tracking is very smooth, but that by no means indicates a lack of vibration. I've had this set up with a scale fuse on it for about 9 months and I've been pretty happy with it, but that doesn't mean it can't get better.

The oscillation does occur after a stick imput and return to center. So I'll try adjusting my bell gain and see how that works. As it is now, I've been making adjustments from the Scale file that comes with the set up software. So my Bell gain in the elevator axis is at 25, about how much lower to you think will help reduce or eliminate this erratic response?

Thanks!

[jvthert]

I have no idea other that to gradually decrease until it goes away. If you lower it too much there will be some delay between input and reaction. The heli wil react slower to stick movement.

[GunPilot]

The reduction in Bell gain worked like a charm, thanks a million!