Swash strange behaviour

Attached Video showing the Swash on my Trex 550E during tests in my garden. Has anybody had the same happen to them? Heli has a Pixhawk mounted at 270deg running V3.5.2 firmware

Hi Geoff,
That is strange. I didn’t see your post in the other thread until just now and I looked at your params. Honestly I cannot see anything wrong with the settings. I thought it might have something to do with the Simple Mode but I see you have that set to zero, disabled.

But in the video the swash action still did not look right with the helicopter facing to the right? I had to play that back a couple times. But looks like when you moved to the left the #1 servo went down and the elevator servo went up. The #2 servo did not move properly for full left aileron.

When you moved the stick full right deflection, the elevator servo goes down and the #1 servo (left one) goes up. That is not right.

And when you applied pitch (elevator) then the right side servo moves. So it looks like possibly the #2 and #3 servos are switched around. Since you have AHRS_ORIENTATION set to 20 for 270 roll, that is right. Verify you have the servos plugged into the proper ports on the pixhawk. And also verify your radio is set to straight outputs and not sending a helicopter CCPM mix to the Pixhawk, which would confuse it.

Hi Chris
Your mention of ‘Simple Mode’ I set simple up just before I noticed the funny swash movement. I did it as I was testing in a confined area. This morning I switched it off, and all OK now. I checked the same on my 450 FBL but that was fine with and without simple switched on. Is there any cure, to correct this happenning?
I’ve attached a log file doing a bit of loitering, but sorrounded by buildings, which I put down to the movement.
Also a video

HI Geoff,
I myself do not like Simple Mode. I think it was invented for multi pilots where it is hard to tell the yaw orientation of the aircraft because it looks the same from all directions. I don’t think it has a valid application in helis. My opinion on the best cure is to not use it for helicopters :grinning:

You are bound to get some multi-pathing of GPS signals and considerable drift around buildings, especially close to the ground where the GPS receiver cannot get more satellites on the horizon.

You are evidently using the settings I saw in your param file, set using the tuning method I outlined. It looks really nice and stable. And you arrived at pretty close to what I am using in a 500 stretch. That Trex 550E is a VERY nice heli. Good size for decent performance, not so big as to be intimidating or overly dangerous flying in close quarters.

Hi Chris
Big day to-morrow, going to try Auto up on the field, forecast low winds. Thanks agian for your help

Is simple mode on by default? I better check that!!!

It should not be on by default. That would be a disaster for helicopters.

Hi Chris
Well I got my first Auto flight on this Heli, to-day. I still seem to be getting some shakes. On the Video, first part filmed from site and the second half filmed from the heli fixed camera. Part way through the flight it does a 15sec loiter.
I’ve turned simple off, and that is how it will stay. All other settings as yesterday.
Bin file attached by Dropbox

Attitude doesn’t look too bad, Geoff. But you do have some vibration affecting the camera and the flight controller. You have significant aliasing of the IMU’s, which will make the heli overly twitchy. And X axis vibes around 27 m/s/s.

You need to address the vibration levels - it is likely from the geartrain in your helicopter. I see you have the white main and autorotation gear, so they are probably the straight cut gears. When you rotate the head turning the motor note how much axial and radial runout the main gear has. Those gears are notorious for having runout.

You can check axial runout by observing the gear and see how much it “wobbles” up and down as it turns. You can check radial runout by checking the backlash between the pinion and main gear at like 6 points around the circumference of the gear. If the backlash is not the same, then you have radial runout as well.

I have not flown one of those Trex helis without modifying the gear mounting to the hub. They have a recess in the gear that sort of presses onto the hub. And self-tap screws that screw into the Delrin gear that tend to warp it. I take those screws out and drill the gear and use bolts, nuts, flat washers and loctite to reassemble (the bolts have to be fairly short so they don’t hit the auto gear). And when I put the gear on the hub I heat it with my wife’s Super Blaster hair dryer to expand it so you don’t have to force it onto the hub. As the gear cools it shrinks onto the hub without warping. And then bolt it with thru-bolts instead of the self-tap screws they come with.

I would venture a 90% certainty that is where your vibration is coming from. I have not built a Trex yet that did not require some work to that main gear to get it to run smooth.

Hi Chris
I checked the gear, and found a wobble, but the radial runout is negative. The gear has helical teeth. I’ve removed the hub and found the self tappers have distorted the the seating face. I’ve ordered some button headed bolts to give a clearance between the two gears. and will bed the two faces of the hub and the gear together.

That will help. That self-tapping screw setup they have in there is not all that good. I’ve never seen one that doesn’t warp that gear.

Another thing that might help is noise damping. I think sometimes the noise from the gears causes problems with the IMU’s - the sound waves. So when I use a frame side mount like that I put this insulation over the Pixhawk. On my Synergy 626 I have the Pixhawk in the same location, Roll 270

New photo by Chris & Kristin Olson

That insulation that is over the Pixhawk is attached to it with double sided mounting tape. It the stuff that folks use in the windshield of their car when it is parked to keep the sun out of the car. The Pixhawk is mounted with two round 3M tissue tape mounting pads. Putting that insulation stuff on the Pixhawk reduced the vibes from ~17-19 down to around 7-8. I attibute it to noise damping with the foam in the insulation being in contact with the Pixhawk’s case damping the vibrations of the plastic case, which in turn gets transferred to the IMU’s.