Weird Roll and Pitch Coupling

[BMPnumber1]

I just brought my K4 out of storage and was double checking blade tracking. I noticed that there is a significant left (port) roll with forward pitch and right (starboard) roll with nose up. Even cranking the bias all the way doesn't cure this. I'm wondering if something's up with the swash plate or if it's from ground effects. Are there any links on how I can troubleshoot this?

Could improper pitch of the fly-bar blades cause this issue? What about blades? I've had a few crashes (as have most new pilots), but does ensuring blade tracking require a blade pitch guide/tool?

(PS I haven't brought it out sooner because I was working on graduating with a BS in Aerospace and Ocean Engineering. Anyone want to hire a neophyte engineer?)

[dmcn2010@hotmail.com]

Somewhere your problem lies,

DISCONNECT THE MOTER BLUE AND RED WIRES
Set all transmitter trims to center
Hover knob to mid travel
Select trainer to idle up 3d mode
Gyro off
Throttle to mid stick
Turn on transmitter

Connect heli battery.
All servos should center to 90deg.
Swash plate should be level both 90deg and parallel to the tail shaft.
Cross link wash-out arms above the swash plate should 90deg to the main shaft and both should be parallel to each other.
With a pitch gauge on the main blades, main blades should be 0deg and the fly bar should be parallel with the top of the pitch gauge.
Paddles should parallel with the main blades, 0deg from each other.


To obtain this,,,,,, transmitter set as above.
Remove main blades.
Remove both long links from the swash plate to the main blade rocker arms.
Set servo arms to as close to 90deg.
Adjust allservo three links to obtain level swash plate.
Adjust all servo three link lengths equally to obtain 90deg wash-out cross links .
I use a main blade cross shaft through the blade grip holes as a aligning tool to zero the main blade grips. Tool should be parallel with the main shaft. Now with the blade grips at 0deg, adjust the blade short links with flybar level so that the rocker arm is 90deg to the main shaft and parallel to the flybar. Do both blades. Make the make sure short links are now the same length. The blade links should not require adjusting again.
Install main blades and place pitch gauge on the blade. Re-Install long links from the swash plate to the blade rocker arm adjusting link so that the top of the pitch gauge is parallel with the fly bar, zero blade pitch.
Next paddles, the fly bar threaded ends and the paddles should be measured equally out from the rotor head. Viewing from paddle ends they should set 0 pitch parallel to the main blades

That’s it.

Recheck all
Set idle up switch to training
Throttle to off
Gyro to on
Connect motor wires
Set bade tracking by adjusting long link never the short blade links. Best up grade was the adjustable eight dollar long link, acts like a real aircraft turnbuckle, not having to pop off a ball link each time.

Hope this helps and not confusing. good luck

[BMPnumber1]

I've done a little more homework and it looks like I have to balance/level the swash plate and ensure that the AOA of the flybar blades are set correctly. Strange thing is it still hasn't taken out all the roll/pitch coupling.

Edit: Talk about great minds dmcn. Thanks for the write up. It sounds exactly like what I've read on the intertubes.

[dmcn2010@hotmail.com]

mine has a tendancy to roll left and backwards, i have to trim right and forward.

how bad is it?

[BMPnumber1]

It's nothing that the bias can't correct; but I've noticed if I correct it via bias then the other roll/pitch coupling is larger. (it's diagonal from nose down pitch/roll) For instance if I correct for a nose down pitch and port roll I get a larger nose up pitch-starboard roll so I definitely think it's with swash, mixing, and flybar.

[BMPnumber1]

So I ran through and did as you wrote; it cured a lot of it but it still rolls with change in pitch. Nose down-port roll and Nose up-starboard roll coupling still works. I'm thinking it might be flybar paddles causing most of the issues.

[dmcn2010@hotmail.com]

when you pitch forward, the forward elevator servo arm goes down and the two back servos arms go up equally

[BMPnumber1]

Looks that way--but my eyeballs might be off.

[dmcn2010@hotmail.com]

Are you using the esky transmitter?

take a 8-10" rod like a piece of coat hanger, place it on the swash plate 90 and parallel to the tail rotor, rod should be level as the swash plate rises and lowers?

something is not symetrical,

Tail and tail rotor is straight? no belt rub?

How tight are your blade grips, how much cross shaft pivot?

Are your blade grip bearings good? the races can get ball bearing indentions.

[BMPnumber1]

I am using the esky transmitter.

I was thinking of doing that just with a scale/ruler.

Tail and tail rotor look straight. I'm not 100% that the belt isn't rubbing; but I don't hear anything particular from it.

Blade grips are properly set. I haven't noticed any cross shaft pivot play. The blades are easily rotated; bearings seem good as well.

[dmcn2010@hotmail.com]

Running out of thoughts,,,, since this a minor event, you may have to take actual measurements rather than by eye,

Blade travel, blades 90deg to the tail. With a scale ruler measuring from the table up to the blade, Starting at 0deg fly bar level measure the travel up and down travel full stick movement. Up and down should be the same, both blades. Pitch up and down and should not be any blade travel.

Is this a constant roll meaning if you climb and level off the tendency to roll stops? Constant roll would be more likely rigging (doesn’t change) and roll during altitude or direction change would be control.

[BMPnumber1]

Don't know. I'm still learning to fly the thing so I haven't checked to see if it's ground effects causing this. I've gotten airborne twice so far; both times prior to the last wreck when I noticed this issue. I do remember having to put in some forward/right collective to ensure it doesn't start walking sideways.

[likeakite]

I cant quite wrap my head around what your problem is from the description. Are you saying the heli drifts in one direction while hovering or after lifting off? Every time i set up my K-3 it has a tendacy to drift left and back a bit, with a level swash. Its not bad, and shortening the front servo to swash link (elevator) by one full turn is all it takes to get it to hold in place in calm conditions. You do need to get it off the ground a few feet to get out of ground effect too. Even a video of it spooling up on the ground would be a help to see what it does.

[BMPnumber1]

Well, I took it out today and attempted to fly it. The first go around didn't work so well. It was touchy and the tail was a blur of incomprehensible particles. Needless to say that the helicopter tipping while trying to control the beast caused the blades to explode.

I pulled off the busted blades and put on some new ones (that I had balanced a few days before). Checked blade tracking; about 1 inch of gap between tips. Cranked the lower blade up to get proper tracking with the upper blade. Suddenly helicopter was much easier to control and operate.

I think my gyro is incorrectly set as when I fly with it on it just wants to yaw left. Easy enough fix.

The rest of the flying is spot on! Thanks for everyone's help.

The result of all this appears to indicate it was a compound issue between improperly set swash plate and ground effects (with probably incorrectly tracking helicopter blades).

[dmcn2010@hotmail.com]

Glad!! that was a lot of typing,

Mine wants to scoot left on takeoff, I’m used to it. HBK3 is also tail heavy. I should receive my xtreme carbon frame tomorrow so I’ll be building from scratch.

Reading about the fly bar less head really has my interest.

Also I did a mod to replace the tail rotor drive belt gear a few months ago i'll be posting. It has been working great. With a new belt and tail rotor shaft gear, the belt would ride up and jump the gear like a streched chain. You could hear it with sharp tail movements. No matter how tight the belt was. So I cut off the plastic gear and replaced it with moded brass ten tooth motor drive gear.

[likeakite]

If you have not changed either of these thru the tip-overs and blade strikes i can guarantee that they are both bent. Those two things HAVE to be straight from the get-go or it will magnify any other problems.

You mentioning that the tracking was off by 1" Holy Cow! That tells me the feathering shaft is bent. If you adjust just one link to compensate for 1" off-tracking, its going to throw the entire rotor off level. Its inportant to keep all your links the same length as its partner. That will keep the swashplate and the washout arms level.

[BMPnumber1]

The feathering shaft appears to be straight. I haven't double checked since the last crash; but most of these tipping crashes haven't damaged the feather shaft before (out of all my crashes only one has really bent the feathering shaft).

[dmcn2010@hotmail.com]

Bent or wormy bars from many straightenings can cause all kinds of issues.

How did or how do you check to see if they are straight?

[BMPnumber1]

I usually roll the bar on a flat surface to verify that it isn't bent if I suspect the feather shaft is the culprit.

(I don't attempt to straighten them; I just replace them.) If I need more accuracy I use feeler gauges to verify that the bar is indeed close to, if not, spec.

[dmcn2010@hotmail.com]

Rollering works good for long flybars. I have a good cordless drill I rotate the cross shaft in and even the slightest bend will show up as a wobble.