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mystery roll force with helicopter blade


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Do you have your blades deploying at the same level, and in opposite directions?

I'm not so good with the physics involved, but possibly it has to do with the fact that the two rotors are not at the same distance from the CoM, so you're getting different leverage effects?

[EDIT - thinking about it more I think the latter makes sense.   Tail rotors are mounted as far back as possible so that they can be as small as possible, and still counter the torque from the main rotor.  You may be inadvertently getting the same effect, but with equal size rotors the one further from COM overpowers the other one.  See generally https://en.wikipedia.org/wiki/Moment_of_inertia]

Edited by Aegolius13
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It looks like the helicopter blades are responding to the roll control.  By default they do move in response to pitch/yaw/roll, in the same way as the ailerons from the main game, except that their roll response is reversed (bug #23244).    When the blades are put on a rotor, the pitch/yaw/roll deflections that would work on a craft moving forward are not often effective in hover.  For your craft, it looks like the roll response is effective, but in the wrong direction.

You might want to turn off the roll response on the helicopter blades. Or,if your coal is to control the craft in hover using aero-forces only, with no reaction wheels,  try it without SAS and use reverse roll-control by hand.

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The mystery roll is likely being cause by asymmetry of lift. As the craft moves forward, on coming "air" hits the advancing blades faster than the retreating blades producing more lift on one side of the aircraft. This is an affect experienced by real helicopters, and it is usually compensated for by flapping and feathering the rotor blades using a automatic fly by wire system. 

For some reason it seams like the KSP devs haven't accounted for this in the design of the rotor system (perhaps in an update), but they have set them up correctly for pitch, roll and yaw. In your case the asymmetry of each rotor ought to be mirrored, but this would likely result in a pitching, rolling then yawing motion that results in a crash. I might be wrong in this case but I have experienced this myself with my own craft (more investigation is needed), however, you ought to have relative control in a hover, and rapidly lose control as airspeed increases. It should also happen even when not making any control inputs, so long as the craft is moving.

edit:

It is asymmetry acting on the each rotor but it's not something Squad overlooked. @Aegolius13 nailed it, with this edit:

"[EDIT - thinking about it more I think the latter makes sense.   Tail rotors are mounted as far back as possible so that they can be as small as possible, and still counter the torque from the main rotor.  You may be inadvertently getting the same effect, but with equal size rotors the one further from COM overpowers the other one.  See generally https://en.wikipedia.org/wiki/Moment_of_inertia]"

Both rotor do experience dissymmetry of lift, which ideally  should be cancelled out by them spinning in opposite directions but if either rotor is slightly further from centre of mass. Then that rotor has slightly more leverage than the other and asymmetry of that rotor wins out and results in a net roll-force. So, this isn't just an effect of the horizontal distance from centre of mass but all vertical height over centre of mass.

If you need to keep the rotors where they are. You'd have to tune the torque, such that the rotor causing the roll is proportionally weaker than the other. That way, the rolling moment generated by the asymmetry causes more drag; which slows the rotor rpm proportionally to the torque setting.

Or you could add feathering/flapping hinges. 

I've recently written a comprehensive guide for single rotors. If anyone is still working on dual rotors like this, you can likely glean a lot from it, that would also apply here.

here's a link: 

 

Edited by Rpatto92
(I got a few things wrong in this original post) 
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I have a similar design that I got to work super well- but it involves the KAL-1000...  Assign your rotor torques and blade authority to the KAL, change the duration from "5" to "100" (to simulate 0%-100%).  Then, assign that KAL to your main throttle.  Now- here's the tricky part: one blade set's authority should always be set to negative, and the other positive.  (You'll have to experiment to find which is which... sorry!)  Also, With a rear rotor higher than the front one (how I built mine), the front blades are usually 7-10% lower than the back ones.  

This video helped me a lot- but it's long.  If you want to skip to the part about how to set up the KAL, skip to about 28 min.
 

 

I should also add: the only time I've had an issue with rolling (not pitch) is from an unbalanced craft.  Even just the .3 tons of a large science arm were enough to force my craft to veer to the side if left alone.  The only way I could think to effectively fix it was to attach a radial ore tank to the opposite side, then fiddle with the ore-load to get the trim correct.

Good luck!

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