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Can't seem to counter the torque from this rotor


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So I've never played around with Breaking Ground stuff before and thought I'd experiment a bit with helicopters. I asked on Reddit and discord but didn't really get any substantial answers.

 

I've got a rotor with helicopter blades and a freewheeling unmotorized servo beneath it, with reversed blades on that. Rotor torque on main throttle, blade angle on up/down axis. The thing lifts off fine, but it consistently spins counterclockwise. SAS help a little but not completely.

 

The coaxial blades on the servo *should* serve to counteract the torque from the main rotor, and they do spin the correct way and provide lift, but for some reason they're not countering all of the rotation. I know this configuration should work as I've seen it done before. Image below.

 

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alas, i also have been playing with helicopter in the last few weeks, and it is apalling how few people will answer about those. and few of those answers will be good. as any other kind of question get answered quickly and in depth, i can only conclude that few people in this subforum know helicopters. so, i'll try to help you with my imperfect knowledge, because it is unlikely there will be somebody more expert who will. and i must premise i never used a rotation servo, nor do i know what they are supposed to do that's different from a rotor.

but, unless gthere is some artifact of clipping that's hiding it, putting one rotor on top of another does not work. as the upper rotor is attached on top of the lower one, it will start rotating with the lower one. while it will rotate its own blades in a different direction. it may be possible that there is some way to make it work by carefully calibrating torques and rotational speeds, but i would not try.

the way to put two rotors on top of each other, instead, is to attach both rotors to something else underneat, and then use clipping to make them look good. You generally want a structural octagon or cube; you place the first rotor on it, then you use clipping to move it upwards, so now you can place the second rotor (which must be equal, but rotating in the opposite direction) also attached to the top of the structural piece. then you move the rotors up and down until they look natural. the structural piece gets hidden in the process.

this way, the two rotors are truly applying equal and opposite torques, and your helicopter won't spin

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16 minutes ago, king of nowhere said:

 i must premise i never used a rotation servo, nor do i know what they are supposed to do that's different from a rotor.

but, unless gthere is some artifact of clipping that's hiding it, putting one rotor on top of another does not work. as the upper rotor is attached on top of the lower one, it will start rotating with the lower one. while it will rotate its own blades in a different direction. it may be possible that there is some way to make it work by carefully calibrating torques and rotational speeds, but i would not try.

 

I'm not using a second rotor or actively producing an opposing torque. Servos are essentially low RPM rotators used for more precise control when motorized, but when unmotorized, as I'm using them, they're able to simply spin independently of the parts beneath them.

The rotation of the rotor is providing an opposite rotation to the parts beneath it. Normally this will be passed to the body of the craft, but the servo is able to spin independently of the body and so rotates opposite to the rotor; this should result in zero net torque on the body of the helicopter, without needing to actively power a second rotor.

 

It does do this, but apparently not enough. As far as my (limited) understanding of physics goes this should be working and an actual powered countertorque shouldn't be necessary, but for some reason this isn't working the way I think it should be.

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3 minutes ago, Panzerbeard said:

It does do this, but apparently not enough. As far as my (limited) understanding of physics goes this should be working and an actual powered countertorque shouldn't be necessary

huh. according to my (pretty good) understanding of physics, this would never work in reality, because attrition would always cause some torque to be passed, but i can't speak for the game physics. maybe it would be possible, maybe not; someone more experienced should confirm it.

you could try setting "damping" on the servo to 0. maybe it is trying to counteract the torque for that reason.

in case it cannot be done with a servo, you can use two rotors and reduce the maximum power to 50%. this will mostly eliminate the extra weight.

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15 minutes ago, king of nowhere said:

huh. according to my (pretty good) understanding of physics, this would never work in reality, because attrition would always cause some torque to be passed, but i can't speak for the game physics. maybe it would be possible, maybe not; someone more experienced should confirm it.

you could try setting "damping" on the servo to 0. maybe it is trying to counteract the torque for that reason.

in case it cannot be done with a servo, you can use two rotors and reduce the maximum power to 50%. this will mostly eliminate the extra weight.

I'll try it with a twin rotor design tonight after work. An unpowered servo does indeed work on KSP though, as the main tutorial on helicopter rotors I can find demonstrates:

I've tried reducing damping but I'll rebuild the assembly from scratch; possibly something is just attached weirdly. Cheers.

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2 hours ago, king of nowhere said:

but, unless gthere is some artifact of clipping that's hiding it, putting one rotor on top of another does not work. as the upper rotor is attached on top of the lower one, it will start rotating with the lower one. while it will rotate its own blades in a different direction. it may be possible that there is some way to make it work by carefully calibrating torques and rotational speeds, but i would not try.

Clipping is how most people make their coaxial helicopters. I showed you on Reddit how I attached both rotors to a cubic strut and clipped them together. The values for each rotor were set the same so that their torque is canceled out by the other.  In this video I only have one set of blades, but the torque is canceled out by another motor aligned coaxially and spinning parts of equal mass to the blades.

 

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If it helps, Panzerbeard, I can give you a plausible reason why it spins, but of course your desired correction is up to how you wish to handle it.

In the case of two identical coaxial motors/rotor hubs ( A & B ) spinning in opposite directions, A - B = 0 and there is no rotational force imparted on the other parts of the vessel we'll call "C".

In the case of a single motor turning a rotor blade, the whole unit that is attached to them will 'absorb' the reactionary torque force - just because your rotor hubs are identical doesn't mean that they have the ability to isolate the torque within them - EVERYTHING attached to torque-producing moment will tend to react to counter, i.e. A - ( B + C ) = 0

The only way to keep the base of the vessel from spinning in your configuration would be to ensure its mass was proportionately larger than the mass of either rotor hub - it would still spin, but at a reduced rate that might be controllable with, say, a reaction wheel.

Edited by Wobbly Av8r
Clarification...
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