How do I make a stable helicopter?

[MAFman]

I've been trying to make a stable helicopter with a single main rotor and a single tail rotor, but the angular momentum of the rotors adds in a way that causes the thing to spin wildly around an axis that itself spins wildly. How do I fix this?

[Vanamonde]

Isn't that just realistic helicopter physics? I think the solution lies in piloting rather than design, though loading the thing with SAS parts would help a little. 

[MAFman]

1 minute ago, Vanamonde said:

Isn't that just realistic helicopter physics? I think the solution lies in piloting rather than design, though loading the thing with SAS parts would help a little. 

Spamming reaction wheels would be unrealistic...Is there a way to tweak the motors or control scheme that would make it not spin?

[Vanamonde]

I'm sure it's possible but it would be a pretty major undertaking, llikely involving complex use of the game's robotics parts. Why don't we slip your question over to the Gameplay sub where someone might be able to give you hints about how to build something like that. 

[18Watt]

You need to counteract the torque from the main rotor.  The easy way is to use dual counter-rotating rotors. However, sounds like you are insisting on building a single rotor.

I’d start with reducing the torque on the main rotor.  Most players tie torque to the throttle, then mash the throttle to full blast.  That much torque probably isn’t needed, so try using less throttle, only as much as you need.

Then you need a tail rotor, with the thrust pointed left or right to offset the torque of the main rotor.

Controlling both rotors is going to be extremely difficult.  On top of controlling torque and blade angle for two separate rotors, you also need to actually fly the thing too.  KSP is not ideally set up for rotorcraft, but it is possible.

In a real helicopter, the tail rotor speed is directly linked to the main rotor, so all you need to adjust is the blade angle.  

[MAFman]

I've successfully made a sort-of-stable helicopter using a single rotor and tail rotor! I just had to move the tail rotor as far away from the center of mass and as far up as possible, and enable yaw control on the tail rotor blades.