Coaxial helis

[markamas]

I sorta got into "flying" robots recently and I have found answers concerning most of my questions, but there is a particular thing I don't understand about helicopters; how do the coaxial system work (I mean, how can I make one from scratch). I understand that each rotor cancels the other's torque, but how does all this mechanicaly work? Are the rotors dependant on each other for speed (I suppose that's not true because the heli turns probably by increasing torque generated by one rotor and decreasing torque on the other). The coaxial rotor design is my favorite heli design because it's compact and more reliable than having a tail rotor.

PS- What is the third rotor (stabilising rotor???) for? How does this one work?

[rogue]

"how can I make one from

"how can I make one from scratch"

I would personally buy the parts from somewhere like xheli.com and build one to get an idea of how to design one, assuming your goal is to design and fabricate all the parts yourself.

[GroG]

In some two rotor systems

In some two rotor systems the speeds of the 2 rotors can vary, you can control spin this way - speeding up on set of blades vs the other.  In coaxial rotor systems where the rotors are mechanically tied to one another you can add a 3rd rotor for spin to change heading.  I saw a clever coaxial rotor toy once and it had a design like this.

The motor drove a wheel which turned both an inner and outer shaft - in this configuration the rotors are mechanically tied to one another, so a third rotor is necessary to control spin.  You might want to look into the "Hiller Flying Platform".  I always wanted to build one of these (smaller) for a bot.  Although I don't believe the hiller platform uses motors facing in opposite directions, I do believe this might be the easiest to make one.  Basically , just mount two motors back to back in a ducted fan, with one fan mounted backwards.  No gears, no exotic shafts, simple and the duct protects you from lobbing off one or two of your fingers.

[GroG]

Hey rogue, just noticed how

Hey rogue, just noticed how much our avatars look alike

[sebathorus]

Coaxial rotors

Hi 

I will try to answer your questions from what I understand.

There are 2 major cases of coaxial counter-rotating rotors:

 - the ones unsed in full scale helicopters

 - the ones used in toy helis.

The full scale ones, use one or many (usual two) engines coupled together to provide power to rotors.

Rotors are turning in oposite directions to cancel angular momentum present in single rotor configurations.

Unlike toy helis, real ones have blades articulated to rotor hub and conected to a device called swashplate. With the help of swashplate, angle of blades (or pitch) can be changed, for all at once - collective pitch or blades can change their pitch during a complete revolution around rotor hub - cyclic pitch.

A good starting point is here:

http://en.wikipedia.org/wiki/Helicopter_rotor

and here:

http://en.wikipedia.org/wiki/Coaxial_rotor

As you said before, to make a turn to left or right, pilot have to create a difference between torque of two rotos, and that is achieved by modify collective pitch of one or both rotors.

Due to complexity of swashplate, toy helis use simplified mechanisms. At toy heli I saw 2 or 3 cases, I’m sure are more:

  - each rotor is powered independently by its own motor, turn is acheived by slowing one motor and accelerating the other. Forward movement is obtained by make front of heli heavier to force it lean forward (flight at fixed point is not possible) or using a third motor mounted as a pusher or to lift the tail (in last case main rotors will push the air either downwards and backwards ).

  - both rotors are powered by same powerful motor and left-right turn is made by a third smaller motor with a vertical propeller.

Because of small parts, a full functional coaxial rotor is either hard, expensive or unreliable to make.

From practice, for small RC models, a multirotor configuration tri or quadcopter is much cheap and reliable and without limitation of simplified coax rotors. Mechanical complexity is very low (propeller attached directly to motor), no need for high precision shafts, ball bearings, servos, linkages, etc all mixing between motors(rotors) is done with cheap, widespread electronics.

Hope it helps.