autorotation with contrarotating blades
 

Any Rooskie aviators here? Investigating contra rotating passive blades and wondered how the Helix/Hornet cabs behaved in autorotation.

The main rotor behaves nicely in autorotation, no magic there.
At issue is the lack of a tail rotor. The coax uses differential collective pitch to adjust the torque balance and create yaw control. Want to yaw in one direction? Just increase the collective to one rotor, and decrease it on the other. This upsets the torque balance, and makes the aircraft yaw, while holding the total lift constant. Works great in powered flight, but in auto, there is no torque driving the main rotor, so little effect from differential collective.
That is why coax helos have big rudders, in auto, the rudders do the yaw control work.

c/r blade and auto
 

Many thanks for that, Google had me confused with rumours of unstable autos and much theory but no actual accounts. does this mean autorotative descent would be at the mercy of every wicked gust and shear?

Rotor tip brakes
 

CoaX Helicopters use rotor tip brakes for yaw control, providing differential torque between the rotors.

Coax I understand bought the design rights to the original Gyrodyne Rotorcycle, developed in the 1950's

Hot and Hi, thanks for that link. An interesting method of directional control.

The web site linked has no response to "rotor tip brakes" when queried, but here are some useful links to systems in that family:

Patent US3015363 - Helicopter-reaction tip brake - Google Patents
Patent US20120282094 - Wind Turbine Blade Tip Brake Apparatus and Method - Google Patents
Gyrodyne RON Rotorcycle - Wikipedia, the free encyclopedia

This link has pictures of the brakes

XRON History

It reminds me a little bit of the Kaman servo flap system, in terms of the linkage going through the blade to create movement of an airfoil (or something) while the rotors are turning.

The patent number 2,835,331 explains and has some figures that help understand the concept.

Patent US2835331 - Directional control system for rotary wing aircraft - Google Patents

From a customer point of view, this looks like a nice feature in terms of lowering noise profile, with the lack of noise from any tail rotor / fenestron.

Flapper Valves
 

Lonewolf,

Peter Batten and Richard Woodward of Coax Helicopters have spoken about this method of yaw control on several occasions. For example in this interview on Mick Cullen's Rotary Wing Show.

It is mentioned in the written summary available at the hyperlink above:

"Yaw control in coaxial rotor designs can be problematic but is overcome in Coax Helicopter’s designs by drag flaps on the end of the rotor blades that provide a torque differential between the blades."

Or go to 26' 20" in the podcast available under this link and hear Richard talk about it.

ca-ax
 

Most interesting replies, thanks Lonewolf for very good links. If I could ask one more, in many of the pictures of 2 bladed co axial cabs, it appears the blade sets are not indexed, ie both discs are free wheeling, If yaw control drags one disc there has to be a de coupling of the engine torque, where does this happen in the transmission? Feel free to ignore if this question is facile, single malts do stimulate the porridge.

Triskele,
I think the only coaxes that are uncoupled are the fixed pitch RC toy ones. The gyrodyne had a little tip brake which stuck out on the airstream, which causes aerodynamic drag out at the end of a long lever arm. This requires more torque delivered from the engine and through the transmission. This difference in torque delivered to the rotors yields a net torque at the shaft, and you have yaw moment. Conventional coax helos achieve differential torque by using differential collective. This gives more pitch, more drag, and more thrust from one rotor and less of those three from the other. Same overall thrust but there is an imbalance in torque. The only place this breaks down is in auto. Ray Prouty discussed this phenomenon in a very good article once.
Bryan

co-ax
 

Thanks IFMU, I thought they may be uncoupled by image searching, it may be a photo effect but some of the twin blade sets appear as though each has its own speed. This may be because most of the pics were early designs. I would have thought the vibration and ground resonance would shake your teeth if they weren't indexed.