Coaxial RC helicopters
Electric coaxial rc helicopters, sometimes referred to as dual-rotor or contra-rotating helicopters, have been responsible for bringing thousands of new people into the hobby of radio control flying in the last few years, and it's easy to see why.
Coaxial rc helicopters are very easy to fly, and their inherent stability in the air makes them perfect first-time helicopters.
Of course, they're not limited to new pilots; very experienced rc helicopter pilots are having a great deal of fun with coaxial helicopters too!
Shown below are a couple of the most popular coaxials currently available, the Blade CX3, left, and its mega-successful predecessor the Blade CX 2, right, both from E-flite, a well respected name in beginner electric rc helicopters:
Coaxial rc helicopters like the Blade CX2 and CX3 come RTF, or 'Ready To Fly', and can be flown with confidence pretty much straight from the box. They are much much easier to master than a conventional helicopter that has a single main rotor and tail rotor, and are equally capable of holding a steady hover - steadier, in fact. Flying them indoors is a realistic option, because their stability makes them easy to control within confined spaces.
See the Blade CX2 and CX3 in more detail here.
How coaxial RC helicopters work
A conventional helicopter has a single main rotor consisting of two or more blades. When the rotor turns, a natural force called torque is generated. This torque makes the helicopter fuselage turn in the opposite direction to the spinning blades in a similar way to when you twist something up, it naturally wants to untwist itself.
To counteract this force, a tail rotor is used to generate sideways thrust to push against the fuselage rotation. This prevents the helicopter from spinning wildly out of control.
The amount of thrust generated by the tail rotor can be changed, either by altering the pitch angle of the tail rotor blades or, on smaller models, by changing the speed of the tail rotor motor. This change in thrust controls the yaw of the helicopter ie which direction the nose is pointing, by either giving in to the natural reaction against the torque (lessening tail rotor thrust) or by pushing the helicopter round in the same direction as the main blades (increasing the thrust).
The illustration below shows these basic forces at work:
However, coaxial rc helicopters don't have a tail rotor, and instead of a single rotor they have two main rotors, one mounted directly above the other. These 2 main rotors spin in opposite directions to each other, as the illustration below shows:
Because the blades are spinning against each other, each one cancels out any torque generated by the other one. As a result, there is no tendency for the fuselage of the helicopter to spin round one way or the other.
This is only the case, however, so long as both sets of blades are spinning at exactly the same speed. As soon as one set changes speed relative to the other one, then torque immediately appears.
This is exactly how yaw is controlled in coaxial rc helicopters, by making one set of blades spin faster or slower then the other set, to purposely generate torque which will cause the helicopter to change direction.
In most coaxial rc helicopters, the top blades are mounted on the main shaft and the lower blades are mounted on a larger diameter hollow shaft that runs up outside of the main one.
Twin side-by-side electric motors control one shaft each, and hence independent rotor speed control is possible. The picture to the right shows a typical coaxial setup for the main drive gear, with each motor cog driving one of the main sprockets. This photo is of the Blade mCX.
Coaxial rc helicopters are, without doubt, the easiest and safest way of getting into the hobby of flying radio control helicopters, and they're suitable for anyone, regardless of helicopter-flying experience. They can easily be flown indoors, but are equally suited to outdoor flying also.
Full size coaxial helicopters
You could be forgiven for thinking that rc helicopters with two sets of rotors are completely fictitious designs, and unique to the radio control world, but there are a number of such full size helicopters. The Russian helicopter manufacturer Kamov have produced several dual rotor helicopters for both civilian and military use, the KA32 shown below is one such example:
There's also a safety factor with coaxial helicopters; no tail rotor means less danger when the helicopter is on the ground. And in flight, a failing tail rotor will almost definitely cause the helicopter to come down out of control. No tail rotor eliminates this risk.
See some coaxial rc helicopters here, if you're browsing to buy.
Related pages
Electric rc helicopters - general overview of electric helicopters.
Gas rc helicopters - if you prefer more noise, smell and a steeper learning curve!
Micro rc helicopters - when mini helicopters get a lot smaller.
RTF rc helicopters - what to expect with a Ready To Fly helicopter.
RC flight simulators - the safest way to learn to fly any rc helicopter.
How helicopters fly - read how these machines stay in the air.
RC helicopter controls - a basic look at how a model helicopter is controlled.
See the E-flite Blade CX2, Blade CX3 MD 520N and Blade mCX.
