Coaxial RC helicopters

Electric coaxial rc helicopters, often referred to as dual-rotor or contra-rotating helicopters, have been responsible for bringing thousands of new people in to 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 popular coaxials currently available, the Blade CX3, left, and its micro-size cousin the Blade mCX, right, both from E-flite, a well respected name in electric rc helicopters:

The E-flite Blade CX3 and mCX2 coaxial rc helicopters

See the Blade coaxial rc helicopter range.
(use code RCAPW for $5 off any order )

Coaxial rc helicopters like those from E-flite's Blade CX range can be purchased in Ready To Fly (RTF) form and can be flown with confidence pretty much straight from the box. Such helis are much easier to master than a conventional helicopter that has a single main rotor and tail rotor and when trimmed correctly are very capable of holding a steady hover even with thumbs off the transmitter sticks!

The E-flite range of coaxials has proved to be a very popular one; the helicopters are backed up by a full range of spare parts and upgrade options are available for some of the models. These 'hobby grade' helicopters are a great choice for anyone looking for a gentle and easy introduction to rc heli flying.

How coaxial RC helicopters work

A conventional helicopter has a single main rotor consisting of two or more separate blades. When the motor powers up and turns the main blades a force called torque is naturally generated. This torque makes the helicopter fuselage spin in the opposite direction to the spinning blades, in a similar way to when you twist something up (for example a length of cord) it naturally wants to untwist itself.

To counteract this torque a tail rotor is used to generate sideways thrust. The tail rotor is a vertically mounted rotor (essentially a small airplane propeller) which pushes air against the direction of fuselage rotation, thus preventing 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 by changing the speed of the tail rotor motor; rc helicopter tail rotors are either variable pitch with servo control or fixed pitch with motor control, the latter option found on the cheaper helicopters.
Whichever method is used, the 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:

Controlling helicopter yaw by tail rotor

Coaxial rc helicopters, however, don't have a tail rotor and instead of a single main rotor they have two sets of main rotors, one mounted directly above the other. The two 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 is immediately generated. This is exactly how yaw is controlled in coaxial rc helicopters, by making one set of blades spin faster or slower than the other set to purposely generate torque which will cause the helicopter to change direction.

Drivegear of a coaxial helicopter 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 pinion driving one of the main drive gear wheels. This photo is of my little 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.

It should be mentioned at this point that flying a coaxial heli isn't as rewarding as a conventional single rotor helicopter. A typical radio control coaxial helicopter cannot be banked in a turn and the forward flying speed is relatively low. So flat, steady flight and easy hovering is what you can expect from the majority of coaxials - forget any aerobatics!

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:

Dual main rotors give huge amounts of lifting power, and there's a notable safety factor with coaxial helicopters; no tail rotor means less danger when the helicopter is on or close to 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 the range of E-flite Blade coaxial rc helicopters here.
(use code RCAPW for $5 off any order )

Beginner rc helicopters - see some popular examples.