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 helis too!

Below is shown one of the better examples currently available, the Blade CX 2 from E-flite, a well respected manufacturer of quality electric rc helicopters:

Coxial rc helicopters like the Blade CX2 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. Flying them indoors is a realistic option too, because their stability makes them easy to control within confined spaces.

How coaxial RC helicopters work

A conventional helicopter has a single main rotor, which when turns generates a natural force called torque. The natural reaction to this force is for the fuselage of the helicopter to spin in the opposite direction to the rotor blades.

So, a tail rotor is used to generate sideways thrust which prevents the helicopter from spinning wildly out of control. The amount of thrust generated by the tail rotor can be changed by the pilot, either by changing 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 in, 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 the 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.
However, this is only the case so long as both sets of blades are spinning at exactly the same speed. As soon as one set changes speed 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 to purposely generate the torque that causes the helicopter to change direction.

In most coaxial helis, the top blades are mounted on the main shaft and the lower blades are mounted on a larger diameter shaft that immediately surrounds 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 vertically mounted sprockets (view is from under the helicopter).

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 on calmer days.

Full size coaxial helicopters

You could be forgiven for thinking that rc helicopters with dual rotors are completely made-up designs, 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:

Red Rocket Hobby Shop have some popular coaxial rc helicopters.

Related pages

Electric rc helicopters - general info on electric helicopters.

Gas rc helicopters - if you prefer more noise, smell and a steeper learning curve!

Mini rc helicopters - see some of the smaller production models available.

Micro rc helicopters - when mini helicopters get a lot smaller.

Toy rc helicopters - low cost, unbreakable fun for all ages.

RTF rc helicopters - what to expect with a Ready To Fly helcopter.

How helicopters fly - read how these machines stay in the air.

RC helicopter controls - a basic look at how a model helicopter is controlled.

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