A coaxial helicopter is one that has two sets of rotors one above the other, rotating on the same axis.
How they work
A hollow outer shaft is connected to the lower set of rotors, and a smaller inner shaft is connected to the upper rotors. They rotate in opposite directions but both produce downwards thrust.
yaw torque from one rotor is canceled by the other so no tail rotor is required.
For full scale helicopters, coaxial rotors have several advantages:
- No power is wasted on a tail rotor (this can be as much as 10-15%)
- Reduced problems with high speed forward flight, due to lower tip speeds and the cancelling out of rotor blade airspeed differential.
- Decreased rotor diameter gives improved manoueverability and reduced storage space.
- Removing the tail rotor removes one of the key vulnerabilities of combat helicopters.
However, they are considerably more complex due to the need for a second swash plate between the two sets of rotors to provide control of upper rotor pitch (cyclic and collective) and the need for a differential collective pitch mechanism to provide yaw control. (Differential head speed would be unable to provide rapid reaction on full scale machines.)
In model helicopters, the complexity described above is usually avoided by using Fixed Pitch rotors and only having one swash plate, with only the lower rotor disc having cyclic control. The upper rotor is usually controlled only by a flybar to provide stabilisation (often at 45 degrees). Yaw control is provided by differential rotor speed.
The flying characteristics of this kind of model helicopter are very different from dual-swash CCPM coaxial helicopters. The upper rotor always resists changes in the angle of the rotor disc - effectively the upper rotor keeps the rotor disc level at all times. This limits forward flight to approximately walking pace, and makes models unable to counteract wind by tilting into the wind. Therefore such models get blown around by even very light winds and are most suitable for indoor flying. Due to the self-stabilising nature and limited speed, these helicopters are often regarded as beginners' machines. This type of helicopter would best suit someone who wants to dabble in helicopters without making a big commitment of money or time. They are great for flying around the house where the limited top speed isn't an issue. They are also a great for practicing mastering the basic controls in different orientations.
Global Hobby Distributors of Fountain Valley, California USA presently markets the Extreme-Flyers X350, an example of which is shown at right. This particular model overcomes the hyper-stable nature of coaxials by utilizing a horizontal tail rotor. This rotor, switchable at the transmitter, engages with forward cyclic. This gives the X350 a sharper angle of attack, allowing for outdoor fast forward flight. Switching off the tail rotor allows the X350 to hover and fly indoors like standard coaxials.
A few CCPM dual swash coaxial model helicopters have been made, and have the benefits of full-scale coaxial helicopters mentioned above without the drawbacks of fixed pitch models mentioned above. The mechanics tend to be very complex - for example see this set of coaxial mechanics modelled on the full scale Kamov Ka50. These models tend to be extremely expensive.