مکانیک پرواز (هواپیمای مدل و سرنشین دار) - Coaxial RC helicopters
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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 is one of the most popular examples currently available, the Blade CX 2 from E-flite, a well respected name in quality electric rc helicopters:

The E-flite Blade CX 2 helicopter

Coaxial 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 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 the same way that when you wind something up, or twist something, it naturally wants to unwind.

To counteract this force, a tail rotor is used to generate sideways thrust to push against the direction of 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:

Controlling helicopter yaw by tail rotor

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:

Blades of a coaxial rc helicopter

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.

Drivegear of a coxial helicopterIn 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 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 also.

Full size coaxial helicopters

You could be forgiven for thinking that rc helicopters with two sets of rotors are completely fictitious 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:

A dual rotor Kamov helicopter

There's also a safety factor with co-axial 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.


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