INTRODUCTION TO SMALL R/C HELICOPTERS

Electric coaxial R/C 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 mCX2, right, both from E-flite, a well respected name in electric rc helicopters:


Coaxial R/C 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 R/C 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:


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.

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 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.

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.

How helicopters fly and are controlled

Helicopters truly are amazing aircraft, and how helicopters fly is what makes them such versatile machines, being perfectly suited to roles ranging from military use to fire fighting and search and rescue.

Helicopters have been around for centuries - well, the principle anyway - but it was Russian aircraft pioneer Igor Sikorsky who designed, built and in 1939 flew the first fully controllable single rotor / tail rotor helicopter - the fundamental concept that would shape all future helicopters.

Why helicopters are so versatile

A normal airplane can fly forward, up, down, left and right. A helicopter can do all this plus has the ability to fly backwards, rotate 360 degrees on the spot and hover ie stay airborne with no directional movement at all.

Helicopters may be limited in their speed, but the incredible maneuverability mentioned above is what makes them so useful in so many situations.


Above, the directions a helicopter can move in and the associated name of control

Controlling a helicopter

Helicopters require a completely different method of control than airplanes and are much harder to master. Flying a helicopter requires constant concentration by the pilot, and a near-continuous flow of control corrections.

A conventional helicopter has its main rotor above the fuselage which consists of 2 or more rotor blades extending out from a central rotor head, or hub, assembly.

The primary component is the swash plate, located at the base of the rotor head. This swash plate consists of one non-revolving disc and one revolving disc mounted directly on top. The swash plate is connected to the cockpit control sticks and can be made to tilt in any direction, according to the cyclic stick movement made by the pilot, or moved up and down according to the collective lever movement.

But first, to explain how the main rotor blades are moved by the pilot to control the movement of the helicopter, we need to understand pitch...

The basics of pitch

Each rotor blade has an airfoil profile similar to that of an airplane wing, and as the blades rotate through the air they generate lift in exactly the same way as an airplane wing does [read about that here]. The amount of lift generated is determined by the pitch angle (and speed) of each rotor blade as it moves through the air. Pitch angle is known as the Angle of Attack when the rotors are in motion, as shown below:


This pitch angle of the blades is controlled in two ways - collective and cyclic....

Collective control

The collective control is made by moving a lever that rises up from the cockpit floor to the left of the pilot's seat, which in turn raises or lowers the swash plate on the main rotor shaft, without tilting it.

This lever only moves up and down and corresponds directly to the desired movement of the helicopter; lifting the lever will result in the helicopter rising while lowering it will cause the helicopter to sink. At the end of the collective lever is the throttle control, explained further down the page.

As the swash plate rises or falls, so it changes the pitch of all rotor blades at the same time and to the same degree. Because all blades are changing pitch together, or 'collectively', the change in lift remains constant throughout every full rotation of the blades. Therefore, there is no tendency for the helicopter to move in any direction other than straight up or down.

The illustrations below show the effect of raising the collective control on the swash plate and rotor blades. The connecting rods run from the swash plate to the leading edge of the rotor blades; as the plate rises or falls, so all blades are tilted exactly the same way and amount. Of course, real rotor head systems are far more complicated than this picture shows, but the basics are the same.


Cyclic control

The cyclic control is made by moving the control stick that rises up from the cockpit floor between the pilot's knees, and can be moved in all directions other than up and down.

Like the collective control, these cyclic stick movements correspond to the directional movement of the helicopter; moving the cyclic stick forward makes the helicopter fly forwards while bringing the stick back slows the helicopter and even makes it fly backwards. Moving the stick to the left or right makes the helicopter roll and turn in these directions.

The cyclic control works by tilting the swash plate and increasing the pitch angle of a rotor blade at a given point in the rotation, while decreasing the angle when the blade has spun through 180 degrees.

As the pitch angle changes, so the lift generated by each blade changes and as a result the helicopter becomes 'unbalanced' and so tips towards whichever side is experiencing the lesser amount of lift.

The illustrations below show the effect of cyclic control on the swash plate and rotor blades. As the swash plate is tilted, the opposing rods move in opposite directions. The position of the rods - and hence the pitch of the individual blades - is different at any given point of rotation, thus generating different amounts of lift around the rotor disc.


To understand cyclic control another way is to picture the rotor disc, which is the imaginary circle above the helicopter created by the spinning blades, and to imagine a plate sat flat on top of the cyclic stick. As the stick is leaned over in any direction, so the angle of the plate changes very slightly. This change of angle corresponds directly to what is happening to the rotor disc at the same time ie the side of the plate that is higher represents the side of the rotor disc generating more lift.


Above, the layout of helicopter controls in relation to the pilot's seat

Rotational (yaw) control

At the very rear of the helicopter's tail boom is the tail rotor - a vertically mounted blade very similar to a conventional airplane propeller. This tail rotor is used to control the yaw, or rotation, of the helicopter (ie which way the nose is pointing) and to explain this we first need to understand torque.

Torque is a natural force that causes rotational movement, and in a helicopter it is caused by the spinning main rotor blades; when the blades are spinning then the natural reaction to that is for the fuselage of the helicopter to start spinning in the opposite direction to the rotors. If this torque isn't controlled, the helicopter would just spin round hopelessly!

So to beat the reaction of the torque, the tail rotor is used and is connected by rods and gears to the main rotor so that it turns whenever the main rotor is spinning.

As the tail rotor spins it generates thrust in exactly the same way as an airplane propeller does. This sideways thrust prevents the helicopter fuselage from trying to spin against the main rotor, and the pitch angle of the tail rotor blades can be changed by the pilot to control the amount of thrust produced.


Increasing the pitch angle of the tail rotor blades will increase the thrust, which in turn will push the helicopter round in the same direction as the main rotor blades. Decreasing the pitch angle decreases the amount of thrust and so the natural torque takes over, letting the helicopter rotate in the opposite direction to the main rotors.

The pilot controls the pitch angle of the tail rotor blades by two pedals at his feet, in exactly the same way as the rudder movement is controlled in an airplane.

NOTAR is an alternative method of yaw control on some helicopters - instead of a tail rotor to generate thrust, compressed air is blown out of the tail boom through moveable slots. These slots are controlled by the pilot's pedals in the same way as a tail rotor is. To generate more thrust, the slots are opened to let out more air, and vice versa.

NOTAR helicopters respond to yaw control in exactly the same way as tail rotor models and have a big safety advantage - tail rotors can be very hazardous while operating on or close to the ground and in flight a failing tail rotor will almost always result in a crash.

Throttle control

The throttle control is a 'twist-grip' on the end of the collective lever and is linked directly to the movement of the lever so that engine RPM is always correct at any given collective setting. Because the cyclic and collective pitch control determines the movement of the helicopter, the engine RPM does not need to be adjusted like an airplane engine does. So during normal flying, constant engine speed (RPM) is maintained and the pilot only needs to 'fine tune' the throttle settings when necessary.

There is, however, a direct correlation between engine power and yaw control in a helicopter - faster spinning main rotor blades generate more torque, so greater pitch is needed in the tail rotor blades to generate more thrust.

It's worth noting that each separate control of a helicopter is easy to understand and operate; the difficulty comes in using all controls together, where the co-ordination has to be perfect! Moving one control drastically effects the other controls, and so they too have to be moved to compensate.

This continuous correction of all controls together is what makes flying a helicopter so intense. Indeed, as a helicopter pilot once said... "You don't fly a helicopter, you just stop it from crashing"!

Helicopter Reading

Three Channel Coaxial RC Helis

In some coaxial helicopters, a tail rotor is still added. The tail rotor is oriented in the same direction as the main rotors. The forward and reverse rate of the helicopter can be changed, because this system allows pitch control. You can fly forwards, backwards and also hover with this kind of helicopter.

Four Channel Coaxial RC Helis

Even though the coaxial design is most frequently used in small, fixed pitch RC helicopters, it can also be used for models with cyclic collective pitch. The only difference between these models and the conventional designs is their lack of a tail rotor. As with the models above, yaw is controlled by changing the rate at which each rotor spins, but the rotors can change their pitch. This allows the helicopter all four degrees of freedom: pitch, bank, throttle, and yaw. The Esky Lama V4 is good example of the four channel coaxial design.


How The Coaxial Design Benefits RC Helicopters

• Yaw can be controlled by changing the speed of the main rotors, so the pitch can be controlled with the tail rotor.
• The main rotors spin in opposite directions, which gives the helicopter stability.
• Coaxial RC helicopters are a good way to learn how to fly, because they are very stable.

SUITABLE VIDEOS & LINKS

http://www.toykin.com/3-Channel-Mini-RC-Helicopter-H707-H707.html VIDEO

www.electric-rc-helicopter.com/

http://www.xheli.com/s031-heli-yellow.html VIDEO

http://www.xheli.com/wa5me4xcorar.html VIDEO

http://www.youtube.com/watch?v=HUVXiZNfbEY&feature=related BR6008A HELI VIDEO

USE OF GYROS IN R/C MODEL HELICOPTERS

The gyro is an essential component in many rc helicopters. Gyros in rc helicopters are most commonly used to control unwanted movement on the yaw axis. When an rc helicopter rotates on its yaw axis, the direction the nose points (the heading) changes. The yaw gyro’s job is to sense any undesired rotation around the yaw axis (clockwise or counterclockwise rotation when viewed from above), and to automatically correct the orientation of the rc helicopter. Without a yaw gyro, even if the rc helicopter was trimmed out to fly straight initially, it would eventually begin to drift and rotate right or left. Normal maneuvering of the rc helicopter and external forces can result in undesired yaw rotation. When this occurs, the gyroscope senses this change in yaw and corrects it by controlling the thrust generated by the tail rotor to compensate for the rotation. This results in stable flight for the rc helicopter and no undesired changes in yaw.

Old rc helicopter gyros operated by using the inertia of a spinning weighted wheel. The wheel would resist changes in orientation, due to it’s angular momentum. A sensor would monitor the orientation of the spinning wheel and use it as a reference to compare to the rest of the rc helicopter. However, these gyros were heavy and consumed energy to keep the wheel spinning. As technology improved new rc helicopter gyros were developed. The solid state gyro has no moving parts, consumes less electricity than its mechanical counterpart, and is more crash resistant.

There are two important types of rc helicopter gyro. The rate gyro senses changes in yaw, and applies corrective action. When the motion stops, the gyro stops correcting. There are two disadvantages to this type: first, although this gyro stops the motion of the rc helicopter, it does not return it to it’s original heading. In other words, if a force were applied to an rc helicopter in level flight, it would turn and then the gyro would stop this motion. The end result would be that the rc helicopter has been turned to a new heading. A second disadvantage is that since the gyro only corrects after the motion has been detected, and the corrective action is always a little late.

The heading hold gyro operates in the same way as the rate gyro, with the exception being that after yaw movement has been corrected it returns the nose of the rc helicopter to it’s original position. This type of gyro does not stop giving commands to the tail rotor when motion stops, but will continue giving these commands to hold the nose of the rc helcopter in a certain orientation. Even more advanced gyros of this type will interpret the yaw requests that the pilot is sending through the radio control system, and will make whatever corrections are necessary to cause the rc helicopter to yaw at the desired rate. With this type of gyro, an rc helicopter will turn equally even in a crosswind.

Even though the heading hold gyro has many advantages, it places several demands on the rest of the rc helicopter system. It will require a very fast tail rotor servo, and a powerful battery to supply the servo, which will be required to make very fast corrections. This can strain the servo, and consume more power. The rc helicopter battery will need to supply the gyro and servos, so a higher capacity battery is better. However, the larger the battery, the larger the weight. An rc helicopter with a heading hold gyro and fast servos can use significantly more power than a less aggressive rc helicopter with a rate gyro.

Clearly, there are many choices of gyros for the rc helicopter pilot, each with it’s own advantages and disadvantages. When choosing a gyro system for your rc helicopter, be sure to consider battery capacity and weight, how you will be flying, and the type of servos with your radio system.

BR6008A R/C HELICOPTER

Quick Details

Style: Radio Control Toy Type: Helicopter Power: Battery Material: metal Scale: 1:16 Radio Control Style: RC Model Place of Origin: China (Mainland) Brand Name: Borong BR6008A Gyroscope Helicopter Model Number: BR6008A       Packaging & Delivery     Packaging Detail: boxed     Delivery Detail 15-30 days           Specifications     1 unit 55USD 10 units 30USD 100units 25USD 1000units 25USD

a. Featuresi. Function: Ascend/Descend, Left/Right, Forward/Backwardii. With Built-in GYROSCOPEiii. FULLY ASSEMBLED, READY TO FLYiv. Full scale remote controlv. 360 degrees exact directionvi. AM radio controlvii. Extremely easy to controlb. Specificationsi. Size: 392(L) x 72(W) x 190(H)mm; Main rotor diameter: 340mmii. Battery for controller: 6 x AA batteriesiii. Battery: 3.7V 1000mAh Li-Polyiv. Charge Time: about 120minutesv. Flight Time: 8-10 minutesvi. Aviate Height: approx. 30mvii. Control Radius: approx. 30mc. Package Contenti. 1 x 3-Channel RC Helicopter (battery included)ii. 1 x Remote Control (battery not included)iii. 1 x 110V-240V AC Charger iv. 1 x Spare Tail Rotor Bladev. Operation Manual


We provide 1 imported 3 Channel, R/C Helicoptors as part of the Transfer of Technology Package.