Learning And Moving On From A Counter-Rotator
Posted on November 21, 2008 by aaronredbaron
One of the trends in this hobby has been the emergence of micro indoor helicopters that utilize a co-axial counter rotating blade design. The popularity was kick started in 2001 when the tethered Hirobo XRB was released. Later in 2004, Hirobo made the XRB available with full radio control. In the years since the market has been flooded with models from a variety of manufactures with the most popular models from ESky and Eflite. These small helicopters are unique in that they offer inherently stable flight characteristics. The cyclic, which tilts the rotor disk to provide directional control, has a neutral feel and will actually return to a hover without pilot input. Because of this stability, these models make a perfect segue way into a hobby that is often intimidating. These models also have their limitations; knowing what to expect from these little indoor wonders will help you make the most out of them and prevent overconfidence when you are prepared to make the next step.
How they do what they do
Where is the Tail rotor? Why doesn’t it need one? These are the first questions that are likely to cross your mind when you first look at one of these flying machines. There is no tail rotor because these models use differential torque, or in other words, they spin the two sets of rotor blades opposite directions to cancel the others torque effect. When the rotor blades are spinning the same speed, the tail stays put. When the pilot applies a tail (yaw) input, one set of rotor blades will speed up, and the other slows down, so the body yaws with the faster set of blades. There is even a gyro, to counter unwanted yaw, the gyro applies more power to one set and less to the other so the model doesn’t drop or climb when correcting. When climbing or descending, both rotor sets are speeding up and slowing down together, so there is much less torque to correct for than a conventional helicopter because the blades cancel each other out.
For the most part, the rotor heads these models incorporate all utilize the same basic design principal. The bottom rotor head gives cyclic control through a very simple direct control system that has no mixing arms or stability augmentation. The top rotor head features a design similar to rubber band powered helicopters whcih offers inherent stability. The top rotor head is a free floating design that has a stabilizer bar with weights at the tip. The stabilizer bar is not mounted 90′ to the feathering axis like the flybar found on most model helis, rather it is mounted so that it is approximately 45′ toward the leading edge of the blades. When the helicopter tilts, this stabilizer bar wants to remain in the same plane. It is connected to the top rotor blades so when it is tilted, it applies a blade pitch angle change which fights back with a force in the opposite direction. This is what allows for the stability that is so sought after by beginners.
Now we know that we speed up both motors to climb, speed one up and one down to yaw, there is cyclic control on the bottom rotor set to tilt the heli in the desired direction, and inherent stability built into the top rotor set to make the heli super stable. Why then are these referred to as indoor only? Why are they limited outdoors to flying in the most calm conditions? The reason goes back to the heli’s inherent stability. When the heli moves forward in still air, the pilot must keep a rather large amount of forward cyclic to keep it moving. It is not uncommon to fly these little things around with full forward cyclic control just to keep up forward speed. One may notice that these little buggers just won’t go very fast. Once they get to a certain speed they either start to climb, or, if you force them to go faster than they want, they will start to tip over and a crash is inevitable. What happens in wind is similar. The heli wants to stay in its place in the air mass, so it travels with the wind. If the wind is blowing faster than the heli’s top speed, the heli will just drift with the wind. Since these models have very slow top speeds, they are only capable of flying in very calm conditions. These helis fly great indoors, and they can be a lot of fun outdoors on a very still day, but they are no good for even the slightest of wind.
“I can tear up the living room with my Blade MCX, now I want to fly 3D!”
Whoa there! Time to slow it down a bit! The counter rotator is the perfect first heli because it offers excellent stability, and with that stability comes confidence. However, full function RC helicopters offer none of the inherent stability built into indoor counter-rotators. The most important things you can learn from one of these whirly birds are orientation, feeling comfortable flying the model around with the nose facing you, the side facing you, moving the heli around where you want to and holding it in a hover in uncomfortable positions. The thing to remember as you fly these little guys is simple, once trimmed for a level hover, the model will hold a hover almost hands off. All you have to concentrate on is throttle and tail (yaw). Assuming you aren’t going to bump into something, the cyclic really only needs to be used to move the heli forward or sideways. The best way to return to a hover with one of these is to simply let go of the cyclic stick, the top rotor head will do the rest. This is all fine and dandy for those first steps, but it is important to remember these helis virtually fly themselves when compared against a conventional rotary contraption. That being said, the counter rotator does use the same basic controls as its bigger brothers, and will teach a novice many useful things. A conventional heli design will not hold a hover hands off, it will not return to a hover automatically from forward flight, and if you hold full forward or roll to the cyclic, the model will continue to flip or roll as it follows your stick inputs. It may be a wise idea to buy a simulator and start flying the sim and the counter rotator to work out what to expect before committing a more complicated model to flight. Finding a local expert is always helpful, and they can help explain what to expect and how to make the most of your little indoor heli.
