A Look at an RC Helicopter
Posted by on 08 October 2012 03:29 PM
RC Helicopter at a Glance
Congratulations on buying your new RC Helicopter. Pat yourself on the back, because you have taken your first step into the world of flight. The air is your playground, and the sky is the limit. But before you start, you should get to know your Helicopter a bit more. What's an airframe? Gyro? Swashplate? Well, let's get started!Airframe
Airframes are usually injection moulds. But there are more expensive models constructed from carbon fibre and/or Aluminium. The Airframe is a frame that hold all of the components of a helicopter, from the motor, electronics, and gyros that allows the helicopter to fly. Modern pod and boom sport models, are semi exposed except for a fiberglass painted canopy that covers the first half of the airframe via some simple body clips.Tail
The tail unit are the two smaller blades on the back of the helicopter. The tail unit helps control the motion of the helicopter, and also helps with stability control. Between the tail unit and main frame is the tail boom. The tail boom provides a route for the tail drive system and control links. It is necessary to have the rear rotor blades at a larger diameter than the main rotor blades away from the main frame. Usually there are also two support structures coming from the base of the helicopter frame, to a fixed point along the boom to aid structural rigidity. This is also where the horizontal stabilizer fins are located, a bit further towards the rear we find the vertical stabilizer connected to the tail drive unit. The power is normally transferred to the tail from the main motor via a belt drive, but it is also possible to have torque tube drives, that have a fixed drive shaft powering it. The rudder control on the radio controls the yaw of the helicopter via changes in pitch of the tail blades.Swashplate
This is one of the most important parts of a RC helicopter, it is how the movements from the servos are transferred to the rotor blades, and finally translated into a movement of the helicopter in the direction required. It allows the stationary helicopter frame and servos to apply movement to the spinning blades pitch. In order to produce propulsion in a certain direction, as the blades spin, at certain points along the circumference of the rotation, the pitch of the blade changes, thereby creating more lift on the left side than the right for example. This will then cause the helicopter to move to the right. This is the Aileron control on the radio, and produces a sideways banking motion. It is the same theory for the Elevator control, it rocks the swashplate forwards and backwards, like the Aileron moves it from side to side.Rotorhead
Relatively simple compared to the swashplate, as most of the control mixing has been done there. The main components here are the blade grips, they are able to rotate to allow for the changes in pitch of the individual blade. Also on the rotor head is the flybar, this is to provide a mechanical form of stabilization to the inherently unstable aircraft. To provide control to the rotor head, we have a complex mixture of washout arms, bell crank arms, mixer arms, pitch control arms and flybar control arms. All working together to keep the helicopter in the air. I wont go into detail on these in this section, as the particular setup varies from one helicopter to another. I will however go into more detail in the mechanical setup section of this website.
A combination of rotor pitch and head speed gives the helicopter lift. When applying increasing pitch on both blades at the same time, this will produce lift, but also increased drag effects will reduce the head speed. On a modern transmitter, the throttle, and pitch are mixed to counter act this and keep a constant head speed, this is also where a engine governer comes into force on a nitro model.
To calculate head speed, we look at the rating of the motor, so a 3000 Kv motor will rotate at 3000 rpm per volt. So the T-Rex 500 runs at 22.2 v. This means the motor will spin at 66,600 rpm when the full voltage is applied, it then makes sense that at half throttle, we will have 33,300 rpm. It is then a simple case of working out the ratio between the teeth on the motor pinion and main gear to give us our head speed. So as an example, if for every one turn of the main gear, the motor turns 40 times, we would have a head speed of 1665 rpm.Speed Controller and Motor
Unlike a nitro model where we have a mechanical mixture control and servo, on a electric helicopter we have a brushless motor and electronic speed controller [ESC]. The speed controller acts as many units in one. Supplying the appropriate voltage to the motor, providing power to the electronics, controlling headspeed, battery level warnings, and many more features. Most ESC's will have a built in battery eliminator circuit (BEC) to maintain the correct voltages to the receiver and ultimately the servos and gyro. If you have a nitro model, this will be a separate unit, and the helicopter electronics will have there own, smaller Li-Po batter pack. The ESC usually will connect to the motor via 3 cables, positive, negative and data. it is important when working on your helicopter on the ground, to disconnect the motor from the speed controller, to avoid accidental arming and spooling up.Servo's and Gyro
If you have a 120 degree swashplate you will have 3 servos mixed to control the swashplate movement. This is known as eCCPM, the controls are mixed on the transmitter, to allow the three servos to control the four directions of motion applied on the cyclic control. Some helicopters use 90 degree non-ccpm swashplates, but on the majority of current helicopters these are much less common. A quick visual inspection of your swashplate will allow you to identify easily which type you have.
Lastly we have the tail servo, usually attached directly to the tail boom, or tucked away inside the main frame below were the tail boom interfaces with the frame. This provides the control for the pitch of the rear tail blades, to allow the model to yaw and rudder control. However, it is necessary to incorporate a gyro between the receiver and servo, to control the position of the tail. Due to the force of the tail rotor, it will generally always want to move. Also looking at the laws of motion, once moving it will want to continue on that path until operated on by an external force. So a gyro counteracts this. It keeps the tail locked steady in one position, when you give some left rudder, it moves left, but the moment you stop the control input the tail comes to sharp stop. Without this you would have to be constantly balancing the tail, and the other controls. No easy task.