RC Helicopter

Introduction

Helicopter are one of the most versatile modes of transportation in existence and give the pilot complete access to a fully 3-dimensional space. Because of this, helicopter, whether RC or full size, are one of the most difficult vehicle to learn how to control it. Before i move more detail about my project, let me introduce first, the basis of the RC helicopter.

Radio controlled (RC) is the use of radio signals to remotely control a device, while, the helicopter is a type of rotorcraft in which lift and thrust are supplied by engine-driven rotors. Combination of this two thing provides a new one product known as Radio-controlled helicopter (RC helicopter).

So, how does an RC helicopter fly? How does it work? after take a few time for searching to the best solution to this question, i got that answer. :) I'll discuss this topic in the next chapter. 

How Do RC Helicopters Fly?

So, how does an RC helicopter fly? Someone once said that it beats the air into submission-and in fact, it kind of does.

RC helicopter flight can be categorized into five different parts, the main rotor, tail rotor, swash plate assembly, collective control and cyclic control.

Figure 1: The part of RC helicopter

Now, let move to the first part. The first part would be the main rotor. It is made of two or more rotor blades. This is what gives the RC helicopter its lifting force, which makes it fly. Without this, RC helicopter will not lift off or take off from the ground. They are shaped like the airfoils that airplanes have. Except they differ in certain ways such as their width and size. The speed of the rotor blades are what determines how fast the RC helicopter is going to go.

Figure 2: Main Rotor Assembly for a Fixed Pitch Main Rotor

Second is the Tail Rotor. This is what stops the RC helicopter from spinning around in circles once its lifts off from the ground. It counter acts the force of the main rotor in order to stop it from spinning. It has a gyro, which measures the difference of the force between the main rotor and the tail rotor, and it automatically adjusts so the RC helicopter will be steady. The sideways thrust of the tail rotor that goes against the force, makes it spin in circles, holding it steady in a straight position.

Figure 3: The Tail Rotor viewed from on the top

Figure 3: The Tail Rotor viewed from on the top

The third thing that makes an RC helicopter work and fly is the Swash Plate Assembly. This is the part that translates the command of the pilot on the controller into the main rotor blades. It consists of two different disks. One rotating, and one non-rotating disk. The swash plate can tilt to any direction. This is what allows the pilot to control the RC helicopter in a 3-dimensional area.

The fourth is the Collective Control. This is what raises the entire swash plate assembly. Whether the swash plate assembly is rising or falling, it changes the angle of all the rotor blades and adjusts it to the same degree. Since it changes the angle of all the blades together, the change in the lift stays constant all throughout the full turns of the blades.

Figure 4: The collective pitch type of main rotor and the swash plate's location

Figure 4: The collective pitch type of main rotor and the swash plate's location

Finally, there is the Cyclic Control. It tilts the swash plate up or down, and increases the angle of the rotor blades individually. Whichever side has a higher pitch angle, that is where the weight shifts too and tilts to, and this is what allows the RC helicopter to move in a 360-degree circle, forwards, backwards, left, right, or a combination of any of the four. Figure 5 shows the radio components of the RC helicopter that will be discuss in the next chapter. 

Figure 5: Radio components of the a RC helicopter

Radio Control Helicopter

RC radios are very important. Other than the RC helicopter, the radio is the most important piece of equipment that will be own. In fact; most RC heli pilots will say the radio is more important, at least once you progress to collective pitch RC helicopters.

After all, without a radio – there is no radio control. The radio basically puts you (or at least your commands) inside your model aircraft.

A good radio will serve you for years and computerized ones with model memory can be used for many models of aircraft from heli's to fixed wing. In short, one radio for everything plus computerized radios are the only option if you want to properly setup your collective pitch heli - again why they are so important.

Figure 6 : Direction of remote control that controlling RC Helicopter

Figure 6 : Direction of remote control that controlling RC Helicopter

The following equipments are needed for an electric RC helicopter:

• FM/PCM radio transmitter and receiver ( 4 channel or more )
• servo
• gyro
• ESC ( electronic speed controller )

FM/PCM Radio Transmitter and Receiver

2.4 GHz spread spectrum radios have taken the RC world by storm. In a few years time this could be the only technology that will be offered on everything but the cheapest toy grade RC radios. So why is spread spectrum modulation so good? To answer this question, let’s first look at the other “narrow band” modulation methods: AM, FM, PCM.

Figure 7 : Wave Forms of RC R/F Signals

AM, FM, and PCM use what is called narrow band radio transmission. Simply put this means they transmit a signal on a specific frequency within the radio spectrum. The same way AM and FM radio stations broadcast their signal. The RC world uses radio frequencies in the 27 MHz to 75 MHz range. Most RC models used the 72 and 75 MHz band range of the radio spectrum.

For your RC radio system to work the receiver must be tuned into the exact same frequency as the RC radio, this allows several models to be flying at once provided they are all on different frequencies within the allowed band range. However, space is limited on this band, and there lies the problem.

With only a limited amount of channels available, it’s important that only one person be transmitting on a specific channel at a time. This is why you see frequency boards and pins at RC clubs. When it is your turn to drive or fly, you take the frequency pin off the board and this lets others know that frequency is in use.

Simple enough, but with more people getting in to the RC hobby, there is always the possibility that another RC radio on the same frequency will accidentally be turn on creating interference.

Spread spectrum radios are not bound by narrow band frequencies they spread their signals out over a large radio spectrum. They also use the frequency range 2.4 GHz. That is a frequency of 2.4 billion cycles per second. This is well beyond the range of most RF making 2.4 GHz much more immune to interference. Let’s now look at how narrow band RC radios transmit their signals.

AM RC Radios

AM stands for amplitude modulation. This is the first and most basic method used for controlling RC models. The problem is it is really easy for the AM signal to be affected by almost any electrical noise generating device.

Any type of electrical or metal on metal noise from lighting to car ignition systems will result in interference (if you ever listen to an AM station when it’s raining out you get the idea). It is all these sources of interference that will cause loss of control on your RC model.

FM RC Radios

FM stands for frequency modulation. It is also referred to as PPM – Pulse Position Modulation and was introduced to the RC world in the early 80’s.
FM receivers are less prone to electrical or metal noise from outside, but there are times when moving parts can send out electrical noise that can be interpreted by the receiver as a legitimate signal and cause a “glitch”. So every now and then your model might twitch. If the glitch was bad enough or lasted more than a few seconds your model could end up in a heap of broken parts.

Figure 8 : AM and FM Signal

PCM RC Radios

PCM stands for Pulse Code Modulation and works by embedding a digital signal within the basic FM radio wave. A digital processor chip inside the RC radio encodes a digital transmission and sends it out on a standard narrow band FM carrier wave. The receiver also has a processor chip that decodes this digital data back into a usable analog signal for the servos.

This method all but eliminates any glitching caused by electrical noise because unless the receiver “hears” a digital command that it understands, it won’t respond. It is this ability to ignore outside interference that makes PCM so perfect for all kinds of RC control.

PCM RC radio receiver can ride out interference because it doesn’t understand it and simply ignores it. This makes it possible to add a secondary feature to that ability called “Fail Safe”. Fail Safe is a safety function that allows you to tell the receiver what to do if it no longer sees or understands the radio signals it receives.

No, this doesn’t mean the receiver is capable of flying and then landing your helicopter if there is radio signal corruption, but it will move the servos to a predetermined value. For safety reasons that usually means throttle off and all other control functions at neutral. This is a good feature to have to insure if you do loose radio communication with your RC helicopter or plane, as to cause excessive damage to your model or hurt people. This is not to say it will absolutely crash if the radio signal is lost, the receiver will continue listening for the signal and if reacquired, control will be regained. But again much like FM and AM PCM is still not immune to interference. If another PCM receiver transmitting on the same frequency you will still have problems.

Spread Spectrum RC Radios.

This brings us to the 2.4GHz spread spectrum radio. No other advancement in RC radio technology has changed the hobby in such a profound and positive way. Interference issues are all gone! No more frequency conflicts! No more Worries! 2.4GHz spread spectrum technology has been widely available since the 90’s with cordless phones, cell phones, and later wireless computer technologies such as Wi-Fi and now Blue-tooth.

The main idea behind spread spectrum is to spread the radio transmission out over a wide range of the radio spectrum. This makes a spread spectrum signal much less likely to run into interference or glitching issues that are common with all narrow band radio transmissions.Even with many spread spectrum radios all transmitting at the same time they are very unlikely to interfere with each. In most cases any signal conflict would happen for such a brief moment – you would never notice it.

Figure 9 : Spectrum of RC Radio

So how does it work?

RC spread spectrum radios use the same type of digital signal that is used in PCM giving all the same advantages that PCM has. The difference is how that digital signal is transmitted and received.

There are essentially two different types of spread spectrum modulation methods that have been developed, FHSS and DSSS.

(1)   Frequency Hopping Spread Spectrum (FHSS)

Frequency hopping, as the name suggests, transmits on a narrow band frequency, but changes the frequency of the transmission hundreds of times a second. For FHSS to work, the receiver has to know the frequency changing pattern so it can hop to the different frequencies in the same order and time frame as the transmitter does.

(2)   Direct Sequence Spread Spectrum (DSSS)

Invented later and is harder to achieve. Unlike frequency hopping, direct sequence uses a special code sequence and spreads data over a wide band width on a select frequency. DSSS is said to provide somewhat faster data transmission and shorter delays because the transmitters and receivers don’t have to spend time switching to different frequencies. However, with the high speed micro processors of today, this is really not the issue it once was.

SERVO

A servo’s job is to convert the angular movement of a servo arm to the linear movement of a control surface. This is done by attaching linkages, called control rods to the servo arm and the associated control surface. When the servo head rotates, it pushes the control rod back and forth. The rod is linked to a control surface, and can move it up or down as the servo rotates.

Three wires control a RC helicopter servo: two to provide the DC electricity that the motor needs, and one that sends the signal, controlling the servo. The signal wire works by sending the servo a series of pulses, which are interpreted by it’s internal circuitry. By varying the timing of each pulse, the servo knows exactly which position to move to.

Figure 10 : The Servo

GYRO 

Figure 11 : The Gyro

The RC helicopter gyro is a small device that detects any yaw (left or right swinging) movement of the helicopter and automatically sends a command to the tail rotor servo to correct and stop/limit the yaw movement.

The gyros that were available had a small motor inside them that spun two heavy brass discs. Any change in yaw movement caused the motor to pivot as the spinning discs resisted the change due to gyroscopic effect.

The movement of the motor was picked up by a magnetic sensor and converted into an electrical command to the tail rotor servo to limit the amount of yaw movement.

This type of gyro is now called a mechanical gyro and has gone the way of the dinosaur. That small motor spinning the heavy weights used up a considerable amount of battery power not to mention the added weight of these heavy brass discs.

Most of the good RC helicopter gyros available today use a non-mechanical piezo crystal yaw detector. These new gyros use up much less battery power and weigh at least half of what the old style mechanical ones did. With no moving parts, the reliability and lifespan is a huge improvement over the older mechanical gyros.

The gyro itself simply plugs into your rudder/tail rotor channel of your receiver and your tail rotor servo then plugs into the gyro. In short, the gyro is in full control of your tail rotor and interprets tail rotor commands from the receiver. Only when the pilot sends a tail rotor command to the helicopter, the gyro will allow the helicopter to turn, how fast it turns depends on the amount of transmitter stick movement.

Figure 12 : An old style one on the back and the newer one in front of it.

Figure 12 : An old style one on the back and the newer one in front of it.

ESC (Electronic Speed Controller)

These are widely used to regulate the speed motors in electric models. What do they do and how they work? The speed control generally plugs into the throttle channel of a 3 or more channel radio. It also has connections to the motor and to the battery that powers the motor. Nearly all speed controls offer a BEC (battery eliminator circuit) function which removes the need to also use the 4 cell battery to power the receiver.
The power to the receiver is provided via the 3 wire lead that you have plugged into the throttle channel. Most speed controllers vary the speed of the motor by turning the power on and off at a varying frequency of ON and OFF times. The more the controller is ON for example, the more power reaches the motor. There are lots of different controllers on the market for different types of motors will not work on Brushless motors and 98% of Brushless controllers will not work on Brushed motors. To choose the right type of controller for your application, you need to know how much current the motor will draw under maximum load and how many cells you will be using.