Introduction to R.C. Helicopters

The following is courtesy of "East Coast Modeling Center." By:Jim Ewing

Welcome to the world of R/C Helicopters. The helicopter is probably the most challenging form of radio control model, being mechanically complex in nature, and requiring 100% concentration while operating.

Flying a model chopper is not unlike balancing a metal bearing or a marble on a piece of glass. If the mechanics of the chopper have been adjusted and aligned very well, it is similar to having a flat piece of glass. If the chopper is not set up just right, it is like having a convex piece of glass where the bearing wants to keep rolling off to one side.

The first thing the budding helicopter pilot must realize is that the model works on the very same principles as the full-size and controlling the chopper is just as difficult, if not more-so due to size and orientation. It is not simply a matter of pushing one button for up, and another for forward flight, etc. Flying a helicopter, just like flying a model aircraft, is a skill that must be learned and that can only happen with practice . . . now that I have you thoroughly discouraged! Although building and flying a model chopper can be complex, it is also extremely satisfying. Being able to accurately control a vehicle which you can hover, fly forward, backward, sideways, and do all kinds of interesting manoeuvres and aerobatics, as well as land at your feet, is very exciting.

Unlike learning to fly a model airplane where flying with an instructor is a must, you basically learn to fly helicopters by yourself. Before you start flying, however, some time with an experienced helicopter pilot will be invaluable. He can help you set up your helicopter (it is extremely important to have the mechanics set up accurately for safe and easy flying) as well as give you some tips on flying; what to expect from your model and how to operate the controls.

How does a Chopper Work?

There are basically two different types of helicopters, those that have collective pitch and those that do not. Collective pitch is where the pitch of the main rotor blades may be simultaneously increased or decreased to change the amount of lift. This gives a quicker response to changes in vertical thrust as controlled by the pilot. On helicopters without collective pitch, the amount of lift is controlled entirely by the speed of the rotor blades or in other words the speed of the engine (throttle control). The reaction time is longer and thus the control is less responsive.

There is a trade-off, however, and that is the cost and complexity of the rotor head. There are a great deal more moving parts in a collective pitch rotor head and thus, they are more expensive. Most current choppers are of the collective pitch variety.

On a standard, collective pitch helicopter there are four controls and these are operated by five channels of your radio system. These controls are the collective pitch, the fore and aft cyclic pitch, the side to side cyclic pitch, and the tail rotor pitch. The collective pitch must also be coupled with the throttle of the engine so that when more load is put on the main rotor blades by increasing the pitch, more throttle is applied to help overcome the additional drag.

The left stick of your radio transmitter controls the collective and throttle in the vertical direction and the tail rotor pitch in the side to side direction. Your right stick controls both cyclic operations; up and down for fore and aft control and side to side for the cyclic side to side control. There are also mixing functions which mix the throttle and collective functions, and the throttle/collective and tail rotor functions.

Operation

The engine of a helicopter drives both the main rotor shaft and the tail rotor via a series of gears and a clutch. As the motor comes to speed, the clutch engages and begins to turn both rotor systems. Generally, at this point, there is no pitch on the main rotor blades and thus no lift. The throttle is increased until the main rotor blades are brought up to speed. To lift the helicopter collective pitch is applied.

Because, for every action there is an equal and opposite reaction, when the engine is forcing the rotor blades to turn in one direction, the body of the helicopter will want to rotate in the opposite direction. The function of the tail rotor is to correct this tendency. The tail rotor blades provide enough thrust to the side to keep the helicopter pointing in one direction. By increasing or decreasing the pitch of the tail rotor blades the direction the helicopter is pointing can be changed.

The cyclic control permits the main rotor blades to be varied independently making the helicopter move in a horizontal direction. If one of the rotor blades increases pitch as it approaches the rear while the opposite blade decreases in pitch while approaching the front during its rotation, more lift will be produced in the rear, tilting the helicopter forward, and thus moving the helicopter in a forward direction. The same principle applies for side to side and rearward, allowing the helicopter to fly in any direction.

The control of the cyclic and collective pitch is transferred from the radio servos to the rotor blades via the swash plate. Part of the swash plate is stationary while the other part is allowed to rotate with the rotor head. Control linkage is connected from the servos to the stationary part of the swash plate as well as from the rotating part of the swash plate to the rotor head.

When flying a chopper, small control inputs are continually required by the pilot to correct for deviations in the flight path. That is why 100% concentration is required in chopper operation. The more accurately the chopper is set-up, the fewer the number of corrections that are required by the pilot.

What Happens if the Engine Stops?

Auto-rotation is a way for helicopters to land successfully after a loss of power from the engine to the rotor drive systems. This is accomplished with the aid of a special device known as an auto-rotation clutch which allows the rotor blades to free-wheel. As soon as power has been cut, the throttle/collective control is brought back all the way. This will usually bring the main rotor blades to have slightly negative pitch. As the helicopter starts to descend, the air moving through the blades will keep them spinning. The spinning blades will act like a parachute in reducing the helicopters decent. When the helicopter nears the ground, the pilot increases the collective pitch making the pitch of the blades again positive. The momentum of the blades is converted to lift, slowing the descent of the helicopter down further, enabling it to land softly.

What Will I Need?

The following is a description of the items you will require to start flying a radio controlled model helicopter.

The Helicopter

When choosing your first helicopter you first have to decide which type of chopper you wish to purchase, a collective pitch machine, or a non-collective pitch machine. If you are starting the R/C hobby from scratch and must buy everything, I would strongly suggest the collective pitch machine. Although it is a bit more expensive initially, it is by far the more versatile of the two and will help you enjoy more advanced stages of the hobby after you have learned to fly. You will also find the learning process easier with a collective pitch machine.

On the other hand, if you are already into radio controlled aircraft and have an aircraft radio, and an aircraft engine in the range that would fit a non-collective machine, this route may be desired to drastically reduce the start-up cost and to get you involved in the sport with less money. Once hooked however, you are bound to want to move up to a collective pitch machine. I am going to assume you are starting from scratch and will continue with the idea of starting with a collective pitch helicopter.

Unlike aircraft where there are specific trainer models geared solely for the first time kit, the differences between helicopters is more suttle with the major difference being size and precision. The larger and more precision a machine is, the better it will fly. These machines would make great training helicopters but they are usually alot more expensive, with high precision parts, and would be very costly to repair in the case of a crash. They are better suited toward competition and the experienced modeler where accidents are usually minimal.

Keep in mind, when learning to fly a helicopter you are bound to have a crash or two and parts are going to have to be replaced. Replacing parts on a precision machine could really run you a bill! A good place to start would be a machine designed for a .30 size engine, a stable flyer with collective pitch, and one with a good avialability of parts. The Radio

As mentioned earlier in this introduction, a proper helicopter radio differs from a standard aircraft radio. There are certain functions that must be mixed electronically and these are found only in radios designed for this purpose. More and more often, one is able to find radio systems that have functions suitable to both aircraft and helicopters. If you think you may be involved in both aspects of the hobby, you may wish to choose one of these for your system.

Other things to think about when purchasing a helicopter system are the servos and the battery pack. It is desirable to choose servos that have output shafts that are ball bearing supported. Since the pressure and vibration on the servos in a helicopter is usually greater than in a plane, bushing servos tend to wear out very quickly and lose their precision which is extremely important in the controlling of a helicopter. Because you are using a minimum of five servos and a gyro (to be discussed later) in a helicopter, and the higher frequency of control input, it is very desirable to have a larger battery pack than the standard 600 maH pack that comes with most aircraft radios. A pack in the range of 1000 to 1200 maH is a better pack to consider. Many helicopter radios take both of these concerns into consideration and come packaged with five BB servos and a large batter pack.

Gyroscope

A gyroscope, or gyro, is an electro-mechanical device used in a helicopter to help semi-automate the response of the tail rotor. In the case of an R/C chopper, the device fits electrically between the receiver and the servo that controls the pitch of the tail rotor blades. A sensor measures any unwanted change in yaw of the aircraft and will correct the situation by increasing or decreasing the tail rotor pitch to stabilize the movement.

Although not absolutely necessary, it is strongly recommended that a gyro be used in every helicopter. Controlling an overly sensitive tail of a helicopter is most frustrating and the gyro can help make the flying much easier and more pleasurable. Gyros are made by most radio manufacturers for operation compatable with their systems.

The Engine

The helicopter engine is similar to a 2-cycle aircraft glow engine except that it has a larger heat sink head for better cooling, and a carburettor with improved mid-range adjustment. When purchasing the helicopter engine, the muffler is not included. Usually the muffler comes with the helicopter kit.

Different motors facilitate different starting methods. Some come with a pull start system for very simple starting. Others use a cone start where an electric starter is used in a manner similar to starting an aircraft. Still others use an electric starter and a belt for starting.

Tools

Since the helicopter is purely a mechanical device, tools for assembly usually include items such as screw drivers, ball drivers, nut drivers, wrenches, pliers, etc. In addition to these there are a couple of specialty tools that come in handy when assembling and setting up the mechanics of your helicopter.

One tool you may want to consider to assist in assembly is a set of ball link pliers. The ball link is the most popular linkage piece on a helicopter and virtually all choppers use them. The ball link pliers greatly assist in the removing and adjusting of these links. A second tool that is extremely valuable during set-up is the rotor blade pitch guage. This device can help you line up your rotor blades so that your pitch is correct. A blade pitch guage can go along way to helping avoid costly crashes and frustration down the road as so much of your chopper's well being depends on how well it is set up initially and maintained throughout its lifetime.

Field Equipment

The field equipment you require will depend largely on the type of motor you have chosen for your helicopter. If you have a glow motor with a recoil pull-starter, you will only require the basics of a fuel can and pump for carrying and pumping fuel to your chopper, and a glow plug igniter to supply current to the glow plug during starting. If you do not have a pull start, in addition you will undoubtedly need an electric starter powered by a 12 volt battery to turn the engine over.



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