Although they will often see it mentioned in their manual, most people don’t understand what the fly bar ratio is, let alone how to measure it. This is a shame, as the manufacturer will have gone to great lengths to set up the best ratio for each of their RC Helicopters – all to no avail.

Many things about the fly bar will affect the way your helicopter performs. The length of the bar, size of the paddles, weight of the bar, and its alignment to the main blades will all affect how responsive and accurate it is to cyclic control. This last alignment is called the fly bar ratio.

The ratio measures the number of degrees the rotor blades move, per degree of fly bar tilt. For example, a ratio of 1:1 means that for every degree the fly bar tilts, the rotor blade will pitch by one degree as well. This is quite a high figure – most Electric Helicopters will have a smaller ratio than this. The lower the ratio, the less distance the main blades will move per degree of fly bar tilt.

All RC Helicopters have at least two fly bar ratio settings, which are adjusted depending on the work you want the helicopter to do. Look at the linkages, and you may see the location points.

It is very easy to work out the ratio, so long as you have a pitch gauge, protractor and spirit level. The April 2008 edition of RC Heli Mag (www.rchelimag.com) has a detailed explanation of how to do this, but briefly it’s as follows:

The fly bar is first tilted to its furthest movement, and the angle of tilt for the fly bar and main blades are measured with the protractor and pitch gauge. This is repeated on the other side.

The total degrees of travel is worked out for the fly bar and blades (for example, a measurement of -4 and +3 would give a total difference of 7 degrees).

The main blade figure is divided by that of the fly bar, to give the ratio.

This ratio is often overlooked, or crudely guessed at. However, it is crucial to the performance of all RC Helicopters, and well worth checking from time to time.

The design and mechanics of RC Helicopters are based on those of their full-size cousins. Most Electric Helicopters have a fly bar, meaning they are either Hiller or Bell-Hiller construction. The fly bar works to both stabilize and control the helicopter.

The fly bar connects to the swash plate via a fly bar control linkage. In collective pitch Electric Helicopters, this linkage connects to a washout assembly, sited between the swash plate and the fly bar itself.

What does the washout ‘wash out’, exactly? The answer is, any unwanted collective input to the fly bar.

The swash plate inputs both collective and cyclic controls to the rotor blades. In Hiller RC Helicopters, the cyclic is input via the fly bar, whilst in Bell-Hiller models some of the cyclic is input direct from swash to rotor. In both cases, however, the fly bar is ‘immune’ from collective input; it responds to cyclic only. This is the reason for the washout assembly. All CP Electric Helicopters have some form of washout – naturally, FP helis don’t need it.

When the swash plate moves cyclically and/or collectively, the movement inputs to the washout. This then ‘mixes out’ the collective input before passing the resultant ‘clean’ cyclic info to the fly bar via the control arm.

The rotor blades still receive their collective input from the swash. The way the washout works, is by sliding down the main mast as the swash plate is raised – cyclic tilting is unimpeded.

If all this sounds confusing - don’t worry! Your main job is to keep regular checks on the assembly and linkages to make sure they don’t loosen or become too stiff. The helicopter will take care of the rest.

No two Electric Helicopters are the same, but linkages don’t vary much. Neither does the way they are set up.

Since the Bell-Hiller mixing head is fast becoming the ‘weapon of choice’ for all serious fliers, it is what we are using in all articles relating to the rotor head. Hence, some components – such as the BH mixer arm – won’t be present on beginner’s RC Electric Helicopters. However, the same prime linkages are present on even the simplest of 3 channel models.

Electric Helicopters should always be set up for maximum response and control, with minimum wear-and-tear. Linkages are an important factor in this.

For example, servo horns have several settings, allowing the pilot to vary the deflection of the servo linkages. For example, the horn linkages can be adjusted if there is any collective (lifting) movement of the swash plate during cyclic operations. If there is, then the ball-links can be moved outwards on the servo horn to compensate (or different length linkages employed if you’ve reached the outermost hole).

However, it must be remembered that rotor head components are often designed to fit several RC Helicopters, so not all the settings may be appropriate for your model. Incorrectly set servo adjustment can cause the servo to bind, and eventually wear out – just as car brakes do. You will know if the servos are binding, as they emit a buzzing noise when cyclic is applied.

Another essential factor is the ball links. All Electric Helicopters have these, and they must be free to move, without being too loose. Check how the ball-links feel: if they are too tight or too loose, squeeze gently with your ball-link pliers to adjust them. It’s important all adjustments are equally set.

The fly bar control arm linkage is present in all RC Electric Helicopters. Because it is responsible for changing the pitch of the fly bar paddles (and thus, the pitch of the rotor disc) it is essential it is regularly checked for tightness and alignment. As a rule, the bell-crank leverages should be at right angles to the deck. A small amount of play is allowed, however.

Ditto the linkages to the rotor blade grips – check for tightness and stability.

Never over tighten plastic components or you will strip the threads. It is best to upgrade any plastic to metal, wherever possible.

These are only a few of the linkages employed in RC Helicopters, but they are the ones most likely to need regular checks.

If you are intending to build RC Electric Helicopters from ‘scratch’, i.e. from a kit or plan – you will need to know how to adjust linkages.

If you’ve decided to upgrade to a new rotor head, tail or blades – you will need to know how to adjust linkages.

From impromptu crashes, to badly shipped, ‘ready-to-fly’ RC Helicopters that aren’t – there are a hundred reasons why you will need to know about linkage adjustments.

Whenever you read about RC Helicopters, the linkages will inevitably come into it. To get Electric Helicopters to fly well the blades must be balanced, the tail rotor level, the centre of gravity aligned accurately, and so on. All rely on linkage adjustments.

Tightening the last screw on a kit build is only the start of it. Electric Helicopters are a complicated labyrinth of bell-cranks, push-rods and interconnecting arms – and we haven’t left the swash plate yet! Your bird may look completed, but rest assured you have several more hours of work ahead of you - loosening, realigning, measuring and adjusting – before you can safely think about flying her.

Any toolbox that services the linkages on RC Helicopters needs a few specialised items. Aside from watchmaker’s screwdrivers, ball-link pliers and so on, you will need: electrical tape, protractors, spirit levels (large and small) engineering rulers, a tape measure and some watch-maker’s forceps.

The protractor is the most important tool, and you can pick one up for a few pence from any dime store. If you can’t remember what it does – revise your school maths! In RC Electric Helicopters, angles are important.

This is most often seen as an upgrade to a normal fly bar head, although some of the more expensive Electric Helicopters do fit it as standard.

As the name suggests, the B-H takes the best features from both types of rotor head: the stability of the Hiller (fly bar) mechanism, coupled with the responsiveness of the Bell (fly bar less).

As a result, you still have the stabilizing effect of the fly bar. However, instead of fly bar linkages passing on the swash plate data to the main blades, the swash plate links to the blades directly via an interface called the mixing arm. The gyroscopic data from the fly bar also inputs to the mixing arm. This allows for instant response to the cyclic input data, together with gyroscopic stability, at reasonably low cost.

As RC electric Helicopters shrink in size, they get more sensitive anyway, so for most people a standard Hiller head works just fine on smaller choppers. However, once you reach 350 size and beyond you may want to think about upgrading to a Bell-Hiller combo.

The mixing arm is located below the main rotors. Both the swash plate and the fly bar input directly into this. The combined output is then relayed to the blade grips. This means instant cyclic input from the swash plate, mixed with stability and control input from the fly bar.

As the fly bar tilts, the mixing arm is raised or lowered. This has the effect of altering the pitch of the main blades accordingly. The pilot can make fine adjustments to the fly bar input of the mixing arm, varying the influence the fly bar has over cyclic control i.e. the pitch of the main blades. The Bell, or swash plate component is pre-set at the factory and normally cannot be adjusted.

On conventional RC Helicopters, the fly bar acts to both amplify the swash plate movements, and keep the helicopter balanced during flight. To effect a cyclic change in the rotor disc, the fly bar transmits these cyclic changes to the main blades via linkages or rods.

In a fly bar less (Bell) rotor head, the swash plate inputs directly to the main blades. Higher torque servos replace the amplification effect of the bar. This allows for an instant, sensitive response – some people would say, a bit too sensitive.

But the fly bar has a very important primary function – that of gyroscopic balance. Without this dampening effect, Electric Helicopters would flip about all over the place during high speed manoeuvres. So, how does a fly bar less rotor head correct this?

Well, when they first came out it was by adding extra weight and blade adjustment. But the latest development is Electronic Stabilization, otherwise known as an Electronic or Virtual Fly bar.

ES heads have a series of electronic gyros, and an electronic mixer unit, in place of the fly bar.

The pilot transmits a signal in the usual way. This is then passed from the receiver to the mixing unit, which takes the pilot’s commands and the information from the gyros, mixes them and sends the resultant data to the swash plate and servos as a CCPM input.

The sophisticated software also adds other little ‘touches’ to make the heli fly as much like a fly bar model as possible. Lag is minimum, and response and stability superb.

Fly bar less RC Helicopters used to be restricted to 1000mm giants. However, Electronic Stabilization heads can be fitted to the most micro of RC helicopters, albeit at a price. Hopefully, cost will come down as they become more popular.

Even in a basic, fixed pitch beginner’s heli, the rotor head is a nightmare of mechanical and electronics gadgetry. However it is also the singular, most important part of your craft; if you intend to make a hobby out of RC Electric Helicopters, it is essential you know what all these parts do and how they go together.

The working parts of Electric Helicopters connect via linkages, all of which must be adjusted and aligned correctly to get the best performance. So, let’s have a look at them.

Push rods are an integral part of all RC Electric Helicopters. These are stiff, metal bars forming part of the linkage between the servos and other moving parts. As their name suggests, they have a piston-like action.

The ends of the rods are threaded to accept a ball link. Ball links are round ended, and the whole assembly works like a ball-and-socket joint, allowing for a wide range of motion. They are essential in areas where 360o movement is required, e.g. the swash plate and servos. If you intend doing a lot of work on RC helicopters, it is wise to invest in a set of ball link pliers and maybe a ball link reamer as well.

Servos never connect to the push rod directly. Instead, a nylon interface – the servo horn – connects the output end of the servo to the ball linkage in RC Helicopters.

Another type of linkage is the bell crank. This is similar in action to the old pull-rope and bell mechanism of country houses (which is where it gets its name from) One area it is found is the fly bar control arm.

These linkages are an essential part of all Electric Helicopters. Badly aligned linkage geometry can severely shorten the life of your rotor head; so make sure you understand the concepts of linear alignment – and put them into practice regularly.