Fly bars are found on all RC helicopters with a Hiller or Bell-Hiller rotor head. The fly bar has a dual role:

1) It assists the servos to change cyclic pitch by exaggerating swash plate movements, thus effecting a tilting effect on the main blades and reducing the load on the servos and swash plate mechanism.

2) It stabilizes the helicopter by acting as a gyroscope. This is the main reason the fly bar is set perpendicular to the main blades, rather than in line with them.

In Electric Helicopters the fly bar spins just as the main blades do, but at 90 degrees to them. This creates a gyroscopic effect, stabilizing the helicopter whilst at the same time exaggerating the cyclic effects of the servos.

How is this achieved?

When a gyroscope spins, it wants to remain in its rotational path (the angle it was at when it first started spinning) RC helicopters always start up at zero pitch, thus the gyroscopic fly bar wants to remain in a horizontal position, irrespective of any cyclic force being applied after take-off.

The way this is achieved in RC Helicopters, where the fly bar is working to increase the cyclic pitch as well as dampening it, is by something called Gyroscopic Precession.

Although the fly bar affects cyclic pitch, its main function is one of stability – something the swash plate can’t control. To achieve both, there is a 90 degree differential between the cyclic response of the main blades to the fly bar input, and the gyroscopic effect of the fly bar itself.

The fly bar linkages allow it to tilt horizontally. Whenever a pilot is performing exaggerated cyclic movements, or it’s a windy day, the fly bar is constantly compensating for the cyclic changes it has just enforced! If this sounds a bit contrary, it must be remembered that cyclic movements mean deliberately disobeying the laws of physics.

RC Electric Helicopters are much lighter than their full-size cousins, and if extreme pitch changes were not dampened down in some way, the machine would be very difficult to control, especially outdoors. This is the reason beginners are advised to add weights to the paddle ends, to further dampen the cyclic and thus increase stability.

It’s a term that’s bandied about a lot with Electric Helicopters, so it must be important, but what exactly is the rotor disc?

Well, it’s not a mechanical part (and yes, we’ve all heard about the Saturday lad who was sent out to buy a spare one!) In both full-size and Radio Control Helicopters, it is the imaginary circle drawn in the air by the spinning main blades.

Most Radio Control Helicopters are fitted with a Hiller rotor head. This has a fly bar with paddle attachments, set at 90 degrees to the main blades and linked directly to the swash plate via control rods (linkages).

In all Electric Helicopters, the cyclic controls work on mechanical servos on the swash plate, causing it to tip front to back or side-to-side. Evidently, anything that reduces the load on the cyclic mechanism is a plus, and this is where the Hiller fly bar system comes in.

When the swash plate tilts, the linkages transfer the movement to the paddles. The fly bar then exaggerates this tilting action before passing the ‘message’ on to the main rotor blade, which is not linked to the swash plate directly but to the fly bar itself.

The Hiller system in RC Electric Helicopters is very similar to the full-size one it was developed from. It has been likened to the power steering in a car.

The effect of the fly bar and paddles, therefore, is to translate a small movement into a much larger one, effecting a maximum effect on the rotor disc from a minimum input from the cyclic servos.

The entire rotor disc therefore tilts enough to unbalance the helicopter, which leans over to compensate – and a cyclic change in direction is effected. Naturally, there is a time lag before the helicopter responds to these changes – the one downside of fly bar mechanics.

The more manoeuvrable RC Helicopters are, the more balanced they must be; so the fly bar has another role – that of gyroscopic stabilisation.

Even palm-size, Electric Helicopters can be complicated things. This is hardly surprising, since they are, in all respects, just scaled-down copies of the originals.

However far you want to take your interest in RC helicopters, you will inevitably need to replace, repair or upgrade parts. You might even decide to build one from a kit. Thus, it helps to know about those parts in a bit more detail.

Here, we’ll cover a few of the parts associated with the rotor head (and before you ask, a ‘wash-out assembly’ isn’t when a packed summer fly-in event suddenly gets rained off!)

By now (we hope!) you know the relationship that exists between the ground transmitter and the on-board receiver unit. You understand that RC Helicopters rely on changes in pitch to change direction, and the relationship that exists between the swash plate, servos, fly bar, rotor blades and mixer unit (where fitted).

But how do these messages get passed across; what mechanics are involved?

Take a close look at the rotor head – there are various complicated linkages between the blades, fly bar and swash plate. But when your blades start to wobble, it isn’t enough to just Google up ‘linkages’ and then hope to get the correct spare part. You have to be far more specific than that.

Linkage is a general term, used to describe any one of the push-rods, bell cranks, mixer arms and washouts that RC Helicopters employ to transfer actions from one part of the rotor head to another. The parts themselves are very specific to the jobs they do – hence the need for some clarification.

A washout assembly, by the way, is a set of linkages which transfer cyclic input to the fly bar control arm.

It also describes an event in Bristol that I went to last year.

Until recently, fly bars were a necessary part of RC helicopter design, offering stability as well as augmenting the movements of the swash plate.

The one downside of this was the lag in responsiveness between the cyclic input and the reaction of the rotor disc. This is down to two effects:

1) The gyroscopic precession effect caused by the 90 degree alignment of the fly bar, necessary to achieve stability.
2) The fact that changes in cyclic from the swash plate are transmitted to the rotor blades from fly bar linkages, rather than from the swash plate itself.

Fly bars were originally used in passenger helicopters (Hiller design). With improvements in helicopter technology, this was dropped in favour of the Bell head – it is rare to see a modern helicopter still displaying its fly bar.

RC Helicopter designers have at last followed this lead; there are now Bell headed RC helicopters available, called fly bar less helicopters. They look good, but have their limitations.

There is a good reason why RC Electric Helicopters kept the old fly bar system. Unlike passenger aircraft, Electric Helicopters are very light, and the slightest gust of wind (or mistake on the pilot’s part) can cause them to become unbalanced. This lightness also makes them very sensitive to changes in pitch control – far more sensitive than the full-sized version. The fly bar helps dampen some of these effects, allowing fast but balanced flying.

For this reason, you are unlikely to see a Bell head on small (400 size or less) Electric Helicopters. It would simply make the heli too sensitive to control. However, larger models such as the T Rex 600 are now available as fly bar less versions.

It is possible to buy conversion kits for some fly barred Electric Helicopters. Although they are tricky to manoeuvre at first, those who have made the switch say it’s the best thing they’ve ever done, not least because it cuts down on a hell of a lot of spares!

This article tells you how to check the pitch settings using a pitch gauge tool like the EK1-0348. Although it seems there are hundreds of brands on the market (almost as many as there are RC helicopters themselves!) they all work in the same way.

Helicopter blades are always aligned with the fly bar. It is essential, therefore, that the fly bar itself is horizontally aligned - but first you need to know what the fly bar is.

All RC helicopters - FP, CP or co-axial - have one of these, so it should be familiar to you. If you really have no idea, check out the article on fly bars. Basically, it’s the rod with paddles on, located just above or below the main rotor blade or blades. For more on this, check out the article on fly bars.

Start off by ensuring your helicopter is standing on a flat, level surface, with the tail boom parallel to the ground. Check the fly bar is aligned with the boom and is totally horizontal (check with a small spirit level). This will mean the rotor blades are at ninety degrees to the bar.

Connect up the helicopter battery and switch on the transmitter, setting the throttle to its mid-point, while making sure the throttle hold is on. People have had nasty accidents, checking the settings on radio control helicopters!

Slide the pitch gauge onto one of the rotor blades, about half-way along, and align the top edge with the fly bar. Checking the transmitter is still in the neutral pitch position; adjust the gauge until it is level with the fly bar. It should read 0 or something similar.

Now repeat the exercise twice more, with the throttle stick at its top setting. This will give you the maximum positive angle. Repeat for the negative maximum (i.e. throttle at its bottom position.)

Repeat for each of the other main rotor blades, adjusting where necessary.

How do I know what the max/min angles should be?

This will be in your RC helicopter’s manual. However, many people “tweak” these settings to give them the best performance. So experiment until it feels right for you. The important thing is that the settings should be equal for all the blades.

What do I do if the settings are out?
You will need to adjust the control linkages on the affected blades. The adjustment nut is found at the base of the linkage itself, and is usually covered by a plastic cap. This fiddly job is best approached with a pair of ball-nosed pliers - another essential tool you should always have to hand.