Flybarless helicopters and FBL Systems

CHOPPERADDICT SPECIALISES IN THE BUILDING, REPAIRING & 
SETTING UP OF R/C HELICOPTERS AND RADIOS

   

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It is a fact of life that over the last year or so, we have heard a lot more about flybarless helicopters, more commonly known today as just FBL helicopters.  Because of this it has meant that we heli builders and repairers have had to go back to school a bit and find out just what this is all about. That has of course included me !

So what is all the fuss about ?

Well firstly, for those who like to build and fly scale models, it means that they are able to have a rotor head that looks almost identical to the real model they are copying. Equally, the FBL rotor heads are far cleaner, with many less parts in them, so they can accept crashes with far less cost than the damage that can occur to a standard flybarred head with all it's mixer arms and washout systems.

But that is not all, going FBL also means that you can have more than just the standard 2 rotor blades we are familiar with.  We can now have rotor heads with 2, 3, 4,5 and even 6 blades on them, which means the scale builder has the chance to match the real heli he is copying exactly.

Ah ha you say, but you need all sorts of extra expensive electronics to make the multi blade and FBL heads work.  Well, that is true in most cases, although rather strangely the 4 bladed head works just fine without any electronics to help it.  There is a good reason for this that we will cover later on.

Basically, there are 2 issues with FBL heads (excluding the 4 blade one if you wish to). These are that the mechanical setup that is crucial to all decent helicopters is very different to the flybarred helicopter.  I do not want to get into great technical depth about all of this, but one thing that you do need to understand is that the rotor blades of all helicopters (even flybarred ones) have to be set up in the correct position to make the helicopter move away from the straight and level exactly 90 degrees before you would assume it should be.  

Let me clarify that with an example using a standard flybarred 2 bladed helicopter.  If we want to put the nose down, it would be totally reasonable for us to assume that when the front rotor is pointing directly ahead of the helicopter, it would have negative pitch on it to pull the nose down while the rear rotor would have the opposite, and therefore positive pitch on it to lift the tail up. This would give us the required nose down attitude.

Well, you are almost right except for a nasty little bit of Physics called PRECESSION. I will not go into what causes PRECESSION, but I will tell you that because of it, our rotor blades have to do the pitch changes that we expect them to do to get the nose down, but they have to do it at exactly 90 degrees BEFORE the helicopter reacts to that input by the pilot.  In other words the blade pointing out at right angles to the right of the helicopter will have positive pitch on it and vice versa for the left hand blade, so that they are actually performing the required action 90 degrees before the helicopter reacts to them.

Now this is fine with a 4 blade head, because each rotor blade just happens to be at 90 degrees to the next one, so all we have to do is to set it up so that the transmitter commands to put the nose down occur for the blade that is positioned correctly, which as we now know, is 90 degrees before the expected position.  That is no real problem to set up with our 4 blade head, so I can go ahead now and explain the second important part of setting up FBL helicopters.

This is called setting up the PHASING.  What's PHASING I hear you asking ? Well, as described above, it is all to do with getting the active blade(s) in the correct position (yes, 90 degrees before the expected action).  We can do this easily with our 4 blade head, so all we have to do is position the head so that one of the blades is pointing exactly down the boom.  Keeping it in that position, we look at the connecting rods from the swash to the blade that is 90 degrees behind the rear facing blade.  All we want is for the lower ball connection on that rod to be exactly in line with the anti rotation pin on the swash plate. This will ensure the 90 degree positioning.  To do this all FBL heads have locking collar that has a drop arm to connect it to the swash plate. We are able to rotate this collar so that we can position the connecting ball exactly where we want it.  So we just slacken it off and while ensuring the rear facing blade doesn't move, we rotate the collar until our 90 degree ball is directly in line with the Anti Rotation pin.  We then tighten the collar and check that each of the blades also end up in the correct position.

Basically, that's it. and the process described above also applies to every other type of multi blade head apart from the 2 blade head, which you clearly could not achieve a 90 degree separation with as the blades are at 180 degree placement opposite each other.

So, having used the 4 blade head to make the description above a little easier for you to understand, we must now cover the other multi blade heads. Well, this is where the electronics have to come in. There are quite a few different systems available today, such as the Align 3G, Skookum, Mikado V-Bar, Helicommand Rigid and other lesser known systems.

All of these have one thing in common, and that is that they have THREE small solid state gyros inside them. positioned in such a way that they can react to pitch roll or yaw.  Yaw is not an issue for FBL or any other helicopter, it is just there to handle the tail and rudder like a normal gyro, so it does mean that you do not need to have another gyro on board to do this as the FBL system can do it for you.

The two important areas of control are Pitch (nose up and down along the central axis of the helicopter) and Roll (raising/dropping either side around the central axis of the helicopter).  The FBL units have a single gyro assigned to each of these movements, and when they sense a movement that was not commanded by the radio, they send signals to the Rx (radio receiver) to counter those unwanted movements.  Now this is of course a very simple description of a pretty complex task, which means the FBL controllers have computing power on board to work out what is needed in terms of amount of corrective movement needed and even the duration of that movement.

So if you think about a 5 bladed head. where each blade is physically 72 degrees away for any other blade, the FBL system has quite a bit of work to do as it is it's job to send the right corrective command to the Rx so that the swash plate will move correctly just when the next blade is going to be positioned 90 degrees before the helicopters reaction will take place.

The same applies to a 3 and 6 blade head, which have blades physically positioned at 120 degrees and 60 degrees apart respectively.  You can see from these differences that there must be some form of possible complicated setup that is needed to tell the FBL system where the true level is, how much swash plate movement is available, how much pitch on the blades is available, and many other factors.  Well yes indeed, they do all have their own setup routines that must be followed very carefully by the person doing the helicopter setup.

FLYING AN FBL EQUIPPED HELICOPTER.

Well, as you might expect, a helicopter equipped with an FBL system does fly and feel rather different to a normal flybarred helicopter.  The major difference most pilots report is that they have a lot more lift power, and unless setup very carefully in the Transmitter, can be extremely twitchy (lively) in so far as cyclic movements are concerned. The lift is easy, as we limit the pitch strange quite a bit as multi blade heads generate more lift as they have more blades to do so with.  To soften the cyclic we use two or even three different ways. First off we can limit the Aileron and Elevator maximum range of movement using the Swash Mix settings.  Then we can use the End Point or ATV settings to further limit the swash plate movement, and finally most FBL systems use quite a large amount of Dual Rate and Exponential (Expo) to really soften the cyclic stick around the centre point.

If all this scares the hell out of you, relax, you can send your FBL helicopter 
to me and I will do all the setups needed to make it fly as you want it to fly.