How do ordinary flybarred helis work, very roughly...

[cjcj1949]

Someone who doesn't fly helis recently asked me how they work and what the flybar does. I started to explain as I thought I knew but things didn't seem to work I thought they did. I know the servos move the swashplate and the heli moves in the direction of the swashplate, but does the flybar stay still, at least to begin with, or does the rotor stay still and the flybar twist. Or is it so complicated that it's not easy to give a simple answer. Perhaps someone has taken a video to show what happens when the heli rolls.

[challis349]

What happens is the flybar moves, so the air flowing over the flybar has changed direction, this combined with the blades hitting the air, which has been displaced by the flybar, causes the movement,


At least, this is what I have learnt and picked up along the way

[Steve]

The rotor disk is a giant gyroscope which is inherently stable. The flybar and paddles are used to overcome this stability and encourage the disk to change direction.

I think.......

[nitros]

Someone who doesn't fly helis recently asked me how they work and what the flybar does. I started to explain as I thought I knew but things didn't seem to work I thought they did. I know the servos move the swashplate and the heli moves in the direction of the swashplate, but does the flybar stay still, at least to begin with, or does the rotor stay still and the flybar twist. Or is it so complicated that it's not easy to give a simple answer. Perhaps someone has taken a video to show what happens when the heli rolls.

The FB does like the main blades. When the rotor turn. If you do it no bar style u have 90' degree of delay in the movement. I don't not if I been clear is a bit confusing

[cjcj1949]

So the flybar and the blades together make the heli disk turn. Sounds good to me.

[pchristy]

Model Aircraft and Model Flying - W3MH - Home Page

[nitros]

So the flybar and the blades together make the heli disk turn. Sounds good to me.

The FB is like a second set of blade. That's why a quad blades head doesn't has FB. The fbl system anticipate the movements electronically. I hope this help to answer your question

[zoombolt]

I noticed on my 250 recently, that if you spool up a bit & put in right cyclic control, (right side stick to the right) the flybar actually drops left, against the angle of the stick input!

This has confused me & led me to seek out the fluff from my navel.

It also doesn't tie in with the main blade & flybar paddles working in unison theory.....

I also tried to read the linked post by pchristy..........
I'm now taking on board alcohol & have started seeing girls in fields dressed as sheep!!!!

[gorber]

I look at it relatively simplisticly. Lots of words, but not complicated.

If there were no flybar at all, what would happen?
Well, as air passes over the heli, either because of wind, or forward flight (or any other lateral flight for that matter), it causes air to go over one blade faster than the other. This causes a force that tries to tilt the rotor disc down on the blade going away from the wind and up on the blade travelling into the wind. So it tries to roll the disc.
But that rotor disc is a gyro, which translates forces by nearly 90 degrees in the direction of the rotation, called gyroscopic precession. The higher the angular momentum, the nearer to 90 degrees the force is translated, which because the rotor disc is spinning fast, and the blades aren't that light, means it effectively IS 90 degrees as far as we are concerned.
So the rolling force due differential airspeeds over the blades gets turned into a pitching force on the rotating disc. Gyroscopic precession means the blade that is travelling into the wind now pushes the leading edge of the disc up, and the blade going away from the wind pushes the disc down (relatively) at the trailing edge of the disc.
So the disc will always want to pitch away from the wind (note that it is not dependent on the rotational direction of the blades). This is bad if we are talking about a heli hovering in wind because it exacerbates the effect the wind has on the helicopter. Not only is the wind moving the heli, but the heli is helping it by pitching with it and pushing in the same direction.

Now the flybar.

There are a few parts to this.
The first is how movement of the flybar affects the rotor disc. They are set up so that as a paddle moves up or down, it tilts the blade trailing it by 90 degrees the same way. Now, again because of gyroscopic precession, this means that trailing blade will force the disc down 90 degrees ahead of it, ie the same way as the flybar disc.
So the rotor disc will always follow the flybar disc.

The second is how does the flybar make the heli more stable? If there is no cyclic input being exerted by the pilot, the paddles are not providing lift. This means there is no effect mentioned earlier whereby the paddle going away from wind produces more lift than the one travelling into the wind. And because the flybar itself has inertia, it resists the pitching movement that results because of the wind, wanting to maintain a flat disc. As the rotor disc tilts, the flybar discs gives the opposite force back, not necessarilly equal, but it does mitigate it, and therefore provides stability.

In terms of the flybar actively controlling the heli, it is just a matter of the swash making the flybar disc pitch in the direction you want the heli to go, because as described above, the rotor disc follows the flybar disc. Again, gyroscoping precession is in play, so the paddles are pitched at a 90 degree lag to where the disc needs to move.

Andrew

[Nick B]

Yay,im now qaulified to make a scilly remark

Whats a flybar...is it a pub at the flying site?

[zoombolt]

I look at it relatively simplisticly. Lots of words, but not complicated.

If there were no flybar at all, what would happen?
Well, as air passes over the heli, either because of wind, or forward flight (or any other lateral flight for that matter), it causes air to go over one blade faster than the other. This causes a force that tries to tilt the rotor disc down on the blade going away from the wind and up on the blade travelling into the wind. So it tries to roll the disc.
But that rotor disc is a gyro, which translates forces by nearly 90 degrees in the direction of the rotation, called gyroscopic precession. The higher the angular momentum, the nearer to 90 degrees the force is translated, which because the rotor disc is spinning fast, and the blades aren't that light, means it effectively IS 90 degrees as far as we are concerned.
So the rolling force due differential airspeeds over the blades gets turned into a pitching force on the rotating disc. Gyroscopic precession means the blade that is travelling into the wind now pushes the leading edge of the disc up, and the blade going away from the wind pushes the disc down (relatively) at the trailing edge of the disc.
So the disc will always want to pitch away from the wind (note that it is not dependent on the rotational direction of the blades). This is bad if we are talking about a heli hovering in wind because it exacerbates the effect the wind has on the helicopter. Not only is the wind moving the heli, but the heli is helping it by pitching with it and pushing in the same direction.

Now the flybar.

There are a few parts to this.
The first is how movement of the flybar affects the rotor disc. They are set up so that as a paddle moves up or down, it tilts the blade trailing it by 90 degrees the same way. Now, again because of gyroscopic precession, this means that trailing blade will force the disc down 90 degrees ahead of it, ie the same way as the flybar disc.
So the rotor disc will always follow the flybar disc.

The second is how does the flybar make the heli more stable? If there is no cyclic input being exerted by the pilot, the paddles are not providing lift. This means there is no effect mentioned earlier whereby the paddle going away from wind produces more lift than the one travelling into the wind. And because the flybar itself has inertia, it resists the pitching movement that results because of the wind, wanting to maintain a flat disc. As the rotor disc tilts, the flybar discs gives the opposite force back, not necessarilly equal, but it does mitigate it, and therefore provides stability.

In terms of the flybar actively controlling the heli, it is just a matter of the swash making the flybar disc pitch in the direction you want the heli to go, because as described above, the rotor disc follows the flybar disc. Again, gyroscoping precession is in play, so the paddles are pitched at a 90 degree lag to where the disc needs to move.

Andrew

I've had a whiskey.....


And you've just given me a headache.




Seriously, very good reply. Thanks for taking the time to explain that little lot.
That's what I'm talking bout.

[nitros]

Yay,im now qaulified to make a scilly remark

Whats a flybar...is it a pub at the flying site?

Could be a very nice name

[Snaaakeey]

I know that when a heli has no fb it becomes almost uncontrollable. The fb (it has no pitch variable so wants to continuously spin in the same plane. This is why different paddles and weights change the flight characteristics) is the gyroscope which dampens the inherently uncontrollable (variable pitch/ uneven pitch) rotor disk. Different off sets of 45 and 90 degrees (of the rotor and fb axis) increases or decreases the fb input to the rotor disk as well as the ratios or leaver length so of the mixing arms which increase or decrease the mechanical leverage of the fb. Bell hillier it thinks it’s called. This is only what I have learned from the net and I expect it’s not very accurate. Quite a large topic!

[pchristy]

Gorber's post is a very good description of what is happening in a rotor disk. Our models differ somewhat from full-size practice for a number of reasons however.

First consider a full-size flybar-less rotor-head - like a Jet Ranger, for instance. If you want the heli to go forward, you increase the pitch on the retreating blade. This increases the lift on the "retreating" side of the heli - but because the blade disk is a gyro, it doesn't actually tilt until 90 degrees later, when its over the tail boom. At this point it will rise. The advancing blade has its pitch simultaneously decreased, and pitches down when its over the nose for the same reasons. Thus the disk tilts forward and off goes the heli.

BUT - the rate at which the disk responds is VERY fast - far too fast for human reactions to cope with! You would have crashed before you had a chance to apply a correction! This is known as the "following rate". Try balancing a broom on an outstretched hand. Its not too difficult. Now try it with a pencil! Its almost impossible, because the pencil responds too quickly!

To slow down the following rate, a Jet Ranger carries about 24 pounds of ballast in each of its rotor blades! My flybar-less Lark works the same way - heavy blades! Most flybar-less scale models do the same - no need for electronics!

The downside is that this system is very sensitive to head speed. The trim changes quite dramatically as the head speeds up or slows down. Also the rotor disk will be affected by gusts, which to the blades will be indistinguishable from control inputs.

In a pure Hiller system, a second "rotor" system is added at 90 degrees to the main rotors. This is what we call the fly-bar. It has small paddles that are quite heavy. The combination of heavy blades on a small diameter bar gives a much slower following rate than that of the main rotor disk. Otherwise, it behaves exactly the same as the main rotor disk as outlined above. The pilot flies the flybar - NOT the main rotor blades. Because the two are physically linked together, they can only move at the rate of the slowest component. So the pilot controls the flybar, and the flybar controls the rotor disk.

This gives a good slow following rate for the pilot, and is relatively resistant to gusts, due to the small paddle area. The downside is that the initial control response is sluggish. The main rotor disk won't budge until the flybar starts to move, and that has been designed to be slow. Its the rotor disk that actually tilts the heli, not the flybar, so there is quite a bit of delay in the system.

A pure Bell system takes a different approach. The pilot controls the main rotors directly, so the initial response is quite sharp. But the flybar - which in a pure system is just weights, not paddles, is also connected to the main blades via the mixing levers, and resists the disk's attempt to move. The flybar will eventually track the rotor disk - centrifugal force will try and keep the flybar at right angles to the rotor shaft. But the fact that the pilot is pulling the disk one way, and the flybar the other, reduces the power of the controls. So although a Bell system responds more quickly, the response is much less than in a Hiller system.

Also, a full size Bell system uses hydraulic dampers to stop the Bell-bar "wobbling". You really can't make effective hydraulic dampers at model sizes, so most designs use - yes, paddles! The problem with this is that it reduces the immunity to gusts that a pure Bell system enjoys.

Nearly all modern aerobatic helis use a mix of Bell and Hiller systems. The Bell bit gives the instant response, whilst the Hiller bit provides the control power. And that is why a model rotor head seems much more complex than a full-size.....!

[cjcj1949]

Model Aircraft and Model Flying - W3MH - Home Page

That was what I meant by simply. So the flybar is there to make it work right. I did think the pitch of the blades changed during rotation, but somebody else said they didn't and when I checked they didn't seem to. Looks like a good article. And it tells you about Bell and Hiller.