A question on flybars if I may?

[gazmk2]

I have been studying what happens to the flybar when you give a cyclic input, mainly Becuase I am fascinated by the small scale engineering in these helis.

One thing I noticed was that all the cyclic inputs move the flybar and not the rotors, so am I correct in saying that you basically control the flybar which in turn tilts the rotors in the necessary direction for any given movement?

So when the heli moves to your inputs it's effectively responding to the flybar? And hence this is why flybarless helis are more responsive as you control the rotor movements directly?

One last quicky (oooh errr!!) how is that most companies have the flybar above the main rotor and Align have it underneath?

Sorry if this has been covered before.

Cheers

Gary

[trillian]

On a flybarred heli the swash movement is shared between the flybar and the blades, some input goes to the flybar in the form of deflecting the paddles and some input applies cyclic pitch to the blade grips.

I think the majority of flybarred heads have the flybar underneath. Only Thunder Tiger, Next-D (Rave ENV and Rave 3D), The Ely.Q Ultimate, some Varios and scale Hirobos (and the early Shuttles) had the flybar on top. (there are others but in general the majority is the underslung flybar arrangement).

[gazmk2]

Is there any advantage of having the flybar underneath over having it above? Or vise versa

[p4ddy]

Logo's have them on top too but not a lot of people know that lol.

[milly0812]

A question on flybars if I may?

Logo's have them on top too but not a lot of people know that lol.

Can't see one ImageUploadedByTapatalk1385581193.788656.jpg maybe it's under the cloak of invisibility lol

[trillian]

Is there any advantage of having the flybar underneath over having it above? Or vise versa

Each design has its pros and cons. There are several different designs for how the mixing between the flybar and the blades is done and when you start to examine what's going on mechanically you begin to see that one of the challenges is keeping the movement as linear as possible and also how many degrees of cyclic pitch can be had without making the swash a strange perportion etc. The design of the Raptors and Rave ENV has a few advantages in that a lot of pitch is available because of how long you can make the mixer arms. The problem is that the headblock and grips are in the way of the linkage, so on the Raptor you have the plastic link that goes around the grips. The Rave ENV has a similar thing only it just goes around one side.

Basically they're all trying to achieve the same goal and all have compromises in one area or another so it's a matter of the designers choosing where they felt the compromises were least and where the advantages were greater.

[tomatwalden]

You might find this page useful ...

Bell Mixing and RC Rotor Heads

[electricsceadu]

Is there any advantage of having the flybar underneath over having it above? Or vise versa

with a flybar underslung you can fly lower inverted and grass cut
with an overslung flybar the bar tangles up in the grass

[jwatts007]

Think the fly bar is on top so it is not in the rotors wash....makes no difference too a 3D machine.....

[cjcj1949]

Thanks for the reference. I'd like to know exactly what a flybar does. There should be some equations that predict the performance of a 450 with different weights on the flybar. There should be a calculable metric in terms of required pilot response and another for wind stability. It should be easy to do make a model for Phoenix or perhaps the numbers are already changeable?

[pchristy]

......so am I correct in saying that you basically control the flybar which in turn tilts the rotors in the necessary direction for any given movement?

So when the heli moves to your inputs it's effectively responding to the flybar? And hence this is why flybarless helis are more responsive as you control the rotor movements directly?

One last quicky (oooh errr!!) how is that most companies have the flybar above the main rotor and Align have it underneath?

Most helis use a combination of the systems used by the full-size Hiller and Bell flybar systems. On a Hiller system, the pilot controls the flybar, which in turn controls the rotors. At model sizes, this gives very powerful, but rather slow acting response. All the early model helicopters bar one used the Hiller system.

On a Bell system, the pilot controls the rotors directly, but through a mixing lever which also provides input to the rotors from the flybar. Because the pilot is controlling the rotors directly, the response is much quicker. However, because he is fighting the flybar for control of the rotor, there is less response. This is something of an over-simplification of what is actually going on, but will do for the sake of illustration!

Excluding the small contra-rotators, the only model that ever entered serious production with a pure Bell system was the original Kavan JetRanger. It proved to have some serious control deficiencies, which were only cured when it was modified to have Hiller style paddles (Bell systems just use bob-weights with no aerodynamic input), and extra control to the paddles as well. This Bell/Hiller hybrid proved so successful, combining the best of both worlds, that it became almost universally adopted.

The flybar is a gyroscopic reference that reduces the "following rate" (the rate at which the rotors respond to human input) to something that the human brain can cope with. A full size rotor (say 30ft diameter) without any rate reduction, will have a response rate of something like 720 degrees a second! This is so fast that no human reactions can cope with it. The pilot applies a control input, and the helicopter has flipped over and crashed before the pilot even realises what has happened!

To illustrate the point, try balancing a broom on your outstretched hand. The brush head at the top, the broom will have a fairly slow following rate. and it should be fairly easy to balance. Now try it with a pencil! That is the problem!

Putting to one side electronic stabilisation systems, it is possible to reduce the following rate of a flybarless rotor to make it controllable. This means adding a LOT of weight to the rotor-blades. A full-size JetRanger carries something like 25lbs of ballast in each rotor blade to allow it to fly without a flybar. The problem with this approach is that it is very RPM sensitive. Its not too much of a problem in full-size machines, as the head speed is fairly tightly controlled for other reasons, but in a model, you get quit serious trim changes with varying head-speed.

Electronic stabilisation systems enable you to have what is essentially an unstable (and therefore very rapid response) rotor, which is controlled by a computer. The pilot flies the computer, and the computer controls the rotor. In that sense, it is not unlike a mechanical Hiller system! Large amounts of negative feedback (via gyro sensors) are used to make the system as-a-whole stable. It is much easier to tailor the desired response from this kind of system than from a purely mechanical system. However. there are drawbacks for some types of flying.

A properly set up flybar can be made very gust resistant. It isn't easy, but if you get it right, when hit by a gust, the model will simply bob up and down a bit, but not move off the spot. This is because the flybar and the model both get hit by the gust together, and the flybar can be adjusted to have the opposite response to the model. You can't do this with an electronic flybarless systems, which can only react once the model has started to move. By then it is too late!

So its horses for courses. I hope that helps explain the situation a bit more clearly!



As far as flybar on top or underneath goes, having the flybar underneath usually allows shorter (and hence more rigid) control rods. This is at the expense of having either less clearance between the flybar and canopy or having a taller mainshaft, and hence more pendulum effect.

cjcj1949: There is a book called "Model Helicopters" By John Drake - probably long out of print now - that contains all the equations for rotor systems and flybars. Its all in imperial units, and is very heavy maths, but everything you need to know is in there! Sadly my copy is packed away pending a house move, but your local library should be able to get a copy.

EDIT: Just corrected that ballast weight for the JetRanger blade! It should, of course, have read 25lbs, not 75lbs! The whole blade weighs about 94 lbs. The ballast is split into two weights, one about 2/3 of the way out, the other in the tip, for damping purposes.

[dillwhacker]

I bought the John Drake book in 1976 and have been fascinated ever since.
CJCJ, you're welcome to borrow it if you like.

[trillian]

This is an oldy but a goody, Colin Mill (of CSM fame) on heli flight. Scroll down a bit and he gets into flybars and bell/hiller mixing etc; Model Aircraft and Model Flying - W3MH - Home Page

[cjcj1949]

This bit looks really useful. Some of the effects are the opposite of what you'd think:-

It is best to think of the flybar as a gyroscope that can be steered by the cyclic controls but when not being steered tends to maintain its axis of rotation relative to the ground rather than the helicopter body or the air. The speed of response of the flybar to commands can be adjusted as follows:

• [*=left]Increasing the weight of the paddles slows it down.
  [*=left]Increasing the area of the paddles speeds it up
  [*=left]Increasing the rotor RPM speeds it up
  [*=left]Increasing the aspect ratio (span/chord) of the paddles speeds it up.
  [*=left]Increasing the length of the flybar speeds it up.

[scallybert]

It's worth reading up on how the Bell-Young (Arthur Young's) flybar works on the Bell 47, UH-1, etc. This has weights and viscous dampers (the dampers make it precess to follow the heli). Once you've got this sussed; the Bell-Hiller flybars make more sense.