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HelicopterFlybar Rotor Head Systems › What and why of a flybar.
11-25-2015 08:10 PM  19 months agoPost 1
Aaron29

rrProfessor

USA

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First. Lol.

But seriously, an introduction to the flybar is in order.

You'll probably notice that most full size helicopters don't have a flybar. Some do. But not all?

So why do/did models have them? The answer is in scaling. All helicopters have slight pitching characteristics that are either corrected by the pilot or a gyro. When there is relative wind, the advancing blade gets more lift than the retreating. This creates a slight roll tendency but due to gyroscopic procession, the effect is 90 degrees of blade rotation later, resulting in a pitch up tendency.

On a full size, the pilot can apply control pressure or trim to correct it. However, due to scaling it is a bit more effort to keep up with on a model. It can be done, as no bar people will attest to. But does take attention and skill.

Enter the flybar. It sees the same oncoming wind and essentially tilts the cyclic in the proper direction to account for this tendency.

This is a VERY simplified explanation, but a good start for those interested in what it does.

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11-25-2015 09:06 PM  19 months agoPost 2
Heli Fanatix

Veteran

Fountain Valley, CA

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Very nicely explained

- Scott

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11-26-2015 01:52 AM  19 months agoPost 3
don s

Key Veteran

Chesapeake, VA

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FB is front-end stabilization, FBL is back-end stabilization.

E820, Raptor G4N, X50F/E, E620, Forza 450, and some planks.

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11-26-2015 08:42 PM  19 months agoPost 4
dkshema

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Cedar Rapids, IA

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The flybar provides stability to the rotor disc, but more importantly, due to the way the linkage is arranged, it acts as a mechanical servo force amplifier.

The flybar is controlled by the swashplate, in exactly the same manner as the rotor blades, and will fly as directed by the pilot, not the "relative wind." Since it is controlled by the swashplate and NOT the relative wind, it cannot "essentially tilt the cyclic in the proper direction to account for" the roll or pitching tendency of the helicopter.

When RC helicopters were first introduced, and prior to the entry of the digital servo, the majority of servos on the market were slow, and had limited torque.

To control/change the pitch of the flybar paddles requires little torque from the servo. The flybar, reacting to the paddle input, becomes a mechanical force amplifier, allowing a relatively wimpy servo to easily control the cyclic and collective pitch of the rotor blades.

Note that in flybar equipped helis, there is no direct-to-swash connection between the swashplate and the main rotor blade grip pitch change links. There is a mixing lever involved.

Generally the lever is mounted off-center on the head, with the longer side driven by the swashplate, the shorter side connected to the pitch change link on the grip. The longer lever moment arm from the swash provides a mechanical advantage to the servo (in changing main rotor blade pitch).

As the flybar changes the plane in which it is currently tracking (due to pilot input via the swashplate), as it tilts, it also, through the same mixing lever, directly changes main rotor blade pitch. This requires NO servo torque.

The mechanical mixing ratio of the flybar to the main rotors can be changed to change the overall behavior of the heli with regard to cyclic inputs. If you look at flybar head evolution to the point just before the electronic 3-axis FBL gyros were introduced, you will see that many major brands were selling a "programmable" head. Mechanically, one could change how the head "flew", based on which holes one used to insert the pivot bolts. The mixing arms, too, had multiple holes to allow one to mount the pivot balls at different distances from the pivot point.

The rotating mass of the flybar, the weight of the paddles, the length of the flybar, the mixing lever pivot placement all affect how the flybar interacts with the main rotor system and overall control feel and sensitivity, and overall stability.

As was noted in the original post, the flybar also sees the same relative wind as the main rotor disc. That really makes no difference, as the flybar and its paddles are controlled by the swashplate, not the relative wind, and will behave in the same manner as the main rotor blades.

The flybar maintains the plane of rotation prescribed by the swashplate, not the relative wind.

The flybar does not correct the tendency of the helicopter to pitch up, or roll as was explained. Its advancing paddle, too will see more lift than that of its retreating paddle. Where it actually flies depends upon the swashplate input.

People successfully flew RC helis without flybars, and without the aid of electronics long before the introduction of the 3-axis gyro FBL controller.

The Horizon 60 from many years back was a pioneer in FBL, as were some of the GMP helis introduced just prior to the exit of GMP from the heli scene.

Scaling has little, if anything to do with the need for a flybar.

Please take a look at the Hiller helicopter designs, and Stanley Hiller's "Rotormatic" design in particular.

Bell Helicopter chose to use a direct-to swash linkage, and to get the main rotors to do their thing requires either a lot of mechanical mixing or hydraulic assist to allow the pilot to control the heli without needing the arms of a gorilla. What is gained, however, is a crisp control input to control response output.

Hiller's rotor system used a flybar to control the main rotor head. but there is a small amount of lag from control input, to control response.

When Dieter Schluter produced his first successful production model helis, he took the flybar system from the Hiller head, and married it to the direct to swash head of the Bell system.

He patented that arrangement, and called it the "Beller" system as it was part Bell, part Hiller.

The Hiller flybar and paddles provide stability and ease the load on the servos. The direct to swash portion taken from the Bell system provides a quick control response. The Hiller portion of the system provides stability.

-----

As evidenced by all those videos from days gone by of Alan and Danny Szabo, Jason Krause, Curis Youngblood, and Todd Bennett, a well tuned and set up flybar heli is just as capable as a flybarless heli. The feel may be somewhat different, but both are capable of doing the same maneuvers.

-----
Dave

* Making the World Better -- One Helicopter at a time! *

Team Heliproz

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11-27-2015 04:44 AM  19 months agoPost 5
Chris Bergen

Elite Veteran

cassopolis, MI USA

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A question, if I may, or maybe a request for clarification...

If this statement is true,
The flybar does not correct the tendency of the helicopter to pitch up, or roll as was explained. Its advancing paddle, too will see more lift than that of its retreating paddle. Where it actually flies depends upon the swashplate input.
Then how can it be stated that a flybar adds stability?

A flybar, as was stated, can be tuned for differing wind conditions, which ABSOLUTELY correct pitching and rolling, and if done incorrectly, can actually make the heli UNstable.

I inadvertently built a raptor 30 head backwards in some way (bah, who needs directions...), whereupon liftoff with a slight breeze, the darn thing flipped over itself! It took a second flip (and another set of blades) to finally grab the instructions to find what I had done wrong. I cannot fully describe the exact problem except to say that the linkages from the flybar to the pitch arms were bassackwards and upon "feeling" the wind, instead of correcting for it actually added MORE input the wrong way, kinda like having the gyro sensing backwards, which in effect is what a flybar is, a mechanical gyro.

Saying a flybar doesn't correct any pitch up tendency is the same as saying a FBL electronic gyro doesn't correct any pitch up tendency, and for anyone who HAS flown FBL WOE, knows that just isn't the case.

I think maybe what you missed is that the flybar also has the ability to tilt, flapping as it's known, and this tilt is what adds or removes pitch in the blades, giving this ability to correct pitching and rolling tendencies from the relative wind. How much it affects the rotor disc depends on all the factors you mentioned, weight, paddle airfoil,etc, as well as flybar length and stiffness, and the designed in geometry of the rotor head system.

Oh, and BTW, so called "programmable" heads were out LONG before, how did you put it, "just before electronic gyros were introduced"?, notably the head used on our Intrepid starting in 1996/1997... Of course that's not what it was called then, but certainly had all the adjustable features that were later "marketed" and hyped by the flavor of the day heli manufacturers..

Chris D. Bergen

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11-27-2015 05:44 AM  19 months agoPost 6
dkshema

rrMaster

Cedar Rapids, IA

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Chris,

You're quite right about adjustable heads being around much earlier than I had suggested, but they weren't really advertised as such. My Hirobo Freya and Sceadu had some alternate mounting holes for the mixing levers and balls, even the Trex 600N has a small amount of alternate mounting hokes built in.

-----
Dave

* Making the World Better -- One Helicopter at a time! *

Team Heliproz

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11-27-2015 06:03 AM  19 months agoPost 7
Aaron29

rrProfessor

USA

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A flybar absolutely sees wind and responds accordingly. Curtis Youngblood's Q&A book talks about using response to gusts to determine optimal flybar ratios. And FAI guys once made an art out of trimming for this.

That said, what dkshema said regarding a flybar attempting to have the head follow the swash is ALSO true and also has a stabilizing effect.

When the turkey is more digested and families have departed, I'll expand my explanations and add some links.

But effectively, again oversimplified, we're talking about two systems operating: Bell and Hiller. The "modern" FB model is a combination of two designs.

Everyone is correct.

So now that we're clear on that...I'll depart without full explanation. Lol Sorry. I promise references later. But Holidays first. Lol.

Flybars rock!

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11-27-2015 12:44 PM  19 months agoPost 8
Pistol_Pete

rrProfessor

Seffner, FL

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A must read for FB fans...

http://www.w3mh.co.uk/articles/html/csm9-11.htm

~~Enjoying the hobby one flight at a time~~

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11-27-2015 05:01 PM  19 months agoPost 9
Chris Bergen

Elite Veteran

cassopolis, MI USA

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I was hoping those articles would be posted! Thanks Pete!

Those are some of the best articles written on the subject, very concise and easily understood.

Chris D. Bergen

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11-28-2015 08:35 PM  19 months agoPost 10
oldfart

rrProfessor

Vancouver, Canada

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I agree - those w3mh articles were/are the best on the subject.

Phil

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12-06-2015 01:29 PM  18 months agoPost 11
870heli

Senior Heliman

Monson Ma. USA

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Yes I read them years back and they were very good. I remember back when James Wang started writing I said wow. His articles were very informative. Now everything seams quick and not very fulfilling if you want to know how a new heli works, or how it works! It's like " I used this battery and my favorite FBL controller and it flue great". Ya I just spent $7 so I could read that.
I have just built my first FBL heli's and they do fly well. Maybe to "well". It's like I didn't do it. They were to EZ to build, not to many parts. And no real setup required. CCPM no worry of bad mixing or anything.
Yes thanks for posting the articles.
I think this will be a good forum.
Thanks Mark
Buz 870Heli

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12-07-2015 03:37 AM  18 months agoPost 12
sideng

Heliman

Cedar City, UT - USA

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rigid rotor

American RC was flying FBL "rigid rotor" machines in the late 70's with no gyros or programmable radios.

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HelicopterFlybar Rotor Head Systems › What and why of a flybar.
12-07-2015 07:10 AM  18 months ago •• Post 13 ••
Aaron29

rrProfessor

USA

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Colin Mill had a few interesting articles on flybars

I see Pete beat me to that one. That was the first link that came to mind.

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HelicopterFlybar Rotor Head Systems › What and why of a flybar.
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