FBL vs Flybar - general questions from a noob

[spiff72]

Hey everyone,

I figured this question should go in this section, but if not feel free to move it, mods!

I am new to helis, and loving it! I started out with a 120 SR, and got pretty good at flying it. The self-correcting nature of the flybar on this FP heli probably saved my *** more times than I realized.

I just got a Nano CPx and it became immediately apparent how much I relied the flybar on the 120 SR! I can hover the Nano quite comfortably, but I am still learning how to do simple banked coordinated turns with it.

So now the question:

Does a FBL CP heli like the Nano (or more generally speaking - ANY FBL CP heli) perform differently than a CP heli with a flybar? Does the flybar still tend to self-level itself in the same way that the flybar on the FP 120 SR does? Or is the flybar on the 120 SR different because of its 45 degree angle, while most of the CP flybar helis I have seen have a 90 degree flybar angle?

My understanding is that the 45 degree flybar tends to self-level more aggressively than other designs.

So if you had a flybar CP heli, and give it forward cyclic to get it moving forward - and then return the cyclic to a neutral position, will the forward motion tend to dissipate and the heli just returns approximately level attitude? With the Nano - it seems like the above situation results in essentially a fixed forward motion which doesn't self-level. (It would keep going forward until you applied a back-cyclic command to bring it back to level.)

I thought the FBL systems were supposed to be like an electronic flybar, but maybe I was mistaken!

Don't get me wrong here - I like the Nano, and I am anxious to advance my skills, but it wasn't quite what I was expecting a FBL heli to be. (Maybe the bigger ones are a bit different)?

Thanks!

[extrapilot]

Spiff

Self-level is what you mean, but it is not what is happening. That is, upright, the helicopter can act like a pendulum, where its mass being below the rotor causes it to want to seek vertical/level. Inverted, this is the opposite, and it wants to roll/pitch 180. Level in this context just means- CG hanging like a pendulum vertically below the rotor hub

By mixing the FB at 45 deg instead of 90deg, there is a phase error (complicated subject, search for posts on rotor phasing if you care to know more) between the rotor and the FB. This results in some of the increased stability you talk about (i.e. in forward flight, without forward cyclic, the rotor tips back and the heli slows). But, it also creates the wobbles people have termed ‘swinging pendulum’ and ‘toilet bowl effect.’ These are not beneficial, and makes acro ugly and imprecise.

This is not a function of CP vs FP, but it is seen in FP more frequently as these machines are typically indoor or trainer machines, and (again, for complex reasons involving relative mass/stiffness of the rotor) tend to have a phase lag much less than 90deg already.

FB and FBL both do a similar task- they create a reference plane (i.e. the FB spins as a gyroscope, the FBL has internal gyros), and when the machine’s rotor moves out of the reference plane, the control system is implemented such that the rotor is biased back into the original plane via cyclic input. This makes the rotor stable. It does not make it seek level, because that implies the FB or the FBL know what ‘level’ is. They don’t.

There is a fundamental difference between FB and FBL. It is that FBL is about angular rate, and FB is about angular displacement. That is to say, with FBL, if your control system happens to be set so that 10deg cyclic stick yields a cyclic rate of 10deg/sec, it will maintain that regardless of rotor speed, winds, CG, etc. So in that example, if you push the stick 50deg forwards, the FBL machine should have pitched 50deg forwards in 1 second, 100deg in 2 seconds, etc.

With FB, the impact that 50deg stick input had has depends on a lot of things- rotor blades, paddles, RPM, mass, etc. So at 3000RPM, maybe your roll rate is X, and at 4000RPM maybe it is X*2- not super consistent. But, FB tends to react more quickly, since it is analog and has no delays in the control loop. So, talented pilots with FB can do amazing things- but they tend to leverage governors and be very careful about maintaining consistent mass and CG to minimize the amount of variability FB can cause.

[spiff72]

Thanks, extrapilot!

That was in interesting read!

[mrmarc]

Originally Posted by spiff72

Does a FBL CP heli like the Nano (or more generally speaking - ANY FBL CP heli) perform differently than a CP heli with a flybar?

Extrapilot made a great reply on the technical aspects, but to answer your question: yes, in a way. I say "in a way" because you can adjust this, to some degree and depending on the FBL controller in question, to suit your flying style.

I am by no means a seasoned pilot, but here's a little insight into my experience. I first bought my 450 as a FB heli. Then I got the mCPX and Nano CPX. I then converted my 450 to FBL, and it flies a lot more like my mCPX/Nano now. I feel like I am flying now more than I did back with the flybar. Granted, I had gained about a month of flying experience from my last FB crash to completing my FBL conversion. I just really liked how the FBL helis flew the way they did compared to my flybarred helis. In a way, I sometimes feel like it's cheating, but I like not having to make all those tiny corrections I needed to with the flybar.

[Mercuriell]

You can set most FBL units up to behave like FB - the difference in flight is mainly due to the gyros locking in attitude which means the heli tracks better through maneuvers requiring less input from the pilot. FB also requires less work from the servos due to the mechanical input from the flybar. Autos are also a it more tricky in FBL as cyclic changes tend to bleed off HS and there is a little less inertia, especially n the smaller heli.

[hANDYman]

Originally Posted by Mercuriell

You can set most FBL units up to behave like FB - the difference in flight is mainly due to the gyros locking in attitude which means the heli tracks better through maneuvers requiring less input from the pilot. FB also requires less work from the servos due to the mechanical input from the flybar. Autos are also a it more tricky in FBL as cyclic changes tend to bleed off HS and there is a little less inertia, especially n the smaller heli.

An interesting comment... but if I recall correctly it's the opposite to what some of the pro pilots have to say. On an auto-rotation clinic video that Curtis Youngblood did, I believe he mentioned the a FBL heli will maintain HS better during an auto than a FB heli. The additional mechanics of the FB head does provide a bit more rotational inertia (which doesn't help much as it is so close to the point of rotation) but it also increases the aerodynamic drag on the system..

Both of my heli's are FBL (450 & 550) and they are both easy to auto. I don't think that they lose HS any faster because they are not FB'd.

[BustedRaptor77]

Originally Posted by hANDYman

An interesting comment... but if I recall correctly it's the opposite to what some of the pro pilots have to say. On an auto-rotation clinic video that Curtis Youngblood did, I believe he mentioned the a FBL heli will maintain HS better during an auto than a FB heli. The additional mechanics of the FB head does provide a bit more rotational inertia (which doesn't help much as it is so close to the point of rotation) but it also increases the aerodynamic drag on the system..

Both of my heli's are FBL (450 & 550) and they are both easy to auto. I don't think that they lose HS any faster because they are not FB'd.

Blades with span moment closer to the tip will be easier to auto because of inertia. Arguing easier to auto FB vs FBL is quite subjective. In my opinion the blades center of gravity is what changes things. Most FBL blades have an outboard CG compared to the same blade in the FB version.

[Felix the cat]

FB maintains momentum but is also moving "extra" weight / mechanics on autos.

FBL doesn't have this extra weight / mechanics but either has the "extra" inertia.

However, learning on FB will sharpen your flying skills much better than starting off with FBL. IMHO

[Boeingpete]

Originally Posted by extrapilot

Spiff

Self-level is what you mean, but it is not what is happening. That is, upright, the helicopter can act like a pendulum, where its mass being below the rotor causes it to want to seek vertical/level. Inverted, this is the opposite, and it wants to roll/pitch 180. Level in this context just means- CG hanging like a pendulum vertically below the rotor hub

By mixing the FB at 45 deg instead of 90deg, there is a phase error (complicated subject, search for posts on rotor phasing if you care to know more) between the rotor and the FB. This results in some of the increased stability you talk about (i.e. in forward flight, without forward cyclic, the rotor tips back and the heli slows). But, it also creates the wobbles people have termed ‘swinging pendulum’ and ‘toilet bowl effect.’ These are not beneficial, and makes acro ugly and imprecise.

This is not a function of CP vs FP, but it is seen in FP more frequently as these machines are typically indoor or trainer machines, and (again, for complex reasons involving relative mass/stiffness of the rotor) tend to have a phase lag much less than 90deg already.

FB and FBL both do a similar task- they create a reference plane (i.e. the FB spins as a gyroscope, the FBL has internal gyros), and when the machine’s rotor moves out of the reference plane, the control system is implemented such that the rotor is biased back into the original plane via cyclic input. This makes the rotor stable. It does not make it seek level, because that implies the FB or the FBL know what ‘level’ is. They don’t.

There is a fundamental difference between FB and FBL. It is that FBL is about angular rate, and FB is about angular displacement. That is to say, with FBL, if your control system happens to be set so that 10deg cyclic stick yields a cyclic rate of 10deg/sec, it will maintain that regardless of rotor speed, winds, CG, etc. So in that example, if you push the stick 50deg forwards, the FBL machine should have pitched 50deg forwards in 1 second, 100deg in 2 seconds, etc.

With FB, the impact that 50deg stick input had has depends on a lot of things- rotor blades, paddles, RPM, mass, etc. So at 3000RPM, maybe your roll rate is X, and at 4000RPM maybe it is X*2- not super consistent. But, FB tends to react more quickly, since it is analog and has no delays in the control loop. So, talented pilots with FB can do amazing things- but they tend to leverage governors and be very careful about maintaining consistent mass and CG to minimize the amount of variability FB can cause.

I'm just transitioning from my CP Nano and FP 200SRX to a FB 450 clone, and while the above won't make me a better pilot it sure puts things in perspective for me as my head was going 'tilt' when people talk about converting to FBL. now I understand what is going on. Excellent explanation. As my old flying instructor would say, "explanation equals understanding".

[npomeroy]

And of course there are FBL units that have an additional true self-leveling function.