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Old May 13, 2009, 01:22 AM
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Lead Wings's Avatar
Perth, Australia
Joined Apr 2007
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Question
How do the self-stabilising Helis work??

One of the greatest revolutions in RC helis in the last few years is the self-stabilising head. They appear on single rotor choppers through to the coaxial helis.

Now from what I can tell, the stabilising system just consists of a fly bar with some weights on it at about 20-30 degrees leading the main blades connected by some linkages. Simple.

But how does it work?? This system somehow keeps the heli stable almost like a pendulum effect the heli comes back to a stable hover. So gravity is involved, but it seems to be much more than the fact the mass is below the centre of lift the blades / fly bar system are somehow adding stability.

Someone care to explain in simple to understand English (ie no complicated physics formula please!!)?? What are the critical design parameters angle of the fly bar to the blades, weight of the weights on the fly bar, shape of the main blades??
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Old May 13, 2009, 12:02 PM
Fly Runaway Fans
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From fellow member Markelew: http://www.rcgroups.com/forums/member.php?u=253694

It's not 'simple' but it's not formulas either. Probly have to read it more than once.
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Old May 13, 2009, 12:38 PM
I'm Ginger & called Adam
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Joined Oct 2002
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I think (not sure) that the angle is 45deg. The best way to understand how these self-stabilise it to spin a bicycle wheel whilst holding the axle ends in each hand - now try to change the angle and feel what forces and direction result. On a chopper these forces are used to change pitch to correct that movement.

The above is guesswork on my part - I'd love to see a real explanation

Adam
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Old May 13, 2009, 12:45 PM
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The reasons a Coax trys to return to a neutral (to the lateral airflow around it) hover, is that the advancing and retreating blades have different "relative" airspeeds, and thus generate different lift (the blade advancing into the lateral airflow making more lift, the one retreating making less). Because the "lift differential" forces are imparted through the spinning blades/head, the forces are delayed by gyroscopic precession (at less than the conventional 90deg because of the low RPM).

The two rotors are rotating opposite directions to each other, so their phasing differential cancels each other out and causes the rotors to want to make the heli "pitch up" away from the source of the lateral airflow (at greater than 2x the amount of a conventional single rotor fixed pitch, 2x because of double the rotors, greater than that because of the lower headspeed involved creating increased lift differential).

My little Blade MCX for example, has a 7.5" rotor diameter and hovers at 1360rpm (that works out to barely 30mph rotor tip speed). The mainblades are a simple undercambered airfoil because of two reasons, 1, that works well at low speed (think slowstick), and 2, that shape is cheap to mold/produce.

A single rotor heli does the same thing, but is countered by the flybar paddle's angle of attack (the mainblades want to "pitch up" but the flybar paddles are angled down by the angle of attack of the body of the heli and the control inputs to counter this force to some degree). A properly designed, setup, and tuned 3D heli, remains neutral by this balance of forces (so whatever angle you set it at, it stays there, at least until it's going so fast the retreating blade is stalling).

-Kai

P.S. The flybar on our typical coaxial fixed pitch helis only acts as a damping gyroscope upon the upper rotor, to dampen out turbulence and radical control movement effects (and the reason it's only at 45deg advance instead of 90deg like larger single rotor helis, is because the mass and RPM's involved in coaxials are too low to fully engage gyroscopic precession).
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Old May 13, 2009, 01:00 PM
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Sorry that my explanation is geared mostly towards coaxials, I just figure that the easiest place to start (and I copied/pasted my explanation from another thread, LOL).

-Kai
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Old May 13, 2009, 06:26 PM
How does it work? How did it?
mareklew's Avatar
Karlsruhe, Germany
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Quote:
Originally Posted by Kai_Shiden
The reasons a Coax trys to return to a neutral (to the lateral airflow around it) hover, is that the advancing and retreating blades have different "relative" airspeeds, and thus generate different lift (the blade advancing into the lateral airflow making more lift, the one retreating making less). Because the "lift differential" forces are imparted through the spinning blades/head, the forces are delayed by gyroscopic precession (at less than the conventional 90deg because of the low RPM).

The two rotors are rotating opposite directions to each other, so their phasing differential cancels each other out and causes the rotors to want to make the heli "pitch up" away from the source of the lateral airflow (at greater than 2x the amount of a conventional single rotor fixed pitch, 2x because of double the rotors, greater than that because of the lower headspeed involved creating increased lift differential).

My little Blade MCX for example, has a 7.5" rotor diameter and hovers at 1360rpm (that works out to barely 30mph rotor tip speed). The mainblades are a simple undercambered airfoil because of two reasons, 1, that works well at low speed (think slowstick), and 2, that shape is cheap to mold/produce.

A single rotor heli does the same thing, but is countered by the flybar paddle's angle of attack (the mainblades want to "pitch up" but the flybar paddles are angled down by the angle of attack of the body of the heli and the control inputs to counter this force to some degree). A properly designed, setup, and tuned 3D heli, remains neutral by this balance of forces (so whatever angle you set it at, it stays there, at least until it's going so fast the retreating blade is stalling).

-Kai

P.S. The flybar on our typical coaxial fixed pitch helis only acts as a damping gyroscope upon the upper rotor, to dampen out turbulence and radical control movement effects (and the reason it's only at 45deg advance instead of 90deg like larger single rotor helis, is because the mass and RPM's involved in coaxials are too low to fully engage gyroscopic precession).
I'm sorry, but what you wrote isn't true.
1. 45 advance on coaxials is due to TWO rotors. Consider that both rotors result in precession forces.
2. Same about flybar advance
3. Differential lift isn't giving the required stability. Combination of flybar dampener with a really low COG is. If differential lift was enough to stabilize a heli, single rotors would fly as easy as coaxials.
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Old May 13, 2009, 06:55 PM
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Quote:
Originally Posted by mareklew
I'm sorry, but what you wrote isn't true.
1. 45 advance on coaxials is due to TWO rotors. Consider that both rotors result in precession forces.
2. Same about flybar advance
3. Differential lift isn't giving the required stability. Combination of flybar dampener with a really low COG is. If differential lift was enough to stabilize a heli, single rotors would fly as easy as coaxials.
Welcome to the conversation, too bad your manners leave a bit to be desired.

Gyroscopic forces require enough rotational speed to have full effect, just because something is rotating, does not mean that it suddenly has full 90deg gyroscopic precession. Spin a bell/hiller heli up only a little bit and give it some control inputs, you'll note the flybar's plane of movement is not perfectly on axis until you reach adequate headspeed and the axis of the flybar plane will vary depending upon the rotor rpm (up until you reach enough speed that it's at proper 90deg gyroscopic precession, after which it will behave "close" to properly). I say "close" to properly because while on the ground the head system is restrained and cannot free form react perfectly (it's fine once in the air).

Note, that one of the models pictured, is a single rotor heli with significant flybar advance to account for the lack of full gyroscopic precession having been reached. Single rotor stable systems are not as stable as dual rotors, but can still be quite stable at the right RPM's (that's why they tend to walk circles when descending, the gyroscopic precession changes the "stabilization advance").

PiccoZs and Hirobo Quarks are examples of self stable single rotor helis, try doing some research on your own next time instead of applying your own assumptions of how things work.

Pendulum effect as a stability factor in helis is a complete misconception. Low slung weight once set in a pendulum motion will want to continue rocking back and forth (the factors that get rid of it are the flybar weights acting as a gyroscope and giving input to the upper rotor of a Coax, and air drag on the airframe).

Damping is only that, it damps out changes of movement, it does not have a gravity seeking factor to itself. If you want proof that self stable helis don't rely upon gravity, just go fly one out in a steady wind and you'll note that it wants to move laterally at the same speed as the wind.

Go ahead and bring up that if you hang a weight from the tail the heli will fly backwards. That is not proof that gravity is a factor, that just proves that if you imbalance an aircraft it will affect it's flight characteristics. Make sure that when you do that, you also positively identify where the CG is in relation to the rotors, you'll find that the rotors do not sit straight above the new CG that you've created (the angle will be less because the heli's rotor will resist the imbalance through dissemetry of lift).

I stand by everything that I wrote as being the truth, if not, prove me wrong . If you choose not to believe me, at least read up on some sound helicopter theory, Colin Mill's articles are a great read no matter your interest level: http://www.w3mh.co.uk/articles/html/csm9-11.htm

-Kai

P.S. All of this has been discussed before, but the thread kept going off topic (in fact, coaxial stability was off topic from the original question), read the thread before you start making acusations: http://www.rcgroups.com/forums/showthread.php?t=965321
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Old May 13, 2009, 07:28 PM
How does it work? How did it?
mareklew's Avatar
Karlsruhe, Germany
Joined Mar 2009
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> Welcome to the conversation, too bad your manners leave a bit to be desired.

But matched by yours, as it seems.

> Gyroscopic forces require enough rotational speed to have full effect,
> just because something is rotating, does not mean that it suddenly has full
> 90deg gyroscopic precession.

I believe we can agree, that precession ALWAYS happens at exactly 90 to both rotation vectors - one for the mass spinning (axle) and the one for attempted tilt. The only thing that varies is the proportion between the speed at which all these rotations in concern.

I believe also, that we can agree on the fact, that on counterrotating helis the rotor rpms as well as inertias can be assumed to be close enought to be considered equal. So regardless of how small they are, the sum of both needs to be taken under consideration when discussing controls of a coaxial heli.

> Spin a bell/hiller heli up only a little bit and give it some control inputs,
>you'll note the flybar's plane of movement is not perfectly on axis until you
> reach adequate headspeed and the axis of the flybar plane will vary
> depending upon the rotor rpm (up until you reach enough speed that it's at
> proper 90deg gyroscopic precession,

Actually, if you want to watch it real close, it NEVER moves at exactly 90. However it moves at an angle close ento 90. But that's not the point here.

> Note, that one of the models pictured, is a single rotor heli with significant
> flybar advance to account for the lack of full gyroscopic precession having
> been reached.

Note, that that advance of flybar on that astronaut model is close to 90 if not at 90.

> PiccoZs and Hirobo Quarks are examples of self stable single rotor helis,

I believe I contradicted your statements related to coaxials. I do not have experience as to how Quarks are stabilized.

> try doing some research on your own next time instead of applying your
> own assumptions of how things work.

Actually I did, thank you.

> Pendulum effect as a stability factor in helis is a complete misconception.

Yet it is responsible for the limited FF speed of flybared coaxials. Check it, move battery up and see. Sure, the heli will be still stable, but will have much larger "throws".

> Low slung weight once set in a pendulum motion will want to continue
> rocking back and forth

Try flying e.g. a Lama in a quick forward-reverse motion, and if you manage to aviod blade clash you will observe exactly that pendulum effect. I didn't say it's perfect, I said it's used.

> Damping is only that, it damps out changes of movement, it does not have
> a gravity seeking factor to itself. If you want proof that self stable helis
> don't rely upon gravity, just go fly one out in a steady wind and you'll note
> that it wants to move laterally at the same speed as the wind.


It would be real nice, if you didn't attempt to contradict statements I have never made. Actually, quite the opposite, I have suggested, that low COG contributes to stability, which you tried to ridicule a paragraph above.

> Go ahead and bring up that if you hang a weight from the tail the heli will
> fly backwards. [...]

Do you want to make up still something I might say that you find easy to counter, or might you stay within what I actually have written?
And yes, an imbalanced the heli will fly back/side/forward depending where the imbalance lies.

> Make sure that when you do that, you also positively
> identify where the CG is in relation to the rotors, you'll find that the rotors
> do not sit straight above the new CG that you've created

The vertical component of lift will pass exactly through the COG unless the heli can't keep itself in the air. If COG isn't on the main shaft axis, rotor plane will have to effectively tilt and thus produce horizontal thrust. As the main shaft will cease to be vertical at this point, a definition of COG being "exactly under the rotors" is no longer precise. Where IS exactly undet the rotors? Under the centre of the lower, the upper rotor, or at mid-point in between? Or somewhere else, like where the effective thrust vector is placed? BEcause in the last case, yes, it will stay exactly under the effective anchor point for vertical thrust component.

> I stand by everything that I wrote as being the truth, if not, prove me
>wrong . If you choose not to believe me, at least read up on some sound
> helicopter theory, Colin Mill's articles are a great read no matter your
> interest level: [URL]http://www.w3mh.co.uk/articles/html/csm9-11.htm

I know his articles, I have read them all. Now be a good guy, and since I cared to read what you've written, now read up on coaxial controls.

Marek

EDIT: a quoted paragraph strayed at the end of my posting
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Old May 13, 2009, 07:58 PM
Fly Runaway Fans
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I don't recall reading where the angle of precessive force varies with speed. But then there are a great many things I have never read.

The amplitude of precessive force, I'll buy.
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Old May 13, 2009, 08:01 PM
How does it work? How did it?
mareklew's Avatar
Karlsruhe, Germany
Joined Mar 2009
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Angle of precessive torque never varies with speed, is always at 90 but the direction heli tilts does, as the tilt is combination of the tilt that caused precession and one that was caused BY precession. Proportion between these is related to intertias along both roll axes and rpms.

Edit:
think of a bike wheel hung by the end of the axle. If it doesn't spin, the gravity wil rotate it flip-down. The faster it spins, the thighter spiral will the free end of axis encircle on its way down. But the axle will never remain perfectly level indefinitely, even very fast wheel will end up hanging down with it's axle vertical, albeit slowly.
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Old May 13, 2009, 08:31 PM
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Quote:
The faster it spins, the thighter spiral will the free end of axis encircle on its way down.
The amplitude whereof I spoke.
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Old May 13, 2009, 08:55 PM
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Cool, you want to go "tit for tat", I can do that. First thing I want to get out of the way, I have nothing against you personally, we both need to make sure to keep this discussion cordial. If anything I say offends you please let me know in PM and I will review what I've posted and remove it if it crosses the line of what's appropriate. Right up front I will state, I make jokes while posting.

Quote:
Originally Posted by mareklew
But matched by yours, as it seems.
Yup, when matched in kind, I simply stated my explaination of it (note no attack of your ideas until you attacked mine).

Quote:
Originally Posted by mareklew
I believe we can agree, that precession ALWAYS happens at exactly 90 to both rotation vectors - one for the mass spinning (axle) and the one for attempted tilt. The only thing that varies is the proportion between the speed at which all these rotations in concern.
Yup, I can agree with that.

Quote:
Originally Posted by mareklew
I believe also, that we can agree on the fact, that on counterrotating helis the rotor rpms as well as inertias can be assumed to be close enought to be considered equal. So regardless of how small they are, the sum of both needs to be taken under consideration when discussing controls of a coaxial heli.
Of course, that's the primary selling point of coaxials of any scale.

Quote:
Originally Posted by mareklew
Actually, if you want to watch it real close, it NEVER moves at exactly 90. However it moves at an angle close ento 90. But that's not the point here.
Yup, that's why I mentioned "close to properly", so you are also attacking something I didn't say, I never said it would move at exactly 90.

Quote:
Originally Posted by mareklew
Note, that that advance of flybar on that astronaut model is close to 90 if not at 90.
I have seen that astronaut model in person, the rotor on it works a little bit differently since it's aerodynamic in nature as well (not just a neutral aerodynamic weighted bar) and no it's not at 90deg (the rotor system on that is in fact and interchangeable copy of a PiccoZ).

Quote:
Originally Posted by mareklew
I believe I contradicted your statements related to coaxials. I do not have experience as to how Quarks are stabilized.
Quote:
Originally Posted by mareklew
If differential lift was enough to stabilize a heli, single rotors would fly as easy as coaxials.
Nuff said?
Quote:
Originally Posted by mareklew
Actually I did, thank you.
Too bad it didn't stick (that's a joke btw, you have some well thought out ideas in your posts, but unfortunately your theories have some behavioral holes)

Quote:
Originally Posted by mareklew
Yet it is responsible for the limited FF speed of flybared coaxials. Check it, move battery up and see. Sure, the heli will be still stable, but will have much larger "throws".
Only according to your theories... Moving the battery only proves that with CG changed upwards, the heli body will resist input changes to angle less (which is called "polar moment of inertia" and still perfectly within the factors that I have mentioned).

Quote:
Originally Posted by mareklew
Try flying e.g. a Lama in a quick forward-reverse motion, and if you manage to aviod blade clash you will observe exactly that pendulum effect. I didn't say it's perfect, I said it's used.
Blade clash is an effect of the flybar inputting cyclic resistive control to the top rotor, exactly as it is designed to do (that rotor flexes down on one side, while you're forcing the lower rotor to flex up, BANG impact),

Quote:
Originally Posted by mareklew
It would be real nice, if you didn't attempt to contradict statements I have never made. Actually, quite the opposite, I have suggested, that low COG contributes to stability, which you tried to ridicule a paragraph above.
Nope, not ridiculeing, I'm doing what's called "covering my bases", I've heard all the arguments you have to bring to the table before, so I'm saving us a few back and forth posts .

Quote:
Originally Posted by mareklew
The vertical component of lift will pass exactly through the COG unless the heli can't keep itself in the air. If COG isn't on the main shaft axis, rotor plane will have to effectively tilt and thus produce horizontal thrust. As the main shaft will cease to be vertical at this point, a definition of COG being "exactly under the rotors" is no longer precise. Where IS exactly undet the rotors? Under the centre of the lower, the upper rotor, or at mid-point in between? Or somewhere else, like where the effective thrust vector is placed? BEcause in the last case, yes, it will stay exactly under the effective anchor point for vertical thrust component.
Mark your CG on your heli and video it, I can promise you the CG will not be directly below the center of the rotors (there is a force called "torque" btw, which the rotors impart into the chassis, without it you wouldn't have control). Without torque, yes the vertical thrust component would have to be straight above the CG in order to maintain stable flight. I did not say the CG would not be directly under the center of lift, in fact it will be under the "effective" center of where the blades lift (however, at that time, that point will not be at the center of the rotor )

Quote:
Originally Posted by mareklew
I know his articles, I have read them all. Now be a good guy, and since I cared to read what you've written, now read up on coaxial controls.
I provided you my reference material, be kind enough to provide me yours so that I can stop you from spreading bad info to beginners (not referring to this thread but the other one linked to in your blog).

I have read your blog, and you are neglecting a primary factor of flybar behavior called "Chinese weights", the flybar weights try to get as far from the center of the mainshaft as they can over time (centrifugal acceleration dictates this, gyroscopic action resists this), where is the farthest point they can reach? straight out from the mainshaft (ie in the plane dictated by the rotation of the mainshaft). In your blog you wrote it off to friction (yes, I will admit that friction plays a small part, but it is not the dominant reason).

Your theory does not account for steady wind behavior (why a coaxial will move laterally at the same speed as the wind it's being subject to), any theory that cannot account for all observed behaviors cannot be considered complete. I challenge you to come up with any situation of flight that you have observed with a coaxial FP heli, that my explanation does not fit (FYI we went through this process in the other thread I linked to, just not formally declared).

-Kai

P.S. Gyroscopic precession does always occur at 90, however it is not always the dominant factor (it's a different form of inertia, it's a dynamic inertia, where an object not rotating is just regular inertia). You wouldn't expect that bicycle wheel sitting stationary to have gyroscopic precession resistance to movement just because it can spin, so you need to accept that the balance of inertia and gyroscopic precession behavior trades off depending upon the rotational speed attained.
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Old May 13, 2009, 09:09 PM
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Quote:
Originally Posted by arbilab
I don't recall reading where the angle of precessive force varies with speed. But then there are a great many things I have never read.

The amplitude of precessive force, I'll buy.
Sorry about that, I thought the original poster wanted a "simplified" explanation, you and mareklew are correct. I decided not to get into the whole balance of gyroscopic precession force and regular old inertial resistance (which are actually the same force with a rotational aspect behavior, see: http://en.wikipedia.org/wiki/Precession).

An object rotating slowly does not exhibit 90deg gyroscopic precession, it IS acted upon by the rotational offset of gyroscopic precession at exactly 90deg, but the effect of that will be offset by the original input force (ie, combine the two and it can appear to be happening at only a few degrees).

-Kai
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Old May 13, 2009, 09:39 PM
Fly Runaway Fans
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No worries, Kai. I'm fresh off a debate about whether HBFP flybar weights drain the battery. Physicists LOVE to argue. Including the 'real' ones, the PhDs. But we're mostly a civil lot, aren't we?
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Old May 13, 2009, 10:19 PM
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Quote:
Originally Posted by arbilab
No worries, Kai. I'm fresh off a debate about whether HBFP flybar weights drain the battery. Physicists LOVE to argue. Including the 'real' ones, the PhDs. But we're mostly a civil lot, aren't we?
Thanks arbilab, I'm hopeing that this entire discussion can be completely civil. In the end without having an actual engineer who designs these things step in we can only speculate and try to reach a consensus as to which explanation fits what we can observe.

I have participated in these "How does..." discussions quite a few times here on RCG (on all kinds of fun topics, Vortex Ring State, Retreating Blade Stall, etc.), the difference between me now and me then, is that I really don't care enough to do the calculations anymore . If I have something I want to tweak on my helis, I just go for it instead of spending hours calculating (it's so much more fun to just "dive in" and see what happens). At first glance dissymetry of lift seems like a minimal force in play on our helis. I myself wrote it off at first, it took flying an unstabilized flybarless heli for me to realize how much of a factor it is.

I was bored one day and set off a smoke cartridge in my front yard and verified that my MCX in a hands off hover moved right along with the smoke plume in the wind (wish I had video'd that, it was neat seeing the turbulence off the rotors, I'm still trying to find the space to make a wind tunnel to play with ). Even I question my theories until I can prove them in real life .

I abuse the heck out of my MCX, from shooting it out of the sky with a nerf vulcan, to launching it from on top of my Y-ufo (even made a little helipad). I've thrown it into the air with the power off and flown out, landed on a willing friend's hat bill (yes he was wearing safety glasses).

-Kai
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