3D Robotics

3689451270?profile=originalMy kids love the Syma S107 toy helicopters, which cost just $20 and are amazingly stable. The big breakthrough for this generation of toy helicopters was the introduction of gyros, but I just took apart a broken one and was impressed to see that it has just ONE gyro (circled in black above), mounted vertically. 

I know that the coaxial props create a natural mechanical stability in the XY direction, and I assume that gyro is stabilizing in the Z (yaw) direction.  But is it really as simple as that? The stability is astounding. They're just motionless in the air until you budge the sticks. 

BTW, there are no magic sensors on the bottom of the board, either:


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  • Hi, everyone,

        New to the forum.  Life-long flight enthusiast, returning to RC related hobbies after a couple of decades away.  Hope I'm not being rude here, but this is an interesting topic.  I love these little coaxials.

        I agree that the flybar is not integrated with the controls.

        The flybar is definitely stabilizing, and therefore resisting changes in attitude.  But the craft is returning to the vertical because it's center of gravity is aligned directly below it's center of lift.  If you take some stuff off the back, it gets nose-heavy and tends to fly forward instead of hovering.

        The flybar apparently just keeps it from doing wild pendulum swings and then hitting the floor.

        If you hold the s107 tilted away from yourself at 15, 30, or even 45 degrees, throttle it up (without touching the other controls) and release it, it will level itself off, even if the tilt is to it's side (bank).  Make sure you have plenty of room (I flew mine into a wall once doing that little experiment).

        Since there are no control mechanisms for banking, I conclude that the flybar and weight distribution have that effect independent of control mechanisms, and the tail-prop is just fighting that effect when it's engaged.  (also mine had a bad tail-motor when I first did that, so it definitely wasn't the logic board doing it.)

        So the digital gyro is only being used by the board so it can hold back on that tail-motor, and restrict how far forward or backward it will tilt.  That's also the main reason it's so vulnerable to wind.  To fight the wind, it would have to tilt over more than that paltry 5 or 10 degrees.

        I am working on how to put a radio in the s800g without the stock board and see what it might be able to do without that digital leash.  Having trouble getting hard information about the motors and so forth.  Open to suggestions.  Arduino maybe?  Or maybe that needs to be a separate thread?

  • Yeah but, the S107 doesn't have a Bell control system.  The controls do not go through the flybar in anyway.  There is no control mixing.  And in any case, when that article talks about "lag", they don't mean in the gyroscopic manner.

  • It's a combination of blade pitch and gyro effect. I think they call it bell-hiller stabilization.  Check out this e-how page for a little more details. 

    On the Bell system, maneuverability is sacrificed for stability. Because the Bell system hinders gyroscopic forces, control inputs lag slightly while the the main rotor works to catch up with the stabilizer bar. With the Hiller system, control inputs are immediate as this stabilization system uses an aerodynamic method to stabilize the rotor system. This latter feature makes the Hiller-type helicopters more desirable for aerobatic activities.

  • There is no gyroscopic effect in the classical sense.  The flybar is hinged, and is free to deflect relative the main shaft.  It's only effect is the manner in which it controls the cyclic angle of the rotor, and the effect that has on airframe.  And the rotor disk doesn't have near enough mass to create a gyroscopic effect.  It's control input is completely due to aerodynamic effect of the thrust vectoring.  

    If you want to test it, remove the link between the flybar and the rotor blades.  You will have no stability anymore.

  • Basically the top blade is subject to two gyroscopic effects, the blade itself and the stabilizer bar, which is 45 degrees lagged relative to the blade.  This causes a dampening effect on the whole system.

    Ok, this simple graph may help to illuminate how the stabilizer lag/dampening effect works:


    This is just modelling changes in gyro effect, as a simple sin wave.  The blade is the blue line, the stabilizer is the pink.  The yellow line above is the net of the gyroscopic effect due to the blade and the stabilizer bar. Basically the model shows that the 45 degree lag is causing the magnitude of the net gyroscopic effect is lower than each of the effect due to the stabilizer or the blade itself.  That's the dampening effect due to stabilizer lag.

  • A couple comments:

    @Ellison, I'm not quite sure exactly what you're saying.  I don't know if it's correct to say that the flybar causes "lag".  But it dampens everything.  The same reaction speed can be achieved by overdriving the input.  You still benefit from the stability, however.

    I can say that when you combine the IMU with a flybar, a single rotor heli is exactly as stable as one of those Coaxials.  In fact, my "loiter" performance is better in Stab mode than it is in Loiter mode. ;)  It just sits there, and does not require constant corrections.  The only input needed is to counteract the movement of the airmass (wind).  I hope to shoot some video this summer of my 6 year old hovering my big heli.

    A flybar is just a perfect PD stability controller that doesn't require much user input or tuning.  Although, it can probably be said that it required significant tuning, but that was done by the heli designer.

    I believe there's no good reason to remove a flybar on a heli, other than the fact that they do soak up some power, and you can get better aerodynamic performance out of them.  I know I just said that the same reaction speed can be achieved by just overdriving the flybar, but I mean that within the context of "normal" helicopter flying.  3D performance is limited with a flybar, but that's not what we're interested in.  The only other reason to remove the flybar is mechanical simplicity and possible vibration reduction.

    @John, I think the problem of Coax helis in the wind is not necessarily due to anything inherent in the Coax design.  I think it's that they were designed to be super stable, and thus can't pitch and roll enough to fight the wind.  I modded a Blade Cx to make it more responsive, but it got to the point where the rotor disks could move so much that they ended up crashing into eachother...  So there is a limit, but they can be made to fly in light winds.

    @Melih, I don't think it's technically correct that they are a gyroscope.  They do not directly stabilize the heli by direct gyroscopic effect.  The gyroscopic effect is used used to control the rotor disk, which is in turn what stabilizes the heli through thrust vectoring.

  • Chris,  I think you are wrong about "just one gyro". 
    Yes PCB have only one electronic gyro but, it is only for yaw. Main gyro is the weights on top. It is a mechanical gyroscope and stabilizing the helicopter on pitch and roll axis. And of course mechanical gyroscopes better stabilizers :) 

  • Developer

    Coaxial's are great for stability but terrible with wind. Some time ago I bought a 450 size coaxial helicopter, hoping I could use it as a UAV platform not needing a IMU. But even the smallest breeze would carry the copter like a leaf, no matter how much you pulled the sticks. In comparison the Hiller system you find on most R/C helicopters are great for maneuverability and flying in wind, but will not hover without constant corrections.

  • Definitely the stabilizer bars.  They don't call them that for no reason. :-)

    Notice also the pitch of the top blade is governed by the stabilizer bar, which counters any abrupt changes in pitch and roll.  As others have mentioned this causes lag.  In stabilize mode our flight control software should mimic the same behaviour, so we fly flybarless.  We'll loose agility because of the dampening effect.

    Even old Bell 204 helis have them:


  • Chris, now you know why I've been hesitant to take off the flybar on my Heli. ;)

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