How can the Syma toy copters be so stable with just one gyro??

My 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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Comment by Jonathan M on April 1, 2012 at 2:12pm

Chris, I have one of those little guys as well and am astounded at the stability.  I really believe that you gain a lot with the counterbalances in plane with the top rotor.  if you play with the heli in your hand with the throttle on, you can tell it's counteracting outside disturbances pretty well in the pitch and roll rotations.  Remove those little suckers and I bet it wouldn't fly well at all!

Comment by Veikko Vierola on April 1, 2012 at 2:44pm

From internet:

The Hiller control system was invented by Stanley Hiller in the 1940s. This was originally called the "Rotormatic" control system, and was so stable the first prototypes could be hovered hands-off for minutes at a time.

For a Hiller control system, the upper swashplate does not directly control the main blade pitch. Instead, it indirectly controls the main blade pitch by changing the pitch of the flybar paddles. As the flybar seesaws up and down, it changes the main blade pitch.

Two advantages of the Hiller system are:

  • It places less stress on the swashplate servos because they only control the pitch of the flybar paddles.
  • The flybar paddles dampen pitch and roll which improves stability.

One disadvantage of the Hiller system is the lag in control response. The flybar paddles must change their plane of rotation in order to change the main blade pitch.

Two popular helis which implement the Hiller control system are the Piccolo ECO/Fun and the GWS Dragonfly.

Comment by Brad Hughey on April 1, 2012 at 3:08pm

What Veikko said was true, but there are no swashplates or paddles on my Syma.  The flybar on the S107 is gyroscopic, and changes the pitch of the upper rotor to counteract any movement relative to the lower rotor and fuselage.  It is an elegant solution.  That they can sell these at retail for $20 is astounding.

Comment by Doug Weibel on April 1, 2012 at 3:33pm

Many of this style coaxial helicopter have excellent stability, due to the large stabilizer weights and long bar on which the stabilizer weights are suspended.  Also, the tall mast adds to the stability.  Many manufacturers have had great stability with this setup even before the introduction of small/cheap gyros.  Of course, the mechanical stability comes with a price.  They have very limited forward speed and other than yaw control have almost no maneuverability.

Comment by mquintilian on April 1, 2012 at 5:33pm

Doug is correct,

If you use a co-ax heli withough a gyro they are still quite stable but the yaw axis drifts even after trimming

Comment by Michael Zaffuto on April 1, 2012 at 6:38pm

My daughter and I each have a EagleJet from Protocol..pretty much the same copter with RF control.  They are amazingly stable..indoors.  There has to be absolutely no wind to fly it outside.  The winning Pixhawk 2009 machine uses the LAMA 400 counter rotating mechanics,  top blades with stabilizer but with swashplate on lower blades for directional control...Much simpler stability requirements compared to other methods as it helped them win the 2009 indoor autonomy competition :)

Comment by Rob_Lefebvre on April 1, 2012 at 7:43pm

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

Comment by Ellison Chan on April 1, 2012 at 7:49pm

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:

Comment by John Arne Birkeland on April 1, 2012 at 8:58pm

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.

Comment by Melih Karakelle on April 2, 2012 at 4:44am

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 :) 


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