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Simple way to amplify voltage to RC helicopter tail rotor?

  1. Nov 28, 2009 #1
    My brother bought a http://www.centrix-intl.com/details.asp?productid=4909" [Broken] RC helicopter. It has a tail rotor which is mounted parallel with the ground and enables the chopper to go forwards or backwards. The helicopter goes backwards great, but it doesn't go forward too well. I would like to amplify the voltage delivered to the rotor motor. Should I just use a transistor voltage amplifier and use the voltage that is currently driving the rotor motor as the transistor base and then tap directly from VCC for the collector and use the emitter to drive the tail rotor motor?

    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Nov 28, 2009 #2
    To control the gain, you should have a small resistor Remitter between emitter and ground (assuming npn) for voltage control of the output current. The collector current is about (Vbase-0.6)/Remitter. A resistor in series with the base also works, but you have less voltage control. The motor should be between the Vcc and collector. This won't work if you need more than the voltage difference Vcc - gnd. If you need to control motor voltage rather than motor current, use a pnp emitter follower on the collector output of the npn.
    Bob S
  4. Nov 28, 2009 #3


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    You would need to find out more about the schematic of the helicopter.

    If the rotor can make the tail of the helicopter go up or down, then it must have a reversible fan in it. So, it is probably in a H- Bridge configuration.

    This means you will not be able to improve the circuit with simple techniques.

    But, assuming the helicopter was designed properly, maybe it has a fault?
    Why not just take it back to where it came from and ask for another one?
  5. Nov 28, 2009 #4
    Oh yeah. I didn't even think about that.

    Good idea. It seems hobby places are understandably picky about helicopter returns, but this one has never flown forward well.
  6. Nov 28, 2009 #5
    Hypothetically, does it simplify things knowing that the H-Bridge is good for one polarity but not the other? Meaning, couldn't the H-Bridge output drive the chopper when going backward and be used to control the amplifier gain when going forward?
  7. Nov 28, 2009 #6


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    Assuming there is a fault, these symptoms would help to isolate it.
    You would examine the waveform being applied to the motor in each direction and if there was a big difference, then trace the cause of this until you find a faulty or incorrect component.

    There would be almost no chance of someone fixing this with random addition of transistors. This is complex circuitry and improving it would be difficult. It would have a H_Bridge and pulse width modulation.

    Just guessing here, but could there be a mechanical faiult?
    Like the fan blade slipping on the motor shaft in one direction, but getting just enough friction in the opposite direction?
    If the motor was being driven correctly, this would be worth checking.
  8. Nov 28, 2009 #7
    It is PWM driven. I hooked a scope to it to measure Vavg and got:
    Full forward 700mv
    Idle -515mv
    Full back -700mv

    I think you may be right about the slippage. The fan blade pulled right off.
  9. Nov 28, 2009 #8
    Is it a symmetrical blade--no camber?
  10. Nov 28, 2009 #9
    Hi Phrak,

    Correct, there is no camber. It is this one: http://www.centrix-intl.com/details.asp?productid=4909 [Broken].
    Last edited by a moderator: May 4, 2017
  11. Nov 28, 2009 #10


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    A good test might be to reverse the connections to the motor (if this is possible).

    This would reverse "up" and "down", of course, but if the helicopter now flew forward properly but flew backwards poorly, then there would be a problem with the circuitry.

    If it still flew forwards badly, there would be a problem with the motor or the fan.

    If the fan blade is slipping, you can put some sewing cotton or fine fishing line in the hole before you put it on the shaft. This might be enough to let it get a grip.
  12. Nov 28, 2009 #11
    It's impossible to tell from the photographs if the tail prop is cambered, but if the convex surface on one side of the blade is equal to the other side, it's uncambered. Flip the propeller over and see if there's a difference in performance.

    Suspend the helicopter from center of the main rotor between two fingers. Is it slightly nose heavy?

    Is there greater downward drag on the nose rather than tail, due to downwash, such that it would prefer an orientation where the plane of the rotor tilts slightly forward?

    First get these easy issues out of the way, then consider the electronics, if need still be.
    Last edited by a moderator: May 4, 2017
  13. Nov 29, 2009 #12
    Hi vk6kro and Phrak,

    Oh, yeah, the tail rotor is uncambered then.

    Good idea.

    Apparently my brother took it way up and crashed it on someone's roof. Now it just wants to spin and it is impossible to fly. We'll have to get that fixed first! :rolleyes:
  14. Nov 29, 2009 #13
    I should have said that it might be tail heavy. I little weight on the nose might cure the problem whether it's tale heavy or not.
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