Simple way to amplify voltage to RC helicopter tail rotor?

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Discussion Overview

The discussion revolves around the challenges of amplifying voltage to the tail rotor motor of an RC helicopter to improve its forward flight capabilities. Participants explore various electrical configurations, potential faults, and mechanical issues affecting performance.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • Sam suggests using a transistor voltage amplifier to increase the voltage to the rotor motor, asking for advice on the configuration.
  • Bob proposes using a resistor between the emitter and ground for voltage control and discusses the limitations of the circuit if more than the voltage difference Vcc - ground is needed.
  • Some participants speculate that the helicopter may use an H-Bridge configuration, which complicates the circuit and may indicate a fault if performance is lacking.
  • One participant raises the possibility of mechanical faults, such as the fan blade slipping on the motor shaft, affecting performance.
  • Measurements of the PWM signal applied to the motor indicate significant differences in voltage when the helicopter is moving forward versus backward.
  • Suggestions are made to test the motor by reversing connections to diagnose performance issues.
  • Discussion includes considerations of the rotor blade's camber and its impact on flight performance.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the potential causes of the helicopter's performance issues, including electrical configurations and mechanical faults. The discussion remains unresolved with no consensus on the best approach to address the problem.

Contextual Notes

Participants note the complexity of the circuitry involved, including the potential for pulse width modulation and H-Bridge configurations. There are also mentions of mechanical issues that could affect performance, which have not been fully explored.

Who May Find This Useful

Hobbyists and engineers interested in RC helicopter mechanics, electronics, and troubleshooting may find this discussion relevant.

neergmas
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My brother bought a http://www.centrix-intl.com/details.asp?productid=4909" 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?

Thanks!
Sam
 
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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
 
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?
 
vk6kro said:
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.

Oh yeah. I didn't even think about that.

vk6kro said:
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?

Good idea. It seems hobby places are understandably picky about helicopter returns, but this one has never flown forward well.
 
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?
 
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.
 
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.
 
Is it a symmetrical blade--no camber?
 
Hi Phrak,

Correct, there is no camber. It is this one: http://www.centrix-intl.com/details.asp?productid=4909 .
 
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  • #10
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.
 
  • #11
neergmas said:
Hi Phrak,

Correct, there is no camber. It is this one: http://www.centrix-intl.com/details.asp?productid=4909 .

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.
 
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  • #12
Hi vk6kro and Phrak,

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.

Oh, yeah, the tail rotor is uncambered then.

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

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:
 
  • #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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