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Old 04-21-2010, 05:45 AM   #1
Pigskin
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Lightbulb Possible fix for SR tail motor problems

Warning: This modification may or may not be dangerous and I am not recommending you do this, I am simply sharing an ongoing experiment. It is not my experiment.

Having said that, let me tell you about the experiment.Your mileage may vary

This is an experiment to find a potential fix, not a solution, to some common problems we have been having with the SR tail. The fix is cheap, and so far seems to work. It can be applied to the stock ESC and N60 motor setup for the SR.

The SR tail motor appears to suffer from a few problems:

1. Low life expectancy and fast motor burnout
2. Annoying angry bee buzzing sound
3. Excessive tail wagging

The theory behind this experiment is that these are all caused by the same underlying problem, which is that the ESC, in combination with the feedback from the gyro, is driving the tail motor too hard and fast, resulting in too much power being delivered to the tail at peak, and then the gyo compounds the problem by oscillating that power on and off.

Whilst good balancing of the ESC power to the tail and setting the gain on the gyro can go some way to minimising these, we have been unable to eliminate them. Personally my main concern is issue number one, the low life expectancy of the motor. I've burned out two of them and I'm not putting up with it any more.

The fix is to put a 1 ohm wire wound resistor in series to the tail motor, which will act as a dampener between the ESC and the motor. This has the effect of smoothing out the peaks and slightly increasing the timeframe over which the power to the tail is delivered. It reduces the overall load on the motor by delivering the power more smoothly.

As I'm not an electrical engineer my analogy is that the resistor is acting like the oil in a suspension unit. It is dampening the sudden changes in the system. It still allows the spring to reach their full potential but stops the suspension arm from smashing into the top of suspension unit and destroying itself.

So the effect of the resistor is that the tail will be driven with a little less power, and changes requested by the gyro will take a little longer to take effect. This has the desirable side effect that the tail wagginess appears to be removed and the angry bee is not so angry. In tests so far the tail motor itself has stayed cold after a battery worth of flight. Long term this is likely to increase the life expenctancy of the motor.

The heli hovers nicely with the resistor in place. Pitch pumping does not affect the tail orientation using 1 ohm resistor. The gyro gain was increased from 75% to 80%.

As this change will affect the speed of response of the tail it might not suit everyone, particularly those who want to get more aerobatic performance from the SR. But then again, this model is designed more for intermediate flyers so I don't see that as an issue.

Note that the resistor will get HOT, care is required in the placement of it.

Other resistor values were tried along the way:

6 ohms - not enough power to hold the tail in place

2.2 ohms - stable hover. calm smooth running motor, able to pirouette both ways. Motor unable to hold the tail in climbs or gusts of wind.

As I said, this is not a solution, just a possible fix. I think E-Flite needs to reconsider their choice of tail motor in this case, perhaps putting the brushed coreless motor into the model as was originally described in their specification on the box.

If you have any comments on this approach which might help with the experiment please share them and I'll pass them on to the person who is working on this.
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Old 04-21-2010, 07:59 AM   #2
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You are now using your battery power to heat to the resistor instead of driving the tail. The resistor is NOT a "damper" like oil in suspension, it is merely a restriction to the flow of power.
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Old 04-21-2010, 08:25 AM   #3
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You are now using your battery power to heat to the resistor instead of driving the tail. The resistor is NOT a "damper" like oil in suspension, it is merely a restriction to the flow of power.
Thanks for the feedback. Yes, that is a side effect of this. The resistor heats up. It also stops the motor from heating up. If there is to much power being delivered to the motor then heat will be generated. The stock motors on the SR can get very hot after a flight on a battery. That is why they have a great big heat sink attached to them. Stick the resistor in the way and let that heat up instead, as well as smoothing out the power flow to the motor. Given that the heat generation has simply been moved from the motor to the resistor, this is still a pretty efficient mod and should not suck up extra battery life.

Bear in mind that the theory here is that the tail motor is being driven too hard by the ESC and gyro, even or possibly especially when the tail has been well balanced to counter the torque of the main rotor when using the ESC pot. So in fact we actively want to push less power to drive the tail.The aim is to give the motor enough power to do its job, but not so much that it overheats.

So the resistor can take the place of the motor and allow the heat to be safely dissipated without damaging the motor. That is something resisitors do well, and they are alot cheaper to swap than new motors.

With the stock motor and ESC the motor is going to heat up because the motor is not capable of converting all the power into movement, as it is being driven too hard. So one of the things this mod does is move the point at which the heat is generated.

But I agree it is a bit more complicated than like oil in a suspension. Bit it is also more complicated than simply being a restrictor on the flow of power, because of the effect of the feedback from the gyro. I used the example of the oil in a suspension system because it does indeed dampen the effect of the constant oscillation of the gyro feedback. It dampens it to the point where the tail wag can be eliminated. Ignore my analogy if it doesn't make sense to you.

I did say that this was a potential fix not a solution. A solution is likely to come in the form of a new more capable motor or an ESC that delivers less power to the stock motor.
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Old 04-21-2010, 12:12 PM   #4
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I am sorry, but electrically you have no idea what you are talking about.
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Old 04-21-2010, 12:15 PM   #5
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@ Samadhi, perhaps someone would respond better to your rebuttal if you worded it more politely and offered reasons as to why.
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Old 04-21-2010, 12:22 PM   #6
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Default Possible Fix

Electricity in and of itself IS a theory, yet to be proven scientifically! I just gotta put my 5 bob in...

Resistor's reduce the flow of current but can not control the oscillation. To do that you would have to frequency beat it with an oscillation crystal to normalize it.....I think

From Wikipedia, the free encyclopedia:

A resistor is a two-terminal electronic component that produces a voltage across its terminals that is proportional to the electric current passing through it in accordance with Ohm's law:
V = IR

I think I get what you are trying to get at but the resistor only reduces the amount of current flow. Too high or too low a current flow can damage electrical motors....I think
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Old 04-21-2010, 01:03 PM   #7
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Working with the capacitor across the terminals of the motor, the resistor forms a low-pass filter allowing the smoother signals to go through the motor and the high frequency signals (from the gyro switching) to be eaten by the capacitor. This will result in a smoother power signal to the motor, but I don't know where you are going to find a 1 ohm resister, I'm sure the wire to the motor has that much resitance.
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Old 04-21-2010, 01:03 PM   #8
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Quote:
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@ Samadhi, perhaps someone would respond better to your rebuttal if you worded it more politely and offered reasons as to why.
I am not trying to be rude, however I don't have time to write a detailed explanation right now.
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Old 04-21-2010, 03:35 PM   #9
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Samadhi is right, you don't know what you are talking about....But having said that, it doesn't mean you haven't found something of value or that it may work. Many great inventions were the result of hard work that didn't work out as planned. Your resistor is simply dropping the voltage to the tail motor slightly. It may be this lower voltage is really good for the motor. These motors are old technology and were originally made to work on lower voltages. It is posible you have stumbled on the first step of a solution. One of my questions for you is this, how does the new tail handle serious 3D??
Here is what I am thinking about your design and the test you have done so far. It is worth testing a higher ohmed resistor.....in combination with a bigger prop. You may have hit on a solution that would involve dropping the voltage to the motor, there by making the motor happy and then increase the size of the prop to gain back some thrust.
So, good job so far, lets test your theory with a few other mods. BTW, what value resitor were you using?? ......Keep those ideas coming.....J

Last edited by John Shipley; 04-21-2010 at 06:04 PM..
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Old 04-21-2010, 03:51 PM   #10
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Or... try dropping the size of the prop or going to one with reduced pitch. Fewer amps being pulled by the motor, less heat

That's the important thing to remember: you need to think of the motor as PULLING amps, not the ESC pushing them there. The motor pulls as hard as it's going to pull given a certain voltage.
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Old 04-21-2010, 04:22 PM   #11
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Quote:
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I am not trying to be rude, however I don't have time to write a detailed explanation right now.
But evidently you do have time to browse this forum!
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Old 04-21-2010, 04:32 PM   #12
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Some blow hard that needs a life will jump on with absolutely no ideas of his own and proceed to knock down the very people that may help others. This guy would have said the earth is flat as Columbus sailed around it...
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Old 04-21-2010, 05:19 PM   #13
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Originally Posted by dave22250 View Post
Some blow hard that needs a life will jump on with absolutely no ideas of his own and proceed to knock down the very people that may help others. This guy would have said the earth is flat as Columbus sailed around it...
+1 +1 +1 +1+1
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Old 04-21-2010, 06:02 PM   #14
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I have been hearing from some of the old timers that the resistors have been tried on this forum 3 years ago when they were using a geared motored tail. They ended up using diodes instead of resistors as they are a better voltage dropper without as much heat build up. This new motor may have some posibilities with diffrent props, pitches ect. If we can figure out that dropping the voltage will work, then we can use the best device for dropping the voltage. As most of you know, I use a futaba 401 gyro that has a delay knob on it. The dealy is on full delay because the electric motor is so quick. What this means is that we have some wiggle room as far as prop size and pitch.
In addition, there are some brushless tail motors in the test process. Most will work for lite weight 3D, but can not hold up for the serious stuff.....J
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Old 04-22-2010, 02:02 AM   #15
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Hi guys, thanks for the interesting and encouraging feedback. I'll pass on the ideas.

With regards to whether I know what I'm talking about, I said in my post I wasn't an electronics engineer so I guess that puts me in the category of not really knowing what I'm talking about. I'm a software engineer by profession. Luckily for me the person doing this does know what he's talking about.

I'll pass on more information as we progress. The feedback I got last night was that it was still working nice after a few batteries at keeping the motor cool.

One of the posts asked about different value resistors. We tried them early on, the results are posted in the first post.
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Old 04-22-2010, 07:14 AM   #16
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P, I was the one who asked what value resistor you were using. Resistors come with several values. One is the ohm value and an other is the watt rating. So I should have been more clear, we know it is a 1 ohm but how many watts is it rated?? J
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Old 04-22-2010, 07:42 AM   #17
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The resistor is currently a 1 ohm 1 watt wire wound resistor.

The tail motor draws about 1 amp at the stable hover point.

A diode will be tested also. I'll report results.

Last edited by Pigskin; 04-22-2010 at 05:24 PM..
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Old 04-22-2010, 05:31 PM   #18
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I also suggested using a little light instead of a resistor as an aid for orientation.

Last edited by Pigskin; 04-23-2010 at 10:02 AM..
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Old 04-22-2010, 06:35 PM   #19
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Hi
redgiki and John Shipley have the explanation bang on

Ultimately reducing current may help as long as their is sufficient power to hold the tail, a different wind motor or less agressive prop would be the ideal

You are indeed dropping the voltage to the motor a little and that in turn reduces the current. What it won't do is soften or filter the spikes - only reduce them

Your method is probably the easiest way to reduce the current, John's idea of a diode is good, these "block" a set voltage, 1.2v for silicon diodes in my memory is still working

Can you ask your friend if he has considered an "inline inductance", I've not used these in practice, but the theorey goes that they limit transient spikes but pass lower frequencies, they often have some resistance. If the damage is due to spikes - then this may also help. It is possible that a wirewould resistor also has some natural inductance

On a side note,I've a couple of brushless motors on order to try

Ian
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