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Old May 04, 2009, 05:14 PM
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Help!
Converting from Brushed to Brushless Motor

I am relatively a beginner or may be an intermediate or so I thought . I can fly Firebird Phantom (3 ch) pretty nice now. And I bought the RealFlight simulator in which I can fly 4 ch aircrafts with no problems whatsoever. So I thought I should give a real 4 ch airplane a try. So I bought the following (not from raidentech.com but just for your reference):

http://www.raidentech.com/new2035pmurt.html

But my experience was terrible.

The first throw (don't have any runway) it could barely fly 3 to 4 ft above the ground and very unstable (tends to roll left or right). I had to almost immediately (in few seconds) land/crash it. Everything was ok except of course the flimsy propeller that it came with broke.

So I replaced the propeller and gave it another try. This time it was worse than before. The plane immediately turned over upside down and boom! It crashed nose first. The wing came off and the motor shaft bent and of course the prop broke. I had stuck tape very carefully on almost the entire body and the wing, so it did not terribly broke into pieces.

I had taken a lots of care in assembly. I made sure that the rudder and elevator was correctly placed. I tested it before flight and they were moving up and down and left and right just fine.

I have fixed the motor shaft back in its place and have ordered some more props. While I am waiting for the props I am pondering as to what I did wrong or what can I do to make it better.

One option I was considering was to replace the 370 brushed motor it came with. I have a feeling that it does not have enough juice. A youtube video of the exact same plane shows that as well. And I have no idea how that would work. Can I just grab a 370 brushless and solder it in or I have to do something else?

Any other suggestion that anyone else might have?
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Old May 04, 2009, 05:16 PM
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One more thing to note is that the motor itself has a small gear and then on top of it there is another shaft connected to the motor gear via another gear that has atleast 5 times the diameter. The prop goes on that shaft. That really kills the rotational speed of the prop. Is that normally how it is done in other aircrafts or is it the stupidity of this airplane designers? Can anything be done about this, like hooking up the prop directly to the motor shaft, but it is not long enough though? Besides that would bring the prop about one inch down from it designed position (motor is below the other shaft). Just wondering.
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Old May 04, 2009, 05:33 PM
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You would need to change the ESC out to a brushless model as well. What you have is a brushed inrunner motor and the extra gear and shaft are known as the gearbox.
You could buy yourself a 370 inrunner brushless motor, and ESC and drop them right in. You will need to know what KV or speed of motor you need to fit the requirements, and also what pinion to fit your gearbox.

As for your planes reaction, it sounds like the ailerons were out of trim. On the simulator adjust the ailerons out of trim a few notches and see if that is how your plane was reacting. If so, that was your problem. I can't think of anything else that would make a plane roll like that. The fix is simple, hold in some opposite aileron, gain some altitude and start adjusting the trim the opposite direction until it flys level.

I think the motor setup that it came with should be fine. I wouldn't change it out until the brushes have given up.

Good luck and keep trying.
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Old May 04, 2009, 05:35 PM
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beaten to it by 30 seconds
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Old May 04, 2009, 05:47 PM
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The gears inside are known as a gearbox. It basically takes the motors high rpm and lowers the rpms so that a larger prop with a lot of thrust can be used (lots of thrust, but low top speed).

My guess is that you didn't cycle the battery. You need to cycle NiCD/NiMH batteries before they can put out their full capacity. Without cycling, your plane has almost no power to fly. Charge it up and discharge it 3-4 times, and it should be ready to go.

Also, let me highlight some false info here from that page you posted:

"Super easy and super stable electric powered scale R/C park flyer for beginner or middle-level users"

Marketing stuff here. They give false info so people are attracted into buying their junk. They could care less about your skill level, let alone your satisfaction. They care only about your money. Sad, but true. The P-51 Mustang is not a trainer ship. It's an intermediate plane, that would be an excellent first warbird, or low wing aerobat, not really a 2nd plane, although this one is made of EPO (if they are telling some true info here)which is highly durable.
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Old May 04, 2009, 05:59 PM
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I had one of those as my first planes. Stock power system, you will be luck y to get it in the air. If you do, the receiver has a range of about 100 feet, so that will get you.

To really make it fly safely, unfortunately you need a new receiver, transmitter, motor, speed controller, and battery.

You may be able to get away with the stock radio, looking at the pictures, it looks slightly different than mine. But I would do several range checks first to make sure it is safe, and don't fly it near anyone!
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Old May 04, 2009, 06:05 PM
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Thank you guys for the quick response.

Xpress: You are right. I never cycled the battery. In fact I am not even sure if I charged it enough. The stupid charger has no indication LED to tell me if it is charged or not. I will make sure I charge and discharge 3 to 4 times before using it next time. Besides I have to wait for my props to arrive anyway, so the temptation to fly before cycling 3 to 4 times won't be able to overcome me What should be good enough charging time to charge 8 AAA NiMH batteries (that's what it has)? Can I leave it overnight? Are NiMH as dangerous as lipo and I should not leave them overnight.

About the ailerons, visually they looked perfectly aligned. In the first flight, it was going left and right roll (not full just about 40 degrees or so from horizon) as I was moving my stick and trying to balance it. Could it be visually perfect but imperfect enough to cause problems in flight?

Also a stupid question: What is the difference between inrunner and outrunner motors?

Thanks for your time, guys!
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Old May 04, 2009, 06:23 PM
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Quote:
Originally Posted by The_Fuzz
I had one of those as my first planes. Stock power system, you will be luck y to get it in the air. If you do, the receiver has a range of about 100 feet, so that will get you.

To really make it fly safely, unfortunately you need a new receiver, transmitter, motor, speed controller, and battery.

You may be able to get away with the stock radio, looking at the pictures, it looks slightly different than mine. But I would do several range checks first to make sure it is safe, and don't fly it near anyone!
Now that's scary. Thanks for the heads up though.
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Old May 04, 2009, 07:55 PM
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Quote:
Originally Posted by shah
Thank you guys for the quick response.

Xpress: You are right. I never cycled the battery. In fact I am not even sure if I charged it enough. The stupid charger has no indication LED to tell me if it is charged or not. I will make sure I charge and discharge 3 to 4 times before using it next time. Besides I have to wait for my props to arrive anyway, so the temptation to fly before cycling 3 to 4 times won't be able to overcome me What should be good enough charging time to charge 8 AAA NiMH batteries (that's what it has)? Can I leave it overnight? Are NiMH as dangerous as lipo and I should not leave them overnight.
A fully charged NiMH or NiCD pack will be warm to the touch. This is because the cells chemistry is altered or something (you'd have to ask a battery guru on this one).

Quote:
About the ailerons, visually they looked perfectly aligned. In the first flight, it was going left and right roll (not full just about 40 degrees or so from horizon) as I was moving my stick and trying to balance it. Could it be visually perfect but imperfect enough to cause problems in flight?
It could be because the servo doesn't have the torque to center the ailerons, or there is some binding somewhere preventing the servo from centering them. Also warps in the wing can cause funky behavior.

Quote:
Also a stupid question: What is the difference between inrunner and outrunner motors?
Nonsense! No question is stupid here

An inrunner motor has the stator on the inside, and it rotates at a much higher rpm. Usually, inrunners are the motors of choice for wings, pylon racers, or fast planes, due to the fact that they can spin a small prop at high rpms, generating huge ammounts of pitch speed for those fast planes. Also, they're perfect for gearboxes, as the gearing takes the motors high RPM's, and lowers it dramatically so it can turn a bit prop with ease. There are low rpm inrunners however, such as the NEU 19 series inrunner motors.

Outrunners have the stator on the outside. The whole front part of the outter casing rotates, creating greater torque for larger props, but still maintaining low rpms. Outrunners are the motors of choice for 3D planes, as they can swing those big, wide bladed props (some have called them air shovels ) which generate huge ammounts of thrust with low pitch speeds, so essentially they can pull a big load and get it up to speed fairly quickly. Your P-51 mustang would be best with a 9" prop or so with an outrunner with about 900-1000Kv (Kv is Kilovolt, or basically the motors RPM's multiplied by the voltage- the more RPM's there are, the higher the Kv rating). Also, there are high RPM outrunners, such as Lil Screamers, some of which are designed for EDF units, which spin small props at high RPM's, to generate a lot of thrust, at the cost of effeciency.
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Old May 04, 2009, 08:16 PM
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Quote:
Originally Posted by Xpress..
(Kv is Kilovolt, or basically the motors RPM's multiplied by the voltage- the more RPM's there are, the higher the Kv rating).
Slight correction to the above info.

"Kv" (rpm per volt) and "kV" (kilovolt) aren't the same thing. A kilovolt is just 1000 volts...nothing to do with rpm. I assume that "Kv" is an acronym for "rpms per volt" in some language other then English, but I've never heard what the K actually stands for. Maybe its German or Latin?

Okay...I tried looking it up, and apparently the "K" in "Kv" just means "constant." And supposeldy the correct way to write it would be with the v in subscript: K<sub>V</sub>

Or maybe not...just some useless trivia for you, anyway
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Old May 04, 2009, 08:47 PM
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I would not leave that charger unattended. Mine got really hot, I actually stopped using it because I thought it was going to melt. Take a look at the charger, it should say what amp output is, mine says 280ma. If your battery is the same as mine, it should be stamped with its mah rating, mine was 600mah. So it should take a bit over 2 hours for a full charge (280max2h-560mah). Of course it won't be dead, so for my battery, 2 hours is more than sufficient. Again though, after 2 hours it gets VERY hot.
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Old May 04, 2009, 09:41 PM
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Quote:
Originally Posted by bjaffee
Slight correction to the above info.

"Kv" (rpm per volt) and "kV" (kilovolt) aren't the same thing. A kilovolt is just 1000 volts...nothing to do with rpm. I assume that "Kv" is an acronym for "rpms per volt" in some language other then English, but I've never heard what the K actually stands for. Maybe its German or Latin?

Okay...I tried looking it up, and apparently the "K" in "Kv" just means "constant." And supposeldy the correct way to write it would be with the v in subscript: K<sub>V</sub>

Or maybe not...just some useless trivia for you, anyway
Hey thanks for clarification. I was also surprised to know that Kv is kilo volt, particularly when Xpress said that it was a product of rpm and volts (which actually from your explanation is rpm divided by volts) in which case it should have been volts per minute (and I still could not figure out what could it possibly be measuring). Now it makes sense that it is rpm per volt. I guess it is probably a constant evaluated based on the structure of the motor including number of windings, dimension of the rotor etc, and hence is a constant for a given motor.

So as Xpress suggests I would need about 1000 Kv motor, that would mean that such motor would be running at about 10000 rpm since my battery is about 9.6 V, correct?

And The_Fuzz, thanks for evaluating the charging time for me.
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Old May 04, 2009, 09:49 PM
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Quote:
Originally Posted by The_Fuzz
I would not leave that charger unattended. Mine got really hot, I actually stopped using it because I thought it was going to melt. Take a look at the charger, it should say what amp output is, mine says 280ma. If your battery is the same as mine, it should be stamped with its mah rating, mine was 600mah. So it should take a bit over 2 hours for a full charge (280max2h-560mah). Of course it won't be dead, so for my battery, 2 hours is more than sufficient. Again though, after 2 hours it gets VERY hot.
Mine is 650 mAh battery and the charger has a rating of 250 mA. Actually now that I think about it, I am not sure if I agree with your calculations. The current drawn from the charger (which is a transformer) is actually dependent upon the load and the 250 mA rating is the max. it could sustain before starting to over heat and melt down. So it really depends upon the impedance offered by the battery, and based on that it would draw the current. Now that current would be the true current value that we should use to divide the 650 mAh (which is amount of charge that it can hold) with. That will give the true number of hours needed.
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Old May 04, 2009, 09:58 PM
Brett
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Quote:
Originally Posted by shah
So as Xpress suggests I would need about 1000 Kv motor, that would mean that such motor would be running at about 10000 rpm since my battery is about 9.6 V, correct?
Well, about 9600rpm, yeah.

Edit: I should have added that this is the no-load RPM...in other words with no prop.
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Old May 04, 2009, 10:27 PM
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I recommend you read this and also do a search for "GWS P-51".

http://www.rcgroups.com/forums/showthread.php?t=898843

It's pretty much the same plane.

It can fly great with some modifications with brushless and lipos being formost and the CG. With a brushless motor and your experience a brushless will give enough power to get it out of trouble and also cut down the weight.

On the GWS P-51 the CG is critical and it is moved forward from what the maufacturer publishes.

Stick with it and you will get the bugs worked out.

Read as much as you can and learn!
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