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Questions about electric power and rc terminology

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Old 06-02-2005, 08:33 AM
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deli_conker
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Default Questions about electric power and rc terminology

Okay. I understand the relationship between amps and volts because of a good explanation found on this forum. I've also read that amps*volts = watts. And that most planes you'll want 50 watts per pound to fly around, 75 watts per pound for aerobatics, and 100 watts/pound for 3d.

I'm new to ARFs and such and was wondering...
[*]If I know how many watts I need, and know the voltage I am going to use, how do I determine which motor to use?
[*]The rating system for motors, what does it mean (300, 400, 480, etc.)?
[*]I understand that different props will make the motor work at different levels thus changing the amount of amps drawn, is this just a trial and error process using a watt meter to determine real numbers or can it be derived mathematically? I don't want to fry anything when I start buying stuff.
[*]Speaking of props, what do the numbers mean (6-4 for example). I'm thinking it has something to do with pitch and length/radius, but there are other things it could be as well (I guess).

Sorry if my questions have already been answered in an earlier post, I tried to search before posting. Just want to make sure I don't railroad myself by buying the wrong things.

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Old 06-02-2005, 08:57 AM
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Walt Thyng
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Default RE: Questions about electric power and rc terminology

D.C.,
you've taken an important step in learning about electrics --- ASK! I'm self-taught about e-power (I did have 25 years of wet-power experience) and I'm about as far from an engineeer as you can get.

Your understanding of the numbers for a prop are correct: an 8/4 prop is 8 inches in diameter and has a pitch of 4 inches. Pitch is the theoretical distance the prop would move forward in an ideal fluid under ideal conditions.

Most of the motor numbers don't mean much. I recommend that you go to kmyers@aol.com, this is the Ampeer newsletter site. It is one of the best e-power resources around. There is an archive of articles all of which are excellent. Ken Myers recently did one on how to choose a motor. I'm reproducing it in our wet-power club newsletter for which I write an e-power column.

I think you might also do well to invest in one of the performance calculator programs (Motorcalc; E-calc) or go to the Diversity model aircraft site which has a free online calculator.

If the urls I gave are incorrect, just do a Google.

Hope all this helps advance you in e-power and keeps you interested. E-power saved the hobby for me(-:
Walt
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Old 06-02-2005, 01:49 PM
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Default RE: Questions about electric power and rc terminology

Okay.

So what's the difference between prop types? As in folding, Master Screw, APC. Are these just different brands or is there an actual physical difference between them?

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Old 06-02-2005, 03:47 PM
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Default RE: Questions about electric power and rc terminology

just different companies that make them.
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Old 06-02-2005, 10:38 PM
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Default RE: Questions about electric power and rc terminology

There are differences in props. For instance APC makes props for gas and props for electrics. The difference there is, the gas props have to be made much stronger to take the firing impulse loads. Of course the E props can be built much lighter where weight is a big factor with the electrics, and they bend rather than break on my less than perfect landings Hope this helps.
PS Folding props are used on gliders to cut down on drag.
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Old 06-03-2005, 02:42 PM
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Default RE: Questions about electric power and rc terminology

Thanks for the info. I appreciate it.

Now how about ESC's? I understand them having a value that states the max amps you want to run through them. Is there any difference between ones that run brushed motors and ones that run brushless? I have seen that on some labels, but didn't know if there was an "actual" difference.

Also, I read about some that have a low voltage cutoff for Li-po batteries. Does this mean that if I go from a stock Ni-MH battery in my plane to a Li-po battery, that I'll have to change the ESC as well to avoid taking the charge too low?

Sorry about all the questions. It's coming together for me, I swear it is...
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Old 06-03-2005, 08:22 PM
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Default RE: Questions about electric power and rc terminology

Deli:
On a brushed motor the timing of the armature is done by the comutator and brushes. The timing on a brushless is done electronically in the ESC or electronic speed control.
Most speed controllers today are programable to do either nicad, nihms, or Lipo. Keep going Deli, don't stop now
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Old 06-04-2005, 04:29 PM
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Default RE: Questions about electric power and rc terminology

Okay. I won't stop then.

I was reading in a magazine about some motors and they had a KV rating. I can't see that as being "kilovolts". Anybody know what it means?

Also, I'm wondering about about gear boxes. I saw one available that had three gears in it of different ratios. I am assuming that the first number refers to the motor and the second, the prop (i.e. 6.6:1) as far as rotations. This would mean that the higher the first number, the lower the prop rpm if using the same motor (assuming I'm correct in my initial assumption...sorry for all the assumptions). Is there a reason you would want a faster or slower rotating prop? I know that the faster it rotates, the more power you'll have (and probably speed too). I can also see needing extra power if you are doing 3d stuff. What types of ratios would you use for different styles of flying (assuming you are using the same motor)? The plane I am getting (actually waiting for it to arrive) says nothing about a gear box. I am assuming that it is direct drive. Since it's not a 3d plane, should I assume it has a weak motor?
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Old 06-04-2005, 07:50 PM
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Default RE: Questions about electric power and rc terminology

I thought KV was kilovolts too till I read where in electric motor parlance it is the number of RPM per volt. For instance my Himax 2025- 4200 the 4200 is RPM per volt or on a 7.5 volt batt. it should turn 31500 with no prop on it. So far i've found no use for these numbers.
Your right about your assumptions on the gear ratios. lower gear will swing a bigger prop as lower RPM. What you need for hovering stuff where you need some stump pulling. Higher gears for a small prop with high pitch for speed and go like snot. Deli , I'm hoping this stuff is helping some other fellows with the same questions. Hope this helps.
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Old 06-06-2005, 09:06 AM
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Default RE: Questions about electric power and rc terminology

I'm not sure I understand completely. I'd figure that using a smaller gear ratio would make a plane faster when comparing like props and motors. Here's my logic.

Let's say we use you above example for a motor. I know there's going to be a discrepency for added weight as far as RPM's are concerned (or would it just cause it to draw more amps?). It's moving 31500 RPM with no prop. You put it on a gear box #1 (6:1). It will turn the propeller gear at 5250 RPM. Now we put it on gearbox #2 (3:1). This turns the prop at 10500 RPM. If you put the same prop on both gearboxes, wouldn't the second one be a faster plane?

And is there a difference between speed and power? I was thinking that if a setup had more power, then it has more speed. That the gearbox acted like a sort of governer to make sure it didn't go too fast.

And also a thing or two about prop size. There has to be a third number or factor when dealing with props. Otherwise, there's no difference between an 8-4 prop and a 5-4 prop. They both have a pitch of 4 and as stated earlier
Pitch is the theoretical distance the prop would move forward in an ideal fluid under ideal conditions.
I can only think that the greater the diameter, the more air it has to cut through which makes it draw more amps but allows it to push/pull harder as well. Which means that the two props on identical planes wouldn't follow the "rule of pitch (or whatever)" that I posted above. Or I just have no idea what "ideal conditions" really are. The only thing that I can think of for a third number is weight and/or air resistance.
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Old 06-07-2005, 12:43 AM
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Default RE: Questions about electric power and rc terminology

Is there a good book on electric RC that covers the basics?
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Old 06-07-2005, 08:27 AM
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Walt Thyng
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Default RE: Questions about electric power and rc terminology

D.C.
in my opinion most of the books on e-power are badly outdated. Bob Boucher's book is very technical, but okay for basics. The last copy I looked at did nothing with brushless motors and infact was very negative about them. The Harry Higgley book is simply ancient and Bob Aberle's is not much better (I heard he was coming out with an update, but I've never seen it.). Right now your best bet is "Quiet Flyer" magazine (the newer R/C Flyer is pretty good, but is less focused on e-power) and reading the posts on this list. Also, as I suggested above, the Ampeer archives are about as good and as up to date as there is right now. Checking out the E-zone online mag would also be a good thing.

I recently found a copy of a packet that I put together for beginning e-power flyers back in about 94. It was like reading a cartoon or an obituary! For example the best info at the time said geared motors were only for gliders, while direct drive was best for all other types of flying. Today, almost the exact opposite is true. There wa an extended discussion of how to treat the latest, hottest 1,000 mah Nicads and much cautioning about never exceeding 30 amps! The list of vendors for e-power stuff was only one page long and nearly all of them are out of business now. I tell you this, because, unless there have been changes I am not aware of, all of the published e-power books are of the same era.

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Old 06-07-2005, 10:41 AM
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Default RE: Questions about electric power and rc terminology

Deli:
The third factor is Horse power or electric equivalent watts. 1 HP = just under 750 watts. To turn the 8-4 prop at the same RPM as the 5-4 would take much more HP. Or another way to look at it is, the motor driving the 8-4 prop, with a certain HP, will be able to turn the 5-4 prop at a much higher RPM therefore flying the plane at a higher speed even though theyre the same pitch. If it could turn the prop twice as fast, it would cover twice the distance, theoretically. On your gearboxes, your right, the second gearbox would be faster. Sort of like shifting to a higher gear in a car. In a model, with the same motor, for low speed pulling power, you'd use a large prop with low pitch, say for hovering. For a pylon racer you'd use a higher gear or no gears at all and drive a small prop with a high pitch. Both motors would be turning at the same RPM, or the same HP(watts) Hope this helps.
I try to do this without mathematical figures as I flunked math in school
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Old 06-07-2005, 07:19 PM
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Default RE: Questions about electric power and rc terminology

Well, the plane I ordered is an RTF so I am going to assume that everything is set correctly as far as motor-esc-prop is concerned. I might make some changes to it later though, or if nothing else use it for experimentation.

A little background...

I had a firebird or aerobird or something a few years ago and it was about zero challenge. The plane I'm getting is a four channel cessna (link below). I'll hit FMS for a while until I'm comfortable with the ailerons and then I'm off to fly it. I'm a quick learner so it shouldn't take long. Eventually I'm going to get a small electric copter and go from there.

Thanks for the advice. All of my questions are answered to some degree at this point although I'm sure that I'll have more later on. I'll also check into those magazines as well. I've visited their web sites already.

Link to the plane I am anxiously awaiting.
Link to the radio I'm going to get (with spectra module).
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Old 06-08-2005, 08:39 AM
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Default RE: Questions about electric power and rc terminology

Unfortunately, you can't make that assumption. There are many RTF setups out there that are simply designed to get your money, not fly. Whether the companies do this intentionally, or they simply don't understand the basics of electrics is debateable, but the long and short of it is, "buyer beware."

Looking at the plane you bought, I think you'll be okay, but it may turn out to be a little underpowered. You might consider upgrading to an 8-cell battery pack, or adding a gearbox and going to a 10-cell pack. The additional Volts will give the plane a power boost, and the gearbox will allow the motor to spin a larger prop at a lower RPM, giving it a better, more efficient "bite" on the air.
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Old 06-08-2005, 08:47 AM
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Default RE: Questions about electric power and rc terminology

Now I have more questions. That didn't take long.

So do motors have a voltage limit? Or can you run what you want as long as you don't put too many amps through them?

Also, I was considering going to Li-Poly batteries before too long for extended flight times.
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Old 06-08-2005, 09:54 AM
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Default RE: Questions about electric power and rc terminology

DELI,
Brushed motors to an extent do have a limit to voltage you can run through them.
Brushless is a whole nother story, same with amps, with proper cooling amps can be run higher then ratings. Heat is a by product of motor working, the cooler the motor is the more you can run through it. An example Mega brushless 16/25/4 rated at 25 amps, I've been running at 50 amps on occation and motor just keeps humming along, but it's out in the airflow so in flight cooling helps it handle the amps.

As for Lipos, read everything you can get your hands on in relation to them.
And definitely ask lots of questions before trying anything with them.
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Old 06-08-2005, 10:46 AM
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Default RE: Questions about electric power and rc terminology

The more I read on this topic, the more I am seeing that I will eventually need to get a watt meter so I can see what is going on. From there, it may be a little trial and error to figure out what parts I need to fly the way I want to.

Another question about terminology; when somebody says that they "burned up a motor", is this due to excess voltage or excess heat? Do you not want to run too many extra volts through a brushed motor because of the volts killing it or because the extra volts allow more amps and in turn more heat in the motor? I understand that extra cooling can let you get by on a brushless, I guess I don't see where that would be different for a brushed motor. Then again, I only know the basics about the two types of motors...
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