Jul 29, 2012, 05:54 AM
Yes, I know it's upside down.
Sydney, Australia
Joined Dec 2008
3,336 Posts
Quote:
 Originally Posted by .quietguy All these posts are the reason I just 'buy & fly'... simply Easier and less taxing on this old brain.
Well, I'm an electrical engineer by profession, so I guess my brain is wired to enjoy this type of stuff.
Jul 29, 2012, 06:02 AM
Closed Account
Joined May 2012
1,792 Posts
Quote:
 Originally Posted by Harrow Well, I'm an electrical engineer by profession, so I guess my brain is wired to enjoy this type of stuff.
I'm a bean counter (accountant)... y'all are a lot smarter than me.

I'm still trying to learn to fly... but I will admit, when I develop some solid skills in that area, the Apprentice is the bird I want!

... Lennie
Jul 29, 2012, 07:23 AM
Registered User
Joined Sep 2010
4,345 Posts
Quote:
 Originally Posted by Harrow Sorry, I have to respectfully disagree. With all other things being equal (battery and prop) the 'KV' rating of a motor is the single most important factor in determing how many amps it will draw. If you are using the same battery and propellor, but one motor has 30% higher 'kV' rating than another, then provided the motor is powerful enough, that motor will spin the propellor 30% faster and draw roughly 30% more current.
Just one example, using eflight as it was easy for me to find. They have three different 480 brushless motors. All same size and same power (around 250 watts on 3 cell) kv ratings of them are 910 kv, 1020 kv and 4000 kv. All produce roughly the same power and draw roughly the same amperage. All are designed for different applications and require different props or fans to properly operate but illustrate that power does not hinge on kv ratings (KV=rpm per volt with no load as pointed out by xmech2k) you can also get a park 450 that is 890kv and puts out around 200 watts or a park 25 that is 870kv and puts out around 600 watts. WATTS = power always has always will.
 Jul 29, 2012, 07:45 AM Keep Flying Swedesboro NJ Joined May 2008 850 Posts Here is my 2 cents on these motors,i not going to get into the techs of it all, LOL but i have the HK g15 950kv motor and i have the eflite P15 950kv .I thought they would be the same spec but they aren't,the g15 seems like its underrated in kv maybe more like 1200,i have been running the eflite P15 in my t28 with a eflite 40 amp pro for years with a 10x7x3 MAS and it pulls 39 amps static with a 4s 30c bat.,so i thought i would change the corsair to the same setup,actally the corsair had the p 15 first and i yanked it to the t28 so to get the corsair with the same setup so i thought i try a cheaper route this time,i put the g15 950kv motor with a turnigy plush 60 amp with a 10x7x3 MAS using the same 4s 30c battery and it pulls 62 amps static,the motors are supposed to be the same rating but the g15 pulls 23 amps more with the same setup,i have no complaints with this motor been running it for 6 months and it pulls great on 4s,not wing ripping but it moves out,i dont see much difference between the corsair and the t28in the air , they are both reinforce with carbon and running retracts,i just think they are higher kv than stated,i have the p25 1250kv motor on my pulse 25 and with a 10x7 i am right at 61amps so i think that is what this g15 really is more like 1200kv,if i were to get one for the apprentice which i may for my float apprentice,did not get a chance to fly it yet with the stock motor so i have to wait and see how it does,i would buy the lower kv g15 so i can prop it up a bit,unless you always run on 3s Ron
 Jul 29, 2012, 12:59 PM Registered User Joined Mar 2011 212 Posts Supporting Documents Removed by User Last edited by Retiredtech; May 29, 2013 at 03:23 AM.
Jul 29, 2012, 04:51 PM
Registered User
Joined Sep 2010
4,345 Posts
Quote:
 Originally Posted by xmech2k Stock rubberbands are 8" at rest. As an alternative, I also have a box of #64 rubberbands from the hobby shop, which I tie 2 together to make work.
xmech2k - do these hold well enough this way? I have always used the #64's, but do not put two of them together. I just use 8 single ones just like I used the originals. I don't remember the originals well enough to remember how long they were. Just seems like two #64 bands would not be very much pressure. But wouldn't be the first time I had something wrong either.
 Jul 29, 2012, 05:24 PM Registered User United States, CA, San Diego Joined Jun 2007 2,801 Posts The one's I have are only 3-1/4" long at rest, so I don't even know if they'd reach on their own! I'm not a rubberband expert, does the #64 indicate thickness, not length? They're Hobbico.
Jul 29, 2012, 05:37 PM
LiPo-Sucker & Airframe EMT
Charlottesville, Virginia, USA
Joined Aug 2010
4,313 Posts
I use #64s also.

Individually, not tied together.

Makes a nice, tight wing hold-down.

With that much tension, it's a good idea to change them often.

(Won't try to say *how* often -- but I check frequently to make sure they're not getting weak or worn-out.)

Dave

# Images

Jul 30, 2012, 12:17 AM
Yes, I know it's upside down.
Sydney, Australia
Joined Dec 2008
3,336 Posts
Quote:
 Originally Posted by bobly Just one example, using eflight as it was easy for me to find. They have three different 480 brushless motors. All are same size and same power (around 250 watts on 3 cell) kv ratings of them are 910 kv, 1020 kv and 4000 kv. All produce roughly the same power and draw roughly the same amperage. All are designed for different applications and require different props or fans to properly operate but illustrate that power does not hinge on kv ratings (KV=rpm per volt with no load as pointed out by xmech2k) you can also get a park 450 that is 890kv and puts out around 200 watts or a park 25 that is 870kv and puts out around 600 watts. WATTS = power always has always will.
You picked a perfect example with those E-flite 480 motors. Check the specs.

Eflite 480 910kV
250W, 20A

Eflite 480 1020kV
275W, 22A

Notice that the watts and amp ratings go up in proportion with the kV rating. Seems E-flite prescribes to the 'kV' theory.

(I have actually used both those motors. I recently upgraded from the 910kV version to the 1020kV version because I wanted more power.)

Yes, a 25 size motor can deliver more power than a 15 size motor, but only if you give it a larger prop, or higher voltage battery. If you have a 25 size motor with 500kV rating, and a 15 size motor with 1000kV rating, and give them the SAME battery and propellor, then the 15 size motor will spin at twice the RPM, and deliver approximately double the power. (Provided the prop is not so large to overload the 15 size motor.)

Is E-Flite lying about their kV ratings? Why do they give kV ratings if they have no meaning? A 15-size 1000kV motor with 10 volts will spin at 10000rpm. A 25 size 500kV motor with 10 volts will only spin at only 5000rpm. If they both have the same prop and battery, then the 15 size will deliver more power (and use more amps) because it will spin the prop faster. (Again, only if the 15 size motor is not overloaded).

Am I saying that the 25 size is not more powerful than the 15 size? Absolutely not. The whole idea of the 25 size motor is that it will be able to swing a much larger prop (and possibly use a higher number of cells), and that is when it the delivers more power.

Now think about all this in the original context. (someone looking for a replacement motor for their Apprentice.) They are using the same battery and prop, so the kV becomes the dominating factor. Now, a lot of cheap motors have the kV rating stated wrong, so I will never say that you should not use an ammeter to check, but you certainly should not ignore the kV either. If you do that you might very easily buy a motor with too low a kV rating, and be dissappointed when you lack power, or else buy one with too high a kV rating (try the 4000kV 480 in your cub and stand back!), and be dissappointed that it tries to spin to fast and draws much too much current.

In the more general context it is always the combination of kV, prop size, and battery voltage that combine to determine the amount of power (number of watts, amount of current, however you want to describe it) that a motor will deliver. A motor rated at 600W will not give 600 watts of power without the appropriately sized prop and battery. Amps x volts = power and always will.

I hope this all makes sense.
Last edited by Harrow; Jul 30, 2012 at 01:41 AM.
 Jul 30, 2012, 01:53 AM Registered User Joined Mar 2011 212 Posts Your calculator is in error Removed by User Last edited by Retiredtech; May 29, 2013 at 02:19 AM.
Jul 30, 2012, 03:03 AM
Yes, I know it's upside down.
Sydney, Australia
Joined Dec 2008
3,336 Posts
Quote:
 Originally Posted by Retiredtech Come on. Let's have a reliable white-paper, some reliable documentation that supports your theory. Up to this point, nothing! RT EDIT: Why did you so massively change your post 2289 ... are you admitting your Kv theory is flawed???
Sorry, the reference to the E-Flite specs was actually meant to be a joke (thus the smiley). They are simply motor ratings that say how many amps or watts the motor can deliver before it begins to overheat. They have no bearing on the relationship between kV and amps or watts and I was wondering if anyone would spot it.

I edited the post a few times as I was trying to make it read better.

Still not sure what you do not agree with. Do you not believe that the 1020kV 480 brushless will spin faster and give more power than the 910kV 480 brushless for same size prop and battery? Or are you just not happy that it's not accurate to 2 significant figures? (I always said it's an estimate and I also that I advocate using an ammeter, plus the amp/watt rating specs are unrelated anyway as I just said.)

The 4000kV motor is drawing a lot more amps. Therefore the I-squared-R losses increase exponentially and heat up the windings limiting it's power rating. That's one reason why EDF's are so notoriously inefficient. For massive changes in kV the linear estimate can not be used, but hopefully no-one will try to put a 4000kV motor on their Apprentice.

Anyway, back to the original physics:

Power = Torque x Angular Velocity

Larger prop = higher torque
Higher 'kV' rating = higher angular velocity (rpm)
Higher voltage = higher angular velocity (rpm).

Therefore power is a function of 3 variables: Prop size, battery volts and kV rating. No single one of them will give the power output, and the maximum power rating from a motor spec will definitely not tell you how much power a motor will produce in a particular setup.

Now, if you are not happy with my statement of the 15 size motor being able to deliver more power than a 25 size motor, you might like to play with one of the online calculators. Here's one. http://brantuas.com/ezcalc/dma1.asp

You need to enter prop size, so I put in 10 inches diameter and 6 inches pitch, and left everything else untouched. I then selected different motors, but left the battery voltage and prop size the same each time. Try the Eflite 10, 15, 25, 32 and 46. Here's the results.

Eflite 10 (1100kV) 31.4 amps, 324 Watts (in), 263 Watts (out)
Eflite 15 ( 950kV) 24.0 amps, 252 Watts (in), 214 Watts (out)
Eflite 25 ( 870kV) 20.0 amps, 212 Watts (in), 175 Watts (out)
Eflite 32 ( 770kV) 15.6 amps, 167 Watts (in), 137 Watts (out)
Eflite 46 ( 670kV) 12.4 amps, 134 Watts (in), 86 Watts (out)

I hope the role that 'kV' plays in determining power output is clear here. That's all I ever intended to originally indicate.
Last edited by Harrow; Jul 30, 2012 at 06:02 AM.
 Jul 30, 2012, 06:36 AM Registered User Joined Sep 2010 4,345 Posts I sorta give up. Just one more comment. You can't compare different kv motors of the same size with the same prop. They don't work that way. Look at how any electric motor is made. The kv rating is determined by the number of and configuration of the magnets (or fields) around the motor. You can have a 1/10 hp 1000 kv motor or a hundred horsepower 1000kv motor. Take the prop off and they will both turn the same speed with the same voltage. But put the prop for the 100 horsepower motor on the 1/10 hp motor and it probably won't turn at all. Check out ohm's law, where is the kv in the equation? Motors of the same size with different kv ratings will require different props. You must size the props to get the motor to perform within it's specific range of design or the whole train gets messed up.
Jul 30, 2012, 07:09 AM
Yes, I know it's upside down.
Sydney, Australia
Joined Dec 2008
3,336 Posts
Quote:
 Originally Posted by bobly I sorta give up. Just one more comment. You can't compare different kv motors of the same size with the same prop. They don't work that way. Look at how any electric motor is made. The kv rating is determined by the number of and configuration of the magnets (or fields) around the motor.
Actually, that is exactly when you can most easily compare them (especially with close values like 710kV vs 810kV vs 950kV), and that is exactly the situtation that started all of this discussion. You will remember someone was trying to decide which of the Turnigy G15 motors to buy, since there were a few different 'kV' ratings to choose from. I made the simple statement that the 'kV' rating will determine how much power and amps the motor will draw. (Incidently, for motors in the same series, the different 'kV' ratings are generally from having a different number of turns in the winding, not from varying the magnets and poles.)

I'm not really sure which bit of the logic is confusing everyone, as it is pretty straight forward. The 'kV' rating is rpm per volt, so a larger 'kV' rating means the motor will spin faster, which means it is making more power and will draw more amps. That's it, it's simple. I'd honestly like to know which part of that is hard to understand.

Try it with two motors and measure the rpm and the amps (or power). I have dozens of times in setting up different planes. It works, and it is why they tell you the 'kV' rating in the specs. (Pity the cheapy motors often have it wrong though.)

Quote:
 Originally Posted by bobly Motors of the same size with different kv ratings will require different props. You must size the props to get the motor to perform within it's specific range of design or the whole train gets messed up.
Or if the 'kV' ratings are not too different, keep the same prop, and each slightly higher 'kV' motor will spin slightly faster and give you more power. That is the heart of this whole thing that no-one seems to want to see.

Anyway, there it is. Don't think I can explain it any other way, and I'll give you all a break by not trying. Hope it was useful to someone. And just for fun, my Apprentice slope soarer for those who came in late and missed it. No motor, so no 'kV' calculations to mess around with.

 Flight of the Pterodactyl (3 min 3 sec)
Last edited by Harrow; Jul 30, 2012 at 07:34 AM.
 Jul 30, 2012, 08:36 AM Glad to be here... United States, GA, Cleveland Joined Feb 2012 185 Posts Changed the firewall, and now... I have a question about WOT on the Apprentice. I have had mine for several months, flying almost every weekend. A few weeks ago, I landed on the nose wheel. Instead of the clevis breaking, as has happened in the past, the plastic collar that holds the nose gear in place broke. Because of some problems early on with the plane, I have a full fuselage thanks to HH. So, I took the entire firewall off the two and swapped them. I used bathroom calk to re-attache to the fuse. Re-attached the motor mount putting the washers in the same place. I balanced the plane at the recommended COG just as it was before, slightly nose down. The re-maiden was Saturday. Tracked fine on takeoff, flew just as it used to at 50% throttle. But now, I get the same characteristics I have seen others report on in this thread. At WOT, it now noses up almost to vertical. Since I don't think I changed the motor angle, my first thought is to fly with new rubber bands to see if I'm getting leading edge lift. I have been flying with the same ones for months, so maybe it's time. If that does not correct the issue, I will try putting very small washers under the top of the motor mount to give a little more downthrust. That seems to be the preferred way of addressing this. Any other advice?
 Jul 30, 2012, 12:04 PM Registered User Joined Mar 2011 212 Posts Let's call a truce and get back to the Apprentice Removed by User Last edited by Retiredtech; May 29, 2013 at 02:18 AM.