HobbyKing.com New Products Flash Sale
Reply
Thread Tools
Old Jul 15, 2011, 12:07 PM
Slipping the Surly Bonds
dz1sfb's Avatar
Attica, MI
Joined Dec 2006
10,334 Posts
I had one of those 16g 1700 Kv motors once. it was not my favorite, but getting the most out of it meant using an APC 7x5. I first flew it with a 7x6 or 6x5 not sure. That on my first Blazy Bee. Here is a vid of its performance with that set up. Mind you, this is a heavier plane than the BB24.

Ken

BLazy Bee Flight (4 min 6 sec)
dz1sfb is offline Find More Posts by dz1sfb
RCG Plus Member
Last edited by dz1sfb; Jul 15, 2011 at 12:13 PM.
Reply With Quote
Sign up now
to remove ads between posts
Old Jul 15, 2011, 01:10 PM
-Daniel
RCDUD3's Avatar
United States, TX, Ennis
Joined Mar 2008
177 Posts
Yeah, it hasn't been my favorite motor either...but it's the smallest one I have at the moment. It flies the BB24 decently. Like i said, mine must be damaged because it stutters, so it probably isn't putting out max power. And not to mention that my BB24 is overweight as well...the 2s 800mah battery is also the smallest I have.
RCDUD3 is offline Find More Posts by RCDUD3
Reply With Quote
Old Jul 15, 2011, 01:20 PM
Slipping the Surly Bonds
dz1sfb's Avatar
Attica, MI
Joined Dec 2006
10,334 Posts
Quote:
Originally Posted by RCDUD3 View Post
Yeah, it hasn't been my favorite motor either...but it's the smallest one I have at the moment. It flies the BB24 decently. Like i said, mine must be damaged because it stutters, so it probably isn't putting out max power. And not to mention that my BB24 is overweight as well...the 2s 800mah battery is also the smallest I have.
You could easily fly a 1700 Kv BW on it.

Ken
dz1sfb is offline Find More Posts by dz1sfb
RCG Plus Member
Latest blog entry: Mini H Quad Project
Reply With Quote
Old Jul 15, 2011, 01:43 PM
Watt Waster
Tsavah's Avatar
Joined Oct 2010
1,807 Posts
I have a BW type motor also that has a "wable" in the shaft at specific rpm with a certain slow propeller I have been trying to use with it, and the motor gets too hot to touch in just a few minutes. I checked the propeller for balance and it appeared to be off a little, but then I remembered it is better to use a smaller propeller that has been checked for balance before trying other options. I use the basic ESC that aren't programmable, so that isn't an option, or necessity. This peticular motor is rated higher than 1500 kV, but since there is nothing I have seen on it to indicate it's rating, I will have to use a watt meter in the future.

I am still amazed how much more there is to learn using all electric components for an RC model, but I am starting to feel I am getting the hang of it. The best part is once one has a good component combination and has a good feel for the other details, it isn't all that much of a challenge to expand one's hanger and options. I continue to move slowly towards larger, heavier models, which should handle a little more wind than my light, small craft I am cutting my RC pilot/builder teeth on, so to speak. I am sure many here are doing the same thing and that has a lot to do with why we like to share as we gain knowledge and experience. Are you thinking a different motor, propeller, and ESC might improve your aircrafts performance?
Tsavah is offline Find More Posts by Tsavah
Last edited by Tsavah; Jul 15, 2011 at 01:51 PM. Reason: spelling
Reply With Quote
Old Jul 15, 2011, 02:22 PM
-Daniel
RCDUD3's Avatar
United States, TX, Ennis
Joined Mar 2008
177 Posts
Yes, I'm going to put this motor on it that will give it more thrust. http://www.hobbyking.com/hobbyking/s...idProduct=5431

The big thing will be switching to a lighter battery, it should drop about 20 grams on that alone.
RCDUD3 is offline Find More Posts by RCDUD3
Reply With Quote
Old Jul 15, 2011, 03:28 PM
Watt Waster
Tsavah's Avatar
Joined Oct 2010
1,807 Posts
Motor Option - Hobby King

Wow, a 2100kV rated motor is a big jump from 1500 kV. You might want to keep the 800 mAh battery for a motor running so fast. Once you get past 1800 kV you are talking helicopter motors, inducted fan, and inducted propeller models. The high kV motors tend to run warm to very warm pretty much no matter what propeller you use, or the thrust suffers. The lower kV motors, closer to 1000 kV are more suited to a slow flyer, which the BBPT is designed and use a slow flyer propeller. Generally speaking, the slow flyer propeller runs best in the lower kV ranges (900-1600) and are often longer than 8" with a pitch in the 4-7 range.

If my memory serves me, most of the BBPT of the size you are flying use the BW style motors in the 1000-1700 kV range and a long slow flyer propeller. HK has tons of those motors, but from time to time there is a run on the 1300 and 1500 kV motors from folks who buy a dozen or more at a time. I have read some threads about people talking, or rather complaining they ordered a baker's dozen and HK didn't have that many on hand. They had to wait until HK could restock to get their order filled and that could be more than a month of delay. I suspect the reason they were ordering so many was because they had or were working on a dozen craft getting ready for a few orders to fill, or a club event. Others were experimenting with many models within the same weight class and were expecting to burn up a few motors, but at less than $16 each, they didn't sweat the details.

What do you think about this motor? It seems to be rated well for a model in your size and weight, if I have the details correct.


http://www.hobbyking.com/hobbyking/s...er_1500kv.html
Tsavah is offline Find More Posts by Tsavah
Last edited by Tsavah; Jul 15, 2011 at 03:33 PM. Reason: spelling and info
Reply With Quote
Old Jul 15, 2011, 03:31 PM
Slipping the Surly Bonds
dz1sfb's Avatar
Attica, MI
Joined Dec 2006
10,334 Posts
Quote:
Originally Posted by RCDUD3 View Post
Yes, I'm going to put this motor on it that will give it more thrust. http://www.hobbyking.com/hobbyking/s...idProduct=5431

The big thing will be switching to a lighter battery, it should drop about 20 grams on that alone.

That is a very good motor. The 2500 Kv version is even better. On 2s you can swing a 6x3 or 5x4.3. On 3s a 5x3 works nice, but you can get more out of it safely with an APC 5.1 x 4.5. I use this motor on my Oshkosh Special 30 loaded with a 900 mAh 2s. A 600 mAh 2s is plenty though.

Ken
dz1sfb is offline Find More Posts by dz1sfb
RCG Plus Member
Last edited by dz1sfb; Jul 16, 2011 at 08:55 PM.
Reply With Quote
Old Jul 15, 2011, 04:52 PM
-Daniel
RCDUD3's Avatar
United States, TX, Ennis
Joined Mar 2008
177 Posts
Yes Tsavah, i'm aware of the jump in kv. I didn't really understand kv in relation to motor size until recently. But I have mostly figured it out by now, I think.
I know that 1500kv BW motor would fly the BB24 just fine. But I want a motor that I can run smaller props on. I think the 2100kv motor will fly it just fine.

Hmm Ken, now I would think that 2500kv would be pushing it...but i'm sure you know better than me. I have 7x3.5, 6x3, and 5x3 props on order in bulk. I will have to search around for some of those sizes in between.
RCDUD3 is offline Find More Posts by RCDUD3
Reply With Quote
Old Jul 15, 2011, 05:08 PM
Jack
USA, ME, Ellsworth
Joined May 2008
16,684 Posts
You guys are making me crazy...

If you have three motors of the same weight they will produce the same amount of power. And that is about 2.5 to 3 Watts of power for each gram of motor weight.

The difference the Kv's is that the higher Kv's will produce their power with smaller, faster turn props and the lower Kv's will produce their power with larger, slower turning props. Getting that wrong means that you will either have a burned up motor or s plane without enough power.

If you happen to like flying with small screaming props better than with larger quiter props that is OK. And you are also allowed to have an impression that a 1700 Kv motor flies better than a a 1300 Kv motor or that you simply like it better or that it was better for your conditions.

But you are not ever, in any case, allowed to have an impression that the higher Kv motor has more power than the lower Kv motor. That would contradict the laws of physics, electrical engineering, and common sense.

Ending my traditional rant, stepping down from the podium, and walking away not mad or angry, just slightly bemused...

Jack
jackerbes is offline Find More Posts by jackerbes
Reply With Quote
Old Jul 15, 2011, 06:22 PM
-Daniel
RCDUD3's Avatar
United States, TX, Ennis
Joined Mar 2008
177 Posts
Very good words of wisdom! Yes, that is what I understand about kv ratings. I think generally lower kv motors tend to be used for slow flyers/trainers and high kv makes me think jet or something fast, not more powerful.

I do have a question, does a higher kv motor turning a smallish prop not equate to more speed than a low kv motor with a bigish prop? Or is the only difference that the higher kv motor has to turn faster to produce the same amount of thrust?
RCDUD3 is offline Find More Posts by RCDUD3
Reply With Quote
Old Jul 15, 2011, 06:46 PM
In Rc for a LONG TIME FFAA#1
laserman's Avatar
Canada, ON, Ottawa
Joined Jun 2004
1,375 Posts
Quote:
Originally Posted by RCDUD3 View Post
Very good words of wisdom! Yes, that is what I understand about kv ratings. I think generally lower kv motors tend to be used for slow flyers/trainers and high kv makes me think jet or something fast, not more powerful.

I do have a question, does a higher kv motor turning a smallish prop not equate to more speed than a low kv motor with a bigish prop? Or is the only difference that the higher kv motor has to turn faster to produce the same amount of thrust?

props large and props small, the difference is that props develop the thrust from the out 3rd of the blade of the prop, so to to get the efficiency the small prop has to spin faster while a larger prop to get the same amount of air will spin slower.
laserman is offline Find More Posts by laserman
Reply With Quote
Old Jul 15, 2011, 07:04 PM
Jack
USA, ME, Ellsworth
Joined May 2008
16,684 Posts
Quote:
Originally Posted by RCDUD3 View Post
Very good words of wisdom! Yes, that is what I understand about kv ratings. I think generally lower kv motors tend to be used for slow flyers/trainers and high kv makes me think jet or something fast, not more powerful.

I do have a question, does a higher kv motor turning a smallish prop not equate to more speed than a low kv motor with a bigish prop? Or is the only difference that the higher kv motor has to turn faster to produce the same amount of thrust?
It *does* equate to more speed. And it will, generally speaking, need more RPM to fly a plane that flew at a lower RPM with a larger or slow fly prop of the same pitch.

A 4 x 6 pitch prop at 3,000 RPM will "screw itself along" or advance further than a 3.8 pitch prop at the same RPM. And certainly further than a 4 x 6 pitch prop at a lower RPM in the same period of time.

Use a pitch calculator like the one at 2BFly.com (look under Powerplant Calculators and choose RC Airplane Pitch Speed Calculator). That will give you the theoretical pitch speed and also the estimated pitch speed allowing for slippage (-20%) and the estimated flying speed (-10% more for a total of -30% of the theoretical speed).

The reason that a slow fly prop of the same pitch will put the same amount of power into the prop at a lower RPM is that it has a wider blade and more drag (that draws more Amps). So it is moving more air and generating more thrust.

Watts = volts x amps, if you're not drawing the amps you are not putting the power into the prop. It is that simple.

A narrow bladed 6 x 6 direct drive prop at 3,000 RPM is not moving much air or generating much thrust. A wide bladed 6 x 6 slow fly prop at 3,000 RPM is moving a lot more air and generating a lot more thrust.

So the bottom line with the low Kv motor and slow fly prop is more power and control at a lower flying speed. And the high Kv motor at a higher RPM with a different prop can give you the same power, good control, and a higher flying speed. And you may like that better or prefer it.

Jack
jackerbes is offline Find More Posts by jackerbes
Reply With Quote
Old Jul 15, 2011, 07:05 PM
Watt Waster
Tsavah's Avatar
Joined Oct 2010
1,807 Posts
Bemused and Walking Away from the Podium

Quote:
Originally Posted by jackerbes View Post
You guys are making me crazy... But you are not ever, in any case, allowed to have an impression that the higher Kv motor has more power than the lower Kv motor. That would contradict the laws of physics, electrical engineering, and common sense. Ending my traditional rant, stepping down from the podium, and walking away not mad or angry, just slightly bemused ... Jack
I can understand the rant, and how easy it is to misunderstand the laws of physics, electrical engineering, and whatever else one might throw into the ... discussion. Yeah, that's it, ... discussion. Ok, allow me to share a personal experience and a detail or two that might shed a little light on the kV ratings vs. power. I had a NutBall I had modified and called an Arupy, so as to not set aside the influence of the modifications. As I made changes to the craft, it got heavier. I had started out with a 1300 BW style motor on the original NutBall and it flew very well, but not as acrobatic as I was sure the design could provide. I put another BW motor rated for 1500 kV on it without any other changes and even re-used the same propeller (8x4). The extra 200 kV of the 2nd motor made the craft more acrobatic. The same was true with the heavier Arupy with a motor rated at 1700 kV and no other changes.

To understand why this would be true, keep in mind the kV rating is related to voltage and rpm produced by the motor, which is transfered to the propeller. For each volt (pulse) fed to the motor in a higher kV rated motor, more rpm is producted to the shaft than the lower kV rated motor, but torque is reduced. At issue is the available torque per volt, which is what makes the propeller spin faster without heating up the motor due to resistance of the air. As the resistance of the air increases (rpm and propeller), the motor torque either helps spin the propeller faster, or slower. The lower the torque (as in higher kV rated motors) the less likely you will keep the propeller spinning properly without overheating problems. As you can imagine, there are a lot of trade-off issues that start with the battery, pass through to the ESC on to the motor, and propeller.

A battery is the first weak point in a high rpm combination, which is common to helicopter and fan jet designs. The reason the battery packs are growing in C rating has to do with how well a battery pack can handle the increased current drain demanded in high rpm motors. Higher amperage drain has to pass through the ESC before it can get to the motor, but that also means more heat will be generated. Batteries, ESC, and motors are sensitive to heat due to amperage drain. Heat is what shortens the life of the components and can burn stuff out, or up, depending on your thinking. Due to my many years as a military electronics technician, I tend to think burn stuff out, as in shorting wires or components until an "open" prevents further current passage. Technicians call this a broken circuit, as in broken wire.

The LiPo batteries, ESC, and motors we use all have wires and other components that can handle so much heat before something shorts, melts, or otherwise becomes "broken". On Youtube, some folks like to call this "letting the magic smoke out", which makes a lot of sense because smoke is often the first thing you see after something gets overheated and becomes shorted out/broken. In most cases, if we talk about electric motors, you are noticing the thin, plastic like coating on the very thin wire is burning off producing toxic fumes. When an ESC burns out, the smoke is often darker, suggesting more than wire insulation has overheated, and is burning away. All most RC pilots know is the engine won't turn anymore and the ESC has turn black, or part of it is almost black where the heat build up was centered.

If we are talking a LiPo battery pack, the first sign something has been overheated due to too fast a charge, or discharge rate is a "puffy", or ballooned outer wrapping. As long as the heat doesn't cause the outer wrap to breach, and oxygen doesn't get to the hot lithium element in the compound, which would cause a chemical reaction that produces a lot of heat, toxic fumes, and maybe even a fire. Some LiPo packs have a thin wire between the cells that is a fuse wire designed to burn apart if too much amperage is taken from the pack (discharge rate), or someone makes a mistake on the re-charge cycle and stuff gets too hot. The catch is few of us have a safe work/test bench setup to repair LiPo packs, so trying to fix your own is too risky.

Ok, enough about heat damage issues and let's get back to kV rating vs power/torque of an electric motor. First, the size of the coated wire wrapped around the star shapped steel plates, how many windings per branch/arm, and the voltage applied is how kV ratings are calculated. More windings on a branch normally means a higher kV rated motor, but it also means the diameter of the wire is smaller. The only way to do better is with stronger magnets and fancy/expensive steel plate compounds. If you have made your own toy motors using a science kit, a school science project, or reproduced a project/experiment from a video you had watched on Youtube, or another video information source, you will understand the details of electric motor parts better.

As the parts of the motor become more expensive, the magnets stronger, and the coating on the thin wire better able to withstand heat, the more efficent the motor is likely to be. If you change nothing except the number of windings and the diameter of the wire wrapped around the steel plates, the kV will change, but not as dramatically as other improvements. More windings normally translates to higher rpm per volt, until you reach the highest current draw the coated wire can tolerate. Increase the copper wire diameter with fewer windings, the more torque the shaft will provide, but at a lower rpm per volt until the current draw overheats the coating. The coating on the copper wire is very much like a varnish, but silicon paints, or coatings tolerate more heat before they burn off. The maker's of RC electric motors use cheaper/softer coatings to keep the price of their motors competitive, or they might not sell enough to stay in business.

If I have provided enough information about electric motor parts, you may comprehend a 1300 kV rated motor will often have a slightly thicker wire, or coating on the wire than a 2700 kV rated motor. The higher 2700 kV rated motor will spin very fast on a dc "pulse" that averages 5 vdc, but it won't provide enough torque to spin a 8x4 propeller without overheating. To control the heating problem, you will need to reduce the propeller size to a 5x3, or somewhere in that range. How heavy the motor is will also be an issue since it has a lot of parts that tend to be a little heavier than a lower kV rated motor. The key point is a small propeller needs to spin very, very fast to produce enough thrust to almost match a 8x4 or larger propeller. The plastic the propeller is made of has rpm limits which must not be exceeded, hence the purpose of the different designs.

Depending on the design of the model, you may want a smaller propeller and a specific weight motor to produce a near 1:1 thrust ratio. The closer to 1 once of weight to 1 once of thrust the motor and propeller combination can produce, the more likely acrobatic manuvers and higher speed flights will be possible. I hope I have provided enough to think about to help a new RC builder/pilot comprehend the relationship between the components and the necessity to match the motor and propeller combination to the aircraft model. Unless you have a large collection of parts, and don't mind switching out parts until you find a workable combination, it is best to take the time to read what combinations have worked for others. Using the same working successfully combination will save a lot of frustration and expense.

Tsavah is offline Find More Posts by Tsavah
Last edited by Tsavah; Jul 15, 2011 at 07:18 PM. Reason: spelling
Reply With Quote
Old Jul 15, 2011, 10:10 PM
A Day @ a Time - Matt. 6:25-34
ruff1's Avatar
North AL, USA
Joined Nov 2009
1,870 Posts
Quote:
Originally Posted by Tsavah View Post
...it is best to take the time to read what combinations have worked for others. Using the same working successfully combination will save a lot of frustration and expense.
Agreed, that is what I did, and it has worked out well for me.
ruff1 is offline Find More Posts by ruff1
Reply With Quote
Old Jul 17, 2011, 09:06 AM
Enjoy Life B4U Die!
Buzz_Man's Avatar
United States, GA, Powder Springs
Joined Dec 1999
4,210 Posts
I recently decided we'll be building a gaggle of these Blu-Baby's during our regular class meetings and also during our after school "technology club" this fall. Why reinvent the wheel? This trainer works, it's awesome.

Afterward, I will take my high school students into the realm of 3D using something like this --> LINK

I'm excited because this will be my last year teaching and I'm going to ignore as much of the needless paperwork this year as I possibly can and focus on actually spending my time teaching, helping students learn skills that will prepare them for a successful future.

This will be fun!
Buzz_Man is offline Find More Posts by Buzz_Man
Reply With Quote
Reply


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Video Blu-Baby Primary Trainer Tony65x55 Electric Plane Talk 0 May 05, 2007 06:05 AM
Chosing Trainer...Kit?Plans?RTF? Seared Ice Fuel Plane Talk 49 Aug 16, 2004 11:50 AM
Which SPAD Trainer to build?? Plans? VBSuperMaxx SPAD - Simple Plastic Airplane Designs 12 May 05, 2004 09:19 PM
Sold FS: Fireball 550 Trainer "PLANS" Rudderman98 Aircraft - Electric - Airplanes (FS/W) 4 Dec 05, 2002 07:38 PM