HobbyKing.com New Products Flash Sale
Reply
Thread Tools
Old Mar 11, 2012, 09:34 PM
Not as Good as The Kid
Aeroplayin's Avatar
South Pasadena, FL
Joined Sep 2009
6,877 Posts
To clarify, that’s 20 Watts-out.
Aeroplayin is online now Find More Posts by Aeroplayin
RCG Plus Member
Reply With Quote
Sign up now
to remove ads between posts
Old Mar 11, 2012, 09:35 PM
They Call him Dead!
YellowJacketsRC's Avatar
United States, SC, Pawleys Island
Joined Jul 2003
6,919 Posts
Thanks for the info AP.
I did put a smaller (Xoar 13 x 6.5) on it. It did tumble better. I think it is due to the large prop screwing things up. I really notice some strange behavior like precession and prop torque with this larger prop. I have an overkill JR in the elevator and some nice Airtronics digitals for ailerons and rudder so I am pretty sure that is not the issue...
YellowJacketsRC is offline Find More Posts by YellowJacketsRC
Reply With Quote
Old Mar 11, 2012, 09:36 PM
They Call him Dead!
YellowJacketsRC's Avatar
United States, SC, Pawleys Island
Joined Jul 2003
6,919 Posts
What would happen if I put a 13 x 8 on there AND changed to a 2820 with higher KV?

Or just rewound this 2815 to higher KV?
YellowJacketsRC is offline Find More Posts by YellowJacketsRC
Reply With Quote
Old Mar 11, 2012, 09:54 PM
Not as Good as The Kid
Aeroplayin's Avatar
South Pasadena, FL
Joined Sep 2009
6,877 Posts
The question is, will a 13x6.5 on a true 750Kv turn 9000 RPMs? If the answer is yes, then 750 Watt-out is great and the Kv efficiency is only down to 78%. But if you are not getting 9000 RPMs, then I would question the efficiency of the setup. A 13x6 should give you 8980 at 78% of 750Kv and 700W which is also great.

Try to get an RPM reading tomorrow with your current setup first. Volts would be nice too.
Aeroplayin is online now Find More Posts by Aeroplayin
RCG Plus Member
Reply With Quote
Old Mar 11, 2012, 10:03 PM
They Call him Dead!
YellowJacketsRC's Avatar
United States, SC, Pawleys Island
Joined Jul 2003
6,919 Posts
Volts at WOT?
YellowJacketsRC is offline Find More Posts by YellowJacketsRC
Reply With Quote
Old Mar 11, 2012, 10:09 PM
Not as Good as The Kid
Aeroplayin's Avatar
South Pasadena, FL
Joined Sep 2009
6,877 Posts
Yes... BTW - Tom is getting 8430 RPMs, 670W and 43A at 15.64v on his 683Kv 2820-750 with a 14x7 Xoar and you can witness the power in his videos (flying his plane, not mine).

If you are getting additional torque because of the 700Kv wind with the 14x7 prop, then I'm guessing you are getting at least the same efficiency, which would bring your Watts up to about 722, which could be the reason why you are reporting torque that Tom doesn't feel at all. Let's see what your RPMs are tomorrow. His are 8430 and yours will have to be in the 8640 range to get the additional 50 Watts-out with the same 80.4% Kv efficiency.
Aeroplayin is online now Find More Posts by Aeroplayin
RCG Plus Member
Reply With Quote
Old Mar 11, 2012, 10:26 PM
They Call him Dead!
YellowJacketsRC's Avatar
United States, SC, Pawleys Island
Joined Jul 2003
6,919 Posts
Will do. Don't forget I am using the 2815 which should produce less power than the 2820 right? Also, I am not complaining at all about the amount of power/ thrust I am getting out of the current 14 x 7 setup. I just feel like the large prop is causing some unwanted effects in certain aspects of the flight envelope.... I am hoping to get the same useable thrust with a prop that is 1" less in diameter if that is possible. But it is looking like I will have to change something other than just the prop to do so....
YellowJacketsRC is offline Find More Posts by YellowJacketsRC
Reply With Quote
Old Mar 11, 2012, 10:47 PM
Not as Good as The Kid
Aeroplayin's Avatar
South Pasadena, FL
Joined Sep 2009
6,877 Posts
If you are using a 14x7 on a 700Kv with 4S and there is no voltage change and no Kv efficiency change, then to get the same Watts-out from a 13x7, you will need to raise the Kv of the motor to 780Kv. So at 80% Kv efficiency, the 700Kv will swing the 14x7 8640 RPMs which needs 722 Watts-out, and the 780Kv will swing the 13x7 an additional 1000 RPMs (approximately) and 725 Watts-out.

Just remember that the main variable in the gyroscopic formula is RPMs, not the diameter of the rotating body. This smaller prop rotating 1000 more RPMs many not solve your problem if gyroscopic effect is really the cause.
Aeroplayin is online now Find More Posts by Aeroplayin
RCG Plus Member
Reply With Quote
Old Mar 12, 2012, 12:34 PM
They Call him Dead!
YellowJacketsRC's Avatar
United States, SC, Pawleys Island
Joined Jul 2003
6,919 Posts
OK,
I broke my last 14 x 7 xoar, so I used an APC. At the very first time I went WOT I got 8600 RPMs very briefly. After that it would jump to 8300 RPM briefly and then sag. I will have to set up my spc on my Optima RX to check volts at a later time.
YellowJacketsRC is offline Find More Posts by YellowJacketsRC
Reply With Quote
Old Mar 12, 2012, 01:48 PM
Not as Good as The Kid
Aeroplayin's Avatar
South Pasadena, FL
Joined Sep 2009
6,877 Posts
Quote:
Originally Posted by dead View Post
OK,
I broke my last 14 x 7 xoar, so I used an APC. At the very first time I went WOT I got 8600 RPMs very briefly. After that it would jump to 8300 RPM briefly and then sag. I will have to set up my spc on my Optima RX to check volts at a later time.
Right. As I said, 8640 RPMs on 15.35v is 722 Watts-out at 80% Kv efficiency. Considering the same Kv efficiency, you can be registering 3.82v per cell or 15.28v to get 8600 RPMs. If you are sagging to 8300, then the voltage can be dropping to 14.8v, which means 647 Watts-out. This means the 750Kv is the right Watts-out match-up for your 13x8 prop.
Aeroplayin is online now Find More Posts by Aeroplayin
RCG Plus Member
Reply With Quote
Old Mar 12, 2012, 02:16 PM
Not as Good as The Kid
Aeroplayin's Avatar
South Pasadena, FL
Joined Sep 2009
6,877 Posts
14x7 with the 700Kv @ 14.8v @80% Kv efficiency
RPMs = (700Kv x 14.8v) x 0.8
RPMs = 8288

Watts = 1.03 x 0.58 x (8.29^3) x (1.17^4)
Watts = 637

13x8 with the 750Kv @ 14.8v
RPMs = (750Kv x 14.8v) x 0.8
RPMs = 8880

Watts = 1.03 x 0.67 x (8.88^3) x (1.08^4)
Watts = 657

Pretty close.
Aeroplayin is online now Find More Posts by Aeroplayin
RCG Plus Member
Reply With Quote
Old Mar 12, 2012, 03:01 PM
Not as Good as The Kid
Aeroplayin's Avatar
South Pasadena, FL
Joined Sep 2009
6,877 Posts
Another reminder that Ken said the new stock DM2820-750 are really wound to about 720Kv.
Aeroplayin is online now Find More Posts by Aeroplayin
RCG Plus Member
Reply With Quote
Old Mar 12, 2012, 03:33 PM
Registered User
United States, NJ, Roseland
Joined Apr 2009
2,214 Posts
Will a 16x7 or 16x8 on 6s pull vertical faster and stronger out of a hover than say a 15x8?
hidaven is offline Find More Posts by hidaven
Reply With Quote
Old Mar 12, 2012, 03:42 PM
Grumpy old git.. Who me?
JetPlaneFlyer's Avatar
Aberdeen
Joined Mar 2006
11,760 Posts
Quote:
Originally Posted by Aeroplayin View Post
JPF - Getting back to the efficiency discussion.... .
Aero,

thanks for the analysis. I think where we are differing in our approaches is that you are trying to make the data fit an assumed 'normal' kv x volts / RPM ratio of approx 0.82 (I don't like the term efficiency for this ratio) and then assuming the data must be incorrect if it doesn't fit. Also you assume that this RPM ratio is the same as the actual output/input efficiency of the motor when in fact it usually is not.

The relationship of actual RPM vs that calculated by kv x voltage is not a constant. It is a decreasing curve which starts at a ratio of 1.0 on no load and decreases in theory to 0 when the motor stalls (though it would burn out before you ever got there). Yes a good motor with a well selected prop may well end up with a ratio of around 0.8 but it's incorrect to assume all motors with any prop will be anywhere close to this figure, you could very easily be 10% out either way. Bear in mind that mechanical power is proportional to the cube of prop RPM so a 10% error in RPM produces a 33% error in power. Even as a rule of thumb these 'efficiency' assumptions are at best 'shaky'.

Attached is a plot from Motrolfly calc.

Note the following points:
  • The curve for RPM/kv x voltage is shown initially in a pink dotted line tagged 'RPM/RPMO%' It starts off at 1 (100%) when the motor has no load it intersects the motor efficiency curve (the red line) at about 25A and descents to zero at about 45Amps (motor stalled).
  • Over the motors realistic operating range 'RPM/RPMO' (the pink curve) is not the same curve as motor efficiency (the red curve). In the heart of a motors range the difference is very marked, about 10 percentage points at 10A in the attached example. Only when the motor is well past it's realistic operating range do the curves come together. The assumption that 'kv efficiency' is the same as actual motor mechanical output /electrical input efficiency is therefore fatally flawed.

Steve
JetPlaneFlyer is offline Find More Posts by JetPlaneFlyer
Last edited by JetPlaneFlyer; Mar 12, 2012 at 04:34 PM.
Reply With Quote
Old Mar 12, 2012, 04:08 PM
They Call him Dead!
YellowJacketsRC's Avatar
United States, SC, Pawleys Island
Joined Jul 2003
6,919 Posts
Thanks AP!
YellowJacketsRC is offline Find More Posts by YellowJacketsRC
Reply With Quote
Reply


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Question RC kits for experimenting with flight control systems SubZer0 DIY Electronics 11 Oct 12, 2011 08:58 AM
Wanted left wing for extreme flight yellow and black 74 yak hondaboy92hatch Aircraft - Fuel - Airplanes (FS/W) 0 Sep 29, 2011 10:18 AM
Discussion Extreme power systems & Royal Evo 12 rbehrends Radios 5 Oct 11, 2007 05:55 PM
Discussion Battery for Logo 10 - Thunder Power "eXtreme" 5S 4600 or Flight Power 5S 3700? WeatherB Electric Heli Talk 6 Jan 25, 2007 10:29 PM