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Feb 20, 2012, 04:25 PM
Registered User
Aeroplayin's Avatar
This is a reply to a request on another thread, and I would prefer to answer it here as not to talk about power systems and calculations on the 60 inch Edge thread.

Kinetic Watts (sometimes called mechanical Watts or Watts-out) is the diameter of the prop in feet to the forth power, times the RPMs divided by 1000 to the third power, times the prop pitch in feet, times the prop constant.

To do a general calculation I use 1.03 as the prop constant unless I know what the pK of the specific prop brand and size is.

To figure out RPMs, unless you know the efficiency of the motor, you can use a general rule of thumb, which is 80% of the Kv. On my five Motrolfly motors, I know that 82.1% was a better value because my tachometer helped me establish this efficiency value with the DM2820’s and 4315's.

This means that if the Kv of the motor is 570 and the motor efficiency is 82.1%, then I multiply the 570Kv times volts. I then multiply this value time 0.821 to include efficiency. I also use 3.84v per cell as an average on 5S since I typically get a 19.20v reading at WOT on a fully charged battery.

570Kv x 19.2v x 0.821 = 8985RPMs and I divide this by 1000 for the formula

Once I know the RPMs, I have all the pieces of the formula using a 15x8 prop:

Watts = ((1.25 ^4) x (8.985 ^3) x 1.03 x 0.67)
Watts = 1221
Amps = Watts divided by Volts
1221W / 19.2v = 63.6A
Last edited by Aeroplayin; Feb 20, 2012 at 04:31 PM.
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Feb 20, 2012, 04:45 PM
Registered User
Aeroplayin's Avatar
You can then calculate the C value (discharge rate) by dividing the Amps by the Ah rating of the battery. So a 5000mAh battery has a 5.0Ah rating.

64A divided by 5 = about 13C

60 minutes to an hour divided by 13C = 4.6 minutes

If you use half the WOT Amps during the duration of your flight, or average about half-throttle, and you only want to use 75 to 80% of your battery's capacity, you can estimate your flying time by multiplying 5000 time 0.75 (3.75Ah) and divide 32A by this value. The result is an 8.5C rating, which is 8.5 A/Ah.

60 min/hour divided by 8.5 A/Ah equals a flight time of just over 7 minutes using these parameters.
Feb 20, 2012, 07:24 PM
Registered User
Dear God Aero!!! Thats more math than I did going to college!!! lol Thanks for the effort pal!
One last question, i have 45-90c 5s 5000's. Does the fact that i have a 45c as opposed to lets say a 30c change your numbers at all? I would guess that the 45-90c will pump out more watts having less resistance......

Also, id expect a bit more than 7 min from a 5000 lipo if these guys using 6s3300 are getting that.....
Last edited by hidaven; Feb 20, 2012 at 08:08 PM.
Feb 20, 2012, 09:43 PM
Volts>amps
stgdz's Avatar
600kv 5s numbers

14x7 900watts bought 50amps
13x7 around 600watts 42-45 amps


no idea on rpms or thrust, guess I am going to have to get a fish scale

pictures





Also I can't fit the EF spinner and the 14x7 on as the shaft isn't long enough. Anyone run into this issue?
Last edited by stgdz; Feb 20, 2012 at 10:38 PM.
Feb 20, 2012, 09:51 PM
Account Closed
rod.d's Avatar
Quote:
Originally Posted by Aeroplayin
This is a reply to a request on another thread, and I would prefer to answer it here as not to talk about power systems and calculations on the 60 inch Edge thread.

Kinetic Watts (sometimes called mechanical Watts or Watts-out) is the diameter of the prop in feet to the forth power, times the RPMs divided by 1000 to the third power, times the prop pitch in feet, times the prop constant.

To do a general calculation I use 1.03 as the prop constant unless I know what the pK of the specific prop brand and size is.

To figure out RPMs, unless you know the efficiency of the motor, you can use a general rule of thumb, which is 80% of the Kv. On my five Motrolfly motors, I know that 82.1% was a better value because my tachometer helped me establish this efficiency value with the DM2820’s and 4315's.

This means that if the Kv of the motor is 570 and the motor efficiency is 82.1%, then I multiply the 570Kv times volts. I then multiply this value time 0.821 to include efficiency. I also use 3.84v per cell as an average on 5S since I typically get a 19.20v reading at WOT on a fully charged battery.

570Kv x 19.2v x 0.821 = 8985RPMs and I divide this by 1000 for the formula

Once I know the RPMs, I have all the pieces of the formula using a 15x8 prop:

Watts = ((1.25 ^4) x (8.985 ^3) x 1.03 x 0.67)
Watts = 1221
Amps = Watts divided by Volts
1221W / 19.2v = 63.6A
nice. very useful info, thanks!

Just wondering, I read that there is a difference between volt-amps and watts. Is that only for AC? I never see anyone mention it when discussing motor setups, and since you are a smarty pants I figure you might know. Thanks again!

EDIT: oh never mind, a 2 second google search turned up the answer.
Feb 20, 2012, 11:44 PM
Registered User
Aeroplayin's Avatar
I do find that when I begin to overload the motor with a bigger prop, the actual real time values will begin to skew. The only way to make everything jive is to lower the motor efficiency, which will drop the RPMs and the Watts-out, but this makes sense. The more the load, the more difficult it is for the motor to achieve anything near the actual Kv.

The higher C rated batteries will sag less, so by keeping the volts up, there will be a slight power advantage, and the level of power will be maintained longer throughout the flight.

And you're right about getting more flight time, but this is a variable that is very different from pilot to pilot. If you do a lot of floaty 3D, you're probably around 35% of the WOT Amps, and you can also use 80% of the battery capacity, which means 5.6C or 10.7 minutes.
Feb 21, 2012, 01:03 AM
Tarot 550/600 & OMPHobby UK
Smoggie's Avatar
Quote:
Originally Posted by stgdz
Also I can't fit the EF spinner and the 14x7 on as the shaft isn't long enough. Anyone run into this issue?
I came across that problem too. The shaft could do with being maybe 3mm longer (should feed this back to Ken). For me it was 'just about' long enough to take a Xoar 14x7 and a spinner, the shaft didn't quite go all the way through the nut but but it was only short by a fraction. It's not proved to be a problem in practice even with over 1kW going through the motor.

Nice numbers for the 5s set up by the way........

Steve
Feb 21, 2012, 07:17 AM
Volts>amps
stgdz's Avatar
Quote:
Originally Posted by JetPlaneFlyer
I came across that problem too. The shaft could do with being maybe 3mm longer (should feed this back to Ken). For me it was 'just about' long enough to take a Xoar 14x7 and a spinner, the shaft didn't quite go all the way through the nut but but it was only short by a fraction. It's not proved to be a problem in practice even with over 1kW going through the motor.

Nice numbers for the 5s set up by the way........

Steve
The 13x6.5 barely makes it, I have got 2-3 threads on the nut and the screw isn't anywhere near being through the nut. Should have taken a picture but currently its got the 13x6.5 on it, maybe I can show it tonight.
Feb 21, 2012, 07:39 AM
Tarot 550/600 & OMPHobby UK
Smoggie's Avatar
I used an Irvine spinner, which maybe has a thinner back plate?.. Also i swapped the big washer that goes under the nut out for one from a Hacker which was steel and slightly thinner than the Motrolfly one. Small changes but perhaps explains why I was able to get the 14" prop on and you cant.

A 14x8 xoar wouldnt fit as the hub is a shade thicker than the 14x7.

I'll take a pic tonight...

Steve
Feb 21, 2012, 04:33 PM
Volts>amps
stgdz's Avatar
Maybe I should have gotten the electric version of the xoar. At least tower will take some back.
Feb 21, 2012, 06:07 PM
Tarot 550/600 & OMPHobby UK
Smoggie's Avatar
Quote:
Originally Posted by stgdz
Maybe I should have gotten the electric version of the xoar. At least tower will take some back.
Ahhh.. that would explain it, the props I'm using are electric specific versions, the i.c. props are sure to have a much beefier hub.
Feb 21, 2012, 06:13 PM
Team Hillbilly (UK)
Space Bat's Avatar
Have a couple of 12x7 Xoar's hopefully coming in next day or so, should be a good improvement for my setup - now if only I can work out how to get rid of my spinner to cowl gap

Does look like will have to cut my box back - but not that keen, still trying to find a similar motor but with shorter length without any luck....
Feb 21, 2012, 08:22 PM
Volts>amps
stgdz's Avatar
12x7 xoar to much for the sk3 and torque 800 kv's?

max current limit on the torque is 38amp and the sk3 is 42a
Feb 21, 2012, 08:32 PM
Team Hillbilly (UK)
Space Bat's Avatar
Quote:
Originally Posted by stgdz
12x7 xoar to much for the sk3 and torque 800 kv's?

max current limit on the torque is 38amp and the sk3 is 42a
Running a 1130kV 45A(60secs) motor with 50A esc with 65A burst...on a 3s

So should be good I have burst these motors over 70A before so should be comfortably in the Zone - albeit on the ragged edge of it
Feb 21, 2012, 09:39 PM
Let the hating begin
evalith8's Avatar
my 60 extra with a motrofly 4315/714 on 4s 4000 with a 15x8 vess made 1636 watts.Its a blast to fly,full throttle is rocket like,cant stay in it very long at all.Very floaty at lower speeds,7 minutes of ringing it out too.I went with a combo that Ken from subsonic planes suggested based around 4000 4s batteries since I was already using them in my 59 & 71 Slicks.This is one awesome plane to fly


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