Beginners Guide to Motor and Prop Selection - Page 3 - RC Groups
Thread Tools
This thread is privately moderated by NoFlyZone, who may elect to delete unwanted replies.
May 07, 2010, 05:13 PM
Registered User
Originally Posted by NoFlyZone
Hi Touro,

Many times, motors that we buy aren't rated by the manufacturer for their constant current rating. All motors really should have these specs published, but they aren't. So what do we do? Easy! We apply a little rule of thumb to help us along the way. This will allow us to get a good approximation of the constant current rating of motors that are sold cheaply on places like HobbyKing.

Multiply the weight of the motor by 2-3 to find a safe power level in watts for the motor.

Looking at the 60 gram 750 kV motor you linked to:
60 x 2 = 120 watts This is the power level we can run it at all day without overheating it. Our constant current rating, if you will.

60 x 3 = 180 watts This is the power level we can run it at for short periods of time only. Our burst current rating, if you will.

Since the average voltage of a 3s Li-Po over the course of a flight is about 10.8 volts, we can take 120 watts and divide it by 10.8 volts and the answer is about 11 amps. We can safely say that this motor then, has a constant current rating of about 11 amps. So can we use it if we wanted a constant current rating of 12 amps? Heck yes, because 11 is pretty darn close to 12, and our rule of thumb we used has a nice safe margin of error built into it.

Does this help?
This definately helps... Thank you very much. Its been great reading through the information you have posted.
Sign up now
to remove ads between posts
May 07, 2010, 05:36 PM
Suspended Account
Originally Posted by TouroLouco
This definately helps... Thank you very much. Its been great reading through the information you have posted.
You're very welcome!
May 08, 2010, 01:20 AM
Registered User
adams0259's Avatar
Originally Posted by NoFlyZone
Without going into all kinds of scientific hocus pocus about why we even put things like down/right thrust into planes (after all this is a tutorial about motor and prop selection), we can safely say that the only reason we use it is to get rid of behavior from our plane that we don't like or want. So, the bottom line is that if our plane is not exhibiting these tendencies to begin with, then we don't need the corrective down/right thust angles, period.

And the only way to tell if it needs it, is to fly it. If it needs it, start experimenting with the thrust angles to take out the aberant behavior. There is absolutely no formula that tells us how much down/right we need.

thanks for the answer..... I am happy with the simple answer, I am not ready for the long and drawn out explanation just yet...... keep up the good work!
May 08, 2010, 03:59 PM
Suspended Account

Outfitting the AT-6 Texan

Our next example will be to take a low wing warbird type plane and choose a combo for it.

Our plane will be the AT-6 Texan below, with the following specs:

AUW : About 20 ounces
Wingspan: 39.4"
Wing Area: 259 inē
Largest prop diameter that will still have enough ground clearance: 10"

Let's see what we can do with this plane...
May 08, 2010, 04:19 PM
Suspended Account
For this plane, we are going to ask WOC to pick a combo for it that will make it a nice, fast plane, with the ability to do some unlimited high speed aerobatics.

The first step is to enter all our information, and then pick a "Flight Mission" of Fast unlimited aerobatics.

Then we'll let WOC suggest a top speed and the thrust we'll need. And finally we'll run the Voltage and kV Wizards to let it pick the right battery voltage and kV we'll need.

After we hit the "Calculate" button, this is what we see...
Last edited by NoFlyZone; May 08, 2010 at 04:40 PM.
May 08, 2010, 05:38 PM
Suspended Account
Taking WOC's recommendations, we now wrap it up by dialing in that 10x7 prop to a 1.00, which will give us our final specs.

We follow the procedure outlined earlier to:

1) Set the desired top speed to what WOC shows,

2) Noticing that we want to lower the gear ratio from 1.05 to 1.00 we will now start raising the "Desired current" a little at a time.

And here is our final product.

A top speed for our AT-6 Texan of about 54 mph, and an impressive 36 ounces of thrust. This is a plane that will be very capable of some high speed aerobatics. Our motor will be using about 20 amps at wide open throttle.

As always.... All components must be checked with a Watt Meter to ensure our motor, ESC, and battery are operating within their limits.
May 08, 2010, 07:16 PM
I must be doing something wrong here, the program keeps telling me that there is no prop in the range, and the plane won't fly. These are the parameters-
Wingspan 39.4in
Cord 6.5in (average)
Area 256.1in (1.77sq ft)
Wing load 12.12 oz/sqft
Weight 21.16oz
Stall speed 17.46mph
speed range 43.65 -52.38mph
I have a 11.1v 3s 20C lipo
Welgard A2212-15 Brushless 920Kv, Max eff 78%, Max current 12A
This is the plane-
I haven't flown it but statically ran it with a 9X4 prop and it drew 8A and pulled the scales (thrust) to 17oz
Any ideas, thanks Barry
May 08, 2010, 07:24 PM
Suspended Account
Here ya go Barry,

I let WOC make all the recommendations. What is different with yours?
May 08, 2010, 08:17 PM
Suspended Account
And here it is after we put the final touches on the Gear Ratio and make it 1.00

Not a bad little setup!
May 09, 2010, 01:31 AM
I don't think I was holding my mouth right, thanks very much for your help. I have to get a plug adapter to charge my LiPo and I'll get a 9X6 TE prop and give it a fly and tell you how it goes.
Thanks again
May 09, 2010, 01:36 AM
I'm also not sure where you changed the gear ratio?
May 09, 2010, 11:09 AM
Suspended Account
Originally Posted by kakariki
I'm also not sure where you changed the gear ratio?
Hi Barry,

The 2 step process to changing the gear ratio was outlined in Reply #22
May 12, 2010, 09:38 PM
Registered User
HawkerTyphoon's Avatar
This is excellent! Thanks of all the work and explanations, this is a bookmark for sure

May 12, 2010, 10:02 PM
Suspended Account
Originally Posted by HawkerTyphoon
This is excellent! Thanks of all the work and explanations, this is a bookmark for sure
Thank you very much HT. But all the hard work will only pay off for me if you guys can actually work the examples, and try out some different planes, etc.

The real hero is the guy who wrote WebOCalc, however. He's one of my own mentors, and a real great guy.

If you have questions, ask away!!!

May 13, 2010, 07:03 PM
Registered User
HawkerTyphoon's Avatar
OK, here's one,

When giving you your prop selection what does the TE stand for, I get SF stands for Slow Flyer but I can't figure the TE out


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Discussion Prop selection Dannyrevs Electric Plane Talk 5 Feb 20, 2010 01:32 PM
Discussion Seeking Good E Prop Selection mt_100 Hot Online Deals 12 Oct 14, 2009 09:36 PM
Cool Handy 3M adhesive selection guide djacob7 Foamies (Scratchbuilt) 1 Sep 03, 2009 01:47 PM
Discussion Foamy building beginners guide. jbat35 Beginner Training Area (Aircraft-Electric) 1 Aug 29, 2009 11:52 PM
Discussion Mountain Models Lucky ACE prop selection mumblety-peg Parkflyers 4 Aug 27, 2009 10:51 AM