Beginner Series - Power System

by FliteTest | November 27, 2013 | (29 Ratings) Posted in How To

The Flite Test Beginner Series is brought to you by Horizon Hobby

Episode 6: Power System

CLICK HERE for a complete listing of the Beginner Series episode list.

A radio controlled, electric plane’s power system will usually consist of:

  • Propeller

  • Motor

  • ESC (Electronic Speed controller)

  • Battery

Each of these components should be properly match to get good performance and reliability.


Propeller

  • There are two numbers that are important. The first number is the diameter (in inches) and the second number is the pitch. These numbers are usually molded into the front of the propeller. (Example: 10 X 4.5)

    • Diameter - The size of the propeller (total length) measured in inches. The diameter of the propeller depicts the thrust generated.

    • Pitch - Pitch is a little harder to understand - The number indicates how far the propeller wants to travel forward in one revolution. A higher pitch propeller is meant for higher speeds and will be less efficient at slower speeds. It is similar to a bike or car in a higher gear. The acceleration is going to be less but the top speed is going to be greater. A lower Pitch prop is going to grip better and give you a faster acceleration however, it has a lower top speed.

  • Types of Propellers:

    • Electric propellers - Stiff, made for normal to high RPM

    • Slow fly electric propellers - Normally flimsy, made to generate a lot of thrust at low speeds - Used on indoor and slow flying airplanes. Can break at higher RPM’s.

    • Gas propellers - Built very stiff and sturdy to take the abuse of the motor, don't use on electric - not as efficient.

 
  • Choosing the correct prop for your plane:

    • Always try to find the manufacturer recommendation first. If you can’t find it, search the internet, or R/C forums.

    • Use a calculator (Excellent calculator: http://www.ecalc.ch/motorcalc.htm) - Play around and find the best options for the whole power system.

    • SUPER IMPORTANT! - Numbers always face forward! Check the airfoil of your blade the rounded surface points forward. If your prop is on backwards it’ll produce less thrust and draw more current. This is a common mistake.

Motor

There are fuel engines, brushed motors and brushless motors. The most common type of motor for park flyers is the Electric brushless motor.


Two different kinds of brushless electric motors:

  • Inrunner - Only the shafts spins - Normally used in fast airplanes, EDF’s, or cars.

  • Outrunner - Far more common - Has more torque, the whole outside of the motor spins. There are motors of every size and shape.


Choosing the right motor for your plane:

  • Always try to find the manufacturer recommendation first. If you can’t find it, search the internet, or R/C forums.

  • Good rule of thumb: You need 75-100 Watts per pound for a trainer, 150W per pound for pattern or warbirds and 200W+ per pound for 3D and EDF Jets.

  • The 80% rule! If a motor is rated for 100W count that as a 80W motor. Will save you tons of problems down the road and give yourself headroom.

  • Use a calculator (Excellent calculator: http://www.ecalc.ch/motorcalc.htm)


There are 3 different parameters that are important to understand when choosing a motor:

  1. Motor size

  2. Power Rating

  3. kV Rating


Sizing

Determining the motor “size” can be very confusing as different manufacturers measure differently. Some measure the stator size (the inside of the motor), some measure the whole case. Some use glow equivalent numbers. (Example: Power .15) The most common indicator for motor size is the stator size. It’s usually formatted like this: 2210-12 first 2 digits are the diameter in mm. The next two digits are the height in mm. The last number, after the dash is the number of “turns” for each winding. The weight (measured in grams) of the motor is another way to determine the size of your motor.


Power Rating (normally in Watts)

This way of rating a motor can be tricky. The wattage is rated differently at different voltages so be careful. Example: 150W at 15V is only good for 100W at 10V as it’s the current that is important. It’s a basic function of size - Larger motors can produce more power and not overheat as easily as a smaller one.


kV Rating

kV - NOT KILO-VOLT it’s a small k (k represents constant.) kV stands for Revolutions per minute / per volt. A 1000kV motor will spin 10,000 RPM on 10V. This rating is calculated when the motor has no load. kV rating has nothing to do with how much power the motor can produce. It represents how fast the motor wants to spin. The kV rating is useful to help determine the size of the prop.


Important Note:
Whenever you change the propeller diameter or pitch it will change the current draw and performance. You can measure this using a watt meter to ensure you’re not pulling too many amps.



ESC (Electronic Speed Controller):

Important things to consider when choosing an ESC are the Current, Voltage and weather or not you need a BEC.

  • Current Rating - How many amps the ESC can deliver continuously. Many ESC’s will also list a ‘max burst’ (usually 10 seconds or less.)

    • This is NOT how much power it will pump out to the motor. It is how much current the ESC can govern before being destroyed.

    • An ESC capable of more amps is not going to hurt anything. It is only going to weigh more.

  • Voltage Rating

    • ESC’s usually have a voltage range they can operate within. Normally it’ll be printed directly on the ESC. If you exceed this voltage the ESC will most likely not work, or will be destroyed.

  • BEC

    • The Battery Elimination Circuit is what powers the Receiver (RX.) Unless you are powering your receiver from a separate battery, you will need to make sure your ESC has a BEC built in.


Choosing the right speed controller:

Look at the amp (or watt) rating of your motor. Use the 80% rule! If the motor is rated for 10 amps get an ESC capable of 12 amps or more.



Links to useful articles:

Using a Watt Meter

Choose Your Prop Adventure

Noobie Do's Noobie Dont's

Measuring Thrust

Choosing a Propeller 

 

HorizonHobby-BeginnerSeries

COMMENTS

jrbemis on November 27, 2013
Awesome as usual. Using your sessions with a grand nephew who is a second grader. He is starting to catch the jokes. He is also learning to build and fly.
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Ruslan_vranov on November 28, 2013
Oh! My small 8A 2300 kv motor with 7x3.5 prop rated 352g of thrust oh my weights...
Calculator gave 400+... ???
*** Thank you guys! That's really useful!!!!
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Patnaude_av8r on November 28, 2013
ive been using a web based calculator and I like its simplicity more than the calculator you refer to in this article. Please review this WebOCalc and let me know your thoughts. I found it via rcgroups, be aware there are a couple of versions out be sure to use the version 1.7.6 which has a few more added features and recommendations. I like it cause it takes all the guess work out of trying to calculate which prop, motor and esc and battery, this calculator tells you your prop, motor, esc, battery based on a few questions like wing length and area, max prop diameter based on individual airframe, your flying characteristics of yourself and the planes capabilities and how long you want to keep plane in the air. it will spit out the best prop and motor combination and battery size. http://flbeagle.rchomepage.com/software/webocalc_1.7.6/webocalc.html
Id really like to hear your thoughts on this.
Patnaude_av8r
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moofidelity on November 28, 2013
I recently found a podcast called All Things that fly, they had an episode called power systems 101 which was really good. A whole hour of great information that you really need to know. It was based on a lecture that one of the hosts gave at RC-clubs so there's a powerpoint presentation to go with it.

This FT episode was a great intro to the subject but I really wanted to share the above as I found it immensely useful and is more of a complete reference.
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jessawalla on December 4, 2013
Just thankyou
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Hafiz on August 8, 2014
my head was like (?.?) hahaha but a great info thanks for sharing knowledge
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CptCrazyFingers on April 13, 2015
I was watching this video and something is just eating away at me. At 13:36 you recap prop diameter. In writing on screen it says:
Prop Diameter
Smaller number: more thrust/less speed
Greater number: more speed/less efficient

I'm sorry if I am missing something, but isn't it the other way around? Larger diameter props generate more thrust than smaller diameter props. And large diameter props create more drag, therefore limiting speed. Speed is more related to pitch. Even in the case of pitch, the larger the pitch the greater the speed. I even used a prop thrust calculator to support this to make sure I wasn't crazy. I know this video is over 1 year old but can someone clear that up for me? Maybe I misunderstood.

By-the-way, you guys are awesome and the best source of information in this hobby. Thank you for everything you do for us.
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JayArr on May 26, 2015
The new link is available here http://www.ecalc.ch/motorcalc.php?ecalc&lang=en
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SlowJeff on May 4, 2015
Broken Link: eCalc appears to have moved or be gone.
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JayArr on May 26, 2015
see updated link below
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fatoa on September 1, 2015
Im new to the hobby been in it for about 2.5 years and im looking to upgrade some of my store bought models to make them faster and I don't understand the motor and what make a faster motor vs a motor with more torque. Can some one show me where I can go for more info on motors and power system.
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