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Thread: Understanding KV and picking the right motors for your Multirotor build

  1. #1

    Understanding KV and picking the right motors for your Multirotor build

    Just helping to answer some commonly asked questions from people who are new to multirotors/RC.

    "KV" stands for 'RPM per volt'.
    (and 'RPM' stands for Revolutions per minute, for those who aren't familiar with the term)

    Lower KV == More torque, Lifting Power and Thrust
    Higher KV == More Speed, Agility and Acrobatic ability


    In general when designing your new Multirotor airplane, you start of with the "standard/default" KV motor which is a comfortable 1000KV. Then based on your requirements (sports quad? heavy lifter? fpv?) you refine your choices further.

    Ideally, you want to be able to hover your multirotor at around 40-50% throttle. Insufficient thrust and your multirotor starts to lose maneuverability and your FC's autoleveling starts to resemble a fat lady playing with hoola hoops. Give it too much thrust and your rig will be underweight and overpowered, resulting in excessive vibrations. Making this terrible for any kind of video (FPV/AP). However, if you are flying a sport flyer/acrobatic quad via Line-of-Sight then being overpowered could be your goal... who DOESN'T want to fly a rocket?

    If you were building a generic quad (ie. you just want something cheap that flies reasonably well), you can't go wrong with a motor close to 1000KV, this is also ideal for FPV rigs as you might like some speed and agility but can't give up too might thrust because FPV gear has a tendency to be heavy.

    If you were building an Aerial Photography rig, your focus would be flight endurance (aka hover time), lifting power, thrust and stability. Pick a good low KV motor within your budget, the difference in price is usually because of better quality components and efficiency. Which directly translates into better flight endurance and more air time. Getting a bigger frame and adding more motors (Y6/Hexa/Octo) also help to give you greater stability in strong winds, which are really useful for reducing vibrations/jello and keeping your target in frame.

    Lastly if you were building a sport flyer, go for something a little over 1000KV. A good target would be Quad configuration running on 1200KV motors. That will give you a good base to build a nippy little devil for speeding around Tampines Field at top speed while flipping, rolling and tearing up the skies. More motors increase stability (your ability to stay horizontal), reduce flight endurance and adds unnecessary weight (making it harder to do tricks) and that is not what you want. Low KV motors can also be very expensive... and are just wasted on a sport/fun flyer. Put your money where it counts!

    I could probably write 500 pages on this subject but let's just keep this short and sweet so we don't drive all the newbies away!
    Hope you enjoyed reading my guide and maybe this would come in handy when you decide on your next build!


    p.s: ... secretly hopes this helps encourage more multirotor enthusiasts in singapore!

  2. #2
    Hi Kaen, Thanks for the information.

  3. #3
    Quote Originally Posted by laughingbro View Post
    Hi Kaen, Thanks for the information.
    You're welcome! Hope it was education. And I wrote it all without ever mentioning propellers! woot...
    I think that deserves an article all on it's own... at the very least...

  4. #4

    Thumbs up

    Hello Kaen

    Thanks for the inform will keep in mind

  5. #5
    Short, concise, sweet and informative.
    I've rectified my quad with the help of the ppl who reply my thread and a guy call TimAtWork.
    Now, i can fly my quad confidently and intending to get a better frame and relatively better low KV motor..
    "..If you fail, don't fret. Just remember that even a plane go against the wind.."

  6. #6
    Quote Originally Posted by boonyeo View Post
    Hello Kaen

    Thanks for the inform will keep in mind
    Quote Originally Posted by raze02 View Post
    Short, concise, sweet and informative.
    I've rectified my quad with the help of the ppl who reply my thread and a guy call TimAtWork.
    Now, i can fly my quad confidently and intending to get a better frame and relatively better low KV motor..
    Cool. I will write more stuff soon... might be making this a 6 part series covering all the essentials.

  7. #7
    Senior Member feudallordcult's Avatar
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    great effort. Kudos to u!
    Heli-ADDICTION:
    Iron-Man 650 (APM) & 680pro (CC3D)
    Gobs 630, 700, Compass 6hv & Trex600esp (初恋)

    All I need is just one good bird...yah right!

  8. #8
    Quote Originally Posted by feudallordcult View Post
    great effort. Kudos to u!
    Thank you for the props!

  9. #9

    Icon14 Great effort!

    Thanks for spending the time and effort! One question:

    Lower KV == More torque, Lifting Power and Thrust
    Higher KV == More Speed, Agility and Acrobatic ability
    Can you elaborate why lower KV => lower rpm => more thrust? I thought thrust comes from rpm, i.e. higher rpm => higher thrust.

    So in a nutshell,

    < 1000kv = AP
    1000kv = FPV
    >1000kv = sport flying

    Am I right?
    Helicopters: KDS 450QS + Tarot 450FL + ZYX4.0, WLtoys V922, HiSKY FBL80, Nine Eagles Solo Pro 180
    Quads: Skyartec ButterflySold:WLtoys V929 V939 V949 V959, HobbyKing Mi-easyr182, Nine Eagles Xtra 300

  10. #10
    Senior Member
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    Good work Kaen!
    It's about time that we build our own knowledge base.
    A big applause to you and the other excellent contributor, Joe Yap. Our very own rc plane craftsman!

  11. #11

  12. #12
    Quote Originally Posted by Loganbest View Post
    go to the link below - it shares about the type of transmitters available and their differences.

    http://www.rchelicopterfun.com/futaba-radios.html

  13. #13
    Quote Originally Posted by FyreSG View Post
    Thanks for spending the time and effort! One question:

    Can you elaborate why lower KV => lower rpm => more thrust? I thought thrust comes from rpm, i.e. higher rpm => higher thrust.

    So in a nutshell,

    < 1000kv = AP
    1000kv = FPV
    >1000kv = sport flying

    Am I right?
    I didn't want to write a very long article so I left out a lot of details.... The KV recommendations are primarily for 450 sized multirotors lifting about 1kg AUW (All up Weight). If your rig is bigger and/or heavier then you have to adjust accordingly, it gets more complicated so I can't give a 1 size fits all advice.

    This part gets IMHO pretty complicated...

    Motors themselves DO NOT allow you to calculate THRUST. A proper thrust calculation can only be generated from a motor + propeller combination.

    Higher KV == More speed == More Thrust? (to a certain extent).
    However, Higher KV motors also have lower Torque (Power), which is more suitable for spinning smaller propellers.
    Trying to spin bigger/heavier props beyond it's capability and your motors may overheat, stop spinning mid-air and cause a crash.

    If you run the same Low KV motors with the same small propellers... you will get lower thrust than the High KV motors (if all other factors remain the same). So the reason for using Low KV motors is when you have a BIG frame because they have the torque to spin bigger propellers that produce More Thrust.

    Take a look at some of these charts.

    750KV motor
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    530KV motor that costs almost double (chart shows motor running only 4S batteries)
    Name:  nx-4006-thrust.jpg
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    Left side is watts drawn, right side is watts/thrust which is what we call efficiency. Higher efficiency means better flight endurance. But if you are lifting a very heavy load and go for the maximum thrust, you end up with poorer efficiency. The sweet spot is to produce enough thrust to hover at 40-50% throttle. So you tweak your propellers/motors/battery voltage till you hit a sweet spot.

    I like high efficiency because I'm doing AP and want to hover for as long as possible... so I try to get a combination that gives me the highest efficiency that generates sufficient thrust to hover my multirotor at 40-50% throttle. You may have a different goal so adjust accordingly.

    Quote Originally Posted by mave View Post
    Good work Kaen!
    It's about time that we build our own knowledge base.
    A big applause to you and the other excellent contributor, Joe Yap. Our very own rc plane craftsman!
    Thanks!

    Quote Originally Posted by Loganbest View Post
    go to the link below - it shares about the type of transmitters available and their differences.

    http://www.rchelicopterfun.com/futaba-radios.html
    It's quite outdated though... I'll have to say that ER9X running on the Turnigy 9X/9XR has changed the game significantly.
    You no longer have to pay usd$500 for a mid-range 9 channel radio. For usd$50+ you get a crazy deal that gives you a super configurable and powerful 9 channel radio. Yes I know it's cheapo grade and low quality... but for that price and money you save you can now afford to buy a fancy flight controller with failsafe features like return to launch and autoland and you still have plenty of money left over to buy a few more 9X/9XRs for backup if and when the first one ever breaks.

    I'm sure all of you with fancy JR/Futaba radios costing way too much money will disagree with me.
    Just like your BMW is most awesome car in the world but my totoya works great and i like saving money!
    Last edited by Kaen; 28-02-2013 at 01:39 PM.

  14. #14
    Great thread. Thanks for the clarification. I do however have another question.
    Can you explain the rest of the numbers.
    I believe I heard this all somewhere before but like most things, in one ear,,,,.
    Correct me if I am wrong and please help me where I forgot.
    28xx = frame size?
    xx36 = ??
    / 9 = number of windings on the armature?

    And finally,,, why is the number of windings important to know and what do I get with higher/lower numbers?

    Thanks

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