how to tell what prop and motor you need

Discussion in 'Scratchbuilding!' started by MoTheG, Apr 11, 2009.

  1. MoTheG

    MoTheG Airman

    Posts:
    1,172
    Likes:
    12
    Points:
    38
    For those of you who want their own program or like to do the math,
    here are some formulas for drive estimation:

    EofD = n/kv.*StromMotor(Uakku,Io,Rkupfer,Rakku,Ul,nl,kv,n)/Uakku./(StromMotor(Uakku,Io,Rkupfer,Rakku,Ul,nl,kv,n)+Io);

    function Im = StromMotor(Uakku,Io,Rkupfer,Rakku,Ul,nl,kv,n)
    Im = (Uakku - reibspannung(Ul,nl,kv) - n/kv - Rakku*Io)/(Rkupfer+Rakku);

    function Ureib = reibspannung(Ul,nl,kv)
    Ureib = (Ul/nl) - (1/kv);

    pm =< n/kv.*StromMotor(Uakku,Io,Rkupfer,Rakku,Ul,nl,kv,n)


    Io = "current at Uakku without propeller" [1A]
    Ul = "Voltage over Motor without propeller at nl" [10V]
    nl = "RPM at Ul without propeller" [0.8*kv*10V]
    Uakku = "Voltage over Battery without current"
    UakkuLoad = "Voltage over Battery with current"
    Rakku = "inner resistance of battery + ESC" [300 mOhm]
    Rkupfer = "resistance of wires" [100 mOhm]
    kv = "kV-rating of the motor"
    n = "targeted RPM"
    EofD = "Efficiency of Drive"
    pm = "mechanical power that the propeller consumes"

    If you don't know the resistance of the battery nor the ESC but the voltage at load than use: Rakku=(Uakku-UakkuLoad)/(Im+Io)

    this is assuming you don't use a transmission, but it works with minor changes with a transmission too.
    you should target an efficiency of 70% - 75%, if you can not get it, than the motor might simply be to small.

    search cloud: gear gearing rating meaning thrust setup rpm torque propeller
     
  2. MoTheG

    MoTheG Airman

    Posts:
    1,172
    Likes:
    12
    Points:
    38
    How to pick a drive

    I decided to make a how-to because I see many questions regarding drive design and even experienced pilots often don't know exactly how to get it right.

    1. You need to know how fast you want to go.
    2. You need to know how much thrust you want.
    3. You start with the Prop: pick one with the same diameter and pitch
      Fast: small propeller / Slow: large propeller
      Pitch[inches] = V(topspeed)[m/s]/n(RPM, standing)[1/min]*60*100/2,54*(at least two other factors, but they seem to cancel each other out, most of the time)
      -> P[inch]=V[mph]/n[rpm]*1056
      now check if the speed and thrust are right.
      • if both the speed and the thrust are too small increase the RPM
      • if the thrust is too small, but the speed is right use a bigger(both) prop at lower RPM
      • if the thrust is right, but the speed is too low increase the pitch.
      • if you need more thrust but can not fit a bigger propeller reduce the pitch and increase the RPM
      • if the speed is way too high and the thrust is way too low use a propeller with more diameter and less pitch (Only do this, if the above doesn't help)
    4. Pick a motor: Now you know what RPM and power you need, this is what you need to pick the motor.
      1. To get you started use a rule of thumb to find the approximate kV-rating you need: "kV-rating"=RPM/4*5/"Voltage of your battery" (under load a NiMH-cell will only have 1.05V and a LiPo-cell 3.4V) if you can carry the weight use a high voltage battery.
        Minimal current your new motor needs to allow: I=Power/"Voltage of your battery". This is rather difficult because you have to rely on the manufacturers data and often you will find that the motor has an efficiency of below 70% at the given max current, that is why you should look for a max. current that is +50% of what you need, so you don't have to come back and pick another motor after doing the math. There is another rule of thumb that you can use at this stage: weight[g] > power/(3W/g). An 75g motor will not convert 450W efficiently at the voltages and RPM we use even if the manfacturer claims this. Higher kv-rating motors will be able to handle a bit more current and therefor power.
      2. Go and look around to find out what motors you can get with roughly that kV-rating and same or higher max current. Say you have come up with an kv-rating of 1234 then you will not find just that on the market obviously. pick a motor that is in the range of -3% to +7% of what you are looking for. If the kV-rating is below 1000 you need to think about a gearbox, a propeller with lower pitch or less batterycells.
      3. Do the math (computer) to determin what motor is best.
      4. Pick an ESC and battery that can deliver the current that you calculated. (more on this in post no.4)
    If the company does not give you enough information or claims above 90% efficiency for their motors, just ignore them and their products.

    What NOT to do:

    • use a Slowflyer-Propeller (SF) when you can avoid it / use RPM instead of Pitch.
    • pick a higher top speed than you can reach
     
  3. Dredogol

    Dredogol M0DERATOR - (aka "Dred")

    Posts:
    950
    Likes:
    6
    Points:
    0
    Pretty nice post there, I'm going to sticky this for you MoTheG.
     
  4. MoTheG

    MoTheG Airman

    Posts:
    1,172
    Likes:
    12
    Points:
    38
    Battery and ECS

    Now that you know what prop you need and approximatly what motor, it is time to help you with the battery and ESC.
    NiCd or NiHM are no good for anything that flyes.
    If you want to save some money you go for Li-Ion instead of Li-Po, that will not save on the investment but it will save in the long-run. Li-Ion lasts longer and takes more abuse but does not have the same energy/weight benefit.
    Now you know what type to get, it is the question how many cells to get.
    S = squareroot(power/20) will tell you how many cells you need in your pack.
    The ESC you need depends on the cell type, voltage, and current.
    By now you should have established all those numbers and added an uncertanty/savety margin of no less than 20%.
    Many ESC don't support Li-Ion also support for more than 3 cells is not to be assumed.
     
  5. Bowler

    Bowler Cadet

    Posts:
    33
    Likes:
    0
    Points:
    6
    hey wow...good info. ya stickie this for us aye :D
     
  6. DavePowers

    DavePowers Administrator

    Posts:
    4,563
    Likes:
    4,720
    Points:
    133
    Wow Moe, that is some really good tech stuff!
     
  7. MoTheG

    MoTheG Airman

    Posts:
    1,172
    Likes:
    12
    Points:
    38
    Links

    Here a few links on the topic:
    http://findarticles.com/p/articles/mi_qa3819/is_200107/ai_n8978472/
    http://www.hooked-on-rc-airplanes.com/model-airplane-propellers.html
    http://www.stefanv.com/rcstuff/qf200401.html
    http://www.wattflyer.com/forums/archive/index.php/t-3582.html
    http://adamone.rchomepage.com/guide5.htm
    some older guides might mention brushed motors; don't use brushed motors, they are heavy and don't like current.
    Gear/transmission was important with b-motors, lesser now, but don't think they are a thing of the past.
    Although low kv-rating motors are available gearing is still a valuable option if you want to maximise thrust and efficiency.
     
  8. MoTheG

    MoTheG Airman

    Posts:
    1,172
    Likes:
    12
    Points:
    38
    simmilar specs different RPM

    Say you want to put a bigger Propeller and motor on your plane that used to work ok with a small one or the other way around.
    Than you can calculate which Prop is similar to the old:
    a x b -> A x B
    A = a*(B/b)^(3/5)

    let us take Dave's 6"x4", say you don't like it but want to keep the specs.
    you want to use a prop with a Pitch of 6" instead, than the equation tells you to try an 8"x6" Propeller instead.
    [​IMG]
    the values are rounded to the nearest integer, the result is that the Diameter increases linearly with the target pitch until 6" of pitch.
    on the right you can see that a 10"x9" could have the same thrust and topspeed as the 6"x4" just at much lower RPM.
     

    Attached Files:

  9. Jbirky

    Jbirky Ace Pilot

    Posts:
    1,449
    Likes:
    25
    Points:
    48
    I am thinking you should write an eBook and explain things in small steps.

    For instance, I have no idea what "Uakku" is about. The only acronym I recognize is kv
     
    droneguy and cdnrcflyer like this.
  10. MoTheG

    MoTheG Airman

    Posts:
    1,172
    Likes:
    12
    Points:
    38
    Uakku doesn't mean anything much, for this sake I could have called it 'pxsqq' or anything else.
     
  11. Jbirky

    Jbirky Ace Pilot

    Posts:
    1,449
    Likes:
    25
    Points:
    48
    Well, if you are going to make up abbreviations, you need to clue us in or we will be clueless.
     
    droneguy likes this.
  12. MoTheG

    MoTheG Airman

    Posts:
    1,172
    Likes:
    12
    Points:
    38
    In my very first post I wrote:
    all names I used are discribed. You may rename them to your best liking, it does not matter.
     
  13. crewdog927

    crewdog927 Rookie

    Posts:
    39
    Likes:
    2
    Points:
    0
    Like writing computer code..there are better guys at it them me so i will let them do thier thing.

    Thank you for being on of the ones who enjoys the very technical aspect of Rc propulsion/theory and sharing your experience. Its a lot to wrap ones mind around.

    I prefer the simpler approach, it goes like this " Dave/Scott/MotheG said it worked for him" cool i think ill try it! lol

    It gets about this deep for me:
    "If A equals success, then the formula is: A = X + Y + Z,
    X is work.
    Y is play.
    Z is keep your mouth shut."
    Albert Einstein

    Scott
     
    droneguy likes this.
  14. MoTheG

    MoTheG Airman

    Posts:
    1,172
    Likes:
    12
    Points:
    38
    sounds like a sound method. There are online calculators for drives.
    http://brantuas.com/ezcalc/dma1.asp
    http://www.s4a.ch/eflight/motorcalc.htm
    http://www.standschub.de/
    they will get you right to a working drive. but they don't know every motor propeller and are not precise at every point of opperation.
    sadly there is no way to avoid trial+error 100%. if the manufacturers would give more and true information about their products it would be easier.
    I started this: http://www.rcpowers.com/forum/showthread.php?t=495 thread to collect working drives.
    I like the quote but could not find proove for it.
     
  15. RobMac

    RobMac Rookie

    Posts:
    39
    Likes:
    0
    Points:
    0
    Whhoooooossshh.......flyby!!!!! :confused::confused::confused:
     
  16. MoTheG

    MoTheG Airman

    Posts:
    1,172
    Likes:
    12
    Points:
    38
  17. Threedee

    Threedee Cadet

    Posts:
    889
    Likes:
    8
    Points:
    18
    See this is why i only fly 3d. All you need to know is 150-200watts per pound and the rest is nothing.
     
  18. MoTheG

    MoTheG Airman

    Posts:
    1,172
    Likes:
    12
    Points:
    38
    that isn't true.
    It is a matter of how you spend those 200W.
    if you do it wrong, even that much will be to little.
    you can spend it on heat or airspeed and not get the thrust you want.
    Put an EDF on your 3D-plane and put 200W/lb into it, it will not fly the way you want.
     
  19. Threedee

    Threedee Cadet

    Posts:
    889
    Likes:
    8
    Points:
    18
    I have been around 3d my whole life I don't even know a thng about picking out tprops. With me I only know what I need to know. Like I have 3 setups, 2204, 2208 and a 2215. I base 90% of the planes i build and by off that. Like I see a sports pllane hats let's say 1.5 pounds hmm 200wats shuld be good , so I just but a new setup based on my old one, about same weght, kv, ands amps, and mabe a larger lip0.



    Sorry for spelling mistakes I'm dong this on m iPhone and it's hard
     
  20. MoTheG

    MoTheG Airman

    Posts:
    1,172
    Likes:
    12
    Points:
    38
    many are better

    Another thing to consider is whether you can fit more than one propeller.
    If you need more thrust using more propellers is a good idea as long as the weight of the two motors and ESCs is not undoing the advantages. Bigger motors usually have a better power/weight ratio, therefor a single 100g motor might still be more efficient than two 50g motors with smaller props.
    If you can't fit a bigger prop, it likely is a better idea to use an additional prop than to spin a single one faster (which also has its mechanical limit).

    My friends plane uses two brushed motors and a single ESC and has surprisingly much thrust and endurance.
    I think this is a good setup for a >2 lb plane.
     
Similar Threads
Forum Title Date
Scratchbuilding! Satellite receiver UGH! May 11, 2017
Scratchbuilding! Scariest Time Flying Story-Tell your's Mar 29, 2015
Scratchbuilding! Every picture tells a story Nov 25, 2014
Scratchbuilding! Any other german here ?? who can tell me whats wrong? Aug 25, 2014
Scratchbuilding! Hi all, can anyone tell me where I can get pdf plans for v2 f35? Apr 19, 2014
Scratchbuilding! Does a 30A UBEC ESC need a BEC? How do you tell the difference? Sep 3, 2012

Share This Page

string(1) "1"