My new MK heavy lift copter project

Discussion in 'Cinestar 8' started by Dave King, Jan 4, 2014.

  1. Michael McVay

    Michael McVay Active Member

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    @Howard: I was just going to ask what Avroto was similar to on the Tiger site. i could not find any similar specs for Avroto the way Tiger does it for their motors. At one point I think I read that the Avrotos were Tigers with white exteriors, bigger bearings, etc and thought maybe the MN3520s would be a comparable spec. If so, here is the chart for the Tiger 3520s:

    If you want to hover at 50% throttle at 28 pounds you need about 1600 grams per motor of thrust. Using the tiger 3520 specs below you could only guess that an 18 inch prop would do it, but if you see the motor temp column going from 16 to 17 takes a big temp jump. Going to 18 might be too much for this...just a guess. That leaves you with about 60% throttle using 16 inch props - or over 65% with 15x5 props.

    The U7s-420s would get you 1600 at roughly 60% staying with a 15x5 prop and the U7-490s would get you 1600 at 50% with a 15x5 prop. Both of the U7s would be running very cold at that level as well.

    Efficiency while providing 1600 grams of thrust seems to be very similiar for the 3520s, U7 420s and U7 490s. All getting in the upper 7s for g/W and all pulling about 9 amps to do it.

    I like the idea of the U7 490s to have the power to swing an 18 inch prop with no problem or the flexibility to be very efficient with a lighter load and deliver long flight times.

    At 50% / 100% throttle (6 cell battery) the U7 490s kick out:

    15x5 props - 3.5 pounds per motor / 9 pounds per motor
    16x5.4 props - 4 pounds per motor / 10.2 pounds per motor
    17x5.8 props - 4.6 pounds per motor / 11.2 pounds per motor
    18x6.1 props - 5.3 pounds per motor / 11.6 pounds per motor

    The max thrust at 100% throttle may not be something we ever need, but sure is impressive. There is a lot of lift available between 50% and 100% with these.

    MN3520 Specs.jpg
     
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  2. Dave King

    Dave King Well-Known Member

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    Michael great stuff! How are you determining that tit will take about 1600 grams of thrust to hover at 50% at 28 pounds? I was trying to figure out how much trust it takes to lift X weight. What would you say it would take to hover at 50% with 25-26 pounds?
     
  3. Howard Dapp

    Howard Dapp Active Member

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    @Michael here are Avroto 3520 specs posted 1 page back

    MontoRc says the Avroto 3520 are more efficient at heavier loads 27-30lbs

    I would think 490kv at this weight would not be so efficient with very short flight time. Even at 25-27 lbs flight time would be short.

    Avroto 3520:

    Weight approximately 190 grams not including wires.
    Weight with wires 228.4 grams
    Includes accessories to fit 3 hole Carbon Fiber props
    Includes 6mm diameter 4 bolt standard prop adapter
    External Dimensions 42.5mm D x 43mm L
    Stator Dimensions 35mm D x 20mm L
    Stator material .2mm NdFeB sheet
    Internal Shaft diameter 5mm.
    No-load speed: 400 rpm / V
    Ri (Internal Resistance) 58 mΩ
    Io (no load current) 0.3A
    Voltage for Io measurement 10V
    Current capacity 39A/180s
    Max continuos Watts: 1150
    Lipo cell 5s-6s
    Wire length 700mm
    14magnets 12 stator arms 12N 14P

    3520 with 30 lbs payload (AUW 45lbs) I believe he was running 16x5 props
     
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  4. Michael McVay

    Michael McVay Active Member

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    @Dave: Basically just take the all up weight and convert to grams (since this is what Tiger uses for their thrust specs) then divide it by 8. For example, 26 pounds is 11,793 grams. Rounded to 12,000 and divided by 8 is 1500 grams per motor to keep 26 pound hovered in the air (perfect conditions assumed - all motors working equally). If the copter had to pitch into wind all motors would need to put out 1500 grams and the ones providing the extra lift to pitch into the wind would have to put out a little more and I have no idea of the others work a little less or not. We are not trying to be that specific for this.

    Obviously if you wanted to do this with a hex instead of an octo you would divide by 6 and each motor would need to produce 2000 grams of thrust instead of 1500.

    Now that you know what you need, you just have to search through all the published specs from Tiger to find something that works for you.

    I like knowing the thrust I need and then finding a number close to that on the Tiger charts. It seems easier to understand once you know what you need from each motor. Then, if you have a particular prop size limit you can see how that effects the motor. Also, since the charts show amp draw and efficiency you can determine the best motor for your job since there will likely be several motors capable of flying the weight you are after. Getting more thrust per watt means longer flights, lower amps mean lower ESC temps, etc.

    Since thrust is a combination of the propeller RPM, length and pitch you can see what a difference each item makes on the charts published by Tiger. KV gives you your RPM per volt so if you can get a motor with a higher KV rating it will spin faster than one with a lower rating when given the same amount of volts from the throttle input. Smaller motors with higher KVs have not generally had the torque to swing a big prop at the high rpms they spin at. The standard QC motors which spin at around 700KV would obviously spin faster than the 400-490KV motors we are discussing, but are not designed to have the muscle to spin that fast with a large prop on them. I know there are some people on this forum that prefer to go down to a 13 inch prop with more pitch (13x6.5) instead of the standard 14x4.7s or the 14x5.5 multi rotor props on the QC motors. Smaller props are easier for the motor to turn.

    With the older version of the MK stack we were a bit stuck to 4 cell batteries (without some mods) so you needed a motor that could take the 14.8 volts of a 4 cell and get enough RPMs to get the thrust necessary to fly the weight of the CS8 or whatever rig you were flying.

    With the new U series we have some variety of motors that can swing very large props ranging from 100KV on the U8s up to the 490KV on one model of the U7s. They obviously made the U7 with some substantial internal parts because it is bigger than a traditional motor. This must be how it can go to 490KV and still swing 18inch props at almost 7000 rpms without getting hot.

    Hope this helps...
     
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  5. Michael McVay

    Michael McVay Active Member

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    @Howard: The Tiger specs for their 3520 would put them at about 80-85% throttle to hover 45 pounds. At that point flight time would be a function of battery size onboard. Again, I'm not sure about how they upgrade when they make Avrotos so there is probably some difference - I'm just not sure of what.

    I saw the specs you posted, but am not sure what I can do with those. The charts I am posting are copied from www.rctigermotor.com where they publish a variety of performance specs on each motor. It takes a particular battery voltage (4S, 6S, etc) and then compares prop sizes at different throttle percentages. They publish things like the amp draw, rpm, thrust (in grams), efficiency (in thrust grams per watt) and the temp range for the motor. What I have been doing is taking those specs and comparing motor scenarios for various weight copters, etc.

    To see how the Avrotos compare we would need someone to run the same type of bench testing on the motor with different props and measure all the currents, watts, etc. Other than the manufacturer I have only seen a few guys do something like this when testing various props to compare them.

    Sorry.
     
  6. Steve Maller

    Steve Maller UAV Grief Counselor

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    I wrote to Jim from MontoRC and asked him that same question about the Avroto 3515 vs. Tiger MT3515. He said,
    "Yes thrust data from any 3515 400Kv should give you a general performance profile. Compared to MT3515 the Avroto 3515 will have equal or very slightly better efficiency in most cases. The The Avroto tends to run slightly cooler, and can reach slightly higher temps before failure."
     
  7. Michael McVay

    Michael McVay Active Member

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    Thanks Steve - I thought I had read a post about that from someone.
     
  8. Howard Dapp

    Howard Dapp Active Member

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    This comment actually woke me up. A powerful efficient motor rated at over 400kv able to spin a 16-17" prop at 6s. I just may go with the U7! Wow have times changed. Just a year ago we didn't have so many choices!

    Btw, thanks Michael for your input...very helpful.

     
  9. Michael McVay

    Michael McVay Active Member

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    My pleasure Howard - the U7 is a mighty beast of a motor. Seems to be what the heavy lift crowd has been looking for. Plus it claims to be dust proof, water proof, German engineered, self cooling, etc.
     
  10. Steve Maller

    Steve Maller UAV Grief Counselor

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    This is interesting. I wonder if the active breaking of the new BL 3.0 on the Double Quadro 2XL is actually generating power, adding to the current available from the battery itself? Otherwise, it’s just a math error in the software, right?
     
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  11. Chris Fox

    Chris Fox Active Member

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    double counting the same electrons, with the regenerative braking it is pumping charge back into the battery when the props are windmilling ..... it is going to be interesting to watch to see what sort of usage people are getting as you and Dave clock up some more flight time with these setups.
     
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  12. Dave King

    Dave King Well-Known Member

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    Agreed!!!!!
     
  13. Dave King

    Dave King Well-Known Member

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    I saw your post over on MK and Holger responded that there is some math error and battery capacity tolerence. Thanks for posting Steve.
     
  14. Steve Maller

    Steve Maller UAV Grief Counselor

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    Here is Holger’s response:
    Like in the old versions: the BL is measuring the current and the FC is calculating the capacity.
    But the BL3.0 have amplifiers to get a better current signal.
    So I guess we’re back to only really having the DC voltage to determine our optimal, safe flight times. Personally, I very much relied on that current number in evaluating my copter’s performance, and I’m concerned about not having the current consumption be useful now that I’m bringing up a brand new bird.
     
  15. Steve Maller

    Steve Maller UAV Grief Counselor

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    Well, maybe it’s better than I thought...looks like there’s a way to calibrate the BLs to the batteries to get more accurate readings. Here’s Holger’s explanation: http://forum.mikrokopter.de/topic-post502430.html#post502430
    Dave, it’ll be a few more days before my bird’s in the air...wanna give this a go? My LiPo chargers (the EOS 0720i Duo3) can definitely track the number of mAh of charge in a cycle, so it sounds like that’d be a good solution.
     
  16. Dave King

    Dave King Well-Known Member

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    I saw that, and its very promising. It's rain in the forecast and I have to work late this week. Probably won't get back up in the air until at least Friday. BTW: I just ordered the U7 motors and I'm going to give them a try and see what happens.
     
  17. Steve Maller

    Steve Maller UAV Grief Counselor

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    Ka-ching! Wow. Stepping up your game, amigo. :eek:
     
  18. Dave King

    Dave King Well-Known Member

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    I've been talking with Michael and Tiger and I really think its the best way to go. It's looking like my Payload is going to be 11 pounds (Not 10 pounds as I thought) so its going to be even more than I thought. Plus that doesn't take anything like higher winds into account or any other weight I might need to add (different lenses, follow focus etc)

    I could go to bigger booms and props but its not going to give me much head room and they would run hotter with the 16's than the 15 combination. I just don't want to take any chances plus I need to worry about how the copter would run in the summer. Plus if I buy 550mm booms I have to get new NAV lights (not cheap) where as if I stay with the current booms I can sell my existing motors and the cost is half of what it would be if I went with booms/props. Plus everything will fit in my trunk easily with the 500 booms.
     
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  19. Howard Dapp

    Howard Dapp Active Member

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    I think that's a wise choice. I've decided to go U7 as well. I also believe the supposed loss in power using the 12C converter will not be a noticeable issue with this higher KV motor.
     
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  20. Steve Maller

    Steve Maller UAV Grief Counselor

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    What are you using for nav lights? My experience with the LED stuff out there is that there's astonishing power at a good price. And a hair dryer or heat gun can get those sticky strips off your booms quite easily. ;-)
     
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