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  1. #1

    a problem about power system that I can't figure out why

    I'm recently building an aeroquad x8, and I found a very strange problem:

    The motor I used is a small bush less motor which can output 500g of thrust under 12 volt,

    that means 8 motor should give me nearly 4000g of thrust, right? but as soon as I put it to test

    I found the max thrust was no bigger than 2500g. what's the problem?

    btw I am using a quality 5800mAh 30c battery, and quality bush less motor,

    all motors were tested separately before mounting to the quad.

  2. #2
    Moderator AeroQuad Core Software Developer kha's Avatar
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    Are the connecting wires heating up? If they are your power is lost on them.

  3. #3
    Hmm. I'd measure the thrust of 1 engine, then 2... then 4... then 6... then 8. Maybe you have a power issue? Do you have a way to measure RPM of the props or current draw? Those would help a lot also.

  4. #4
    Senior Pilot
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    What configuration are you testing the 8 motors in? Is it X8 (quad with two rotors on each arm, one rotor above the other)? This config is lossy.

  5. #5
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    battery?

    Hey, perhaps an open door but you can check the power consumption versus the supply capabilities of the battery.

  6. #6
    Quote Originally Posted by tori View Post
    What configuration are you testing the 8 motors in? Is it X8 (quad with two rotors on each arm, one rotor above the other)? This config is lossy.
    Yes, it's x8, but why this config is lossy? I thought the backwash will cause the lower set motor to generate more thrust, and I can also save the weight of four extra arms.

    I think I have found the reason of losing power, battery voltage drop to 10v (28A) when all 8 motors spinning at the top speed, the motor I use can only give about 320g of thrust under 10 volt. I guess I have to buy a 4s battery for it to generate enough thrust.

  7. #7
    Moderator AeroQuad Core Software Developer kha's Avatar
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    Quote Originally Posted by jiankkll View Post
    Yes, it's x8, but why this config is lossy? I thought the backwash will cause the lower set motor to generate more thrust,
    The lower propeller will suffer from the turbulence generated by the upper and thus has lower performance.

    Quote Originally Posted by jiankkll View Post
    I think I have found the reason of losing power, battery voltage drop to 10v (28A) when all 8 motors spinning at the top speed, the motor I use can only give about 320g of thrust under 10 volt. I guess I have to buy a 4s battery for it to generate enough thrust.
    Your 5800mAh 30C battery should be able to supply around 170A of current so it shold be enough. If you go for 4S verify that all the parts can handle that i.e. you might need 'high voltage' ESC:s.

  8. #8
    Quote Originally Posted by kha View Post
    The lower propeller will suffer from the turbulence generated by the upper and thus has lower performance.
    If that's true then I'm totally lost the interest of X8, because my goal is to build a quad that can achieve 40 mins of flight time.

    Thank you for your advice, luckily my ESC supports 4S battery. After switching to a new 4s 4000mAh battery I got 4200g of maximum thrust,

    finally, my 2000g x8 quad can takeoff safely.

  9. #9
    Moderator AeroQuad Core Software Developer kha's Avatar
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    For what I have seen 40min would be really hard to achieve...

    From my experience the power needed to keep 1kg in air (for a multirotor) is at least 100W.

    Lipo batteries have energy density of about 150Wh/kg.

    With these numbers a multicopter made 100% of battery would be able to hover for 1.5 hours.

    From this you can calculate that for 40 min hover the battery should be at least about half of the total weight, which will be very hard to reach.

  10. #10
    Senior Pilot elchan's Avatar
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    I think, if you want to achieve higher flight times, you'll have to beam microwaves at the quad, and convert that into electricity.
    Seriously though, if you're looking at high flight times, you will need to be even higher voltage, and higher kv motors. This is why all electric/hybrid car manufacturers use kilovolt drive batteries.


 
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