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Power vs weight ?

Posted by Mike O on September 5, 2011 at 7:54am

I recently got my KK-board based quad to fly. Very few problems after reading the manual :-) The biggest obstacle to longer flights seems to be my lack of flight experience.

 

The copter is a simple homemade 8mm alu-tubing frame of 102gr, four 1400kV BL motors, two sets of 8 x 4.5 props, 30A ESCs and a 1500mAh LiPo. All together 505gr rtf. Arm length app 50cm.

 

Now, my first problem is that the thing just rockets up in the sky. Even with the throttle offset at -100, the thing doesn't want to hover at neutral stick, it still ascends. So before I do anything alse, I struggle to keep the thing hovering, and that pretty much takes up my motoric capacity leaving little for adjusting rotation and direction.

 

What to do? Fit a heavier battery? I've got a couple of 2200mAh (or even 3000mAh) LiPos, I could use. I have also considered using 8 x 3.6 props instead, but most people use much larger props...

 

Suggestions welcome - thanks!

 

Mike

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Replies

  • Mike O September 18, 2011 at 1:26am

    3692271536?profile=originalThank you for your feedback and advice! This is how my quad look today. Still a bit prototype, but I think you get the drift. Total rtf weight 520gr.

     

    Part of the KK board behavior puzzles me: after start-up calibration, the quad sometimes rotate slowly cw. Other times more stable. I have changed the board fixation (now suspended in foam), but without any change. I can compensate on transmitter, but not really satisfied with that, as it's a big off-set.

     

    Would that be a flight contoller / KK-board problem, or perhaps one of the motors behaving erratically?

     

    BTW: I have ordered 1000KV motors to see if that provides a wider choise of props.

     

    thanks,

     

    Mike

  • Mike O September 14, 2011 at 1:52pm

    Just got a set of 7x4 props: That did the trick; Thrust to weight is much more balanced now. Hovering at 40-50% throttle. Now, I just need to learn to fly it :-)

  • Mike O September 5, 2011 at 1:31pm

    Gentlemen,

    thanks for your input - I probably went a bit overboard with the motors, but 1400kV sounded conservative to me, as I'm used to RC cars :-)

     

    The motors are "Turbojet" brand from HobbyKing, and they are not listed in the calculator, but comparing them to an old brushed Permax 450 Turbo, weighing twice as much, they seem to make an equal amount of thrust. The 450 Turbo is rated at around 500gr for that propeller, so I'm probably getting 2kg of thrust for my 500gr rig. That's a non-conservative 4:1 power to weight ratio :-)))

     

    I'm going to fit 8 x 3.6 instead of the 8 x 4.5 and use the 2200mAh as well. I hope that will calm down things a bit, otherwise I just have to get another 4 motors :-(

     

  • Jose Angel September 5, 2011 at 8:41am

    Do you know the thrust of your motors?

    Usually I build copters with a thrust/weight ratio between 2.2:1 and 3:1.

    For example thrust 2,2kg, weight 1kg (45%). This provides a nice flying experience with appropriate power.

    If you have a thrust/weight ratio of say, 6:1 maybe you can reach the space if you are not carefull with the throttle :) Have you tried an expo curve for the throttle?

    Think about a ferrari motor installed in a bicycle...

     

    This calc could also be useful to plan a new copter:

    http://www.ecalc.ch/xcoptercalc_e.htm

     

  • Haidar Abu Bakar September 5, 2011 at 8:08am

    My friends who fly tricopters and quads all use motor less than 1000kv and bigger props but they use 2200mah Lipos.

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