Hi everyone
We're building a multicopter using an X8 setup. Our challenge is that we have to build a very small copter which can lift a heavy weight. The total frame weight is 2kg (with battery but without payload). The payload is 3-4kg. So the max AUW is 6kg.
Total Thrust = 6000g x 2
Total Thrust = 12.000g
We have 8 motors. So 12.000/8 = 1500g thrust each.
Because it has to be a small copter we use 8-10" in props. At this point we have APC SF 8x6 props.
I used http://www.flybrushless.com/tools/thrustCalc to calculate that I need 12.500 RPM for each motor to generate 1600g thrust per motor.
I was thinking about the following motors: http://www.hobbyking.com/hobbyking/store/__9664__hobbyking_ap03_4000kv_brushless_micro_motor_3_1g_.html
With these motors I can use a 10A ESC.
Now, to choose my battery I take 12.500 RPM / 4000kv = 1,45V => So I have enough with a 2S battery.
The max current of each motor is 2,5A. A total of 20A. I select a 5000mAh 30C 2S battery.
If my math is correct (5000mAh/20A*60) I have a 15 min flight.
Is that math correct? With these specs I have a X80 copter lifting a heavy payload for 15 minutes?
Thanks,
Bests
Wouter
Replies
I've not had time to check this properly but it seems your calculations are heading in the wrong direction.
Rotor thrust is essentially a function of power input, motor efficiency, rotor efficiency and rotor diameter. The motors you are looking at are only 12.58W motors (7.4V x 1.7A). That gives you 100W in total.
You could potentially lift your 6kg with 100W, but would need very large (and fine pitch) rotors. The thrust figure quoted by Hobby King is 61g, that gives you a total of 488g of thrust. I know you are suggesting bigger propellers but even if the motors are not overloaded I doubt they are big enough to give you 1.5kg of thrust each.
I'll try and remember to get a calculation to you that gives the relationship that allows you to calculate maximum thrust from power input, rotor size and efficiency.
I think you need to be starting with a larger motor.
Cheers,
Sam
Hi Sam
Thanks for the feedback. What if I take T motors:
http://www.rctigermotor.com/html/2013/Professional_0912/43.html
Or is this not powerful enough?
Bests
Wouter
No problem.
The static thrust / power calculation you need is described here:
http://quadcopterproject.wordpress.com/static-thrust-calculation/
Note that there is no efficiency term in that calculation so you will need more power than quoted. You will have to check actually values but I would have thought 75% best from the propeller and maybe 80% for the brushless motor would be reasonable starting values.
I'm note sure why you say you need 12kgf of thrust? You are right that you need more that your max take-off weight of 6kgf, but you should get away with a bit less than 12kgf. One of the best things to to when your designing an aircraft of any sort is to start with data for existing aircraft. Have a look at other X8 and quadrotors and work out the static thrust to weight ratio (or the specific thrust, thrust per unit weight) that they typically use and aim for something similar.
You are of course best to use actual experimental data. The T motor MT2208 KV1100 link does quote thrust values. If you need to stick with 8" propellers, the value listed is 675gf at 14.8 V. 8 x 675gf gives you 5400gf. you probably need almost double that.