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How to evaluate which motor/prop setup to use?

#1
Hey guys,

Love the H-quad idea - we built one, 1m x 1m in size, to be flown with the APM 2.5+.
Now our problem is the power to take off. It's way heavier than we'd hoped, comes in to 1.3 kg about. This is with APM 2.5, GPS, receiver, 2x 3S 1500mAh batteries in parallel and the power system we pulled off a DJI F350 ARF kit (920kv motors, 18A ESCs, 8x4.5 2-bladed props).

We tried taking off, and even at full power, it couldn't. So here's our question: how do you choose a prop/motor combo (and battery choice) to lift your multi-copter? Is it trial and error, or are there formulas we can use as a guideline?

We're not averse to buying new motor/props/batteries, but if we can reuse the motors we'd like to. We have 9x6 props already, but I don't have a clue whether that will give sufficient lift, or what it will do to power consumption.

Any help appreciated in the matter!
 

Liemavick

New member
Mentor
#2
Double check your motor rotation and prop orientation. Even at that weight and size Its kinda odd your not getting any lift at all.
 

Tritium

Amateur Extra Class K5TWM
#3
The DJI configuration on a 450 uses 4S batteries for the 8" props. For a 3S battery they use a 10" prop. (which is what I ran on mine till I sold it) so you might try some 10" props.
I believe that the 350 thru 550's use the same ESC's and motors.

Thurmond
 
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#4
Double check your motor rotation and prop orientation. Even at that weight and size Its kinda odd your not getting any lift at all.
Thanks for the head-check, but we're getting lift, just not enough to get off the ground... (full throttle and we're just chopping grass)

Thurmond, thanks for the propeller sizes - we actually have a F-330 (not 350, got confused). I believe you may be right about the prop sizes, so we'll try new props and 4S batteries and see where that leads us.

What kind of capacity should we be looking at for 10-12 minute flights? 2200 or more?
 

hansdude

Junior Member
#5
Maybe try...

I know this is kind of late, and I'm not too experienced with this stuff, but I had some ideas that maybe you'd find useful.

There's a lot of stuff going on in a craft like that. Are you sure that the motors are giving it their all? I'd first try to see what you can get out of one individual motor.

Check out FliteTest.com: Measuring Thrust

I'd do this with just one motor connected directly to a receiver or even a servo tester as long as you just use the signal wire. If thrust * 4 < 1.3 kg then you have your answer. I've heard recommendations of having twice as much thrust as weight for good control authority.

Also note that some ESCs can be calibrated to your throttle range. If the ESC is calibrated to a throttle range larger than what you're giving it then that could potentially gimp your output. Check out the manual for the ESC.
 
#6
hansdude,

Thanks for your input. As I mentioned earlier though, the motors can and will sustain flight for a quad up to 1.6kg according to DJI's stats for the F450 (which is the same motors as we have here, only the ESCs differ). The 18A DJI ESCs I have cannot be calibrated or programmed. They're hardwired out of the box, and this isn't making me very happy, obviously, but I have some 30A Turnigy plushes that came in with the new props.

At this point I've upgraded to 30A ESCs, 4S batteries and I have a set of 10x4.7 and 12x4.7 props to try on, and hopefully that will lead to a flying H-Quad. (roughly 1m on the side, it is BIG)

I will let you know what the results are after the flight attempt on Thursday.
 

Tritium

Amateur Extra Class K5TWM
#7
Careful with those 12X4.7 and 10X4.7. The 10X4.7 is rated for 3S batteries on that motor. The 12X4.7 will likely cause overheating of the motor. No way to know absolutely without a wattmeter and testing.

Thurmond
 
#8
Careful with those 12X4.7 and 10X4.7. The 10X4.7 is rated for 3S batteries on that motor. The 12X4.7 will likely cause overheating of the motor. No way to know absolutely without a wattmeter and testing.

Thurmond
Aha, thanks for the information Thurmond! May I ask where you found those ratings? I'm more of a DLG guy myself, just getting into multirotors, so the whole motor-prop tweaking thing is a bit outside my previous experience.

Cheers,
John
 

Tritium

Amateur Extra Class K5TWM
#9
Aha, thanks for the information Thurmond! May I ask where you found those ratings? I'm more of a DLG guy myself, just getting into multirotors, so the whole motor-prop tweaking thing is a bit outside my previous experience.

Cheers,
John
They were from memory of my early days of studying and building on the DJI platform.
Like I indicated a Wattmeter is a very valuable tool. You can try various combinations of power
plant components and the wattmeter will tell you if you are within limits for the components you have chosen.

Thurmond
 
#12
Thanks for the link Mustang!

Thurmond, when you said the props are "rated", does that mean there's an RPM limit for the prop to not go over before it explodes, or is it a matter only of current? That's what had me a bit confused earlier. Re-reading I guess I didn't explain my confusion very well.

At this point we've been having bad weather here, so I'll only get to do a test on Saturday. Keeping my fingers crossed for it to work.
 

Tritium

Amateur Extra Class K5TWM
#13
There is an RPM that a prop comes apart at but I couldn't tell you what it would be for each of the many different props. The DJI factory props have been known to come apart at some very bad times. I have not personally had one fail BUT...............

Coaxial Helicopters with thin blades are the worst. I gave away an Esky Big Lama after the third time the blades exploded about 100' off the ground.

Thurmond
 
#16
Ouch. Yeah that would be bad... Let's hope the DJI props I have don't follow that same pattern.

Good news & bad news : the good news is that with a 4S I easily lift off with the 18A ESCs and the 8x4.5 props.

Bad news : the APM 2.5 seems to have some sort of compass issue - when it's off the ground it starts playing "spinning-top". I still have control over position, just that the yaw is constantly spinning (and yes, I double and triple-checked the prop/motor directions).

Methinks some of the cabling is generating too much of a magnetic field and confusing the compass. Going to change wires from the mono-filament to standard wire. But that's a problem for another thread (if cabling changes don't fix it).
 

Tritium

Amateur Extra Class K5TWM
#17
What software version on APM 2.5? I believe you are in simple mode if spinning and controllable. Check your APM 2.5 by connecting it to your mission planner and check your modes.

Thurmond